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Refrigeration - Introduction to the basics - The scroll compressor

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 e.g. scroll compressors, screw compressors, rotary piston compressors, turbo compressors and reciprocating compressors. Today we want to focus on the scroll compressors. Scroll compressors are very common in air conditioning applications - their classic application is the chiller. Especially in the heat pump sector, the scroll compressor is increasingly conquering market share. Generally there are scroll compressors but also for normal cooling

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Layout and function

 

On Danfoss scroll compressors, the crankshaft is 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. Through this principle, there are various 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 often. This leads to lower pulsations. For the mechanic, this means that less often a muffler 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. Danfoss scroll compressors have two different methods. In the devices painted in blue, the radial sealing (sealing of the scroll flanks with respect to each other) is achieved solely by the oil film, while flexible metallic seals are used for the axial sealing. The two spirals are not pressed against each other, but run completely stationary, even in the compression mode. The situation is different with the black-painted Danfoss scroll compressors. These work according to the "compliance principle". This means that one of the two spirals is pressed against the other by means of medium pressure from one "pocket" of the scroll set, where 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. If one searches for the term "compliance" in a technical dictionary, one finds "translation" as "translation", while the term is translated in a general dictionary with "agreement". In fact, the truth is probably in between. It is a flexible interplay of the two scroll screws while self-optimization of the interaction in practical operation. The installer should note in this context that for these black compliance compressors (compressor type designation always begins with "H" - for example "HRP" etc.), a certain under-performance may initially occur during initial start-up. This point is usually not really noticeable in normal operation, but is very important for power measurements on test benches. The blue scroll compressors call from the first minute on their full cooling capacity.

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Assembly and service

 

These compressors already include the required amount of oil in the scope of delivery. After a certain period of time after installation, it is advisable to check the oil level via the oil sight glass (in the case of the standard blue scroll compressors) in the lower part of the compressor. The ideal oil level is half the height of the oil sight glass, but 1/4 to 3/4 can also be tolerated. All Danfoss scroll compressors are 100% suction gas cooled. This means that a silencer hood may be fitted where necessary, as the compressor releases all excess heat from the refrigerant flowing through it. The refrigeration connections for these compressors are arranged one above the other - suction side at the bottom (large connection) and pressure side at the top (small connection). Both are designed as Rotolock screw connections or as solder sockets integrated directly in the compressor. For initial installation, Rotolock valves should be used - at least for Rotolock threaded fittings - as these greatly simplify service interventions on the compressor or the refrigeration system and provide the ability to easily install a high and low pressure switch. In this context, it should be noted that the connection closest to the spindle of a Rotolock valve can be shut off (connection option for the service manometer). The other connection can be used for a pressure switch (connection can not be locked). In addition, "SZ" or "SM" scrolls on the compressor housing typically provide three additional, smaller connectivity options: a low pressure port, which is typically not used, and an oil overflow port, which is only needed in compound operation. In combined operation, a 10-gauge copper pipe is attached to the oil overflow as an oil balance to the sister compressor and the suction line to the composite compressors as symmetrical as possible. Additional check valves in the individual pressure lines can be dispensed with, since there is already a non-return valve inside this scroll compressor. A special highlight is the third small connection. If the NPT plug of the oil drain port is replaced with a suitable nipple with a 7/16 UNF gauge port, oil change can be performed without tilting the compressor. For this purpose, 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. This is made possible by a small copper tube inside the compressor which, starting from this connection, runs down into the compressor sump. The assembly of the scroll compressors takes place on rubber buffers. In general, when first contacting with scroll compressors, it should be kept in mind that in this type of compressor, the head, ie the upper 20% of the compressor, has a compression end temperature (hot gas temperature). 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. Especially the compression end temperature is always an issue in scroll compressors. For example, Scroll compressors for air conditioning or heat pump use, if they are operated at the usual evaporation temperatures of 0 or 10 ° C, show no abnormalities in the compression end temperature. This means that the value will seldom rise above 100 ° C - no problem for compressors, bearings, scrollset and refrigeration oil. However, if it happens that such a compressor is driven far below its application limits for a long time with the suction pressure, for example due to a permanent strong throttling of an evaporating pressure regulator - or for other reasons, then excessively high pressure nozzle temperatures can occur. For this reason, it is a very good idea to install or even retrofit a Druckgasendtemperaturüberwachung in systems with scroll compressors, if this is not available. This task can be done by a simple mechanical thermostat with remote sensor (eg "KP 81"). It is not necessary that the temperature setting be extremely close to the operating point, 135 ° C maximum, 120 ° C is here a good practice value.

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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 anti-cyclic to the compressor (compressor is running - crankcase heater off, compressor is on - crankcase heater on). The blue scroll compressors are commonly found as three-phase 400 V models in our market. The electrical connection is relatively simple, as the compressors are already internally connected at the neutral point and no jumpers have to be placed in the connection box. There are three connection pins to which the three phases coming from the contactor (or, ideally, the motor protection in the control cabinet) are directly connected. It is now very important that the scroll compressor operates in the correct direction of rotation during operation. 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 everything is in order with the power supply. The phase conductors (measured phase to phase) should always be about 400V. As additional protection against overtemperature and against excessive current load, there are versions of these blue compressors (size "SZ / SM84-110" and "120"), which have a bimetallic protection in the neutral point of the windings. In other words, it can be assumed that the internal motor protection has tripped, if an "infinite resistance" is measured between all three pins during a resistance measurement at the compressor (disconnect supply voltage beforehand). As soon as the compressor has cooled down, the bimetallic protection switches on again. If the electric motor is ready for operation, the three measured resistance values ​​of the pins are approximately equal to one another. The value is between 0.4 and a single-digit ohm value, depending on the capacity of the compressor. In the case of internal motor protection, however, caution is required. For other compressor sizes, such as "SZ / SM115" and "125-185", no internal motor protection is incorporated in the neutral point of the winding. These variants have only one extra bimetallic relFdivease, which switches off at internal overtemperature in the compressor. This bimetallic release is potential-free and must be integrated in the safety chain upstream of the compressor contactor (before "A1"). For the blue Performer compressors, there is even a third engine protection variant. The power ratings "SZ240", "300" and "380" have a "Kriwan INT69" thermistor motor full protection. The contacts "M1" and "M2" on the "INT69" are potential-free and must be integrated into the safety chain upstream of the compressor contactor (before "A1"). This is similar to the connection procedure in the case of the bimetallic release. The "INT69" itself requires a supply voltage of 230 V or 24 V - depending on the version. This information is important for the installer, as not only the usual load cable with the three phases and the protective conductor must be brought to the compressor, but also the "neutral" for the supply voltage of the motor protection module and two wires of the safety chain of the compressor contactor. When the scroll compressor is in operation, the motor full protection permanently checks whether the temperatures in the compressor are OK via thermistors incorporated in the compressor. In the event of overtemperature, it opens the contact between M1 and M2 and thus removes the compressor contactor. The speed of this compressor 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 line frequency (Hz = 1 / s means that 50 Hz alternating current 50 times in the second the current direction is changed).

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Information

The content is based on the booklet series of the same name by Danfoss (www.danfoss.de/kaelte), which deals with the basic relationships in compression refrigeration systems and the associated basic components. The series is aimed at refrigeration engineers in service and plant construction, at new entrants in refrigeration technology, at apprentices to the refrigeration plant manufacturer and at all those who would like to gradually incorporate the practice-oriented basic knowledge of the cold again.

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Tasks of a thermostat - room temperature control

One could almost go so far and claim that the unofficial fifth main component of a refrigeration system next to the compressor, the throttle body and condenser or evaporator is the thermostat. This is because even with the simplest compression refrigeration systems such as the refrigerator, a thermostat is always used for room temperature control. In this case, either the thermostat switches the compressor directly on and off again or a contactor or a relay. Alternatively, with the use of "pump down" or "pump out" circuits, the direct switching of the solenoid valve coil is possible. Thus, too high a contact load, as they would arise in the direct compressor load on the thermostat bypassed. If no solenoid valve is installed in the liquid line in the system, then this circuit variant can not be applied naturally. The use as a room thermostat assumes that it is a system in which the evaporator cools the air.

 

Water and brine temperature control

However, if the evaporator does not cool the air, but cold water or brine, the thermostat can regulate, for example, the water temperature. Such a control thermostat is usually installed in the return (the brine water comes back from the consumer, such as a chiller, back to the evaporator). In the flow you will normally find an additional thermostat, the antifreeze thermostat. It prevents - in addition to a flow switch - the falling below certain thresholds that lead to icing of the water cycle and thus frost blasting, for example. could result in the evaporator. This is therefore a danger, since the volume of water after the aggregate state transition to ice increases by about 10% and thus can quickly lead to greater damage in trapped water. Of course, it is still possible to use a room thermostat even in chilled water systems, even if the room is cooled with chilled water.

 

Defrost termination

Back to the actual refrigeration technology: Another standard application is the defrost limit thermostat. In order to avoid unnecessary energy consumption, the defrost length can be controlled very precisely by a thermostat whose sensor is ideally placed in the most stubborn iron nest of the evaporator. This offers a clear energy advantage over the time-controlled defrost, is always defrosted over a certain, defined time, regardless of whether the evaporator is already free of ice or not. Even better in this context is a demand-controlled defrost control, which, however, is no longer feasible with a simple thermostat. This requires more highly qualified electronics, which can decide on the basis of stored system characteristics whether a defrost must be initiated. However, care should be taken to ensure that certain time windows can be preselected despite the "Demand Defrost" function. This avoids the defrost being initiated just in or just before the loading phase. 

Compression end temperature monitoring

Somewhat rarer you will find Druckrohrthermostate. This application is intended to protect compressors from excessive compression temperatures. At the same time, the refrigerating machine oil is protected against excessive thermal stress and thus from denaturation. Especially with the use of scroll compressors, this protective measure is more common. But it is also recommended for reciprocating compressors.

 

sensor charge

Basically Danfoss thermostats of the "KP" series are delivered with two different sensor fillings. In order to be able to decide which filling and which devices are suitable for the specific application, it is important to know the difference. So it is on the one hand to a steam filling and on the other to an adsorption filling. The most important difference is that with steam filling, the sensor must always be placed colder than the "KP" housing. This fact stems from the fact that a certain amount of liquid is ready to evaporate in the interior of the sensor. If this is evaporated in the sensor, the pressure inside the sensor system increases and can push apart the pressure bellows, which in turn actuates the contact system. However, if this liquid moves towards the thermostat housing, it can no longer be vaporized in the sensor and the function will not work. The phenomenon is comparable to the classic refrigerant transfer, in which the refrigerant always moves to the coldest point. In the case of the adsorption filling, it does not matter if the temperature at the sensor is warmer or colder than at the associated housing.

 

Types Standard thermostat for wall mounting

Roughly speaking, there are two main types of thermostats in refrigeration: the adjustable standard thermostat for wall mounting and the refrigerator thermostat. The standard wall-mounted thermostat (such as the Danfoss "KP" type) with room sensor is often used by plant manufacturers for normal cold rooms. Thermostats of the same design are now also used with the remote sensor for the defrost limit function. It is important to pay attention to the appropriate sensor filling. It is recommended that the adsorption, as this does not shift, so that is switched reliably. 

Refrigerator thermostat

The second main design is the classic refrigerator thermostat. This serves as a room thermostat for refrigerators, but is also used in smaller counter cooling and the like. The advantages of a refrigerator thermostat are the very moderate price and the optimized longevity of the product. In case of replacement in case of service please note the following: Fridge thermostats are available in countless variants, which differ only in very few details. These are the length of the capillary tube sensor, the temperature switching points and the question of whether it is an automatic defrost or constant reclosure temperature in the temperature plus range and whether a signal should be connected in parallel or counter to the direction of action. For this reason, can be used to cover service cases on a few service thermostats, which greatly facilitates the task. At Danfoss, there are eight service thermostats that can replace the vast majority of refrigerator thermostats in the event of repairs, thanks to an extra-wide temperature range and a longer capillary tube. For the refrigeration system manufacturer, the service thermostat no. 3 and 8 are particularly important here. The service thermostat no. 3 is suitable for automatic defrost refrigerators or constant reheat refrigerators, which are currently used in the majority. For the small refrigeration, the service thermostat No. 8 is important, as it is often used for drinks vending machines and spirits cooling in the catering industry. He rather serves the upper evaporation temperature range. Saladettes are also easily adjustable with this thermostat. Of course, there are basically other types of mechanical thermostats. At this point, however, only the most important types, which are used in refrigeration, will be described.

 

Types of sensors

For the various applications of the thermostats, there are also various sensor designs, which are optimized for the appropriate purpose. To measure the room actual value, a fixed spindle is usually mounted under the thermostat (type "KP" or "RT"). For use as an evaporator thermostat, a simple capillary tube sensor or a cylindrical remote sensor with capillary tube connection to the main unit can be used. For mounting to a pipeline - e.g. When monitoring the compression end temperature at the discharge port of a compressor - the cylindrical remote sensor is recommended. Finally, to regulate or monitor the temperature in a ventilation duct, there are special duct sensors.Kontaktbelastung

An important point when using thermostats with potential-free contacts is the contact load. The three different values ​​for the contact load, which manufacturers usually state, have already been mentioned in the article of the last issue "Pressure Switches". These load cases are also valid for thermostats, so here again a short summary. These three values ​​are the purely ohmic value (in this case the highest contact load is classically possible), the partially inductive and the purely inductive load case. An example of an ohmic load (load designation: AC1) is an electric resistance heater for defrosting. This load case is to be selected when selecting a defrost limiting thermostat, via which the heating is switched directly. Partial inductor (AC3) is for example an electric motor. Of course, this also includes a compressor, which is switched directly from the thermostat. By contrast, a coil (AC15), as used in solenoid valves, acts as an inductive load for a thermostatic contact system.

 

Electrical connection

A standard thermostat "KP" with changeover contact system usually has three connection contacts on which the wires of the electric cable can be placed. This is similar to the pinout on "KP" pushbuttons. The three connections are "phase in" (contact designation "1"), "thermal sense" (contact designation "2") and "cold acting" (contact designation "4"). It does not matter if, for a two-person occupancy, "phase in" ("1") and "cold effect" ("4") are confused with each other. The connection "heat sense" ("2") is rarely used in refrigeration. An exception, for example, is when it needs to be heated for certain reasons. Conceivable here is the control of an electric heater. Specifically, a "KP61" from Danfoss (room thermostat) places "phase in" on contact 1 and "cold acting" on contact 4.

 

Settings

The thermostats for wall or sheet-metal console mounting "KP" offer the following setting options. On the front side of the device two separately adjustable scales can be seen. It is on the left to the setpoint and right next to the difference setting (hysteresis). On the left scale the upper switching value is set and on the other side the difference. Switching value minus difference results in the switch-off value. With simultaneous electrical connection on the contacts "1" and "4" is now switched on at the left set value (for example, the compressor switched) and switched off at this value minus the difference set to the right. Example: Value left "-10" ° C, value right "6" ° C - corresponds to a cut-off value of -16 ° C. Since this is a mechanical component, the value for the difference is not always exactly the same. It differs depending on the set switching value. The exact difference value can either be determined empirically at the plant by readjustment or determined exactly by a corresponding nomogram (part of the instructions for "KP"). In practice, the procedure "left value minus difference on the scale right" is usually sufficient.

 

IP-Protection

The level of IP protection may be another important issue, depending on location and environmental factors. IP crash course: The first digit of the two digits that make up the IP rating (IP54, for example) describes protection for touch protection, and the second for water protection. An IP degree of IP 3 * indicates that a wire with a diameter of 2.5 mm may not penetrate into the device so certified. IP * 3 also means suitability for falling water spray up to 60 ° from vertical. All in all, the higher the degree of protection, the better the device is protected against dust, dirt particles and moisture. Standard thermostats "KP" have a degree of protection of IP33 against dust and moisture. If an accessory protective housing is used, IP55 has already been achieved. If an even higher degree of IP protection is desired, the "RT" series, designed for particularly inhospitable environments, is available. These have IP degrees of protection from 54 to 66, depending on the version.

 

Electronic thermostats or refrigeration controller

So far, all versions relate exclusively to mechanical thermostats. With regard to reliability, robustness and simplicity of use, these thermostats will be indispensable in refrigeration even in the future. Nevertheless, it may be useful in some cases to use electronic thermostats or refrigeration controller. Especially by the bundling of functions like e.g. Room temperature control with defrost limitation and display of the actual room temperature, which is available today with all common cold store controllers, often offer these devices advantages. In addition, the desire for specific hysteresis values can lead to the selection of an electronic thermostat, as mechanical devices do not always allow any hysteresis or switching differential value to be realized.

 

Information

The content is based on the booklet series of the same name by Danfoss (www.danfoss.de/kaelte), which deals with the basic relationships in compression refrigeration systems and the associated basic components. The series is aimed at refrigeration engineers in service and plant construction, at new entrants in refrigeration technology, at apprentices to the refrigeration plant manufacturer and at all those who would like to gradually incorporate the practice-oriented basic knowledge of the cold again.

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Ejectors are used to increase efficiency in CO2 booster systems.

The ejector

*c-Ejector is used to increase efficiency in CO2  booster systems. In conjunction with a high-pressure valve, three different types of gas injectors and two types of liquid ejectors can be combined as required.

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Principle mode of action of an ejector

The *c-Ejector utilizes the expansion work existing in the refrigerant at the gas cooler exit to draw in a different partial mass flow and to promote a higher pressure level. The exiting at high pressure level from the gas cooler CO2 is accelerated in the motive nozzle. As a result, the static pressure decreases and the flow exiting the motive nozzle has a lower pressure than the suction pressure of the NK stage. This allows either gas or liquid to be withdrawn from the suction side of the NK compressors. Both partial streams mix in the mixing chamber. In the diffuser, the flow is decelerated again, causing a pressure increase to medium pressure level. After the diffuser, the mixture is passed into the medium pressure separator.

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Integration of the *c-Ejector into a booster system

The starting point for integrating the *c-Ejector into an R744 refrigeration system is a booster system with parallel compaction. The flash gas accumulating in the medium-pressure separator is sucked off by a parallel compressor, thus keeping the pressure in the medium-pressure separator constant. In addition, gas ejectors can be installed, which suck the gas vaporized after the NK-cooling points before the NK-compressors and promote it to a higher pressure back into the medium-pressure separator.

If an accumulator is added after the NK cooling points, the cooling points can be operated quasi-flooded. The accumulating in the accumulator liquid is sucked from the liquid ejectors and conveyed back into the medium pressure.

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Functioning of gas and liquid ejectors

There is a general distinction between two types of ejectors: gas and liquid ejectors. But what exactly is the difference between gas and liquid ejectors? Let's look first at the gas ejector in the log ph diagram:

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The motive flow is relaxed from 1 to 2 in the motive nozzle below the NK pressure level. Saturated gas can be sucked from 3 to 4. Depending on the entrainment ratio, point 5 sets at the end of the mixing chamber. In the diffuser, the mixture is recompressed from 5 to 6 again. The line of action of the gas ejector thus has a higher vapor content in the medium-pressure separator.

 

Of the NK compressors, the gas ejector extracts a refrigerant mass flow and promotes this in the medium pressure. From this, the parallel compressor sucks off the accumulating gas. The efficiency advantage lies in the fact that the parallel compressor must apply a lower pressure difference to the high pressure than the NK compressor and this is thus relieved.

The prerequisite for the effective use of the liquid ejector is the quasi-flooded operating mode of the cooling points. The liquid ejector acts like a refrigerant pump. It conveys liquid refrigerant directly from the suction-side accumulator of the normal cooling stage into the medium-pressure vessel. The accumulator is a container which is installed in front of the NK compressors and separates the liquid from the gas phase. The mass flow drawn in by the ejector can again be fed directly to the cooling points. Furthermore, a conventional injection valve can be used at the cooling points, which is regulated after a very slight overheating below the MSS. An increased heat transfer coefficient causes the same heat transfer surface a lower possible temperature difference between the refrigerant and the refrigerant. As a result, the evaporation temperature can be raised. The NK compressors only have to apply a lower pressure difference to the high pressure.

 

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The motive flow is relaxed from 1 to 2 in the motive nozzle below the NK pressure level. Saturated liquid can be sucked from 3 to 4. Depending on the entrainment ratio, point 5 sets at the end of the mixing chamber. In the diffuser, the mixture is recompressed from 5 to 6 again. The line of action of the liquid ejector thus has a lower vapor content in the medium-pressure separator.

 

With the aid of the liquid ejector, the accumulating liquid can be pumped back to medium pressure "for free". The greatest efficiency gain is achieved when both types of ejectors are combined.

 

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Gas and liquid ejectors suck saturated or saturated liquid from the accumulator. Depending on the load ratio of gas to liquid ejectors, a point 7 sets after mixing the two partial streams, which indicates the entry into the medium-pressure separator.

 

Efficiency gain with  *c-Ejector

Even a booster with flash gas bypass can be operated more efficiently than a conventional two-stage refrigeration system (eg cascade with R134a / R744, satellite network with R404A) at low outside temperatures (<10 ... 15 ° C, depending on the design). Due to the transcritical operation of the booster but has to accept significant losses at high outside temperatures. To counter this, the installation of a parallel compressor is recommended. This ensures that the accumulated in the summer high proportion of flash gas in the collector no longer needs to be sucked through the NK compressor. Depending on the configuration of the parallel compressor, there is a switch-off point under which flash gas bypass mode must be switched again. Nevertheless, savings of well over 10% can be expected on a few days a year.

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With the installation of *c-Ejectors, the cooling system can be operated more efficiently throughout the year if the overall system is designed correctly. Due to the gas ejectors, even in subcritical operation, sufficient flash gas accumulates in the medium-pressure separator, which prolongs the running time of the parallel compressor. At high outside temperatures, the efficiency gains are significantly higher than those of parallel compaction due to the design of the ejectors on a typical summer day.

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Introduction Oil differential pressure switch MP 54, MP 55, MP 55A

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MP 54 and MP 55 Oil differential pressure switches are used as safety switches to protect refrigeration compressors from too low a lubricating oil pressure in the refrigeration compressor. If the oil pressure is too low, the oil differential pressure switch shuts off the compressor after a certain time. MP 54 and MP 55 are used in refrigeration systems where HCFCs and non-flammable HFC refrigerants are used. MP 55A is used in refrigeration systems with R717. MP 55A can also be used in systems with HCFCs and non-flammable HFC refrigerants. MP 54 has a fixed differential pressure and a thermal time relay with a fixed trip time. MP 55 and 55A have adjustable differential pressure and are available with or without thermal time relay.

 

particularities

  • • Large control range
  • • Can be used in freezer, refrigeration and air conditioning systems
  • • Can be used for HCFC and non-flammable HFC refrigerants
  • • Electrical connections on the front of the unit
  • • Suitable for AC and DC
  • • Cable gland for cables with 6 - 14 mm diameter
  • • Small contact difference

approvals

  • CE Zeichen gemäß LVD 2006/95 / EC EN 60947-1, EN 60947-4-1, EN 60947-5-1
  • China Compulsory Certificate, CCC versions with UL and CSA approval are available on request

ship approvals

  • Germanischer Lloyd, GL Registro Italiano Navale, RINA (MP55)

 

Construction of oil differential pressure switch

  • 1. Connection to the pressure side of the lubrication system, OIL
  • 2. Connection to the suction side of the refrigeration system, LP
  • 3. Adjusting washer (MP 55 and MP 55A)
  • 4. Reset button
  • 5. Tester cable
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The function of the pressure switch is only due to the differential pressure (pressure difference) between the two oppositely acting corrugated tube elements, while it is independent of the absolute pressures acting on the corrugated tubes. MP 55 and 55A can be adjusted to different differential pressures by means of the pressure adjusting disc (3). The set differential pressure is read on the inside scale. MP 54 is fixed and has no pressure adjustment disc. The factory-set differential pressure is hammered into the front panel of the device.

 

Technical specifications

 

control voltage 230 V or 115 V AC or DC Permitted voltage tolerance: -15 - 10% Max. Working pressure PS / MWP = 17 bar Max. Test pressure Pe = 22 bar

ambient temperature

The time relay is temperature compensated for the area

230 V or 115 V AC or DC cable glands laterally 13.5 Cable diameter 6 - 14 mm Maximum corrugated pipe temperature: 100 ° C Switch difference max. Δp [bar 0.2 protection class IP20 to EN 60529 / IEC 60529 Contact loads MP with time relay (contacts MS)

AC15 = 2A 250V

DC13 = 0,2A 250V

Contact loads MP without time relay

AC15 = 0,1A 250V

DC13 = 12W 250V

Cable cross sections Rigid cable 0.2 - 1.5 mm2 Cable cross-sections flexible, without wire end ferrules 0.2 - 1.5 mm2 Cable cross-sections flexible, with wire end ferrules 0.2 - 1 mm2 torque Max. 1.2 Nm Nominal pulse voltage 4 kV pollution degree 3 Short circuit protection, fuse 2 A Isolationsspannung 250 V Max. Working pressure PS / MWP = 17 bar Max. Test pressure Pe = 22 bar

function

If the oil pressure fails during start-up or falls below the set value during operation, the compressor comes to a standstill after the tripping time has elapsed. The electrical circuit was divided into two separate circuits, a safety circuit and a differential circuit.

The timing relay (e) in the safety circuit is activated when the effective lubrication oil pressure, the oil differential pressure (the differential pressure between the pump pressure and the suction pressure), is less than the set value. The timer is deactivated when the oil differential pressure is greater than the setpoint plus the contact difference.

 

electrical diagram

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The two terms oil differential pressure and contact difference are shown in the two diagrams below. Both must be taken into account when using oil differential pressure switches.

The first shows the function of the differential pressure switch at start-up, the second shows the function during operation.

 

At the decrease

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Pos. A: Proper start During commissioning, the build-up of the lubricating oil pressure to the set / fixed difference plus the contact difference occurs before the time relay switches off (here after 45 seconds). In point A the contact T1-T2 opens and the time relay is switched off. Normal oil pressure for the compressor is sufficient.

Pos. B: The lubricating oil pressure does not rise to the set / fixed difference plus the contact difference before the timer expires. At point B, the time relay interrupts the operating circuit LM and the compressor switches off. Any signaling device connected to terminal S is activated. A restart can be made after about 2 minutes by pressing the reset button and correcting the causes of the error.

 

Operational

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Pos. C: The lubricating oil pressure falls below the set / fixed difference during operation. At point C, the safety circuit T1-T2 is closed, and the time relay is activated.

Pos. D: The lubricating oil pressure rises to the set / fixed difference plus the contact difference before the timer expires. In section D the safety circuit T1-T2 opens and the time relay is switched off. Normal oil pressure for the compressor is sufficient.

Pos. E: The lubricating oil pressure falls below the set / fixed difference during operation. At point E, the safety circuit T1-T2 is closed and the time relay is activated.

Pos. F: The lubricating oil pressure remains below the set / fixed difference. In point F, the time relay interrupts the operating circuit LM and the compressor switches off. Any signaling device connected to terminal S is activated. A restart can be made after about 2 minutes by pressing the reset button and correcting the causes of the error.

After the start

After commissioning, check whether the differential pressure switch works as desired.

Press the test device (left in the housing) for this check.

Holding the device down turns off the compressor motor after the trip time indicated on the timer has expired.

 

measurements and weight

 

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Liquid separator for ammonia

The liquid separator:

Is a mostly horizontal pressure vessel with a large storage volume, which is mainly used in refrigeration systems with pump circulation.

Larger ammonia refrigeration systems usually use flooded evaporators with multiple refrigerant recirculation. Due to the forced circulation by means of refrigerant pumps, large distances between the refrigeration center (engine room) and the consumers can be bridged.

The circulating number n indicates the ratio of the amount of liquid delivered by the pump to the amount of refrigerant evaporated in the evaporator at full load.

As the refrigerant speed increases, the heat transfer coefficient from the refrigerant to the evaporator inner wall increases sharply. He reaches at n = 2. , , 3 a maximum, which slowly decreases at higher circulation (s) again. Because of the uncertain pressure drop determination in the evaporator circuit n = 3 to 4 is recommended.

For the reliability of such systems, a liquid separator is of crucial importance. The separator is the link between the compressor and the evaporator circuit and thus divides the refrigeration cycle. In it, on the one hand, the liquefied refrigerant is released from the condenser and separated the non-evaporated refrigerant from the evaporator.

 

The separator collector thus has essentially the following tasks to fulfill:

On the one hand, it must provide and collect sufficient liquid refrigerant for all operating states of the refrigeration system.

On the other hand, the non-evaporated liquid refrigerant fraction from the liquid-vapor mixture, which flows back from the evaporator circuit.

The proportion of liquid refrigerant in the separator is very much influenced by the load on the evaporator. The load demand in the evaporator circuit often change so fast that they can not be intercepted by the control.

As the load in the evaporators is reduced, the liquid will boil less and the evaporators will take up more liquid NH3, as the refrigerant pumps will continue to pump evenly, thus decreasing the liquid level in the trap.

Increasing load on the consumer or evaporator, due to increased heat input, creates more refrigerant vapor. As a result, the evaporator contains less liquid refrigerant. The backflowing liquid refrigerant now ensures an increase in the level in the separator.

 

The following criteria should be considered when dimensioning the separator:

  1. Ensure a sufficient volume of the container above the liquid to ensure the separation of the liquid droplets flowing back from the evaporator circuit.
  2. A sufficient distance between the entry of the return line from the evaporator circuit and the suction line connection for the compressor circuit at the separator.
  3. A sufficient total volume of the separator, to compensate for the fluctuating fluid absorption of the evaporator, due to changed load, or during defrosting.
  4. Due to the low solubility of refrigeration oils with ammonia, oil settles at the lowest point in the separator. To remove it from the separator, it is recommended to design an oil sump with appropriate oil return.
  5. Ensuring a minimum fluid level to guarantee a cavitation-free pump operation. The eddy formation at the inlet or the suction pipes to the refrigerant pumps must be avoided at all costs.

Conclusion: The liquid separator is the central organ in the industrial NH3 refrigeration system with pump operation. The design, calculation and trim arrangement is crucial. An incorrectly dimensioned compressor is interchangeable, a wrong sized liquid separator is fatal and can be replaced only by very high effort. Thus one should know very well the requirements of the plant, also with regard to possible plant extensions.

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The deaerator for ammonia (NH3) refrigeration systems

Air in the refrigeration system raises the condensation pressure and makes the system a power hog.

 

Why breather?

 

Air in the refrigeration system raises the condensation pressure and makes the system a power hog.

Every operator of an industrial refrigeration plant knows that this accounts for the majority of its energy consumption. Since most of the engines are powered by 3-dig- it kW engines, every hour the engines are idle is worth a lot of money.

The use of a breather only makes sense for NH3 refrigeration systems in the deep-freeze range, which means that an evaporation temperature below -33 ° C prevails.

(The fact that the air is properly evacuated from the system for new refrigeration systems and for maintenance purposes is taken for granted here, meaning that this air entrapment is not discussed here.)

 

Where does the air come from?

 

If ammonia (NH3) were left open in a container under atmospheric pressure, it would have a temperature of about -33 ° C. In a closed refrigeration cycle, with an evaporating temperature below -33 ° C, the system operates under reduced pressure. No matter how small a leak on the suction side, air is permanently sucked into the system. This clarifies where the air comes from and why it is constantly multiplying.

The air is conveyed via the suction line through the compressor to the high pressure side and thus migrates into the condenser. It remains there now because air can not be liquefied or condensed.

Where air is, there can be no ammonia gas.

This means the air "claws" with its presence condensation surface in the condenser. As a result, the usable capacitor area becomes smaller and smaller.

A small condenser, which can no longer dissipate the heat sufficiently to the environment, leads in logical consequence to a higher condensation temperature and an increasing condensation pressure. As the condensing pressure increases, the drive motor of the compressor must do more work due to the higher differential pressure between suction and high pressure. More compressor work means more power consumption. This results in higher energy costs.

On the other hand, the higher pressure gas temperature due to the increased condensation pressure leads to an increased thermal load of the lubricating oil and thus to a reduction in lubricity. The oxygen content in the air also causes the oil to age prematurely.

In order to ensure perfect function and economic operation, the air has to be permanently removed from the system.

 

At which point of the refrigeration system does air collect?

 

At which point of the refrigeration system does air collect?

The air that enters the refrigeration cycle accumulates in the condenser where it is coldest and where the slightest gas movement occurs. Practical experience shows that the best way to collect air via a small attached dome at the condenser outlet is to have it vented. (Image)

 

 

But even in the subsequent pipelines or in the refrigerant collector itself can be vented.

For any refrigeration system, this venting port must be carefully selected and specified to ensure optimal venting. In case of doubt, several alternative connections should be provided, which can then be controlled alternately by means of solenoid valves.

 

The working principle of a deaerator

 

In order to separate refrigerants, especially NH3 and air, the different density of the gases is used. In contrast to air, NH3 can be liquefied even at relatively low pressures. Air, however, remains longer in the gas state.

In the deaerator, which builds on this principle of density differences, the air-refrigerant mixture is cooled down as much as possible, so that the main amount of NH3 liquefies.

The liquid refrigerant is returned to the refrigeration cycle, while the air with the lowest refrigerant content is drained via a water trap (open water tank).

Compared to a hand-vent, in which up to 50 g of refrigerant per kg of air can escape to the outside, by using a breather the refrigerant loss is reduced to 0.03 kg per kg of air.

 

construction

 

The deaerator is a fairly compact component, which mainly consists of a 1.30 m DN100-DN150 large pipe. In this pipe, a heat exchanger coil is installed and on the breather shell are some DN15-DN25 connections. These include the admission of the air-refrigerant mixture from the condenser, the refrigerant injection, the refrigerant suction, the connection for the air outlet, a level switch and boiling pressure or boiling temperature monitoring (eg RT280A).

 

function

 

The built-in container heat exchange coil is traversed in the ideal case with TK ammonia. Either by partial flow during pump operation or with high-pressure fluid based on DX injection.

The evaporating refrigerant in the heat exchanger coil is again brought to or sucked off via the return line to the TK separator.

The high-pressure air-refrigerant mixture from the condenser is passed through a dip tube to the bottom of the deaerator (below the heat exchanger coil) in a distribution chamber. The built-in nozzle bottom ensures a uniform distribution of the rising gas mixture.

The refrigerant contained in the mixture condenses on the cold heat exchanger coil. The non-condensable air rises in the heat exchanger and collects there. The refrigerant condensed out via the coil slowly causes the liquid level in the deaerator to rise to the level switch.

When the level switch responds, the liquid supply to the heat exchanger coil is closed by a solenoid valve changeover. The emptied solenoid valve at the bottom of the container, the condensed NH3 is brought from the container shell space through the coil to the return line and thus to the TK separator.

If enough air has accumulated in the upper part of the deaerator, the typical NH3 ratio changes from boiling temperature to boiling pressure. The control unit (RT280A or SPS with temperature and pressure transducer) now knows that there is only air in the apparatus and opens the air release solenoid valve.

The air is passed through a small control valve in a water tank, where the remaining portion of NH3, which is still bound in the air, to so-called. "Ammonia Spirit" is converted. If the pressure / temperature ratio or the set discharge time matches, the venting solenoid valve is closed again.

Depending on the amount of air in the system and the number of vents, the water tank will eventually reach a saturation point. It can then perceive the typical biting NH3 smell in the engine room.

If the smell of NH3 is detected, this is no reason to panic. It can be an indication that only the drain tank needs to get new water.

 

Special advantages of a Kreutzträger KALEX breather

Cantilever Refrigeration KALEX

 

  • A KALEX breather is made entirely of stainless steel and therefore can not corrode

  • A large heat exchanger surface ensures a clean separation between refrigerant and air during refrigerant liquefaction

  • The maintenance-friendly, reliable valves and switchgear ensure perfect automatic operation of the device

  • The venting device is completely pre-assembled with all fittings and controls, easy installation can be done with little effort

  • It can subsequently be integrated as an isolated solution with independent control or directly into an existing PLC

The KALEX is also flexible when it comes to supplying refrigerant. Depending on the system design and the installation site, either the pump supply line or the injection of high-pressure liquid can ensure the function.

With the KALEX air vent, the economic and technical disadvantages of air in the NH3 refrigeration system can be avoided, which increases the availability of the system.

Author: Fa. Kreutzträger Kältetechnik

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Hermetic compressors for refrigeration

The nameplate found on every hermetic compressor contains important information for the buyer or user and also for the delivery process.

Among the most important corner data are the type designation of the compressor, the type of refrigerant to be used as well as the electrical connection data.

At the suction port, the gaseous refrigerant is sucked into the compressor, on the opposite side of the compressor housing are the process and the compressed gas connection. The compressed refrigerant is conveyed from the compressor to the condenser via the compressed gas connection. Via the process connection the refrigerant is filled after the installation of the compressor. This is usually done at the end of the assembly line in the factory.

The electrical connection is located on the outside of the compressor. Depending on the model, the starting device is pre-assembled ex works with a suitable plastic protective cover.

The compressor housing is made of sheet steel, while the upper cover part is welded to the lower housing part. This connection is hermetically sealed, thus ensuring that no refrigerant can escape outside. 4 springs securely hold the mechanical unit (motor, cylinder and valve unit) of the hermetic compressor in the housing in the central position. Outside of the housing are two foot plates for mounting the compressor in the unit.

The engine is a very important part of the compressor, it consists of stator, rotor and power cable. To avoid a Schwingunsübertragung from the engine of the compressor to the compressor housing, the compressor motor is mounted on spring elements. This principle of construction keeps the working noise level low.

The stator consists of an iron package with two copper windings. The package consists of sheets. The windings are well protected, so that any loose wire pieces can not cause any damage. In contrast to the stator, the rotor has an iron core cast around with aluminum.

Each compressor type receives a matching electrical starting device. It serves to start the hermetic compressor; For this purpose, it supplies the necessary power to the auxiliary winding (starting winding) of the motor.

Once the compressor motor has reached a certain speed, the current consumption of the main winding decreases and the starting winding is switched off by the starting device. The motor receives its power via a flexible cable.

The unit of piston and cylinder consists of four parts:

  • Block
  • outlet pipe
  • crankshaft
  • piston

The output pipe is fixed on top of the block, at the bottom of the crankshaft is an oil pump. The piston includes a piston pin connected to it by connecting rod.

Inside the block are two outlet chambers that allow the refrigerant to reach the pressure port. The turns of the output tube make this even more flexible.

The crankshaft is fixed to the rotor, it transforms the rotary motion of the engine into piston strokes. Through these piston strokes, the cylinder moves up and down. The movement makes it possible for the refrigerant to be sucked in, compressed and finally pushed out at the pressure connection.

The valve unit includes an outlet valve and a suction unit, both mounted on the main valve plate. The valves open and close the valve discs during the intake and exhaust strokes.

As a result, the compressed refrigerant can reach the outlet chambers. Finally, there is also a silencer to dampen the intake noise. It sits between the suction connection and the suction side of the pump unit.

Components of a hermetic compressor

  • 1 housing with connections and foot plates
  • 2 upper lid
  • 3 block with stator holder
  • 4 stator (with screws)
  • 5 rotor
  • 6 valve unit (screws, cylinder cover, seals, valve plate)
  • 7 crankshaft with sleeve
  • 8 connecting rod with piston
  • 9 Ölansaugrohr
  • 10 springs with suspensions
  • 11 pressure tube (screw, washer, seal)
  • 12 Starting device (PTC starter, cover, strain relief for cables)

"With kind permission of Secop GmbH"

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Refrigeration - Introduction to the basics - Thermostatic expansion valve

1 Introduction 

A thermostatic expansion valve has a thermostatic element (1), which is separated from the valve housing by a membrane.

The element is connected by a capillary tube with a sensor (2), a valve housing with valve seat (3) and a spring (4).

 

Mode of action of a thermostatic expansion valve

The function of a thermostatic expansion valve is determined by three basic pressures:

P1:

Sensor pressure that acts on the top of the diaphragm and opens the valve.

P2:

Evaporator pressure acting on the underside of the diaphragm and closing the valve.

P3:

Spring pressure, which also acts on the underside of the diaphragm and closes the valve.

When the expansion valve controls, there is a balance between the sensor pressure at the top of the diaphragm and the evaporator pressure plus spring pressure at the bottom of the diaphragm.

The static overheating is set by means of the spring.

 

 

2. overheating

Overheating is the difference between the temperature measured at the sensor of the thermostatic expansion valve and the evaporation temperature.

The evaporation temperature is determined via pressure gauges on the suction side.

Overheating is indicated in Kelvin [K]

 

 

3. hypothermia

 The subcooling is defined as the difference between liquid temperature and condenser pressure / temperature at the inlet of the expansion valve.

The subcooling is given in Kelvin [K]. Hypothermia of the refrigerant liquid is necessary to avoid vapor bubbles in front of the expansion valve.

Steam bubbles reduce the performance of the expansion valve or reduce the liquid supply to the evaporator. A supercooling of 4-5 K is sufficient in most cases.

 

4. External pressure equalization

 Expansion valves with external pressure equalization must always be used when using liquid distributors. The use of manifolds generally results in a 1 bar pressure drop across the manifold and manifold.

Expansion valves with external pressure equalization should always be used in refrigeration systems with large evaporators or plate heat exchangers.

In these, the pressure drop is usually greater than the pressure corresponding to 2K.

 

 

5. Fillings

 A thermostatic expansion valve can have three different fillings:

1. Universal filling

2nd MOP filling

3. MOP filling with ballast (standard)

 

1. Universal filling

Universal fill expansion valves are used in most refrigeration systems. Operating conditions:

  • Pressure limitation (MOP) not required
  • Systems with high evaporation temperatures
  • Element may be colder than the probe

 

 

 

2nd MOP filling

Expansion valves with MOP filling are used in systems where it is necessary to limit the suction pressure during start-up, eg in freezer systems. All expansion valves with MOP have a very small filling in the sensor.

 

This means that the thermostatic element has to be warmer than the sensor. Otherwise, a charge shift may occur from the probe to the element, which inhibits the function of the expansion valve.

 

3. MOP filling with ballast (standard)

Expansion valves with MOP ballast fillings are preferably used in refrigeration systems with "highly dynamic" evaporators, eg in air conditioning systems and plate heat exchangers, which have a high transmission capacity with small internal volumes.

 

With the MOP ballast filling, 2 to 4 K less overheating can be achieved than with other types of filling.

 

 

6. Choice of thermostatic expansion valve 

The thermostatic expansion valve may be determined if the following is known:

  • - Refrigerants
  • - Evaporator power
  • - Evaporation temperature
  • - Condensing temperature
  • - hypothermia
  • - Pressure drop across the valve
  • - Internal or external pressure equalization

 

7. Designation 

The thermostatic element is provided on the top with a shield. The letters refer to the refrigerant intended for the valve:

  • X = R 22
  • Z = R 407C
  • N = R 134a
  • 50 R 410A
  • S = R 404A/ R 507

On the label are valve type, evaporation temperature range, possibly MOP point, refrigerant and the max. Working pressure PS / MWP specified.

 

The nozzle insert for T / TE 2 is marked with the nozzle size (eg 06) and the calendar week + the last digit of the calendar year (eg 279).

The number of the nozzle insert is also indicated on the lid of the plastic container for use.

 

For TE 5 and TE 12, the upper marking (TE 12) indicates for which valve type the insert is intended.

The lower marking (01) refers to the nozzle size. For TE 20 and TE 55, the upper marking (N / B 50/35 TR) refers to the nominal power in the two evaporation temperature ranges N and B as well as on the

(50/35 TR = 175 kW in the N range and 123 kW in the B range). The lower marking (TEX 55) indicates for which valve type the insert can be used.

 

8. Assembly 

The expansion valve must be mounted in the liquid line in front of the evaporator and its sensor mounted as close as possible to the suction line behind the evaporator.

In the case of external pressure compensated valves, the equalizing line must be located immediately near the sensor on the suction line.

 

The sensor is mounted on a horizontal pipe on the suction pipe, in a position that, when compared to the dial of a clock, corresponds to the time between 1 and 4 o'clock. The attachment depends on the outside diameter of the pipe.

 

Attention!

The probe should never be attached to the bottom of the suction line as it will pick up the wrong signals when there is oil in the bottom of the tube.

 

The sensor should determine the temperature of the superheated suction steam and must therefore not be installed so that it can be influenced by external heat and cold.

The Danfoss sensor clamp allows a firm and secure mounting of the sensor on the pipe. This ensures that the sensor has the best possible thermal contact with the suction line. The TORX design of the screw makes it easy for the installer to transfer the torque from the screwdriver to the screw without forcing the tool into the notch of the notch and damaging the notch.

 

The sensor should not be mounted behind an additional heat exchanger as this attachment will result in a distorted signal to the expansion valve.

 

The sensor should not be installed near large components as this will also result in a distorted signal to the expansion valve.

 

The sensor is, as mentioned above, to attach to the horizontal part of the suction pipe immediately behind the evaporator and must not be mounted on a manifold or riser behind an oil sack.

 

The sensor must always be installed in front of an oil lifting bow.

 

9. Setting the static overheating 

The expansion valve comes with a factory setting, which in most cases does not need to be corrected.

If a readjustment is necessary, this is done by means of the adjusting spindle on the expansion valve.

Turning clockwise increases the expansion valve overheating and decreases it by turning it to the left (counterclockwise).

For T / TE 2, one revolution of the spindle results in a change in overheating at 0 ° C evaporation temperature by about 4K.

 

For TE 5 and subsequent sizes, one revolution of the spindle will result in a change in overheat at 0 ° C evaporation temperature of about 0.5 K.

For TUA and TUB, one revolution of the spindle results in a change of overheating at 0 ° C evaporation temperature by about 3K.

 

Oscillations in the evaporator can be eliminated by the following procedure: Increase overheating by turning the adjusting spindle to the right so that oscillation ceases. Then turn the adjusting spindle stepwise to the left until the oscillation starts again.

From this position, turn the spindle clockwise approximately once (but only 1/4 turn for T / TE2 valves).

The plant is no longer commuting, the evaporator is fully utilized. A fluctuation in overheating by ± 1 K is not considered as commuting.

 

Excessive overheating in the evaporator may be due to insufficient refrigerant fluid. The overheating is reduced by gradually turning the adjusting spindle to the left until oscillation is detected.

From this position, turn the spindle clockwise approximately once (but only 1/4 turn for T / TE2 valves).

In this setting, the evaporator is fully utilized. A fluctuation in overheating by ± 1 K is not considered as commuting.

 

10. Replacing the nozzle insert 

If no steady-state condition is reached in the procedure described above, the nozzle insert is to be replaced by a smaller one.

If the overheating of the evaporator is too great, the valve capacity is too low and the nozzle insert must be replaced with a larger one.

TE, T2, TUA and TCAE are supplied with replaceable nozzle insert.

 

11. Hunting 

With a significant oversizing of an expansion valve, it can come to the so-called "hunting". "Hunting" is the English word for "hunt" and describes the commotion of overheating and concomitantly the suction pressure. Hunting shows that the overheating, although on average, terminates at an acceptable level, but increases constantly and then decreases again, so that the system does not reach the steady state.

The main reason for this is an oversized power nozzle. Remedy can bring the replacement of the nozzle towards a smaller capacity. However, the same phenomenon also occurs, for example, if the actual values ​​at the valve are set too small, if they are smaller than the MSS (minimum stable signal) of the evaporator.

For example, an overheat setpoint of 4K combined with a plate heat exchanger type evaporator will almost always result in commutation of overheating. A corresponding increase in the superheat setpoint can remedy this situation.

 

12. Too much overheating 

Causes of excessive overheating on the evaporator can be, for example: flash gas upstream of the expansion valve, MOP charge transfer, too small a nozzle, loss of refrigerant or sensor loss. Flash gas formation can be easily detected in the sight glass. The sight glass should be free of bubbles under normal operating conditions. With strong flash gas formation, it may be necessary, for example, to install an internal subcooler to introduce additional subcooling into the liquid reservoir upstream of the expansion valve. Supercooling is the additional sensible temperature reduction below the wet steam level at the appropriate pressure.

Example: The service manometer reads 10.6 bar. This corresponds to 45 ° C at R134a. If one measures now before the expansion valve at the liquid line 40 ° C and neglected for the sake of simplicity, any pressure drops, then there is a supercooling of 5 K (Kelvin pressure differences are always in Kelvin (K)). Flash gas is formed by partial evaporation of the liquid by overcoming differences in height as liquid riser or by pressure drops in the liquid line. MOP charge transfer can occur when the thermal head of the expansion valve is colder than the sensor.

Thus, the refrigerant of the sensor charge shifts to the colder place, in the thermal head and the sensor no liquid refrigerant is left over to vaporize and thus increase the pressure in the sensor. The "MOP restocking test" is simple: just warm the thermal head. If the problem disappeared after that, then there was a MOP-filling shift.

 

13. Design example 

Now we come to the interpretation of an expansion valve by means of data sheet. The following classic example should be considered: a cooling chamber with 7 kW at -10 ° C evaporation, 45 ° C condensation and 10 K subcooling, the refrigerant R134a is used. For the selection of the data sheet refrigerant and evaporation temperature are crucial.

In our case this means: Use of the data sheet for R134a and evaporation temperature range +10 to -40 ° C. First, the cooling capacity must be corrected with the overcooling correction factor, which can be read on the small table below (see table "Design example" on this page above). At 10K it is 1.08. 7 (kW) divided by 1.08 gives 6.5 (kW).

Since "-10 ° C" is explicitly stated as a separate table, the appropriate nozzle size for the cooling capacity of 6.5 kW can be read directly. In addition, the pressure drop across the expansion valve is needed because the condensing temperature is not specified directly in the data sheet.

The refrigerant spool remedies this situation: Here we can see that the liquefaction and evaporation temperatures are 9.6 bar (10.6 bar - 1 bar) apart

In order to meet the pressure drop across the distributor on the evaporator, we deduct a further 1 bar from the result. This results in 8.6 bar. We are guided by 8 bar Δp, 6.5 kW cooling capacity, -10 ° C evaporation temperature and can now look for the right nozzle size. The value is 6.9 and the associated nozzle size is 5.

Thus, a "TEN 2" valve ("E" stands for external pressure equalization and "N" stands for the refrigerant R134a) can be selected with nozzle 5 for our refrigerator in this example.

 

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Electronic Expansion Valves (EEV) for refrigeration systems

Electronic expansion valves in a refrigeration system

  01. Introduction 02. small overheating higher evaporation pressures better COP 03. permanent optimization of overheating 04. any MOP point 05. Operation 06. Service 07. manually switch output relay 08. steady electronic valves 09. Pulse width modulation 10. Dimensioning of the liquid line 11. Humidity  

1 Introduction

  Optimum evaporator filling, even in the event of heavy load fluctuations, flexible MOP points and the highest possible evaporation temperatures to increase energy efficiency, are always an issue for plant engineers and operators in refrigeration technology. These requirements can often not be sufficiently taken into account with the usual thermostatic expansion valves. Electronic expansion valves, on the other hand, are ideal for this purpose.   

2. small overheating higher evaporation pressures better COP

  The advantages of electronic overheating control are obvious. The evaporator is always optimally filled with refrigerant. Even with strong power fluctuations, so the most diverse partial load cases, the amount of refrigerant to be injected can be precisely metered. This is done by passing the current overheating in the evaporator via a pressure transmitter (marked "P" in Figure 1) and a very sensitive temperature sensor (Figure 1 "S2") to the controller "EKC 315A" in a timely manner.

 

Der Regler kann nun Maßnahmen ergreifen, um optimal kleine Überhitzungen zu erreichen. Diese adaptive Reglung der Kältemitteleinspritzung führt zu einer optimalen Nutzung des Verdampfers und damit zu den höchst möglichen Verdampfungsdrücken, die in dieser spezifischen Anlage realisierbar sind. Dies führt nicht nur zu höheren COP-Werten, sondern auch zu Energieeinsparungen, dennder COP-Wert ergibt sich aus dem Quotienten aus Kälte- und Antriebsleistung.

 

3. permanente Optimierung der Überhitzung

 

Die Überhitzung passt sich immer dem minimal stabilen Signal (MSS-Linie) des Verdampfers an, so dass es zu keinem Abdriften des Signals in den instabilen Bereich (Abbildung 2 – links der MSS-Linie) kommen kann. Der Regler „EKC315A“ pickt sich dabei zunächst einen beliebigen Überhitzungssollwert heraus, z.B. 8 K. Dann versucht er, diese 8 K als Sollwert in der Anlage zu realisieren. Da hier alle Informationen, d.h. Überhitzungstemperaturwert ,,S2“ und momentaner Verdampfungsdruck ,,P“, zusammenlaufen und außerdem zur Optimierung der Regelungsfunktion zu diesen beiden Werten eine aufgelaufene Historie abgespeichert ist, kann der Regler leicht entscheiden, ob der momentan angestrebte Wert bei den gerade herrschenden Lastverhältnissen machbar ist.

Schwankt beispielsweise der Verdampfungsdruck stark und ergeben sichschnell wechselnde Überhitzungswerte, so ist das eine Zeichen dafür, dass ein höherer Überhitzungssollwert angestrebt werden sollte. Bleiben allerdings Überhitzung und Verdampfungsdruck weitgehend konstant, so kann mit einem niedrigeren Überhitzungssollwert, z.B. 7 K (dann 6 K, 5 K usw.), fortgefahren werden. Die permanente Überprüfung der optimalen Überhitzung ist ein entscheidender Vorteil elektronischer Expansionsregelung gegenüber rein thermostatischen Ventilen. Diese müssten schon im Voraus auf den maximalen Überhitzungssollwert, den die individuelle MSS-Kennlinie der Anlage beschreibt, eingestellt werden. Dieser Wert ist jedoch nicht so einfach festzustellen, so dass bei einem mechanisch-thermostatischen System diese ohnehin schon schlechtere Ausgangsposition meist noch dadurch verschlechtert wird, dass der Monteur bei der Inbetriebnahme einen ,,Sicherheitszuschlag“ zu dem nötigen minimalen Überhitzungswert addiert. Im Hinblick auf die Funktionssicherheit einer Anlage ist das nicht falsch, denn eine etwashöhere Überhitzung ist einem zeitweisen ,,Durchschießen“ sicher vorzuziehen. Allerdings beeinflusst diese Maßnahme die energetischen Sachverhalte in der Anlage negativ. Bei dem elektronischen „EKC 315A“- Einspritzsystem fällt dieser „Sicherheitszuschlag“ weg, da sich das System hinsichtlich der Überhitzung, wie beschrieben, selbst einregelt.

 

4. beliebiger MOP Punkt

 

Ein wichtiger Punkt, der bei elektronischer Einspritzregelung regelmäßig unterschlagen wird, ist die freie Wahl des MOP-Punktes. Bei dem MOP-Punkt handelt es sich wie schonbei denthermostatischen Expansionsventilen beschrieben um den maximalen Verdampfungsdruck („maximum operating pressure“), mit dem das Expansionsventil arbeitet. Während es bei thermostatischen Expansionsventilen grundsätzlich nur ganz bestimmte MOP-Punkte gibt, für die auch jedes Mal ein anderes Bauteil ausgewählt werden muss (z.B. -20 °C für Tiefkühlanwendungen oder +15 °C als „Klima-MOP“), ist dieser Punkt bei elektronischen Expansionsventilen frei wählbar und kann im Bedarfsfall auch nachjustiert oder komplett abgeändert werden. So kann in den meisten Fällen komplett auf den Einsatz eines Startreglers verzichtet werden, was speziell bei größeren Anlagen einer deutlichen Kostenersparnis gleichkommt. Gleichzeitig ist die Einstellung des gewünschten Sollwertes bei der elektronischen Variante schneller und eleganter zu bewerkstelligen als bei einem mechanischen Startregler.

 

5. Bedienung

 

Die Bedienung des Reglers erfolgt über zwei Drucktasten. Über diese beiden Tasten, kombiniert mit einem dreistelligen Display, lässt sich der Regler komplett programmieren, wobei alle wichtigen Daten angezeigt werden. Somit ist es jedem Monteur an der Anlage möglich, in den Regelkreis einzugreifen oder sich relevante Daten anzeigen zu lassen. In dem Menü für den Regler erscheinen nicht nur grundsätzlich einstellbare Werte wiezum Beispiel der Kältemitteltyp, sondern es ist außerdem auch möglich, über das Eingreifen in Stabilitäts- und Verstärkungsfaktoren genau Einfuss auf bestimmte Abläufe zu nehmen. Das gilt zum Beispiel für die Überhitzungsregelung, so dass ein Pendeln der Überhitzung verhindert werden kann. Weiterhin lässt sich zwischen adaptiver und belastungsabhängiger Überhitzungsregelung wählen. Die adaptive Überhitzungsregelung wurde hier bereits ausführlich beschrieben. Bei der belastungsabhängigen Überhitzungsregelung werden in bestimmten Teillastfällen absichtlich höhere Überhitzungen gefahren, um beispielsweiselängere Verdichtermindestlaufzeiten zu gewährleisten oder um das Bereifungsbild des Verdampfers positiv zu beeinflussen. Damit könnte dann auf die eine oder andere Abtauung verzichtet werden.

 

6. Service

 

Besonders interessant für den Monteur bei der Inbetriebnahme aber auch beim Service an der Anlage ist das Servicemenü der elektronischen Überhitzungsregler. Alle Parameterwerte, die mit „u“ beginnen, zeigen Anlagen-Istwerte an, die für alle Arten von Fehlerdiagnosenbzw. für die Bewertung von Anlagenzuständen wichtig sind. Dabei sind besonders die drei Parameterwerte „Anzeige der Überhitzung“, „Anzeige der Temperatur am S2- Fühler“ (bedeutet am Verdampferausgang) und „Anzeige der Verdampfungstemperatur“ zu beachten. Diese drei Werte geben Aufschluss über den Anlagenzustand. Zum einen sind sie schnell auslesbar und müssen nicht mühsam mit dem Servicemanometer und dem Monteur-Temperaturmessgerät ermittelt werden. Zum anderen sieht man sofort, welche Werte der Regler als gegeben annimmt. So gehört es zur Standardvorgehensweise eines erfahrenen Monteurs, bei elektronischen Systemen vor der eigentlichen Inbetriebnahme zunächst alle Fühler zu überprüfen (bei den üblichen Widerstandsfühlern ist dies recht einfach mit einem Ohm-Messgerät möglich. So hat ein P T1000-Fühler bei 0 °C einen Widerstand von 1000 Ohm), um langwierigen Fehlersuchen bei falsch durch den Sensor aufgenommenen Istwerten vorzubeugen. Durch einen Blick in das Servicemenü entfällt diese Vorgehensweise, denn hier kann direkt beurteilt werden (im Zweifelsfall selber mit dem Thermometer oder Manometer nachmessen), ob der Wert realistisch ist oder nicht.

 

7. manuell Ausgangsrelais schalten

 

Ähnlich wichtig wie die Eingänge sind natürlich auch die Ausgänge des Reglers. Um speziell diesen Punkt bei der Inbetriebnahme zu vereinfachen, bietet das Reglermenü die Möglichkeit, die Ausgänge für das „AKV“- Ventil, das Magnetventil und den Alarmausgang manuell zu übersteuern. Typisch für Regelprobleme ist die Frage, ob der Regler den Ausgang nicht schaltet, weil er dies aus irgendwelchen Gründen nicht für erforderlich hält, oder weil der den Ausgang z.B. wegen eines Defekts nicht schalten kann. Dieser Punkt hat selbst erfahrene Monteure schon Stunden und Tage an Fehlersuche gekostet. Aus diesem Grundempfiehlt es sich, bei einer Inbetriebnahme grundsätzlich die entsprechenden Ausgangsrelais einmal gezielt einzeln auszuprobieren. So werden auch schnell Verdrahtungs- und Zuordnungsfehler aufgeklärt.

 

8. stetige elektronische Ventile

 

Grundsätzlich besteht die Möglichkeit, mit einem „ETS“-, „ICM“- oder einem „AKV“- Ventil zu arbeiten. Diese Stellglieder unterscheiden sich wie folgt:

 

Das „ETS“- und „ICM“-Gerät ist ein stetiges Ventil, das z.B. gerne bei Kaltwassersätzen mit dem Kältemittel R407C eingesetzt wird, wenn jedes Grad Überhitzung zählt und selbst geringste Schwankungen des Verdampfungsdrucks vermieden werden sollen. Der Regler kann als P-, PI- oder PID-Regler eingesetzt werden. Bei der P-Regelung handelt es sich um eine Standardregelung gemäß der Abweichung (Beispiel: Wird die Überhitzung zu groß, wird der Öffnungsgrad des Ventils immer mit der gleichen Geschwindigkeit erhöht). Bei der PI-Regelung kann die „Nachstellzeit“ („I- Anteil) gesondert verändert werden. Das heißt, die Reaktionsgeschwindigkeit der Regelung kann verändert werden, mit anderen Worten, die Regelung wird nervöser oder träger. Beides kann erforderlich sein. Der „D-Anteil“ bei der PID-Regelung optimiert außerdem die Regeleigenschaften bei plötz- licher Sollwertänderung. Dieser Regelmodus ist besonders dann ratsam, wenn das Einspritzsystem mit einer externen Überhitzungssollwertschiebung – z.B. von einer übergeordneten Regelung – betrieben wird. Beim Einsatz eines „ETS“- bzw. „ICM“-Ventils sollte zusätzlich ein Magnetventil in der Flüssigkeitsleitung vorgesehen werden, das ebenfalls vom „EKC“ ansteuerbar ist. Falls tatsächlich auf das Magnetventil verzichtet werden muss, dann ist es unabdingbar, eine USV (unabhängige Spannungsversorgung)an das Stellglied („ICM“) bzw. an den Regler („ETS“) anzuschließen. Das ist unbedingt notwendig, da ein „ICM“ oder „ETS“ im Fall eines plötzlichen Spannungsausfalls auf seiner momentanen Öffnungsposition stehen bleibt und damit weiter Kältemittel in den Verdampfer einspritzt, was zu größeren Schäden bis hin zum Verdichterausfall führen kann. Mit einer USV kann das Ventil selbst in einem solchen Fall immer noch geschlossen werden.

 

9. Pulsweitenmodulation

 

Bei Pulsweitenmodulation und dem Einsatz eines „AKV“-Ventils kann auf ein zusätzliches Magnetventil in der Flüssigkeitsleitung verzichtet werden, da dies den Durchfluss der Flüssigkeitsleitung auch dauerhaft schließenkann und bei Spannungsausfall automatisch in die geschlossene Position zurückfällt.

Die Kombination „EKC-AKV“ arbeitet nach dem Prinzip der Pulsweitenmodulation. Das bedeutet, dass das „AKV“-Ventil je nach Öffnungsgrad des Ventils für eine bestimmte Zeit komplett geöffnet und für den Rest der Periodenzeit wieder vollständig geschlossen wird. Bei einem Öffnungsgrad von 50 % und der Standard-Periodendauer von 6 s (diese Größe ist veränderbar) hieße das beispielsweise, 3 Sekunden offen und 3 Sekunden zu (bei 20 % Öffnungsgrad entsprechend 1,2 s auf und 4,8s zu).

 

10. Dimensionierung der Fluessigkeitsleitung

 

Speziell beim Einsatz pulsweitenmodulierter Ventile sollte der Dimensionierung der Flüssigkeitsleitung ein hoher Stellenwert eingeräumt werden. Allgemein wird in der einschlägigen Fachliteratur bei der Auslegung des Durchmessers der Flüssigkeitsleitung eine Geschwindigkeit von 0,5 m/s als Richtwert genannt. Schaut man sich jedoch real existierende Kälteanlagen an, so liegt dieser Wert eher bei ca. 1 m/s. Da dies für normale (stetige) thermostatische Expansionsventile in der Regel kein Problem darstellt, hat sich dieser Richtwert weitgehend eingebürgert. Bei pulsweitenmodulierten Ventilen sieht es jedoch anders aus. Hier sollte man sogar soweit gehen und die 0,5 m/s auf die maximale Ventilleistung und nicht nur auf die Verdampferleistung auslegen. So würden die meisten Anlagenbauer wohl bei einem 7 kW-Verdampfer mit R404A und -10 °C Verdampfung (ohne besondere Unterkühlung) ein 12er-Kupferrohr als Flüs- sigkeitsleitung auslegen. Die Strömungsgeschwindigkeit wäre mit 0,92 m/s durchaus im üblichen Rahmen. Bei „strikt 0,5 m/s“ wäre dann jedoch ein 15 mm- bzw. 16 mm- Rohr mit ca. 0,5 m/s erforderlich. Wird zu guter Letzt noch die Ventilleistung statt der Verdampferleistung berücksichtigt, könnte sogar ein 18 mm-Rohr notwendig sein (Beispiel: Ventilleistung 10 kW. „AKV“-Ventile werden nie auf 100 % Öffnungsgrad ausgelegt – Öffnungsgrade zwischen 30 und 70 % sind anzustreben). Berücksichtigt man diesen Grundsatz, so treten in der Regel keine beschleunigten Flüssigkeiten im System auf, welche sonst gerne zu röhrenden Geräuschen und zu schwingenden Rohrleitungen führen können. Dann steht ein solches System elektronischer Einspritzung auch in Punkto Langlebigkeit und Zuverlässigkeit einem traditionell thermostatischen System in nichts nach.

 

11. Feuchte

 

Kältefachleute wissen, dass das Thema „Feuchte“ in der Kältetechnik und ganz besonders bei unverpackter Ware, Fleisch, Gemüse und Obst eine wichtige Rolle spielt. Trotzdem wird dieses Thema hier nicht so offensiv angepackt wie in der Komfort-Klimatechnik, in der entsprechende Hygrostate und besonders Dampfbefeuchter bei zu geringer Feuchte fächendeckend genutzt werden. Dennoch kann es auch in Kälteanlagen je nach Situation erforderlich sein zu entfeuchten. Hierfür werden üblicherweise indirekte Maßnahmen eingesetzt wie z. B. die Veränderung der Verdampferlüfterstufen bzw. der Verdampfergeschwindigkeit. (D.h. langsamere Ventilatorgeschwindigkeit = tiefere Verdampfungstemperatur = Entfeuchtung und umgekehrt). Mit einem elektronischen Einspritzsystem kann dieser Punkt direkt beeinflusst werden: Einfach den Überhitzungssollwert mit einem externen Signal von 4-20 mA schieben und schon wird bei kleinstmöglichen Überhitzungswerten keine oder kaum Entfeuchtung bzw. bei großen Überhitzungswerten hohe Entfeuchtung erzielt. Natürlich gilt auch hier, dass der Taupunkt für eine Entfeuchtung immer unterschritten werden muss. Die Einjustierung eines solchen Systems kann durch die ausgeschiedene Kondensatmenge am Verdampfer stets recht einfach vorgenommen werden. Ein solches System eignet sich neben Gemüse- und Obstlagerung im Übrigen auch für Komfortklima-RLT-Anlagen und Klimaschränke.

 

Stephan Bachmann,

Danfoss Kältetechnik, Offenbach

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Magnetventile für Kälteanlagen

Magnetventile für kälteanlagen

 

  1. Aufbau
  2. Dimensionierung
  3. Anordnung
  4. Montage
  5. Anwendung
  6. MOPD
  7. Die Spule
  8. NC / NO

 

 

1. Aufbau

 

Wie ist ein solches Magnetventil aufgebaut? Allgemein setzt sich ein Magnetventil aus einer Spule und aus einem Ventilgehäuse zusammen. Die Spule ist auf einem Ankerrohr montiert. Bei kleineren, direktgesteuerten Ventilen öffnet und schließt der bewegliche Anker das Ventil, indem er direkt den Ventilsitz freigibt oder schließt. Um bessere interne Dichtigkeit zu erreichen, ist der Teil des Ankers, der auf den Ventilsitz trifft, mit einem Teflondichtplättchen versehen. Beispiele für solche direktgesteuerten Magnetventile sind die Danfoss-Typen „EVR 2“ und „3“. Bei servogesteuerten Magnetventilen erfolgt die Ankerbewegung in gleicher Weise. Allerdings wird nun anstelle des gesamten Ventilsitzes eine Servobohrung geschlossen oder geöffnet. Bei Servoventilen mit Membrane führt dies zu einer Bewegung der Membrane über die am Ventil anstehenden Differenzdrücke, die dann dem Öffnungs- bzw. Schließvorgang des Ventils entspricht. Bei Servomagnetventilen mit Kolben und ohne Membrane ist das Prinzip gleich. Auch hier wird das Ventil über die Servobohrung geöffnet und geschlossen – allerdings mittels Kolbenmechanismus und nicht mit Membrane. eine grobe Clusterung nach Leistungsgrößen ist durchaus sinnvoll. So sind die Magnetventile kleinster Leistungsgröße, wie beispiels- weise „EVR 2“ und „3“, direktgesteuert. Beiden größeren Anlagen schließen sich dann die Baugrößen „EVR 6“ bis „EVR 22“ an, die allesamt mit Membrane ausgestattet und servogesteuert sind. Schließlich ndet man bei ganz großen Leistungen mit Trockenexpansion die Magnetventile „EVR 25“ bis „40“. Dieses sind dann servogesteuerte Kolbenventile. Sollten diese Größen auch nicht mehr ausreichen, so werden Hauptventile (z.B. Danfoss „ICS“- oder „PM-Ventile“) einfach mittels Magnetventilaufsatz (EVM) zu einem Magnetventil gemacht. Diese Kombinationen lassen dann hinsichtlich der Größe der Anlage kaum mehr Wünsche offen.

Ein praktischer Tipp:

Öffnet man ein Magnetventil und findet darin weder eine Membrane noch einen Kolben, dann handelt es sich in aller Regel um ein direktgesteuertes Ventil. Dies wird dann auch kleinere Anschlüsse haben, wie 6, 8 oder 10 mm.

 

2. Dimensionierung

 

Warum ist es für den Praktiker überhaupt wichtig zu wissen, ob er es mit einem direkt- oder servogesteuerten Magnetventil zu tun hat? Tatsächlich ist dieser Punkt für die Dimensionierung der Ventile von entscheidender Bedeutung. Direktgesteuerte Magnetventile benötigen keinen Mindestdruckabfall für den Betrieb. Aus diesem Grund haben diese Ventile eine extrem gute Teillastfähigkeit, die es ermöglicht, einen moderaten Druckabfall für den Volllastfall zu projektieren (Ventile „EVR 2“ und „EVR 3 “). Bei zwangsservogesteuerten Ventilen (z.B. Typen wie „EVRAT“ oder „EVRST“ – der Buchstabe „T“ steht hier für die Zwangsservosteuerung) gelten die gleichen Auslegungskriterien wie für die direktgesteuerten. Auch hier gibt es keinen Mindestdruckabfall, dem Rechnung getragen werden muss. Bei servogesteuerten Ventilen („EVR 6“, „10“, „15“, „20“, „22“, „25“, „32“ und „40“) hingegen muss neben dem maximalen Druckabfall auch der minimaleTeillastfall betrachtet werden. So darf bei minimaler Teillast die minimale Druckdifferenz, die das Ventil braucht, um stabil arbeiten zu können, nicht unterschritten werden. Diese Mindestdruckdifferenz des Ventils ist aus den entsprechenden technischen Datenblättern ersichtlich. Beispiel: ein „EVR 10“ hat einen benötigten Mindestdruckabfall von 0, 05 bar. Bei 20 kW kälteleistung, R134a und -10 °CVerdampfung, also Normalkühlung, und einem Einbau in der Flüssigkeitsleitung wäre das „EVR 10“ zunächst keine schlechte Wahl, denn 0,06 bar Druckabfall in der Volllast liegt über 0,05 bar und ist somit in Ordnung. Sollten jetzt jedoch z.B. zwei gleich große 10 kW-Verdichter im Verbund geschaltet auf diesen Kältekreis drücken, dann wird der Mindestdruckabfall bei Betrieb von nur einem Verdichter unterschritten. Rechnerisch wäre dann nur noch 0,02 bar Druckabfall gegeben. Somit sollte in diesem Fallbeispiel dem „EVR 6“ der Vorzug gegeben werden. Bei „eVR 6“ ist der Mindestdruckabfall des Ventils auch 0, 05 bar. Der Volllastdruckabfall ist 0,36 bar und der Teillastdruckabfall 0,09 bar. Beide Werte sind größer als 0,05 bar. Somit arbeitet das Ventil in jedem anzunehmenden Betriebszustand stabil. Sollten einmal trotz intensiver Anstrengungen für Volllastkälteleistungen, bei denen üblicherweise Magnetventile ab der Größe „EVR 6“ einzusetzen wären, aufgrund einer zu niedrigen Teillast keine passenden Ventile zu finden sein, so kann technisch auf ein entsprechendes Hauptventil ausgewichen werden.

Praxistipp:

Kleine Hauptventile der Baureihe „PM“ oder „ICS“ mit Pilotventil „EVM“ sind sehr gut teillastfähig und oft auch dann noch einsetzbar, wenn mit den Standard-Magnetventilen „EVR“ die entsprechenden Teillasten nicht mehr gefahren werden können. Nachteil dieser Ventilkombinationen ist der höhere Preis im Vergleich zum Standard „EVR“. eine weitere Lösung für solche Teillastfälle können zwangsservogesteuerte Ventile (Mindestdruckabfall 0 bar) sein. Diese Ventile wie „EVRAT“ und „EVRS T “ wurden ursprünglich für Ammoniak konzipiert, sind aber auch für die „Kupferkälte“ einsetzbar.

 

3. Anordnung

 

 

Der Haupteinsatzbereich von Magnetventilen ist die Flüssigkeitsleitung. Geschätzte 95 % aller Magnetventile in der Kältetechnik werden dort verbaut. Dabei ist eine Platzierung des Magnetventils nahe am Expansionsventil ratsam, aber nicht zwingend erforderlich. Man minimiert so die Gefahr des Auftretens beschleunigter Flüssigkeiten. Da aber dieser Effekt (er macht sich bemerkbar durch schwingende Rohrleitungen und Schlaggeräusche beim Öffnen des Magnetventils) in gewerblichen Kälteanlagen eher selten ist, kann das Magnetventil beliebig angeordnet werden, wenn dies die baulichen Gegebenheiten nahelegen. Die Frage, ob ein Magnetventil zwingend vor oder hinter der Trockner-Schauglas-Gruppe montiert werden sollte, ist eher eine „Glaubensfrage“. Setzt man das Magnetventil in Flussrichtung vor dem Schauglas, so kann man den Absaugvorgang beobachten, falls die Anlage in „Pump down“ oder „Pump out“ geschaltet ist. Diese Anordnung ist aber nicht zwingend so vorgeschrieben.

 

4. Montage

 

 

Magnetventile für die „Kupferkälte “ sind entweder mit Bördelanschlüssen oder mit Lötanschlüssen ausgestattet. Die Rohranschlüsse der Bördelanschlüsse können klassisch mittels Bördelglocke am Montagerohr oder unter Verwendung von Bördeladaptern angeschlossen werden. Bördeladapter bieten den Vorteil, dass diese Schraubverbindung dann nicht mehr als Bördelverbindung gilt und somit gewisse Einschränkungen, die in der norm EN 378 aufgeführt sind, nicht mehr greifen. Außerdem ist im Servicefall ein Austausch ohne Lötvorgang möglich. ein Nachteil ist, dass dann trotzdem noch an beiden Anschlussenden Lötungen ausgeführt werden müssen. Bei Lötanschlüssen wird klassisch direkt am Ventil hartgelötet. Dazu ist ein Zerlegen des Magnetventils in der Regel nicht notwendig. Die Verwendung eines kühlenden nassen Lappens reicht normalerweise aus. Magnetventile „EVR “ sollten in einen horizontalen Rohrabschnitt vorzugsweise mit der Spule (Ankerrohr) nach oben eingebaut werden. Unter schwierigen Montageverhältnissen darf das Ankerrohr auch bis zur horizontalen gedreht werden. (Rohrleitungsanschlusse waagerecht und Richtung Ankerrohr, das zur Seite wegschaut). Zwischenstellungen zwischen diesen beiden Extremstellungen sind auch denkbar.

 

5. Anwendung

 

„EVR“-Magnetventile für Kältemittel können aber nicht nur in der Flüssigkeitsleitung eingesetzt werden. Auch die Anwendung in der Heißgas-, Kondensat-, Saug- und Heißgasbypassleitung ist möglich. im Heißgas-, Heißgasbypassleitungsbetrieb und in Heißgasspeiseventilen für Heißgasabtauung sollte besonders auf die maximal zulässigen Medientemperaturen der Magnetventile geachtet werden. Beim „EVR“ sind dies 105 °C. Bei Magnetventilen für die Saugleitung ist der interessantere Wert die minimale Medientemperatur. Diese ist bei „EVR“ -40 °C. Dabei ist zu berücksichtigen, dass hier selbst Verdampfungstemperaturen von -45 °Ckein Problem sind, da das Kältemittel in der Saugleitung bereits überhitzt ist. Das heißt, zu der Verdampfungstemperatur von -45 °Csind mindestens 7 k zu addieren. Mit diesen rechnerischen -38°Cliegt man wieder voll im Anwendungsbereich. Falls ein Heißgasbypassmagnetventil zusätzlich zu einem Heißgasbypassregler eingebaut werden soll, kann getrost ein „EVR“ ausgewählt werden. Soll dieses Magnetventil jedoch auch Regelaufgaben übernehmen und minütlich getaktet werden, dann sollte ein spezielles Magnetventil Typ „EVRP “ für hohe Taktraten eingesetzt werden.

 

6. MOPD

 

Ein interessanter Punkt bei Magnetventilen ist der „MOPD“. „MOPD“ heißt „maximum opening pressure differential“ und steht für den maximalen Öffnungsdifferenzdruck, der von der betreffenden Ventil- Spulenkombination gehalten werden kann. Dieser „MOPD “ hängt maßgeblich von dem Magnetventiltyp, aber auch von der verwendeten Spule ab. So kann beispielsweise ein „EVR 3 “ mit einer 10 W- Wechselstromspule 21 bar und mit einer 12 W-Wechselstromspule schon 25 bar halten. Dieser Punkt ist beim Einsatz in der Flüssigkeitsleitung und im normalen Kühlbetrieb kein Problem. Wird dann aber z.B. durch Schließen des Magnetventils abgesaugt und über den Niederdruckschalter die Anlage abgeschaltet, muss vom Magnetventil der volle Differenzdruck zwischen Hoch- und Niederdruckseite gehalten werden. ein Beispiel hierfür wäre bei normalkühlung R134a -10 °CVerdampfungstemperatur = 1 bar Manometerdruck(Überdruck) und 45 °C Ver üssigungstemperatur = 10,5 bar. Damit muss das Magnetventil (10,5 – 1 =) 9,5 bar halten können. Das ist meist problemlos möglich. Bei R404 A oder R507 sind diese Druckwerte meist höher. in diesen Fällen sollte man die „MOPD “- Thematik im Hinterkopf behalten. Als Praxistipp kann gelten, im Zweifelsfall eine 10 W-Standardspule durch die „stärkere “ 12 W-Spule zu ersetzen. Dies ist kein großer Aufwand, hat nie negative Auswirkungen und hilft eventuell in einem Grenzfall.

 

7. Spule

 

 

Ein wichtiger Teilaspekt bei Magnetventilen ist die Spule. Die Montage der Spule in der aktuellen „clip on “- Ausführung ist denkbar einfach. Spule einfach auf das Ankerrohr des Magnetventilunterteils aufstecken und einmal nachdrücken, bis es einrastet - fertig. Die Spule ist aus einem Stück und oben geschlossen. Wichtig ist dabei zu kontrollieren, ob der O-Ring am unteren ende des Ankerrohrs (am Übergang vom Ankerrohr zum Gehäuse) montiert und unversehrt ist. Dieser O-Ring dient zur Abdichtung der Spule gegen Feuchtigkeit (auch Luftfeuchtigkeit). Von außen ist der Spulenkörper diffusionsdicht, nur von innen (vom Ankerrohr her) kann Feuchtigkeit eindringen. Diese Feuchtigkeit von innen ist der Hauptfeind von Magnetventilspulen. Sollte man ältere Ausführungen dieser Spulen vor sich haben, dann ist auch die Abdichtung am oberen ende des Ankerrohrs zu prüfen. Diese Ausführungen (Bezeichnung „18Z“ = ältere Version, im Gegensatz zu „18F“ = clip on) sind zunächst oben und unten offen und werden mit entsprechendem Montagematerial befestigt (verschraubt) und abgedichtet.

Praxistipp:

Findet man eine geplatzte Spule vor, bei der sich außen bereits „kunststoffnasen“ gebildet haben, so ist der Grund hierfür meist Feuchtigkeit, die von der Innenseite in die Spule eingedrungen ist. kann man an der Innenseite der Spule braune Rosteckenerkennen, dann ist von einer ungenügenden Abdichtung der Spule auszugehen. Bei der instandsetzung sollte dann besonders diesem Punkt Rechnung getragen werden (eine neue Spule montieren und diese mit dem O-Ring abdichten).

 

8. NC/NO

 

Magnetventile gibt es in NC- („normally closed “ = stromlos geschlossen) und in NO-Ausführung („normally open“ = stromlos offen). Die üblichen Magnetventile in der Flüssigkeitsleitung sind als NC- Ventile ausgeführt. Das hat den Vorteil, dass bei Stillstand der Anlage und bei spannungsloser Magnetventilspule das Ventil geschlossen ist, was Vorteile hinsichtlich der Vermeidung von Kältemittelverlagerungen bietet. Auch ein Stromausfall des Energieversorgers führt aus diesem Grund zu keinerlei Problemen an der Kälteanlage. Der Einsatz von NO- Ventilen hingegen kann besonders ratsam sein, wenn das Ventil immer nur kurze Zeit geschlossen sein soll. Auch wenn Spulen nicht zu den Hauptstromfressern einer Kälteanlage gehören, kann so über Jahre gesehen doch einiges an Energiekosten eingespart werden. Bei „EVR “-Magnetventilen sind alle Leistungsgrößen („EVR 2 - 40“) in Version NCerhältlich, jedoch nur „EVR 6 - 22“ in NO.

Praxistipp:

Wie erkennt man, ob es sich um ein NO- oder NC-Ventil handelt, falls auf dem Ventil keine Typenbezeichnung mehr erkennbar sein sollte? Am oberen Ende des Ankerrohrs befindet sich bei jedem Magnetventil („EVR“) eine umlaufende Nut, die zur Befestigung der Spule dient. Das ist bei NCund NO gleich. Findet man allerdings noch eine weitere umlaufende Nut am unteren Ende (in der Nähe des restlichen Magnetventilgehäuses) vor, dann handelt es sich um ein NO-Ventil. NC-Ventile haben nur eine Nut im Ankerrohr.

 

 

Stephan Bachmann,

Danfoss Kältetechnik, Offenbach

 
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