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Precise media temperature control

Precision refrigeration and air conditioning are essential for many applications and the demand is constantly increasing. An example is the requirement to maintain a temperature of exactly 20 °C in a measuring room in which machine components are measured with an accuracy of fractions of a millimeter. It is important to rule out even the smallest temperature changes so that the measurement results are not falsified. However, applications in the food industry are also conceivable. In such applications, constant air outlet temperatures from the evaporator are the key to success. This can be achieved very well with an electronic suction throttling or media temperature control.

 

Installation in the suction line

With media temperature control for refrigeration systems with a pipe size of up to 54 mm, an electrically controlled valve (e.g. type "KVS") is installed in the suction line. This valve is equipped with a stepper motor and is controlled with an electronic controller (e.g. "EKC 368"). A fast-reacting temperature sensor (e.g. "AKS 11" or "21") is used as a sensor. This sensor records the actual value of the air outlet temperature of the evaporator and forwards it to the controller. The controller now compares its temperature setpoint with the actual value and decides whether the stepper motor valve should be opened further or closed.

 

 

A completely open valve ensures maximum cooling and thus the lowest possible blow-out temperature in the specific load situation. An almost closed valve means a high discharge temperature. Thus, the controller will command the valve to "open the valve" in situations where the setpoint is well below the actual value. This would be the case, for example, with a setpoint of 0 °C and an actual value of 15 °C. Even with a temperature difference of just 1 K, the controller specifies a small degree of opening in order to prevent the controller from overshooting. With such a system, temperature fluctuations of maximum +/- 0.5 K can be expected.

 

Different transient curves in the starting process

Particular attention should be paid to the start-up process of the controller. The "EKC 368" controller offers three different transient responses to choose from. The first option is to cool down as quickly as possible.

 

 

With this setting, a very significant undershoot in the discharge temperature of the evaporator is accepted in order to enable rapid cooling and thus faster reaching of the setpoint temperature. The second option allows only slight undershoot. The last option is a complete omission of blowdown values ​​below the target value during the start-up phase.

 

 

 

This offers the advantage of very gentle cooling of unpackaged goods, avoiding any unnecessary dehumidification. In this case, however, more time must be allowed for reaching the desired setpoint.

 

service

The controller is operated using two pushbuttons. The controller can be completely programmed using these two buttons, combined with a three-digit display, with all important data being displayed. This means that every fitter on the system can intervene in the control loop or have relevant data displayed. In the controller menu, not only do values ​​that can be set appear, but it is also possible to precisely influence certain processes by intervening in stability and amplification factors.

 

service

The service menu of the electronic accumulation controller is particularly interesting for the fitter during commissioning or service on the system. All parameter values ​​that begin with "u" indicate actual system values ​​that are important for all types of error diagnostics or for evaluating system states. These values ​​provide information about the system status.

 

 

On the one hand, they can be read out quickly and do not have to be laboriously determined with the temperature measuring device. On the other hand, you can see immediately which values ​​the controller assumes as given. For example, it is part of the standard procedure for an experienced fitter to check the control sensor and, if necessary, the defrost sensor in electronic systems before the actual commissioning (with the usual resistance sensors this is quite easy to do with an ohm measuring device. This is how a "PT1000" sensor has at 0 °C a resistance of 1000 ohms) to prevent lengthy troubleshooting if the actual values ​​recorded by the sensor are incorrect. This procedure is no longer necessary when you look at the service menu, because here you can directly assess (if in doubt, measure yourself with the thermometer) whether the value is realistic or not.

 

Steady electronic valves

The "KVS" is a continuous valve that can be used to avoid even the slightest fluctuations in the evaporation pressure. The "EKC 368" can be used as a P, PI or PID controller. The P control is a standard control according to the deviation (Example: If the air outlet temperature is too high, the opening degree of the valve is always increased at the same speed).

 

 

With the PI control, the "reset time" (I component) can be changed individually, which at the same time leads to an adjustment of the reaction speed of the control. In other words: the regulation becomes more nervous or sluggish. Both may be required. The "D component" in PID control also optimizes the control properties in the event of a sudden setpoint change. This control mode is particularly advisable if the system is operated with an external setpoint shift, eg by a higher-level controller.

 

humidity

Refrigeration experts know that the subject of "moisture" plays an important role in refrigeration technology and particularly in the case of unpackaged goods, meat, vegetables and fruit. Due to the constantly high discharge temperature of the "EKC-KVS" system, any undesired dehumidification can be avoided. Nevertheless, there can also be situations in refrigeration systems where dehumidification is required. Indirect measures are usually used for this purpose, e.g. B. changing the evaporator fan stages or the evaporator speed (ie slower fan speed = lower evaporation temperature = dehumidification and vice versa). This point can be influenced directly with an electronic suction throttle system: Simply shift the target value of the air outlet temperature with an external signal from 0 to 10 V and no or hardly any dehumidification is achieved with high evaporation temperature values ​​or high dehumidification with low evaporation. Of course, it also applies here that the dew point must always be undershot for dehumidification. The adjustment of such a system can always be carried out quite easily due to the amount of condensate that is separated out on the evaporator. In addition to storing vegetables and fruit, such a system is also suitable for comfort air conditioning HVAC systems and climate cabinets. The adjustment of such a system can always be carried out quite easily due to the amount of condensate that is separated out on the evaporator. In addition to storing vegetables and fruit, such a system is also suitable for comfort air conditioning HVAC systems and climate cabinets. The adjustment of such a system can always be carried out quite easily due to the amount of condensate that is separated out on the evaporator. In addition to storing vegetables and fruit, such a system is also suitable for comfort air conditioning HVAC systems and climate cabinets.

 

defrost

The "EKC 368" also offers the possibility of defrost control through external initiation via an input contact. After defrosting has been initiated, its standard sequence must be defined by setting the controller accordingly. With hot gas defrosting, the "KVS" valve closes during the defrosting process. After the end of defrosting, the "KVS" valve does not open suddenly - like a standard solenoid valve. This is particularly beneficial in terms of relieving the high pressure (and possibly liquid) in the evaporator. In addition, it is not necessary to install an additional valve in the suction line, since the "KVS" can also be used for this shut-off function.

 

 

With electrical defrosting, the "KVS" is open. A defrost sensor in the evaporator pack ends the defrost as soon as the temperature of the evaporator pack has reached the temperature set on the sensor. As a safety precaution, a maximum defrosting time can be set in the refrigeration controller, which ends defrosting in the event of a defective defrosting sensor or similar malfunction. After defrosting comes the dripping time. Once this has been completed, the cooling process is restarted. To put it simply, the dripping time simply serves to drain off the ice that has turned to water on the evaporator via the condensate drain.

 

Solution for large systems

If larger connection sizes than 54 mm are required for the suction line, it is recommended to use a solution with a master valve. This system consists of an "ICS" main valve, a screwed-on pilot valve "CVQ" and the associated controller "EKC 361" (without defrosting functions - the alternative "EKC 367" is equipped with practically the same defrosting functions as "EKC 368"). Here, too, the main valve with pilot is installed in the suction line. The effect of the control and the control-technical functionality correspond to the "KVS-EKC 368" system. However, the "CVQ" uses a pressure cartridge (actuator) to actuate the valve instead of a stepper motor. A "PTC" heating resistor and an "NTC" resistor are installed in this actuator. The "PTC" can be controlled by the controller with 24 V AC, so that the actuator is heated. The valve is closed or opened by this heating. The "NTC" resistor gives the controller feedback on the degree of opening of the valve via the internal actuator temperature. The actual value of this actuator temperature and the current setpoint can be read out in the controller menu of the "EKC 361" ("367"). This is a good troubleshooting tool. If the setpoint (current actuator reference) is, for example, 100 °C and the actual value is also (set/actual value overlap), the control behavior is OK at first glance. If the current setpoint is 100 °C and the actual value is 50 °C, for example, without increasing further, then there is a fault in the controller or in the actuator. In such a case, it should be checked

 

remote service

In principle, this control system also offers the possibility of remote service. It is possible to equip the controller with a LON module and to record the corresponding data via a master unit or to intervene in the control system from anywhere via a modem connection. This can be done using special software (type “Danfoss AKM”). This means that this media temperature control can be integrated into complex Danfoss control system networks if required.

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4-way reversing valves

 

construction

A 4-way reversing valve has four pipe connections. Three of these connections are placed on one side and one on the opposite side. The three copper pipe connections have a larger diameter than the single one on the opposite side. The middle of the three large connections is permanently on the suction side and the single, small connection is always on the pressure side. Since the other two can be either on the suction or on the pressure side - depending on how it is switched - they are designed in the dimension of the permanent suction line connection to take pressure drops into account. A 4-way valve also has a pilot solenoid valve with a coil, which can be used to change the refrigerant flow direction by energizing it.

 

function

For the following description, let's assume that the small (pressure) port is pointing up and the other three ports are pointing down. Here we see the small pilot solenoid valve with its coil. With a standard 4-way valve there are only two switching positions - no intermediate positions. In switching situation one, there is no voltage at the coil of the pilot solenoid valve.

 

3D view - coil energized

Functional diagram – coil energized

 

As a result, high pressure hot gas from the pilot line of the small port (permanent pressure side) is introduced into the slide mechanism chamber from the right. At the same time, the pressure on the left side of the slide chamber can be relieved via the permanent suction connection by outflow to the low-pressure side. This moves the slider to the left and opens the main paths from top to bottom right and far left to center. In switching situation two, the hot gas finds its way from the top to the left, while at the same time the suction gas can flow downwards from the right to the middle.

 

3D view - coil de-energized

Functional diagram – coil de-energized

 

This is achieved by energizing the coil of the pilot solenoid valve with supply voltage and injecting high pressure into the spool chamber from the left. The pressure on the right side can thus be relieved to the middle, lower main connection, which leads to the spool moving to the right.

 

Pressure drops and sizing

Pressure drops are fundamentally important for valve dimensioning. Excessive pressure drops generally have a negative effect on the energy efficiency of the refrigeration system, while pressure drops that are too small can disrupt the stable operating behavior of a servo-solenoid valve, for example. With 4-way valves, both points are less critical. Minimal pressure drops on the pressure or suction side are not a problem for 4-way reversing valves, since the spool mechanism responsible for switching the valve is actuated by the differential pressure between the high and low pressure side of the refrigeration system. This is useful because high and low pressure are present directly at this type of valve, which is not usually the case with other valves in a classic dry expansion refrigeration system. It is therefore not decisive for the functional reliability of the valve which pressure drop actually occurs, for example on the suction side between the refrigerant inlet and outlet. The issue of "excessively high pressure drops" is also hardly an issue with "Saginomiya" 4-way valves from Danfoss if the dimension of the suction line is used as the primary design criterion. If the 4-way valve is selected based on this pipe size, then in the vast majority of cases a valve with very moderate pressure drops is obtained. Of course, it is safer to consult the relevant performance tables to verify that the valve chosen is large enough. But in most cases this check is not really necessary. on the suction side between refrigerant inlet and outlet. The issue of "excessively high pressure drops" is also hardly an issue with "Saginomiya" 4-way valves from Danfoss if the dimension of the suction line is used as the primary design criterion. If the 4-way valve is selected based on this pipe size, then in the vast majority of cases a valve with very moderate pressure drops is obtained. Of course, it is safer to consult the relevant performance tables to verify that the valve chosen is large enough. But in most cases this check is not really necessary. on the suction side between refrigerant inlet and outlet. The issue of "excessively high pressure drops" is also hardly an issue with "Saginomiya" 4-way valves from Danfoss if the dimension of the suction line is used as the primary design criterion. If the 4-way valve is selected based on this pipe size, then in the vast majority of cases a valve with very moderate pressure drops is obtained. Of course, it is safer to consult the relevant performance tables to verify that the valve chosen is large enough. But in most cases this check is not really necessary. if the dimension of the suction line is used as the primary design criterion. If the 4-way valve is selected based on this pipe size, then in the vast majority of cases a valve with very moderate pressure drops is obtained. Of course, it is safer to consult the relevant performance tables to verify that the valve chosen is large enough. But in most cases this check is not really necessary. if the dimension of the suction line is used as the primary design criterion. If the 4-way valve is selected based on this pipe size, then in the vast majority of cases a valve with very moderate pressure drops is obtained. Of course, it is safer to consult the relevant performance tables to verify that the valve chosen is large enough. But in most cases this check is not really necessary.

 

arrangement

Such a valve is integrated into both the hot gas line and the suction line of a refrigeration system. The two permanent lines – i.e. the pipeline that is always the hot gas line, regardless of the switching of the valve, and the pipeline that is always the suction line – are particularly easy to assign for assembly. The hot gas line coming from the compressor is routed to the small connection of the 4-way valve. The suction line leading to the compressor is placed on the middle of the large connections. These two lines between valve and compressor do not change their function. In this context, it should be noted that the external pressure equalization of an expansion valve is always connected to the permanent suction line, i.e. to the line coming from the middle large connection of the 4-way valve must be connected. If this is not heeded, this external pressure equalization will be subjected to a pressure that is far too high, which initially does not allow the expansion valve to function (the valve is closed with all its might) and may even cause permanent damage to the expansion valve. The two outer large connections remain. These can now be temporarily on the high or low pressure side. The two outer large connections remain. These can now be temporarily on the high or low pressure side. The two outer large connections remain. These can now be temporarily on the high or low pressure side.

 

Assembly

When installing the valve, the following must be observed with the standard soldering process (copper/copper solder joint, can be hard-soldered with, for example, "Silfos 15"): Real copper sockets, such as are usually found in this type of valve, must be used very easy to solder with copper lines. Due to the excellent thermal conductivity of copper and the precisely fitting slide mechanism, it is extremely important to limit the heat input on the valve during the soldering process as much as possible. For this purpose, a cooling, wet cloth should be wrapped around the valve during the soldering process. Once this hurdle has been overcome, the 4-way valve is a component that functions reliably throughout the entire service life of a refrigeration system.

 

use

4-way valves are used for circuit reversal of “one-to-one” refrigeration systems. With this reversal, the evaporator becomes the condenser and the condenser becomes the evaporator. This circuit is often used, for example, in air conditioning split devices that are supposed to cool in summer and heat in the transitional period. When heating, these devices then become air-to-air heat pumps. Another application can be the desire for efficient defrosting. With the circuit reversal of "one-to-one" systems, the evaporator, which has now become the condenser, can be defrosted from the inside. This means that the heat does not have to be carried to the ice in the evaporator by electric heaters in the evaporator package, but rather that hot gas is sent directly through the pipe system where ice has previously accumulated.

 

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Compressors for direct current

 

Comfort is becoming more and more important these days - also in refrigeration technology. This applies in particular to the field of mobile refrigeration technology, from mobile cool boxes with compressor technology to vehicle cabin cooling for trucks. For this reason, today we are dealing with compressors for direct current and mobile use.

 

Compressor with EC motor

There are two main points that are particularly important for compressors in mobile applications: First, the mechanical suitability for use in non-stationary applications such as mobile homes, trucks, boats and coolers. Since the mechanical loads on a compressor - for example in a truck - are much higher than in stationary operation, the compressors must be designed for this (types "BD" = battery driven). For example, the internal suspension and various internal parts of BD compressors are mechanically reinforced so that they are resistant to changing centrifugal forces, road bumps, etc. On the other hand, the direct current supply plays a decisive role. In most mobile applications, the on-board electronics work on a direct current basis.

 

 

Different speeds

Because of this, the electronics of BD compressors differ significantly from stationary compressor starting devices designed for AC. In the case of compressors for direct current, the current direction of the supply voltage - in contrast to alternating current - is always the same and does not change. A rotating field must therefore be generated electronically. In other words: a BD compressor basically requires electronics for operation, so it makes sense to equip them with additional features. This includes the possibility of being able to operate the compressor at other speeds.

The standard speed of a BD compressor is 2000 rpm (an AC compressor rotates about 2900 rpm with one pole pair and 50 Hz). With most BD compressors, however, the speed can easily be increased up to 3500 (in some types even up to 4500 rpm). For this purpose, a resistor is connected between the "C" and "T" terminals of the electronics.

Depending on the resistance value, the desired speed can be defined. The resistor can simply be selected as a standard resistor approximately the size of the resistor value table in the BD brochure. The fact that the compressor then runs at 2511 revolutions per minute instead of 2500, for example, is irrelevant in practice. As the compressor speed increases, so does the refrigeration capacity, since the compressor then passes through more volume – or the volume flow increases.

Due to this fact, it is possible under certain circumstances to use a smaller BD type as a service replacement for a slightly larger compressor by increasing the speed slightly. The 2 cm3 compressor BD 35F with a speed of a good 2500 rpm can be used as a replacement for a BD 50F with 2.5 cm3 at 2000 rpm.

R to ΩEngine speed in rpmControl current in mA

02,0005

2772,5004

6923,0003

15233,5002

 

 

battery protection shutdown

In stationary refrigeration systems that are operated at the socket, an undervoltage protection switch-off is not common and would only make sense in exceptional cases. The situation is completely different with dc compressors. It would be extremely annoying if a luxury class car could no longer start due to a dead battery, just because the BD compressor for the cooling compartment in the rear seat bench drained it completely. To prevent this, the standard BD electronics are equipped with a battery protection shutdown function.

This is the "factory setting" at 9.6 V as the switch-off value and 10.6 V as the switch-on value for on-board voltages of 12 V. Since these values ​​are already quite low, they can be changed with resistors - similar to the procedure for speed preselection. The path of the protective shutdown resistor is between “C” (“common”) and “P” (“protection”). This means that higher switch-off values ​​can also be achieved, ie the battery can be discharged to a lesser extent. In the case of special electronics, such as for solar use, this function is usually removed, since a photovoltaic panel supplies different voltages, including undervoltages, without there being "imminent danger".

resistance kΩ12V shutdown V12V turn on V12V maximum voltage24V shutdown V24V turn on V24V maximum voltage

09.610.917.021.322.731.5

1.69.711.017.021.522.931.5

2.49.911.117.021:823.231.5

3.610.011.317.022.023.431.5

4.710.111.417.022.323.731.5

6.210.211.517.022.523.931.5

8.210.411.717.022.824.231.5

1110.511.817.02324.531.5

1410.611.917.023.324.731.5

1810.812.017.023:625.031.5

2410.912.217.023.825.231.5

3311.012.317.024.125.531.5

4711.112.417.024.325.731.5

8211.312.517.024.626.031.5

 

 

thermostat

With DC compressors, the cooling command can be connected directly to the electronics. For this purpose, a potential-free contact must be connected between contacts "C" and "T" (thermostat). A simple thermostat (fridge thermostat or KP thermostat) should be provided here. As soon as the compressor is to be switched on, the thermostat switches through. When the desired temperature is reached, for example in the cool box, the thermostat contact opens and the compressor stops. There are no special requirements in terms of contact loading for these thermostats, as they are not integrated into the load circuit. In general, however, the ampere values ​​in a car / truck are always quite high, since the voltage used is much lower than in the stationary area and the current must therefore be correspondingly higher.

 

 

diagnostic function

A diagnostic function is integrated into the compressor electronics to facilitate service and commissioning. This function distinguishes between 5 different error diagnoses: "Battery protection shutdown", "Fan overcurrent", "Starting with back pressure", "Minimum speed" and "Electronics overtemperature". "Battery protection shutdown" means that the battery protection voltage has fallen below - or the restart voltage of the battery protection has not yet been reached. This means that the compressor must not be started so that the weakened battery is no longer discharged. "Fan overcurrent" means that the fan connected to terminals "+" and "F" is drawing too much current (more than 1A). A 12 V device must always be selected as the fan - regardless of whether the BD compressor is operated on a 12 or 24 V on-board network. In the case of "Starting with back pressure", the diagnostic function tells us that a back pressure in the refrigeration system of more than 5 bar difference between the suction and pressure side prevents the compressor from starting. Complete pressure equalization in the system is ideal for starting up a DC compressor, but even small pressure differences can be overcome during start-up. In extreme cases, this error code can also mean that there is a mechanical defect in the compressor that prevents it from starting (eg seizures in the crankshaft bearing). This error code will also appear without a measurable differential pressure upstream and downstream of the compressor. Another error code is "Underrun minimum speed". This code is issued when too high a load (e.g too high pressure level on the pressure side) acts on the compressor and pushes the speed to below 1850 rpm. The last possibility of fault diagnosis is "Electronics overtemperature". Especially in mobile applications in very small compartments or in the engine compartment in summer, high temperatures can occur in the electronics unit. To protect this unit, it may be necessary to switch it off if the thermal stress is particularly high. These 5 diagnoses are output as an interval signal. If an LED is connected to the BD electronics, the error code can be read directly from the flashing signal. Error code 3 (starting with back pressure) is indicated by three flashes, then a pause, three more flashes, etc. If no LED is connected, the interval impulses between the contacts "+" and "D" (diode) can be measured using a multifunction measuring device (position >24 V dc). Then, for example, two short pulses - pause - two short pulses - pause etc. would be error code 2, i.e. "fan overcurrent".

Number of light pulseserror type

5Overtemperature of the electronics unit (if the cooling system is overloaded or the ambient temperature is too high, the electronics become too hot)

4Too low engine speed (If the cooling system is too heavily loaded, the minimum engine speed of 1,850 rpm cannot be maintained)

3Motor does not start (The rotor is stuck or the differential pressure in the cooling system is too high (>5 bar))

2Fan overcurrent (the fan loads the electronics unit with more than 1 ampere)

1Supply voltage (The supply voltage was outside the set range).

 

 

Compressor cooling and assembly

BD compressors are basically single-cylinder machines with a vertically arranged crankshaft. They are normally supplied with sufficient oil so that an oil additive is generally not necessary. Due to the additional heat radiation, the compressors have oil centrifugal cooling. Static cooling by the ambient air is usually sufficient for the small power sizes, cooling by forced ventilation is necessary for the larger compressors or high evaporation temperatures. For this reason, these compressors should not be equipped with a sound insulation hood. A look at the respective data sheet provides information as to whether static cooling ("S") is sufficient or whether forced ventilation ("F" fan = fan) must be used. The soldered connections of these compressors are sealed with capsolutes in order to avoid unnecessary entry of foreign particles or moisture into the compressor. To remove these capsolutes, there are special capsolute lifters that should be used to open all closures. This means that even if the process socket is not required, it must be freed from the capsolute and soldered shut.

The compressors are mounted on rubber buffers. These mounting rubbers should always be used, as the buffering, together with the internal suspension springs of the inner compressor block, absorbs the forces when the compressor starts and stops, centrifugal forces and vibrations.

 

 

Comparison of technologies

As an alternative to compression refrigeration systems for mobile direct current operation, two other technologies are mainly widespread: cooling using a Peltier element and absorption refrigerators. Peltier elements are attractively priced, but offer only very low cooling capacities compared to BD compressor systems. They are often installed in simple and smaller coolers for private use. Their use is also limited with regard to the maximum differential temperature. An evaporation temperature in the minus range at an outside temperature of 30°C is not feasible with a Peltier element - but with BD technology it is. In terms of energy, the compressor technology in the direct current range is significantly more efficient - not least because of the EC motor technology. As a rule of thumb, only about 1/3 of the power consumption can be assumed here, which a corresponding Peltier element would consume. In terms of energy, the absorption refrigerator is on a similar level to the Peltier element. It also consumes three times more electricity than compressor cooling. Absorption refrigerators are available as minibars in hotels and - as a mobile application - often in mobile homes. The absorption refrigerators in campers are usually operated using a heat source (usually propane burners).

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Infinitely variable compressor capacity control

Infinitely variable speed control using a combination of compressor and frequency converter is the trend. In order to obtain an efficient system, it has been shown that the compressor must be specially adapted to speed control. This is not surprising, since there are special challenges in terms of oil management, electrical and mechanical stresses. But the frequency converter should also be matched to the refrigerant compressor. For example, starting a compressor under back pressure is a difficult task for a standard frequency converter, since full motor torque is required immediately. This requirement differs from standard frequency converter applications such as pump or fan speed control.

 

 

Optimal partial load capacity

Compressors in refrigeration technology are normally only designed for the maximum system load. In fact, however, the systems run at part load for 65% of their operating time, so that the compressor is oversized for long periods of time. Conventional controls that compensate for this excess compressor output are on/off controls, pressure-controlled output controllers or hot-gas bypass controllers. Compared to these methods, a compressor frequency converter package offers superior control performance and is the more energy-efficient solution. The cooling capacity of a conventional hermetic reciprocating compressor is constant, the motor and crankshaft rotate at 2900 revolutions per minute (50 Hz, one pole pair). With a "VTZ Compressor Drive", on the other hand, the speed can be varied in a frequency range from 30 to 90 Hz. Depending on the necessary cooling load, this results in an engine speed of between 1800 and 5400 rpm. Therefore, the compressor is always correctly dimensioned with regard to the cooling requirement. The “VTZ” thus offers the same minimum partial load as a 3-compressor combination. However, the control gradation is not 33, 66 and 100%, but stepless.

 

 

 

Reduced starting currents

built soft starter, which significantly reduces the current peak during the compressor start compared to the direct start. The frequency converter starts with a very low frequency when the compressor starts and adapts it to the actual rotational speed of the rotor. In the case of a direct start of a compressor, on the other hand, 50 Hz is applied directly, even if the rotor has not yet started moving at all. This leads to starting current peaks that do not occur in this form in frequency converter operation. No extra motor protection switch is required to monitor the operating currents and the symmetrical current consumption. This monitoring function is also performed by the "Type CD" (Compressor Drive) frequency converter. A load contactor for the compressor is also superfluous due to the "CD".

 

 

Special frequency converter and compressor

It is not possible to control a one- or two-cylinder compressor for every frequency converter, as different torques occur when the crankshaft rotates 360°. This phenomenon only shows up with compressor control, not with pump or fan speed control, for example, and can therefore lead to malfunctions of an unsuitable frequency converter. In addition, an incorrect parameterization of the acceleration ramp for the operating frequency could result in a compressor failure after a very short time. In particular, ramps that are set too long can have devastating consequences for the build-up of lubrication in the compressor. The correct ramp is around 0.6 seconds. In order not to burden the commissioning technician with these details, the "CD" frequency converter is designed for operation with one and two-cylinder compressors, as well as pre-adjusted to the correct ramp. The "VTZ" size can be selected directly via the "CD" settings menu. The converter thus automatically knows how many cylinders the compressor has and it also knows the correct ramp. The "VTZ-CD" package offers a "VTZ" compressor specially designed for speed control, which harmonizes perfectly with the "CD" frequency converter, which has been optimized for refrigeration technology.

 

 

cabling

When using a frequency converter, a number of things must be observed, particularly with regard to cable routing and shielding.

 

The motor cable (from the converter to the compressor) must always be routed separately from the control and bus cables and even from the mains cable. In addition, the motor cable in particular should be kept as short as possible. If this is not observed, EMC problems will quickly arise, which manifest themselves, for example, in the infl uence of control electronics and bus lines. A shielded sheathed cable must be used as the motor cable, the shield of which must be connected on both sides – i.e. both on the frequency converter and on the compressor. Specifically, the sheathed cable should be stripped all around and a metal clamp should be used for shielding. The motor cable must be laid at a minimum distance of 200 mm from the control or bus lines and the mains cable. Also, the power cord must not be routed directly together with the non-load cords. Particular care should also be taken with the motor cable if this is to be de-energized. Due to the fact that the power electronics of a frequency converter contains high-performance capacitors, dangerous voltages can still occur in the motor cable for a certain time after the mains has been switched off.

 

 

Regulation by means of a pressure sensor

Together with a pressure sensor, the package works in a similar way to a compound controller.

 

The frequency converter receives a pressure setpoint that it tries to keep constant. If the pressure value increases, the compressor speed is increased. If the actual pressure value falls, the speed is reduced. With this control, a very constant suction pressure can be achieved. Pressure transducers with standard signals of 4 to 20 mA (current signal) or 0 to 10 V (voltage signal) can be used, as well as special versions with voltage signals of 1 to 5 V. For converting a factory-set pressure transducer with voltage signal to current signal there is a DIP switch on the frequency converter.

 

 

 

Setpoint from an external controller

If a target value for the speed of the compressor is already supplied via an external controller, the "CD" can also be operated with it. With this variant, the pressure transmitter is omitted. It is replaced by the input signal from the external controller. A standard voltage signal of 0 to 10 V or a 4 to 20 mA current signal can also be used here. This control can be a PLC (programmable logic controller) or a compound controller with an output for a speed-controlled compressor. In terms of control technology, operation with a compound controller can produce very elegant control results. For example, a group of three with two direct start compressors and a "VTZ-CD" would initially start with the "VTZ" at low output. When the "VTZ" is running at full speed, the compound controller switches on another compressor and drives the "VTZ" back down to a minimum speed of 30 Hz. If the suction pressure continues to rise, the "VTZ" increases the speed again until finally the last compressor is switched on and the "VTZ" drives back again. This variant brings a similarly stable suction pressure result as a large speed-controlled compressor in stand-alone operation. In the case of a compound connection of a speed-controlled compressor with direct start compressors, attention must be paid to the oil management. In total, no more than three compressors should be connected in parallel. Oil level regulators are also essential in such a compound configuration. If the suction pressure continues to rise, the "VTZ" increases the speed again until finally the last compressor is switched on and the "VTZ" drives back again. This variant brings a similarly stable suction pressure result as a large speed-controlled compressor in stand-alone operation. In the case of a compound connection of a speed-controlled compressor with direct start compressors, attention must be paid to the oil management. In total, no more than three compressors should be connected in parallel. Oil level regulators are also essential in such a compound configuration. If the suction pressure continues to rise, the "VTZ" increases the speed again until finally the last compressor is switched on and the "VTZ" drives back again. This variant brings a similarly stable suction pressure result as a large speed-controlled compressor in stand-alone operation. In the case of a compound connection of a speed-controlled compressor with direct start compressors, attention must be paid to the oil management. In total, no more than three compressors should be connected in parallel. Oil level regulators are also essential in such a compound configuration. In the case of a compound connection of a speed-controlled compressor with direct start compressors, attention must be paid to the oil management. In total, no more than three compressors should be connected in parallel. Oil level regulators are also essential in such a compound configuration. In the case of a compound connection of a speed-controlled compressor with direct start compressors, attention must be paid to the oil management. In total, no more than three compressors should be connected in parallel. Oil level regulators are also essential in such a compound configuration.

 

 

 

Installation

The commissioning of a "VTZ-CD" package does not take much time. The package solution is already preset at the factory so that the system can be put into operation with just a few settings. Difficult settings, such as the start ramp or important motor characteristics, cannot be easily changed and are already 100% stored in the frequency converter under the corresponding "VTZ" size.

S

 

A control unit with a graphic display, which is normally located on the front of the converter and can be removed if necessary, is used to adjust the setting parameters. The operator display can also be used to collectively save settings from one “CD” and transfer them to another “CD”.

 

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Structure – pressure-guided water valves

Water valves - pressure and temperature controlled

 

Cooling water regulators have two connections for the water circuit and one for the cooling circuit. The two water connections – at least those of the smaller devices with a pipe connection of up to 1 ½” – are designed as G inch internal threads, as has long been the standard in water technology. With connection sizes of 2, 3 or 4 inches, flange connections are standard, which are butt welded to the pipe side. The connection for the high-pressure side of the refrigeration system is designed as a 7/16 UNF flared nipple. This can be connected to the refrigeration system via a 6 mm copper pipe or a pre-assembled capillary pipe with the appropriate union nuts. With type "WVFX" up to size 25, there is a gray adjustment knob opposite the refrigerant side for adjusting the nominal condensing pressure. If this knob is turned in the "+" direction (counterclockwise), the setpoint and thus the desired condensation pressure (or the condensation temperature, respectively, because in the wet steam area of ​​the condenser the pressure and temperature are constant to one another) increases. For example, if the condensation temperature with the refrigerant R404A is to be raised from 35 to 40 °C, the adjusting knob must be turned in the “+” direction, i.e. anti-clockwise. In contrast, turning in the "-" direction causes the condensation pressure or temperature to drop. For example, if the condensation temperature with the refrigerant R404A is to be raised from 35 to 40 °C, the adjusting knob must be turned in the “+” direction, i.e. anti-clockwise. In contrast, turning in the "-" direction causes the condensation pressure or temperature to drop. For example, if the condensation temperature with the refrigerant R404A is to be raised from 35 to 40 °C, the adjusting knob must be turned in the “+” direction, i.e. anti-clockwise. In contrast, turning in the "-" direction causes the condensation pressure or temperature to drop.

 

 

Structure - temperature-controlled water valves

The effect and setting of temperature-controlled water valves are exactly the same as their pressure-controlled relatives. The main difference is that a temperature is actually recorded as the actual value. For this purpose there is a remote sensor on temperature-controlled cooling water controllers (e.g. type "AVTA") that measures the current temperature. This means that even with an "ATVA" valve, it must be turned in the "+" direction (counterclockwise) in order to increase the setpoint, for example from 35 to 40 °C.

“WVFX” – “AVTA” setting

 

 

probe placement

Measuring the condensing pressure with a temperature-controlled device is not easy. While the outlet point of the control line for the refrigerant connection can be selected anywhere on the pressure side of the refrigeration system (hot gas or liquid side) with the "WVFX" pressure version, this is not possible with the "ATVA" temperature version. The value measured when the remote sensor is connected to the hot gas line does not correspond to the condensation temperature, but is significantly higher. This is because the refrigerant here is already fully gaseous and superheated. This point is therefore not suitable for attaching the "AVTA" sensor. The situation is similar with the liquid line, since the liquid refrigerant is already subcooled there. Thus the actual temperature is lower than the value which can be read on the high-pressure gauge as a pressure-temperature correspondence. The condenser outlet is more suitable, but the sensor should be installed in front of the collector. Apart from the problems described, there must also be good heat transfer from the refrigeration pipe to the sensor. Particular attention should be paid to this point if the sensor is mounted on a stainless steel pipe. Stainless steel is a poor conductor of heat. It is therefore possible that the current temperature is forwarded to the sensor and the "AVTA" cooling water controller with a slight delay. This leads to a more sluggish control characteristic and thus often to fluctuations in the condensing pressure. Attaching the sensor to the cooling water line itself is not normally advisable, because in general the sensor should always feel the medium to be cooled. If an "AVTA" is actually used to keep the cooling water temperature constant, then the following must be observed: In order to enable the valve to open again after it has closed, a bypass should be installed via the cooling water regulator. Otherwise there is a risk that the temperature-controlled cooling water controller will no longer open, since the temperature at the sensor point is low (no need to open) and warm water can no longer flow. a bypass should be installed via the cooling water regulator. Otherwise there is a risk that the temperature-controlled cooling water controller will no longer open, since the temperature at the sensor point is low (no need to open) and warm water can no longer flow. a bypass should be installed via the cooling water regulator. Otherwise there is a risk that the temperature-controlled cooling water controller will no longer open, since the temperature at the sensor point is low (no need to open) and warm water can no longer flow.

"AVTA" - temperature-controlled cooling water controller

 

 

temperature range

The control range is also decisive for the right choice of a temperature-controlled cooling water controller. There are "AVTA" and "WVTS" devices with a control range of 0 to 30 °C, 25 to 65 °C and 50 to 90 °C. At "AVTA" there is another special range with 10 to 80 °C. The "AVTA" / "WVTS" versions for ranges from 25 to 65 °C are therefore recommended for standard refrigeration systems with condensation temperatures between 30 and 55 °C. Theoretically, 10 to 80 °C is also conceivable, since the normal liquefaction range is also covered here. In practice, however, preference should be given to the 25/65 variant in this case, since this controller has a better control resolution due to its smaller temperature range. In the case of service, i.e. when replacing a temperature controller whose sensor is located in an immersion sleeve, the diameter of the sensors must also be taken into account. There are versions with a diameter of 9.5 and 18 mm. The explanations show that the pressure-controlled cooling water controller is generally more suitable for use in compression refrigeration systems than the temperature-controlled version. The latter is more recommended for special applications such as special refrigerants with high pressure.

Location of the valve in the system

 

 

Arrangement of the cooling water regulator

A frequently asked question relates to the placement of the chilled water regulator. It can be installed on the water side both before and after the condenser. In water systems, a coarse dirt filter should always be installed in front of the cooling water regulator. This serves to filter out larger foreign particles that are in the water system. Coarse dirt filters must be serviced regularly, with the maintenance intervals depending on the degree of contamination of the cooling water. As a rough guideline, six-monthly maintenance can be assumed. When using treated river water, as is often the case in large companies near a river, an even higher frequency of maintenance may be required. For the use of more aggressive media that would attack the housing of the standard cooling water regulator, there are also special versions of "WV-FX 10 - 25" in stainless steel. The addition of anti-freeze or the use of brine may be necessary during standstill phases in winter operation to prevent the vehicle from freezing

  layout diagram

 

 

pressure drops

When dimensioning temperature and pressure-controlled cooling water controllers, the water volume flow and the performance of the cooling water controller play a role. This results in a certain pressure drop, which must always be taken into account when designing valves. With directly controlled cooling water controllers such as the pressure-controlled "WVFX" or the temperature-controlled "AVTA", the main focus is on avoiding excessive pressure drops, since both valve series can work stably even with the smallest pressure drops. In the case of servo-controlled cooling water controllers such as "WVS" (pressure) and "WVTS" (temperature), the minimum and maximum pressure drops must be observed. A "WV(T)S" requires a minimum water pressure drop of 0.3 bar in order to be able to work stably. If this value is not reached during the design, a smaller power rating must be selected. Otherwise the valve falls into unstable control mode. In pump operation, one should not allow excessive pressure drops. The reference value is a pressure drop well below the 1 bar mark.

Sectional drawing "WVFX" (pressure-guided)

 

The use of a line control valve is recommended to prevent the pump from working against a closed 2-way valve (the cooling water regulator). In such a case (valve closed), the balancing valve allows the water to flow back to the pump via a bypass. This ensures a continuous flow of water and at the same time prevents damage to the pump. If city water is actually present in front of the valve (e.g. with 4 bar water pressure) and then runs out freely, the valve can certainly be designed smaller. In practice, the pressure drop across the valve will always be 4 bar.

Pressure-guided water valve "WVFX" in high-pressure version

 

 

refrigerant suitability

In general, pressure-controlled cooling water controllers are suitable for all common HFC and HCFC refrigerants, as long as the control range at the permissible operating pressure harmonizes with the design criteria of the entire system. For example, a "WVFX 15" has a permissible operating pressure of 26.4 bar for the refrigerant connection and is therefore suitable for a system with the refrigerant R407C and a maximum operating pressure of 25 bar. With a set value of 40 °C (dew point temperature) on the high-pressure side of the refrigeration system, this corresponds to a gauge pressure of approx. 14 bar. "WVFX 15" is available with two different control ranges: 3.5 to 16 bar overpressure and 4 to 23 bar overpressure (gauge pressure). In the case described, the variant with 3.5 to 16 bar overpressure is suitable. In the same plant operated with 50 °C condensation, it would be the variant with 4 to 23 bar overpressure, since 50 °C condensing dew point temperature corresponds to about 19 bar operating pressure. However, the latter is more of a theoretical example, as systems with water-cooled condensers usually have lower condensation temperatures than air-cooled systems. For water-cooled systems, condensation temperatures between 30 and 40 °C are standard, even in summer. In air-cooled systems, this value is often 10 K higher. For particularly high pressures, there are also special versions of the "WVFX 10 - 25" with maximum permissible operating pressures of up to 45.2 bar and a working range of 15 to 29 bar overpressure. These devices are ideal for the refrigerants R410A or R744 (CO2) in subcritical operation. it would be the variant with 4 to 23 bar overpressure, since a condensation dew point temperature of 50 °C corresponds to around 19 bar operating pressure. However, the latter is more of a theoretical example, as systems with water-cooled condensers usually have lower condensation temperatures than air-cooled systems. For water-cooled systems, condensation temperatures between 30 and 40 °C are standard, even in summer. In air-cooled systems, this value is often 10 K higher. For particularly high pressures, there are also special versions of the "WVFX 10 - 25" with maximum permissible operating pressures of up to 45.2 bar and a working range of 15 to 29 bar overpressure. These devices are ideal for the refrigerants R410A or R744 (CO2) in subcritical operation. it would be the variant with 4 to 23 bar overpressure, since a condensation dew point temperature of 50 °C corresponds to around 19 bar operating pressure. However, the latter is more of a theoretical example, as systems with water-cooled condensers usually have lower condensation temperatures than air-cooled systems. For water-cooled systems, condensation temperatures between 30 and 40 °C are standard, even in summer. In air-cooled systems, this value is often 10 K higher. For particularly high pressures, there are also special versions of the "WVFX 10 - 25" with maximum permissible operating pressures of up to 45.2 bar and a working range of 15 to 29 bar overpressure. These devices are ideal for the refrigerants R410A or R744 (CO2) in subcritical operation.

Typecondensing sideliquid sidek value in m³/h

refrigerantAdjustment range of the closing pressure barPermitted working pressure PB barMax. test pressure p' barmediumPermitted working pressure PB barMax. test pressure p' bar

WVFM 10CFC, HCFC, HFC3.5 / 10.015.016.5Fresh water, neutral brine, sea water10102.4

WVFM 163.5 / 10.015.016.510102.4

WVFM 103.5 / 16.026.42916241.4

WVFM 104.0 / 23.026.42916241.4

WVFM 153.5 / 16.026.42916241.9

WVFM 154.0 / 23.026.42916241.9

WVFM 203.5 / 16.026.42916243.4

WVFM 204.0 / 23.026.42916243.4

WVFM 253.5 / 16.026.42916245.5

WVFM 254.0 / 23.026.42916245.5

WVFM 324.0 / 17.024.126.5101011.0

WVFM 404.0 / 17.026.426.5101011.0

WVS 32CFC, HCFC, HFC, R717 ( NH3 )2.2 / 19.026.429.0Fresh water, neutral brine101612.5

WVS 402.2 / 19.026.429.0101021.0

WVS 502.2 / 19.026.429.0101632.0

WVS 652.2 / 10.026.429.0101645.0

WVS 802.2 / 19.026.429.0101680.0

WVS 1002.2 / 19.026.429.01016125.0

Condenser Fan Speed ​​Controller “XGE” – “RGE”

 

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Thermostatic expansion valves

An alternative to thermostatic expansion valves are capillary tubes, although these are mainly used in refrigerator systems and simple, small refrigeration systems. The most important advantage of capillaries is the cost factor. The disadvantage is the inflexible regulation (or no regulation) of the injection into the evaporator.

Another alternative to thermostatic devices is the automatic expansion valve, which, however, hardly plays a role in today's refrigeration system construction. This is mainly because these valves aim to keep the evaporating pressure constant under all circumstances, even in the case of liquid return to the compressor.

Three-part expansion valve

 

 

functionality

Thermostatic expansion valves are designed in such a way that sufficient overheating at the evaporator outlet is guaranteed so that the compressor is not damaged. The overheating is the difference between the measured temperature value at the evaporator outlet and the wet vapor temperature of the pressure read on the pressure gauge (usually a corresponding temperature template is already placed over the pressure scale of the service pressure gauge to facilitate orientation, which varies from refrigerant to refrigerant). At the same time, this ensures that the entire exchange surface of the evaporator is used.

In this context, the refrigeration specialist should always keep an eye on the important role of the aggregate state change in compression refrigeration systems: in the evaporator, which absorbs the heat and in the condenser, which gives off the heat again (outside the room to be cooled). In this way, a significantly smaller volume flow can be used in the heat exchangers than would be possible with our usual refrigerants due to the heat transport through sensible temperature increase (measurable temperature increase). In addition, with this latent (hidden) heat absorption, we also benefit from the fact that there is no temperature change in the wet vapor zone (in the phase change) with single-component refrigerants and azeotropic refrigerant mixtures. This fact is very convenient in terms of heat transfer calculations.

 

But how does such a thermostatic expansion valve work?

an automatic mechanical controller without external energy. This means that it does not need an electrical connection or higher, externally generated pressure to function. In this context, it should be mentioned that there are also a large number of electronic expansion valves that work on the principle of overheating control. A separate article will be dedicated to this product family at a future point in this series in the KKA.

Expansion valve with external pressure equalization and evaporator

 

 

The components

The thermostatic expansion valve consists of a thermostatic element with a sensor, a sensor filling, a membrane, a connection by means of a tappet between the membrane and the valve seat and a housing, which usually offers the option of using power nozzles of different sizes. Coming from the receiver, liquid refrigerant from the liquid line is present at the expansion valve. This liquid is now controlled by the expansion valve and injected into the evaporator via the injection line and, if necessary, a refrigerant distributor. The sensor is attached to the evaporator outlet and it must be ensured that the evaporating pressure is present under the diaphragm in the expansion valve. If thermostatic expansion valves with internal pressure equalization are used – mostly in the case of very low refrigeration capacities and systems without a distributor in the injection line – the evaporation pressure, more precisely the pressure in the injection line, is automatically fed into the valve under the membrane. Most expansion valves, however, are valves with external pressure equalization due to their design. The evaporation pressure must be taken from the evaporator outlet and fed to the valve via a small branch line. A 6-tube (copper tube with an outside diameter of 6 mm) should be used for this and not a capillary tube. Valves with external pressure equalization are to be considered standard as they take into account the pressure drop across a refrigerant manifold (eg venturi manifold). the evaporation pressure, more precisely the pressure in the injection line, is automatically fed into the valve under the diaphragm. Most expansion valves, however, are valves with external pressure equalization due to their design. The evaporation pressure must be taken from the evaporator outlet and fed to the valve via a small branch line. A 6-tube (copper tube with an outside diameter of 6 mm) should be used for this and not a capillary tube. Valves with external pressure equalization are to be considered standard as they take into account the pressure drop across a refrigerant manifold (eg venturi manifold). the evaporation pressure, more precisely the pressure in the injection line, is automatically fed into the valve under the diaphragm. Most expansion valves, however, are valves with external pressure equalization due to their design. The evaporation pressure must be taken from the evaporator outlet and fed to the valve via a small branch line. A 6-tube (copper tube with an outside diameter of 6 mm) should be used for this and not a capillary tube. Valves with external pressure equalization are to be considered standard as they take into account the pressure drop across a refrigerant manifold (eg venturi manifold). The evaporation pressure must be taken from the evaporator outlet and fed to the valve via a small branch line. A 6-tube (copper tube with an outside diameter of 6 mm) should be used for this and not a capillary tube. Valves with external pressure equalization are to be considered standard as they take into account the pressure drop across a refrigerant manifold (eg venturi manifold). The evaporation pressure must be taken from the evaporator outlet and fed to the valve via a small branch line. A 6-tube (copper tube with an outside diameter of 6 mm) should be used for this and not a capillary tube. Valves with external pressure equalization are to be considered standard as they take into account the pressure drop across a refrigerant manifold (eg venturi manifold).

 

 

the membrane

Expansion valve on the evaporator with venturi distributor

 

For the following theoretical considerations, we assume parallel filling of the upper part of the expansion valve. Parallel filling means that there is the same refrigerant as a sensor filling in the expansion valve as in the system. In fact, parallel fillings are only used today for small series of special valves. In large series valves, refrigerant mixtures are used almost exclusively. We also assume that there is sufficient sensor charge in the expansion valve, so that we are always in the wet vapor range of the refrigerant in the sensor charge and never get into the overheated area. Various forces now act on the membrane in the expansion valve: from above, the pressure of the sensor charge and from below the evaporation pressure together with the superheat adjustment spring. If the diaphragm moves downwards, the valve will open and in the opposite direction, the expansion valve will tend to close. Let us now assume that the forces acting from above and below are in equilibrium. If the overheating at the evaporator outlet increases, the pressure and thus the force in the sensor, i.e. from above on the membrane, increases. The result is a downward movement of the diaphragm. As a result, more refrigerant is injected into the evaporator, superheat is reduced and the valve closes. At the same time, the overheating is always constantly controlled. If the overheating at the evaporator outlet increases, the pressure and thus the force in the sensor, i.e. from above on the membrane, increases. The result is a downward movement of the diaphragm. As a result, more refrigerant is injected into the evaporator, superheat is reduced and the valve closes. At the same time, the overheating is always constantly controlled. If the overheating at the evaporator outlet increases, the pressure and thus the force in the sensor, i.e. from above on the membrane, increases. The result is a downward movement of the diaphragm. As a result, more refrigerant is injected into the evaporator, superheat is reduced and the valve closes. At the same time, the overheating is always constantly controlled.

 

 

superheat setting

If you want to increase the superheat, turn the adjusting screw clockwise (this is the case with practically all expansion valves). With smaller compact valves, such as the "T2" or "TU", a fairly large change in superheat can be achieved with just a 360° turn.

 

P1 = bulb pressure P2 = evaporating pressure P3 = spring pressure

 

This can be up to 4 K per revolution, but varies depending on the evaporating temperature range and whether a MOP valve is used. With larger, three-piece valves, the change in superheat per revolution is significantly smaller. The rule of thumb is: 0.5 K per 360°. If in doubt, we recommend reading the relevant setting instructions. If we now start from our point of inertia with equilibrium of forces, the stronger tension of the spring changes the force that acts on the membrane from below. The valve moves towards the closed position. As a result, less refrigerant is injected into the evaporator, which leads to an increase in superheat. If the additional spring force is now equalized by the additional sensor force - due to the higher overheating -

MOP filling

 

 

MOP expansion valves

 

Standard filling (universal filling)

 

If an MOP valve is used instead of a standard expansion valve (MOP = maximum operating pressure or maximum working pressure), the valve aims to avoid excessive evaporation temperatures. Such MOP valves are particularly popular in deep-freeze systems, since deep-freeze compressors have a smaller electric motor than normal refrigeration or air-conditioning compressors and are protected against overload with this measure. The only technical change with a MOP valve compared to a "standard valve" is the limited sensor charge. If the complete sensor filling has evaporated at a certain temperature, the sensor pressure practically no longer increases (or only increases negligibly). This means that the force acting on the expansion valve diaphragm from above

 

Make superheat setting changes in small increments and never in haste

 

This results in an upper limit for the evaporation pressure. For example, at a room temperature of +10 °C, a MOP valve with an MOP point of -10 °C will continue to vaporize at -10 °C, whereas a "standard valve" would already vaporize at 0 °C. It is important for the practitioner to know that higher overheating can occur, especially with MOP valves, without there being a malfunction.

 

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High pressure float control valves in industrial refrigeration

 

 

 

This description is based on high-pressure floats from TH-Witt.

The high-pressure float regulator relieves all refrigerant on the high-pressure side on the low-pressure side, but without letting gas through. This simple mechanical method enables extremely energy-saving operation without electrical control.

The control on the high pressure side fulfills the task of throttling within the system by draining condensate. It is therefore particularly suitable for systems with central separators. Due to the purely mechanical mode of operation, condensate drainage is guaranteed at all times without additional control effort.

 

 

 

In contrast to the low-pressure float control, the fluctuating amount of refrigerant is in the central separator.

 

Principle of a single-stage system                 

Liquid refrigerant accumulating in the condenser reaches the float and is there expanded to the low-pressure side with constant enthalpy. As a result of the expansion in the outlet of the high-pressure float regulator, there is a liquid / gas mixture behind the high-pressure float regulator, which flows to the separator.

From the separator, the gas can be fed back to the compressor or the liquid can be fed to the evaporator.

The condensate temperature can be optimally adapted to the external conditions, which ensures a very energy-saving mode of operation. Undercooling of the liquid is usually impossible.

 

Principle of a two-stage system

Here, too, a high-pressure float regulator is mounted between the condenser and the separator, which relaxes the condensate to mean pressure. A second regulator is used to expand the refrigerant to the low pressure side. Two-stage refrigeration systems with high-pressure float control have improved efficiency and avoid high final temperatures of compression

Since the high-pressure float regulator installed between the MD and LP side diverts the refrigerant from the MD container up to its tapping point on the LP side, the LP container is designed in such a way that the entire fluctuating amount of refrigerant can be absorbed (LP side and excess of the MD side).

 

FLOAT REGULATION principle

The condensate entering the high-pressure float regulator housing causes the float to be lifted. A slider is operated via a lever transmission, which releases a corresponding part of the throttle opening and drains the condensate to the separator. Because the float has to overcome frictional forces, the opening is gradually adjusted.

When the liquid level in the float falls, the slide is moved over the opening and thus closes the outlet. When the float ball reaches the bottom, the lapped surfaces of the slide and outlet ensure a tight seal. The buoyancy of the float depends on the diameter and weight of the ball and the density of the liquid to be drained off.

 

FUNCTION OF THE VACUUM NOZZLE

In order for the condensate to flow independently to the controller, it would actually be necessary to arrange the controller below the condenser. In order to enable an arrangement above the condenser, all high pressure float regulators are equipped with an internal vacuum nozzle (exception from TH-Witt: Type: HR1 BW). This connects the gas space of the housing with the outlet nozzle. Due to the pressure difference between the high-pressure and low-pressure sides, gas is sucked in from the housing to the low-pressure side and a slight negative pressure is created in the housing. This means that a height difference of up to 3 m and a vertical distance of up to 30 m to the condenser can be achieved.

In addition, it is ensured that even a small amount of flash gas, which forms in the feed lines or during a system shutdown, can be discharged via the vacuum nozzle.

When the system is at a standstill, the pressure is slowly equalized so that the entire refrigerant charge can move to the coldest point. (In winter this can be the condenser.) The factory dimensioning of the vacuum nozzle is designed in such a way that the power loss theoretically determined by the gas bypass remains in the range below 1% of the nominal power.

 

FUNCTIONAL CONTROL

All regulators have an externally operated lever that enables the float ball to be lifted. This allows the controller to be opened deliberately in order to check its function.

 

VENTILATION

Air or other non-condensable gases can have a very damaging effect on the entire system and especially the high-pressure float regulator. Almost all problems can be traced back to it. Good ventilation is therefore important. Use a water tank and attach a hose to the vent valve EE3 / EE6. After the vessel has been filled with water, the setting valve EE3 / EE6 can be opened carefully. When no more air bubbles emerge, venting can be stopped.

 

FAILURE ANALYSIS: No.Appearance Causes and remedies 1 Controller does not open in automatic mode
  1. Is the controller too small?
  2. Inlet / outlet valve closed?
  3. Too great a pressure difference?
  4. Vacuum nozzle too small or blocked?
  5. Moisture in the system, vacuum nozzle frozen over?
  6. Air in the system
  7. Slide control blocked, for example, by deposits or corrosion
  8. Float ball defective
2 Controller does not close
  1. wrong float ball (replace if necessary)
  2. Transport lock of the lever (toggle or hole in the lever must point downwards)
  3. Worn slide control (replace if necessary)
  4. Opening of the vacuum nozzle too large (or not closed when connecting a solenoid valve line)
3 Condensing pressure too high without back pressure
  1. lack of heat dissipation at the condenser
  2. Condenser dimensioned too small
  3. Too great a cooling capacity in start-up
4th Too high condensing pressure due to back pressure
  1. Air in the system
  2. Formation of steam in the supply line
  3. Insufficient oil cooler function
  4. Too great a resistance in the supply line
  5. Too great a difference in height in front of the regulator (if necessary, enlarge the vacuum nozzle) S
5 Strongly fluctuating pressure on the LP side
  1. Insufficient refrigerant charge
  2. high frictional forces on the slide control (check internal parts for deposits or corrosion)
  3. Controller was oversized
6th Minimum level alarm on the low pressure side
  1. see point 4
  2. Condenser filled with # refrigerant in winter (shut off individual or all condensers)
  3. Insufficient refrigerant charge

 

 

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Differential pressure regulator

In addition to the receiver pressure control, there is another application for the control of the differential pressure. This applies to systems with hot gas defrosting, since the hot gas has to flow through the evaporator in the direction of the liquid line when defrosting. It should be possible to switch off this artificial differential pressure in operating phases without defrosting. In this case, it is advisable to use an "ICS" (or "PM") main valve with differential pressure pilot attachment "CVPP", as already mentioned. In order to switch off the differential pressure control, which is not required in collector control mode, an additional pilot is required: an EVM solenoid valve for direct mounting on "ICS" (or "PM").

If the two pilot valves "CVPP" (do not forget the branch line to the main line in the flow direction after the valve) and "EVM" are mounted in parallel on an "ICS" for a maximum of three pilot valves, the differential pressure function of the "CVPP" is active when the "EVM" is closed . When the "EVM" is open, there is no (increased) differential pressure. To control a main valve "ICS" with "CVPP" and "EVM", the solenoid valve "EVM" must be closed also normally open. Thus, for example, a normally closed "EVM" ("NC") would be closed when the coil was not energized. The differential pressure can then be set directly while the system is running,

As a measuring point in the direction of flow in front of the valve, the compressor pressure connection, or better still, the side pressure gauge connection on the "ICS" valve (always valve inlet pressure) can be used. After the valve, a measuring point on the condenser or collector or a corresponding T-nipple can be used the branch line to the "CVPP".

 

Fig. 1: Defrosting function Heading 1.01 Connection from the circulating canal Heading 1.02 Circumferential canal !!! The pressure from p1 (evaporator) is fed through this channel to connection P and to the pressure gauge connection. Heading 1.03 CVP (0-7 bar) connection opens when the defrost pressure rises to 5.5 bar in the evaporator and releases the pressure on the piston. Fig. 2: OPEN / CLOSE function Heading 1.05 The hot gas from the external connection presses the main piston and forcibly opens the main valve. external HG connection IN The solenoid valve EVM is located in connection SIl and opens or closes the path from SI (HG connection). So open / close function with HG support. Heading 1.06 By screwing in the ext. The channel to p1 is then closed. Heading 1.07 p1 is the pressure in the evaporator.

Main application in valve stations in the return line

Explanation of the connections and ducts in the PM and ICS main valve The paths for PM and the new ICS are identical

With ICS valves, the head module with the pilot valves can be rotated by 90 °. The function is retained.

 

 

 

 

 
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Mechanical seals

Since the mid-1950s, mechanical seals have become increasingly popular as shaft seals over the traditional stuffing box. Compared to the stuffing boxes, the mechanical seals offer the following advantages:

  • They hold tight to the shaft in the event of minor displacements and vibrations.
  • You don't need to be hired.
  • The sealing surfaces have less friction and therefore minimal power losses.
  • The shaft does not slide along the sealing components and is therefore not damaged by wear (lower repair costs).

 

The mechanical seal is the part of the pump that separates the medium from the atmosphere. Figure 1.3.1 shows different types of pumps that are equipped with mechanical seals. The majority of mechanical seals are manufactured in accordance with the European standard EN 12756. To select a specific mechanical seal, the following information about the properties of the pumped medium and the resistance of the seal to the pumped medium must be determined:

  • Type of medium to be pumped
  • Pressure to which the mechanical seal is exposed
  • Speed ​​to which the mechanical seal is exposed

 

Installation dimensions The following pages explain how a mechanical seal works, the various types of seals, the materials used for mechanical seals and the factors that affect the performance of a mechanical seal.

 

Components and functionality of the mechanical seal

The mechanical seal consists of two main assemblies: a rotating assembly and a stationary assembly. These assemblies consist of the parts listed in Figure 1.3.2. Figure 1.3.3 shows how the various parts of the mechanical seal are arranged.

  • The stationary part is built into the pump housing. The rotating part of the seal sits on the pump shaft and rotates during pump operation.
  • The two primary sealing surfaces are pressed against each other by the spring pressure and the pressure of the pumped medium. During operation, a film of lubricant from the pumped medium forms in the narrow gap between the two sealing surfaces. This film evaporates before it is released into the atmosphere, making the mechanical seal fluid-tight (see Figure 1.3.4).
  • The secondary seals seal the mechanical seal against the shaft.
  • The sealing surfaces are mechanically pressed together by the spring.
  • The driver transfers the torque from the shaft to the seal. With bellows seals, the torque is transmitted directly through the bellows.

 

 

 

 

Sealing gap

During operation, the pumped medium forms a lubricating film between the sealing surfaces. This lubricating film consists of a hydrostatic and a hydrodynamic element.

  • The hydrostatic element is generated by the pumped medium, which is pressed into the gap between the sealing surfaces.
  • The hydrodynamic lubricating film is created by the pressure generated by the rotation of the shaft.

 

The thickness of the lubricating film depends on the pump speed, the liquid temperature, the viscosity of the pumped medium and the axial forces of the mechanical seal. The constant exchange of the pumped medium in the sealing gap is guaranteed by two effects:

  • the evaporation of the pumped medium into the atmosphere
  • the circulating movement of the pumped medium.

 

Figure 1.3.5 shows the optimal relationship between good lubrication and low leakage rate. The optimum ratio is achieved when the lubricating film wets the entire seal gap, except for a very narrow evaporation zone close to the atmospheric side of the mechanical seal.

Often leaks occur due to deposits on the sealing surfaces. When using coolants, deposits quickly build up through evaporation on the atmospheric side of the seal. When the pumped medium evaporates in the evaporation zone, microscopic solid particles from the pumped medium remain as deposits in the sealing gap and cause wear there. These deposits are seen with most types of fluids. Conveying media with a tendency to crystallize are problematic. In order to avoid excessive wear, it is best to choose sealing surfaces made of hard material (e.g. tungsten carbide (WC) or silicon carbide (SiC)).

The narrow sealing gap between these materials (approx. 0.3 μm) minimizes the risk of solid particles penetrating the sealing gap and thus also minimizes deposits.

 

Balanced and unbalanced seals

To achieve a balanced pressure between the primary sealing surfaces, two types of seals are offered: balanced and unbalanced seals.

Relieved seal

Figure 1.3.6 shows a relieved seal and the forces acting on it.

Unbalanced seal

Figure 1.3.7 shows a non-balanced seal and the forces acting on it.

 

Several forces act on the sealing surfaces in the axial direction. Spring force and hydraulic force of the pumped medium compress the seal, while the force of the lubricating film in the seal gap counteracts this. At high pressure of the pumped medium, the hydraulic forces may be so great that the lubricating film in the sealing gap cannot prevent contact between the sealing surfaces. Since the hydraulic force is proportional to the area on which the pressure of the pumped medium acts, the load in the axial direction can only be reduced by reducing the pressure area.

 

The load factor (K) of a mechanical seal is defined as the ratio between area (A) and area (B): K = A / BK = load factor A = area that is exposed to hydraulic pressure B = contact area of ​​the sealing surfaces Load factor around K = 0.8, for those not relieved around K = 1.2.

 

Mechanical seal types

This section describes the main types of mechanical seals: O-ring seal, bellows seal, and cartridge seal.

O-ring seals

In an O-ring seal, the seal between the rotating shaft and the rotating sealing surface is achieved by an O-ring (Figure 1.3.9). The O-ring must be able to slide freely in the axial direction in order to be able to absorb the axial displacement due to temperature changes and wear. Incorrect position of the stationary seat can lead to abrasion and unnecessary wear on the O-ring and shaft. O-rings are made of different elastomers according to their operating conditions (e.g. NBR, EPDM and FKM).

 

Advantages and disadvantages of an O-ring seal

  • Benefits:

  • Suitable for hot
  • Fluids and high
  • Press

 

  • Disadvantage:

  • Deposits on the shaft (e.g. rust) may be a hindrance
  • the movement
  • the O-ring seal in the axial direction

Fig. 1.3.9: O-ring seal

 

Bellows seals

A common feature of bellows seals is the rubber or metal bellows that act as a dynamic sealing element between the rotating ring and the shaft.

 

Rubber bellows seals

The bellows of a rubber bellows seal (see Figure 1.3.10) can be made from different elastomers (e.g. NBR, EPDM and FKM) - depending on the operating conditions. Two geometrical principles are used in the construction of rubber bellows:

  • Bellows
  • Rollbalg

Fig. 1.3.10: Rubber bellows seal

 

Metal bellows seals

In a conventional mechanical seal, the spring generates the force required to close the sealing surfaces. In the case of a metal bellows seal (Figure 1.3.11), the spring is replaced by a metal bellows with the same force. The metal bellows acts as a dynamic seal between the rotating seal ring and the shaft and as a spring. The bellows has several folds with which the required contact pressure is generated.

Advantages and Disadvantages of Metal Bellows Cartridge Seals

  • Benefits:

  • Insensitive to deposits (e.g. rust and lime) on the shaft
  • Suitable for hot media and high pressures
  • Low load factor leads to less wear and a longer service life

 

  • Disadvantage:

  • Possible fatigue failure of the mechanical seal if the pump is not correctly aligned
  • Possible fatigue from excessive temperatures or pressures

 

 

Cartridge seals

With the cartridge mechanical seal, all parts form a compact unit, which is attached to a protective shaft sleeve ready for installation. A cartridge seal offers many advantages over a conventional mechanical seal (Figure 1.3.12).

  • Advantages of the cartridge seal:

  • Easy and quick service
  • Construction protects the sealing surfaces
  • Pre-tensioned spring
  • Safe handling

"With the kind permission of GRUNDFOS GMBH"

Source: GRUNDFOS INDUSTRY PUMP MANUAL

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Scroll compressor for normal cooling

 

The compressor is the heart of every compression refrigeration cycle. It brings vapor refrigerant from a low pressure level (low pressure suction side) to a high level (high pressure side). Compressors are available with different operating principles. These are, for example, scroll compressors, screw compressors, rotary piston compressors, turbo compressors and reciprocating compressors. Today we focus on scroll compressors for refrigeration. Scroll compressors are very common in air conditioning applications - their classic application is the chiller. The scroll compressor is also gaining more and more market share especially in the heat pump sector. Scroll compressors are also ideal for normal cooling applications.

 

Layout and function

 

Danfoss scroll compressors have the crankshaft standing upright. Above the crankshaft is the scrollset. This scrollset consists of a fixed and an orbiting spiral. These two spirals engage and compress the refrigerant by an orbital motion from the outer part of the scrollset toward the center. Due to this principle, there are different stages of compaction at each stage of the compaction process (different sized "pockets" in which compression is taking place). Thus, compared to reciprocating compressors, smaller portions of refrigerant are ejected more frequently. This leads to lower pulsations. For the fitter this means that a muffler rarely has to be used for pulsation damping. Noise problems or malfunction of pressure switches caused by pulsations are not to be expected in systems with scroll compressors. During operation of the compressor, one of the two spirals is pressed against the other by means of a medium pressure from a "pocket" of the scroll set, in which the complete compaction has not yet been completed. As a result, the two spirals are "retracted", so to speak. This "break-in phase" is completed at the latest after 72 hours of operation. It is a flexible interaction of the two scroll screws while self-optimization of the interaction in practical operation. In this context, the installer should note that MLZ compressors may initially experience a certain under-performance during initial start-up.

 

Assembly and service

All Danfoss scroll compressors are 100% suction gas cooled. This means that it may be possible to mount a silencer hood because the compressor releases all the excess heat from the refrigerant that flows through it. The refrigeration connections for these compressors are arranged one above the other - suction side down (large connection) and pressure side up (small connection). Both are designed as solder sockets integrated directly into the compressor. Inside the "scroll compressor head" is a check valve, which avoids a reverse rotation of the scroll set after switching off the compressor. The assembly of the scroll compressors is done on rubber buffers. In general, one should bear in mind when first contacting with scroll compressors that in this type of compressor the head, ie the upper 20% of the compressor, Compression end temperature (hot gas temperature) has. This is not the case with fully hermetic reciprocating compressors. There are all housing sites on the compressor (with the exception of the pressure port) on the suction side and thus have no high temperatures. In particular, the compression end temperature is always an issue in scroll compressors. For example, "MLZ" scroll compressors, when operated at the usual evaporation temperatures of -10 ° C, will show no abnormality in terms of final compression temperature, which means that in the rarest of cases the value will rise above 100 ° C - for compressors, Bearing, scrollset and refrigeration oil are no problem. that such a compressor, for example, by a permanent strong throttling of an evaporative pressure regulator - or for other reasons - a long time with the suction pressure far below its application limits is driven, then it can quickly come to excessive pressure nozzle temperatures. For this reason, it is recommended to install or retrofit a compressed gas end temperature monitor for systems with scroll compressors, if this is not available. This task can be carried out by a simple mechanical thermostat with remote sensor (eg "KP 81"). It is not necessary that the temperature setting is extremely close to the operating point, 135 ° C maximum, 120 ° C is here a good practice value. then it can quickly come to excessive pressure nozzle temperatures. For this reason, it is recommended to install or retrofit a compressed gas end temperature monitor for systems with scroll compressors, if this is not available. This task can be carried out by a simple mechanical thermostat with remote sensor (eg "KP 81"). It is not necessary that the temperature setting is extremely close to the operating point, 135 ° C maximum, 120 ° C is here a good practice value. then it can quickly come to excessive pressure nozzle temperatures. For this reason, it is recommended to install or retrofit a compressed gas end temperature monitor for systems with scroll compressors, if this is not available. This task can be carried out by a simple mechanical thermostat with remote sensor (eg "KP 81"). It is not necessary that the temperature setting is extremely close to the operating point, 135 ° C maximum, 120 ° C is here a good practice value.

 

Multikältemitteltauglichkeit

"MLZ" Scroll Scroll compressors are approved for the standard refrigerants R404A, R507 and R134a. The application limits for R404A and R507 allow evaporation temperatures from -30 ° C (with limited condensing temperature) to +10 ° C with up to 60 ° C liquefaction. A condensation pressure reduction, as is often the case in modern refrigeration technology, is possible with evaporation from -30 to -10 ° C even up to +10 ° C. This value can not be achieved by many reciprocating compressors. "MLZ" compressors with R134a can be operated at evaporating temperatures between -15 and +15 ° C. Since condensing temperatures up to 74 ° C are possible, the "MLZ" is suitable for seasonal heat pump operation as well as for heat recovery.

 

R404A offers a higher volumetric cooling capacity than R134a. Thus, the compressor with R404A at the same evaporation temperature has a higher cooling capacity than the same model with R134a. These differences in the cooling capacities due to the refrigerant used are also commercially exploitable. An important advantage is the greater flexibility of the customer. If, for example, a new system is being built and the customer does not yet know for sure whether there will be an extension in the next few years, it is advisable to install a multi-refrigerant "MLZ" scroll compressor in operation with R134a It will be possible to add a lot of cooling capacity just by changing the refrigerant to R404A, without, however, replacing the scroll compressor. The customer benefits, because the refrigerant R134a is energetically very good and is R404A (R507) only in terms of universality after. For applications of commercial refrigeration, such as petrol station shops, etc., there are the "MLZ" compressor also already integrated in fully equipped condensing sets "Optyma Plus" with fan speed control, weatherproof housing, compressor contactor, emergency stop, dryer and sight glass. The time savings in on-site assembly is one of the main reasons why such ready-to-use condensing units are used.

 

lubrication

The compressors already include the required amount of oil in the scope of delivery. After a certain period of time after installation, a check of the oil level via the oil sight glass in the lower part of the compressor is recommended. The ideal oil level is halfway up the oil sight glass, but 1/4 to 3/4 can also be tolerated. In the lower housing area there is an oil drain port, with which oil can be drained without tilting the compressor. For this, it is sufficient to produce a slight overpressure on the suction side of the compressor and to drain the oil from the compressor via this connection and the service manometer. "MLZ" compressors are pre-filled with a PVE (polyvinyl ether) "oil". PVE has the advantage over conventional POE (polyol ester) lubricants that it does not chemically react with water and form acid. Hygroscopy is comparable to POE oil, but moisture in the system can also be easily removed or evacuated by this property. Also interesting is the compatibility with R22. This makes it possible to respond flexibly to export inquiries, such as those from Latin America, because R22 still dominates as a refrigerant there. To ensure optimum lubrication of the compressor internal parts and good oil return, the scroll compressor should not be started more than twelve times per hour and should be switched off for at least one minute after each stop. react flexibly from Latin America, where R22 still dominates as a refrigerant. To ensure optimum lubrication of the compressor internal parts and good oil return, the scroll compressor should not be started more than twelve times per hour and should be switched off for at least one minute after each stop. react flexibly from Latin America, where R22 still dominates as a refrigerant. To ensure optimum lubrication of the compressor internal parts and good oil return, the scroll compressor should not be started more than twelve times per hour and should be switched off for at least one minute after each stop.

 

Electrical connection

For outdoor installation or if low ambient temperatures at the compressor can not be excluded, a crankcase heater should be used. This should always be switched countercyclically to the compressor (compressor is running - crankcase heater off, compressor is on - crankcase heater on). The "MLZ" scroll compressors are commonly found on the market as three-phase 400V models. The electrical connection is relatively simple, since the compressors are internally already connected in the star point and no bridges are to be placed in the connection box. There are three connection pins to which the three phases coming from the contactor (or, ideally, motor protection in the control cabinet) are directly connected. It is important to note that the scroll compressor runs in the correct direction of rotation. If strong mechanical noises occur and the connected service manometer does not set the usual pressure difference between high and low pressure, the scroll compressor will most likely run in the wrong direction of rotation. Remedy can be created by exchanging two phases on the compressor terminal board. With the help of a voltage tester, it can be checked on the compressor terminal box whether the power supply is in order. The phase conductors (measured phase to phase) should always be about 400V. As an additional protection against overheating and against excessive current load, a bimetallic protection is incorporated in the star point of the windings. That is, in general, the internal motor protection triggers when a resistance measurement at the compressor (supply voltage before disconnect) a " infinite resistance "between all three pins is measured. As soon as the compressor has cooled down, the bimetallic protection switches on again. When the electric motor is ready for operation, the three measured resistance values ​​of the pins approach each other. Depending on the capacity of the compressor, the value is in the single-digit range of ohms. The speed of these compressors is at 50 Hz about 2900 U / min, since the electric motor is wound with a pair of poles. At 60 Hz, for example, the compressor would run correspondingly faster (about 3480 rpm) than at 50 Hz, because the rotor of the compressor is based on the corresponding mains frequency (Hz = 1 / s means that at 50 Hz alternating current 50 times in the Second the current direction is changed). the bimetallic protection switches on again. When the electric motor is ready for operation, the three measured resistance values ​​of the pins approach each other. Depending on the capacity of the compressor, the value is in the single-digit range of ohms. The speed of these compressors is at 50 Hz about 2900 U / min, since the electric motor is wound with a pair of poles. At 60 Hz, for example, the compressor would run correspondingly faster (about 3480 rpm) than at 50 Hz, because the rotor of the compressor is based on the corresponding mains frequency (Hz = 1 / s means that at 50 Hz alternating current 50 times in the Second the current direction is changed). the bimetallic protection switches on again. When the electric motor is ready for operation, the three measured resistance values ​​of the pins approach each other. Depending on the capacity of the compressor, the value is in the single-digit range of ohms. The speed of these compressors is at 50 Hz about 2900 U / min, since the electric motor is wound with a pair of poles. At 60 Hz, for example, the compressor would run correspondingly faster (about 3480 rpm) than at 50 Hz, because the rotor of the compressor is based on the corresponding mains frequency (Hz = 1 / s means that at 50 Hz alternating current 50 times in the Second the current direction is changed). The speed of these compressors is at 50 Hz about 2900 U / min, since the electric motor is wound with a pair of poles. At 60 Hz, for example, the compressor would run correspondingly faster (about 3480 rpm) than at 50 Hz, because the rotor of the compressor is based on the corresponding mains frequency (Hz = 1 / s means that at 50 Hz alternating current 50 times in the Second the current direction is changed). The speed of these compressors is at 50 Hz about 2900 U / min, since the electric motor is wound with a pair of poles. At 60 Hz, for example, the compressor would run correspondingly faster (about 3480 rpm) than at 50 Hz, because the rotor of the compressor is based on the corresponding mains frequency (Hz = 1 / s means that at 50 Hz alternating current 50 times in the Second the current direction is changed).

 

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