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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. 

 

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.

 

The controller can now take measures to achieve optimally small overheating. This adaptive control of the refrigerant injection leads to an optimal use of the evaporator and thus to the highest possible evaporation pressures that can be realized in this specific system. This not only leads to higher COP values, but also to energy savings, because the COP value results from the quotient of cooling and drive power.
 

PERMANENT OPTIMIZATION OF OVERHEATING

The overheating always adapts to the minimum stable signal (MSS line) of the evaporator, so that the signal cannot drift into the unstable area (Figure 2 - left of the MSS line). The "EKC315A" controller first selects any desired overheating setpoint, eg 8 K. It then attempts to implement this 8 K as the setpoint in the system. Since all information, ie overheating temperature value "S2" and current evaporation pressure "P", come together here and, in addition, a history of these two values ​​is saved to optimize the control function, the controller can easily decide whether the currently desired value is at the currently prevailing load conditions is feasible.
 

If, for example, the evaporating pressure fluctuates greatly and the superheat values ​​change rapidly, this is a sign that a higher superheat setpoint should be aimed at. However, if superheat and evaporation pressure remain largely constant, you can continue with a lower superheat setpoint, eg 7 K (then 6 K, 5 K etc.). The permanent monitoring of the optimum superheat is a decisive advantage of electronic expansion control compared to purely thermostatic valves. These would have to be set in advance to the maximum overheating setpoint, which is described by the system's individual MSS characteristic curve. However, this value is not so easy to determine so that with a mechanical-thermostatic system, this already poor starting position is usually made worse by the fact that the fitter adds a "safety margin" to the required minimum overheating value during commissioning. With regard to the functional reliability of a system, this is not wrong, because a slightly higher overheating is certainly preferable to a temporary "shooting through". However, this measure has a negative impact on the energy situation in the plant. With the electronic "EKC 315A" injection system, this "safety margin" does not apply, since the system regulates itself with regard to overheating, as described. With regard to the functional reliability of a system, this is not wrong, because a slightly higher overheating is certainly preferable to a temporary "shooting through". However, this measure has a negative impact on the energy situation in the plant. With the electronic "EKC 315A" injection system, this "safety margin" does not apply, since the system regulates itself with regard to overheating, as described. With regard to the functional reliability of a system, this is not wrong, because a slightly higher overheating is certainly preferable to a temporary "shooting through". However, this measure has a negative impact on the energy situation in the plant. With the electronic "EKC 315A" injection system, this "safety margin" does not apply, since the system regulates itself with regard to overheating, as described.
 

ANY MOP POINT

An important point that is regularly neglected with electronic injection control is the free choice of the MOP point. As already described for the thermostatic expansion valves, the MOP point is the maximum evaporation pressure (“maximum operating pressure”) at which the expansion valve works. While there are basically only very specific MOP points for thermostatic expansion valves, for which a different component must be selected each time (e.g. -20 °C for deep-freeze applications or +15 °C as "climate MOP"), this point is electronic expansion valves can be freely selected and, if necessary, readjusted or completely modified. In most cases, the use of a start controller can be completely dispensed with, which equates to significant cost savings, especially in larger systems. At the same time, the desired setpoint can be set faster and more elegantly with the electronic variant than with a mechanical start controller.
 

Operation

The controller is operated using two pushbuttons. The controller can be completely programmed using these two buttons, combined with a three-digit display, with all important data being displayed. This means that every fitter on the system can intervene in the control loop or display relevant data. In the menu for the controller not only fundamentally adjustable values ​​such as the type of refrigerant appear, but it is also possible to influence certain processes precisely by intervening in stability and amplification factors. This applies, for example, to the overheating control, so that overheating fluctuations can be prevented. You can also choose between adaptive and load-dependent overheating control. The adaptive superheat control has already been described in detail here. In the case of load-dependent overheating control, higher overheating is deliberately run in certain partial load cases, for example to ensure longer minimum compressor runtimes or to positively influence the frosting pattern on the evaporator. This could then be dispensed with one or the other defrost.

 

SERVICES

The service menu for the electronic overheating controller is particularly interesting for the fitter during commissioning and also when servicing the system. All parameter values ​​that begin with "u" indicate actual system values ​​that are used for all types of error diagnostics or are important for the evaluation of plant conditions. The three parameter values ​​"display of overheating", "display of temperature at the S2 sensor" (meaning at the evaporator outlet) and "display of the evaporation temperature" must be observed in particular. These three values ​​provide information about the system status. On the one hand, they can be read out quickly and do not have to be laboriously determined with the service pressure gauge and the fitter's temperature measuring device. On the other hand, you can see immediately which values ​​the controller assumes as given. For example, it is part of the standard procedure for an experienced fitter to check all the sensors on electronic systems before they are actually put into operation (with the usual resistance sensors, this is quite easy to do with an ohm meter. For example, a P T1000 sensor has a resistance at 0 °C of 1000 ohms) to prevent lengthy troubleshooting if the actual values ​​recorded by the sensor are incorrect. This procedure is no longer necessary when you look in the service menu, because you can assess directly (if in doubt, measure yourself with a thermometer or manometer) whether the value is realistic or not. in electronic systems, first checking all the sensors before actually putting them into operation (with the usual resistance sensors, this is quite easy to do with an ohm meter. For example, a P T1000 sensor has a resistance of 1000 ohms at 0 °C), to avoid lengthy troubleshooting prevent incorrectly recorded actual values ​​by the sensor. This procedure is no longer necessary when you look in the service menu, because you can assess directly (if in doubt, measure yourself with a thermometer or manometer) whether the value is realistic or not. in electronic systems, first checking all the sensors before actually putting them into operation (with the usual resistance sensors, this is quite easy to do with an ohm meter. For example, a P T1000 sensor has a resistance of 1000 ohms at 0 °C), to avoid lengthy troubleshooting prevent incorrectly recorded actual values ​​by the sensor. This procedure is no longer necessary when you look in the service menu, because you can assess directly (if in doubt, measure yourself with a thermometer or manometer) whether the value is realistic or not.
 

 SWITCH OUTPUT RELAY MANUALLY

Of course, the outputs of the controller are just as important as the inputs. In order to simplify this particular point during commissioning, the controller menu offers the option of manually overriding the outputs for the "AKV" valve, the solenoid valve and the alarm output. Typical for control problems is the question of whether the controller does not switch the output because it does not consider it necessary for some reason, or because it cannot switch the output, for example due to a defect. This point has cost even experienced fitters hours and days of troubleshooting. For this reason, it is advisable to try out the corresponding output relays individually during commissioning. In this way, wiring and assignment errors are also quickly cleared up.

 

 STEADY ELECTRONIC VALVES

In principle, it is possible to work with an "ETS", "ICM" or an "AKV" valve. These actuators differ as follows:

 

The "ETS" and "ICM" device is a continuous valve that is often used in chillers with the refrigerant R407C, for example, when every degree of overheating counts and even the slightest fluctuations in the evaporation pressure are to be avoided. The controller can be used as a P, PI or PID controller. The P control is a standard control according to the deviation (example: if the superheat becomes too large, the opening degree of the valve is always increased at the same speed). With PI control, the "reset time" ("I component") can be changed separately. That is, the reaction speed of the control can be changed, in other words, the control becomes more nervous or sluggish. Both may be required. The "D component" in PID control also optimizes the control properties in the event of a sudden setpoint change. This control mode is particularly advisable if the injection system is operated with an external overheating setpoint shift, eg from a higher-level controller. When using an "ETS" or "ICM" valve, a solenoid valve should also be provided in the liquid line, which can also be controlled by the "EKC". If the solenoid valve really has to be dispensed with, then it is essential to connect a UPS (independent power supply) to the actuator ("ICM") or to the controller ("ETS"). This is absolutely necessary since an "ICM" or "ETS" remains in its current open position in the event of a sudden power failure and thus continues to inject refrigerant into the evaporator, which can lead to greater damage up to and including compressor failure. With a UPS, the valve can still be closed even in such a case.

 

PULSE WIDTH MODULATION

With pulse width modulation and the use of an "AKV" valve, there is no need for an additional solenoid valve in the liquid line, as this can permanently close the flow of the liquid line and automatically return to the closed position in the event of a power failure.

The "EKC-AKV" combination works on the principle of pulse width modulation. This means that the "AKV" valve is fully opened for a certain time and fully closed again for the rest of the period time, depending on the degree of opening of the valve. For example, with an opening degree of 50% and the standard period of 6 s (this variable can be changed), this would mean 3 seconds open and 3 seconds closed (at 20% opening degree, corresponding to 1.2 s open and 4.8 s closed).
 

 

DIMENSIONING OF THE LIQUID LINE

Especially when using pulse width modulated valves, the dimensioning of the liquid line should be given high priority. In general, a speed of 0.5 m/s is given as a guide value in the relevant specialist literature when designing the diameter of the liquid line. However, if you look at existing refrigeration systems, this value is closer to approx. 1 m/s. Since this is not usually a problem for normal (continuous) thermostatic expansion valves, this guideline value has become widely accepted. With pulse-width modulated valves, however, the situation is different. Here you should even go as far as designing the 0.5 m/s for the maximum valve performance and not just for the evaporator performance. Most plant builders would probably design a 12 mm copper pipe as the liquid line for a 7 kW evaporator with R404A and -10 °C evaporation (without special subcooling). At 0.92 m/s, the flow rate would be well within the usual range. With "strictly 0.5 m/s", however, a 15 mm or 16 mm pipe with approx. 0.5 m/s would then be required. Finally, if the valve capacity is taken into account instead of the evaporator capacity, an 18 mm pipe could even be necessary (example: valve capacity 10 kW. "AKV" valves are never designed for 100% opening - opening degrees between 30 and 70% should be aimed for) . If you take this principle into account, there are usually no accelerated liquids in the system, which can otherwise lead to roaring noises and vibrating pipelines.
 

HUMIDITY

Refrigeration experts know that the subject of "moisture" plays an important role in refrigeration technology and particularly in the case of unpackaged goods, meat, vegetables and fruit. Nevertheless, this topic is not tackled as aggressively here as in comfort air conditioning technology, in which appropriate hygrostats and especially steam humidifiers are used across the board if the humidity is too low. Nevertheless, depending on the situation, it may also be necessary to dehumidify in refrigeration systems. Indirect measures are usually used for this purpose, e.g. B. changing the evaporator fan levels or the evaporator speed. (ie slower fan speed = lower evaporating temperature = dehumidification and vice versa). With an electronic injection system, this point can be influenced directly: Simply shift the superheat setpoint with an external signal of 4-20 mA and no or hardly any dehumidification is achieved with the smallest possible superheat values ​​or high dehumidification with high superheat values. Of course, it also applies here that the dew point must always be undershot for dehumidification. The adjustment of such a system can always be carried out quite easily due to the amount of condensate that is separated out on the evaporator. In addition to storing vegetables and fruit, such a system is also suitable for comfort air conditioning HVAC systems and climate cabinets. that the dew point must always be undercut for dehumidification. The adjustment of such a system can always be carried out quite easily due to the amount of condensate that is separated out on the evaporator. In addition to storing vegetables and fruit, such a system is also suitable for comfort air conditioning HVAC systems and climate cabinets. that the dew point must always be undercut for dehumidification. The adjustment of such a system can always be carried out quite easily due to the amount of condensate that is separated out on the evaporator. In addition to storing vegetables and fruit, such a system is also suitable for comfort air conditioning HVAC systems and climate cabinets.

 

Stephan Bachmann,

Danfoss Kältetechnik, Offenbach

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Construction 

How is such a solenoid valve constructed? In general, a solenoid valve consists of a coil and a valve housing. The coil is mounted on an armature tube. In smaller, direct-acting valves, the moving armature opens and closes the valve by directly releasing or closing the valve seat. In order to achieve better internal tightness, the part of the armature that meets the valve seat is covered with a Teflon sealing pad. Examples of such direct-acting solenoid valves are the Danfoss types "EVR 2" and "3". In the case of servo-controlled solenoid valves, the armature movement takes place in the same way. However, instead of the entire valve seat, a servo bore is now closed or opened. In the case of servo valves with a diaphragm, this leads to a movement of the diaphragm via the differential pressures present at the valve, which then corresponds to the opening or closing process of the valve. The principle is the same for servo solenoid valves with pistons and without diaphragms. Here, too, the valve is opened and closed via the servo bore - but by means of a piston mechanism and not a diaphragm. a rough clustering according to performance variables makes sense. The solenoid valves with the smallest output, such as "EVR 2" and "3", are directly controlled. The two larger systems are then followed by the sizes "EVR 6" to "EVR 22", all of which are equipped with membranes and are servo-controlled. Finally, the solenoid valves "EVR 25" to "40" can be found for very high outputs with dry expansion. These are then servo-controlled piston valves. If these sizes are no longer sufficient, the main valves (e.g. Danfoss "ICS" or "PM valves") are simply made into a solenoid valve using a solenoid valve attachment (EVM). These combinations then leave little to be desired in terms of the size of the system.

A practical tip :

If you open a solenoid valve and find neither a membrane nor a piston in it, then it is usually a directly controlled valve. This will then also have smaller connections, such as 6, 8 or 10 mm.

 

SIZING 

Why is it important for the practitioner to know whether he is dealing with a direct or servo-controlled solenoid valve? In fact, this point is crucial for the sizing of the valves. Direct acting solenoid valves do not require a minimum pressure drop to operate. For this reason, these valves have an extremely good partial load capability, which makes it possible to project a moderate pressure drop for the full load case (valves “EVR 2” and “EVR 3”). In the case of forced servo-controlled valves (e.g. types such as "EVRAT" or "EVRST" - the letter "T" stands for forced servo control) the same design criteria apply as for directly controlled valves. Again, there is no minimum pressure drop that needs to be taken into account. With servo-controlled valves ("EVR 6", "10", "15", "20", "22", "25", "32" and "40"), on the other hand, the minimum partial load case must also be considered in addition to the maximum pressure drop. At minimum partial load, the minimum pressure difference that the valve needs in order to be able to work stably must not be fallen below. This minimum pressure difference of the valve can be seen from the corresponding technical data sheets. Example: an "EVR 10" has a required minimum pressure drop of 0.05 bar. With 20 kW cooling capacity, R134a and -10 °C evaporation, i.e. normal cooling, and installation in the liquid line, the "EVR 10" would initially not be a bad choice, because a pressure drop of 0.06 bar at full load is more than 0.05 bar and is therefore in order. However, if, for example, two 10 kW compressors of the same size are connected together and press on this cooling circuit, then the minimum pressure drop is undershot when only one compressor is operated. Mathematically, there would then only be a pressure drop of 0.02 bar. Thus, in this case study, preference should be given to "EVR 6". With "eVR 6" the minimum pressure drop of the valve is also 0.05 bar. The full load pressure drop is 0.36 bar and the part load pressure drop is 0.09 bar. Both values ​​are greater than 0.05 bar. Thus, the valve works stably in every assumed operating state. If, despite intensive efforts for full-load cooling performance, for which solenoid valves from size "EVR 6" would normally have to be used, no suitable valves can be found due to a partial load that is too low, a corresponding main valve can technically be used instead. Mathematically, there would then only be a pressure drop of 0.02 bar. Thus, in this case study, preference should be given to "EVR 6". With "eVR 6" the minimum pressure drop of the valve is also 0.05 bar. The full load pressure drop is 0.36 bar and the part load pressure drop is 0.09 bar. Both values ​​are greater than 0.05 bar. Thus, the valve works stably in every assumed operating state. If, despite intensive efforts for full-load cooling performance, for which solenoid valves from size "EVR 6" would normally have to be used, no suitable valves can be found due to a partial load that is too low, a corresponding main valve can technically be used instead. Mathematically, there would then only be a pressure drop of 0.02 bar. Thus, in this case study, preference should be given to "EVR 6". With "eVR 6" the minimum pressure drop of the valve is also 0.05 bar. The full load pressure drop is 0.36 bar and the part load pressure drop is 0.09 bar. Both values ​​are greater than 0.05 bar. Thus, the valve works stably in every assumed operating state. If, despite intensive efforts for full-load cooling performance, for which solenoid valves from size "EVR 6" would normally have to be used, no suitable valves can be found due to a partial load that is too low, a corresponding main valve can technically be used instead. With "eVR 6" the minimum pressure drop of the valve is also 0.05 bar. The full load pressure drop is 0.36 bar and the part load pressure drop is 0.09 bar. Both values ​​are greater than 0.05 bar. Thus, the valve works stably in every assumed operating state. If, despite intensive efforts for full-load cooling performance, for which solenoid valves from size "EVR 6" would normally have to be used, no suitable valves can be found due to a partial load that is too low, a corresponding main valve can technically be used instead. With "eVR 6" the minimum pressure drop of the valve is also 0.05 bar. The full load pressure drop is 0.36 bar and the part load pressure drop is 0.09 bar. Both values ​​are greater than 0.05 bar. Thus, the valve works stably in every assumed operating state. If, despite intensive efforts for full-load cooling performance, for which solenoid valves from size "EVR 6" would normally have to be used, no suitable valves can be found due to a partial load that is too low, a corresponding main valve can technically be used instead.

Practical tip:

Small main valves of the "PM" or "ICS" series with pilot valve "EVM" are very suitable for partial loads and can often still be used when the corresponding partial loads can no longer be operated with the standard "EVR" solenoid valves. The disadvantage of these valve combinations is the higher price compared to the standard "EVR". another solution for such partial load cases can be forced servo-controlled valves (minimum pressure drop 0 bar). These valves such as "EVRAT" and "EVRS T" were originally designed for ammonia, but can also be used for "copper refrigeration".

 

ARRANGEMENT

 

The main area of ​​application for solenoid valves is the liquid line. An estimated 95% of all solenoid valves in refrigeration technology are installed there. Placing the solenoid valve close to the expansion valve is advisable, but not absolutely necessary. This minimizes the risk of accelerated liquids occurring. However, since this effect (it becomes noticeable through vibrating pipelines and knocking noises when opening the solenoid valve) is rather rare in commercial refrigeration systems, the solenoid valve can be arranged as desired if the structural conditions suggest it. The question of whether a solenoid valve should be mounted upstream or downstream of the dryer sight glass group is more of a "question of belief". If the solenoid valve is placed in front of the sight glass in the flow direction, in this way you can monitor the suction process if the system is switched to "pump down" or "pump out". However, this arrangement is not mandatory.

 

ASSEMBLY

 

Solenoid valves for "copper refrigeration" are equipped with either flared connections or soldered connections. The tube connections of the flared connections can be connected to the mounting tube in the classic way using a flare bell or using flared adapters. Flared adapters offer the advantage that this screw connection is then no longer considered a flared connection and therefore certain restrictions listed in the EN 378 standard no longer apply. In the event of service, it can also be exchanged without soldering. one disadvantage is that soldering still has to be carried out on both connection ends. In the case of soldered connections, the classic method is to braze directly on the valve. It is usually not necessary to disassemble the solenoid valve for this. Using a cooling wet rag is usually sufficient. "EVR" solenoid valves should be installed in a horizontal pipe section preferably with the coil (armature tube) facing up. Under difficult assembly conditions, the anchor tube may also be turned to the horizontal. (Pipe connections horizontal and towards the anchor tube looking away to the side). Intermediate positions between these two extreme positions are also conceivable.

 

APPLICATION 

However, "EVR" solenoid valves for refrigerants cannot only be used in the liquid line. It can also be used in hot gas, condensate, suction and hot gas bypass lines. in hot gas, hot gas bypass line operation and in hot gas feed valves for hot gas defrosting, particular attention should be paid to the maximum permissible media temperatures of the solenoid valves. With the "EVR" this is 105 °C. In the case of solenoid valves for the suction line, the more interesting value is the minimum media temperature. This is at "EVR" -40 °C. It should be noted that even evaporating temperatures of -45 °C are not a problem as the refrigerant in the suction line is already overheated. This means that at least 7 k must be added to the evaporation temperature of -45 °C. With this calculated -38°C you are again fully within the range of application. If a hot gas bypass solenoid valve is to be installed in addition to a hot gas bypass regulator, an "EVR" can be selected with confidence. However, if this solenoid valve is also to take over control tasks and be clocked every minute, then a special solenoid valve type "EVRP" should be used for high clock rates.

 

MOPD

An interesting point about solenoid valves is the “MOPD”. "MOPD" means " maximum opening pressure differential".' and stands for the maximum opening differential pressure that can be withstood by the valve-spool combination in question. This "MOPD" depends significantly on the type of solenoid valve, but also on the coil used. For example, an "EVR 3" with a 10 W AC coil can hold 21 bar and with a 12 W AC coil 25 bar. This point is not a problem when used in the liquid line and in normal cooling operation. However, if suction is then drawn off by closing the solenoid valve and the system is switched off via the low pressure switch, the solenoid valve must maintain the full differential pressure between the high and low pressure sides. an example of this would be with normal refrigeration R134a -10 °C evaporating temperature = 1 bar gauge pressure (overpressure) and 45 °C condensation temperature = 10.5 bar. This means that the solenoid valve (10.5 – 1 =) must be able to hold 9.5 bar. This is usually possible without any problems. With R404 A or R507, these pressure values ​​are usually higher. in these cases one should keep the “MOPD” issue in mind. As a practical tip, if in doubt, replace a 10 W standard coil with the "stronger" 12 W coil. This is not a big effort, never has negative effects and may help in a borderline case.

 

COIL

 

An important aspect of solenoid valves is the coil. The installation of the coil in the current “clip on” design is extremely easy. Simply attach the coil to the armature tube of the solenoid valve base and press once until it snaps into place - done. The spool is in one piece and closed at the top. It is important to check whether the O-ring at the lower end of the armature tube (at the transition from the armature tube to the housing) is fitted and undamaged. This O-ring is used to seal the coil against moisture (including humidity). From the outside, the coil body is diffusion-tight, moisture can only penetrate from the inside (from the armature tube). This internal moisture is the main enemy of solenoid valve coils. If you have older versions of these coils in front of you, then the seal at the upper end of the anchor tube must also be checked. These versions (designation "18Z" = older version, in contrast to "18F" = clip on) are initially open at the top and bottom and are fastened (screwed) and sealed with appropriate mounting material.

Practical tip:

If you find a burst spool where "plastic noses" have already formed on the outside, the reason for this is usually moisture that has penetrated the spool from the inside. If you can see brown spots of rust on the inside of the coil, then it can be assumed that the coil is not sufficiently sealed. When repairing, this point in particular should be taken into account (mount a new coil and seal it with the O-ring).

 

NC/NO 

Solenoid valves are available in NC (“ normally closed” = normally closed ) and NO versions (“ normally open” = normally open).). The usual solenoid valves in the liquid line are designed as NC valves. This has the advantage that the valve is closed when the system is at a standstill and the solenoid valve coil is de-energized, which offers advantages in terms of avoiding refrigerant migration. For this reason, even a power failure of the energy supplier does not lead to any problems with the refrigeration system. The use of NO valves, on the other hand, can be particularly advisable if the valve should only be closed for a short time. Even if coils are not one of the main power guzzlers in a refrigeration system, quite a bit of energy costs can be saved over the years. With "EVR" solenoid valves, all performance sizes ("EVR 2 - 40") are available in version NC, but only "EVR 6 - 22" in NO.

Practical tip:

How can you tell whether it is an NO or NC valve if the type designation is no longer recognizable on the valve? At the top of the armature tube, each Solenoid Valve (“EVR”) has a circumferential groove that is used to attach the coil. This is the same for NC and NO. However, if there is another circumferential groove at the lower end (near the rest of the solenoid valve housing), then it is an NO valve. NC valves have only one groove in the armature tube.

 

Magnetventil NO    Magnetventil NC

 

 

Stephan Bachmann,

Danfoss Kältetechnik, Offenbach

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General function

Filter dryers are usually installed in liquid lines of dry expansion refrigeration systems and have a dual function there. On the one hand, they are intended to catch coarse dirt particles and copper shavings, but on the other hand also to bind moisture in the system. For this purpose, modern filter dryers are equipped with a dry block of 100% molecular sieve for optimum drying performance and filtration. The filter drier should be replaced with every intervention in the refrigeration system.

 

drying capacity

When looking at the corresponding data sheets, one repeatedly encounters the term "dry performance". It indicates the amount of refrigerant that can be "dried" by the filter drier at a defined refrigerant and temperature. It is based on a certain moisture content of the refrigerant before and after the drying process. For example, a "DML 53" dryer can deliver 8.5 kg refrigerant R134a at 24 ° C from 1050 ppm to 75 ppm. Incidentally, "ppm" stands for "parts per million", ie particles per million particles, and in this specific case refers to the number of water molecules (H2O) present in the refrigerant per million refrigerant molecules. If the refrigerant charge of the system to be planned has not yet been determined, then the design of the suitable filter dryer can also be carried out after the liquid performance. By "liquid power" is meant the cooling capacity in "kW" for the installation of the dryer in the liquid line.

When looking at the corresponding data sheets, one repeatedly encounters the term "dry performance". It indicates the amount of refrigerant that can be "dried" by the filter drier at a defined refrigerant and temperature. It is based on a certain moisture content of the refrigerant before and after the drying process. For example, a "DML 53" dryer can deliver 8.5 kg of refrigerant R134a at 24 ° C from 1050 ppm to 75 ppm. Incidentally, "ppm" stands for "parts per million", ie particles per million particles, and in this specific case refers to the number of water molecules (H2O) present in the refrigerant per million refrigerant molecules. If the refrigerant charge of the system to be planned has not yet been determined, then the design of the suitable filter dryer can also be carried out after the liquid performance. By "liquid power" is meant the cooling capacity in "kW" for the installation of the dryer in the liquid line.

Practical tip: The ppm values ​​stated before and after drying can be taken over by the plant manufacturer as practicable and, in practice, based solely on the refrigerant and the refrigerant charge of the system.

Choice and size

Thus, a suitable filter dryer can already be designed via the drying or liquid power. In general, the internal volume of the standard dryer is always important. Therefore, it is now common for manufacturers to include the volume of the filter driers in the type designation. In the last example we talked about a "Danfoss DML 53". The "53" is an internal volume of 5 inch3 and the connection size 3 (10 mm). The connection size in this type designation is then divided by 8 to get the corresponding connection (eg "DML 53": connection 3, ie 3/8 and corresponds to 10 mm as metric unit). When doing so, it may be necessary to pay attention to the suffix "s" (eg "DML 82s"), which stands for "solder" (English term for soldering) and thus refers to a solder connection. if the suffix "s" does not exist, it is the flanging variant. Practical tip: If you are forced to use the type of dryer that is currently included in the after-sales service vehicle, you can use the last number of the type designation to select a suitable replacement for the connection size.

 

Standard and biflow dryer

Filter dryers are available in various designs. The standard dryer (eg "DML") is most commonly found in commercial plant engineering. It has a solid core and forms an inseparable unit with the compact housing. These standard dryers are only designed for one flow direction (observe arrow direction on the dryer housing). This is usually completely sufficient. However, if you have a real biflow operation - as occurs in a heat pump with a 4-way reversing valve - you can either arrange two standard dryers in parallel with opposite flow direction and mount a check valve in the corresponding flow direction or equal to them select a biflow dryer ("Danfoss DMB").

 

Burnout and housing dryer

If a standard dryer is actually required, but the piping connection sizes are already in the range of 22 mm and larger, then the use of a housing filter dryer for replaceable dry blocks is recommended ("Danfoss DCR"). These housing filter dryers are available for operation with one or more solid inserts. The main advantage of this variant is the very easy replacement of the dry inserts without soldering or disassembly of the piping and the moderate service price, as the housing remains in the system and does not have to be bought again. The main field of application of this housing dryer is also the liquid line. However, when using a case dryer as a burn-out dryer (use block insert "48 DA"), the suction line is favored as the installation location. This may be necessary because too much acid has formed due to a water leak in the refrigeration system or an engine fire. Burnout block inserts or filter driers are predominantly made of aluminum oxide, as it is specially optimized for acid absorption. In such a burn-out case, the block insert should be exchanged after certain time intervals and finally a coarse dirt filter element ("Type 48F") should be used at the last exchange. Such burn-out dryers are also available with non-exchangeable inserts for small pipe connections ("Type DAS"). If you want to be aware of the enormous water absorbency and at the same time the high amount of moisture that is bound in the ambient air, in mind, then you can build the following simple experiment. You open a dryer block insert (eg. "48 DM") and put the block on a letter scale. Now note down the weight and note the color of the dry core (take a photo if necessary). One day later, the color of the dryer has darkened considerably and the weight has increased significantly.

FaceSeal and pen dryer

For smaller systems that are difficult for a collector to use, a collector dryer ("Type DMC") can be used. Basically, it can be said that here simply a large housing as a replacement for the collector and a dry core were combined. The world's most common type of dryer - thank the white goods - are Penciltrockner. These pencil dryers are usually copper colored (and made of copper - other types of driers are painted), filled with silica gel beads as a desiccant and can be found in virtually every refrigerator right in front of the capillary tube. Another variant of this variety of driers are the face-seal dryers. For plant manufacturers who deal exclusively with stationary cold, these are real exotic. The face-seal dryer is usually used in the transport refrigeration and commonly referred to as "O-ring dryer". These are standard dryers but with a different thread connection than the flaring equipment. The sealing of the dryer towards the pipe connections is realized with an O-ring and is flat-sealing, as the name "Face seal", literally means "face-sealing", already says.

 

selection

In order to select the suitable dryer for a specific plant, one orients itself first to the pipe dimension of the liquid line. Since there are usually several filter dryer sizes for a port diameter, the dry performance can be a good further help for the selection. For a quick selection - without further consideration of dry or liquid performance - you should always select the slightly larger dryer. For example, the dryer sizes "DML 83", "DML 163" and "303" are all provided with a 10 mm crimp connection. The coarse choice should tend to be the size "163" or "303". The decision for a slightly larger dryer is never a real disadvantage (except the slightly longer length).

 

pressure drops

In general, always pay attention to the resulting pressure drops when selecting components. In the case of filter driers, this is usually not necessary because they have only a marginal pressure drop when used with the appropriate piping. Practical tip: If you find a filter drier in an existing plant that condenses or even forms ice despite the usual condensing temperatures (eg 45 ° C) (and without a subcooler installed) after the filter drier on the pipe surface, then the Dryer with high probability added with dirt particles. In the case of measuring connections before and after the dryer, the service manometer then displays a correspondingly high pressure drop when checking. in such a case, only the replacement of the dryer helps.

Practical tip: A filter drier with O-ring and the addition FS (eg "DML305 (FS)") is a face-seal dryer.

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The piping network of a refrigeration system is hermetically sealed and connects all necessary components to one system. The refrigerant flowing in the pipes is in different states of aggregation depending on the pressure and temperature. The pipe material used for ammonia refrigeration plants is steel, and for all other refrigerants copper and steel.

The pipelines are classified according to their function in:

  • suction
  • pressure line
  • liquid line
  • Injection line
  • Control and impulse line

 

Dimensioning of pipelines

Careful dimensioning and faultless installation is a prerequisite for the proper functioning of a refrigeration system and its economical operation. Malfunctions in a newly installed refrigeration system, in particular the failure of compressors or if the contractually agreed cooling capacity is not achieved, are often due to incorrectly dimensioned or improperly installed pipelines. The designer is responsible for the dimensioning and laying of the pipelines. Small pipe dimensions lead to a favorable material and assembly price, but also to higher speeds and thus to greater pressure losses . However, the necessary pipe size depends crucially on the refrigerant used and their volumetric cooling capacity.

The dimensioning of the pipelines is therefore always an optimization task

High pressure losses can be avoided if:

  • the speed is small
  • the pipeline is short
  • there are few bends or restrictors

 

For optimal flow velocities, it is recommended depending on the refrigerant:

 

Flow Refrigerant

 

Suction

The pressure loss in the suction line leads to an equivalent temperature drop, which does not exceed 0.5K to max. Should be 1K. This temperature drop is dependent on the saturation temperature of the respective refrigerant. So z. B. a pressure drop in the suction line of an ammonia refrigeration system of 0,1bar, depending on the applied suction pressure, different Saugdrucktemperaturabsenkungen:

 

PRESSURE LOSS SUCTION

 

To compensate for this drop in temperature of the piping, the pressure at the inlet of the compressor must be set to 0.1 bar lower. Lowering the suction pressure also leads to a loss of cooling capacity. By lowering the suction pressure, each refrigerant compressor loses cooling capacity and lowers the COP of the refrigeration system!

 

Liquid line

For pressure drop in fluid lines, due to flow losses (dynamic pressure losses), the static pressure difference of the geodetic height differences (H in m) must be considered!

With a flow from top to bottom (downpipe), the dynamic pressure loss is reduced by the static pressure! Means: it comes practically to a pressure increase (depending on the height difference) which has a rather positive effect.

With a flow from bottom to top (riser), the pressure losses add up. The pressure is thus lower at the highest point in the riser than at the lowest point (depending on the height difference). This pressure reduction results in low subcooling of the refrigerant for pre-evaporation.

If, due to the pressure drop, the saturation pressure (point on the boiling line) is undershot, pre-evaporation of the refrigerant already occurs upstream of the throttle element. Bubbles (flash gas) would already be visible in a sight glass. To prevent this partial evaporation as a result of unavoidable pressure losses, a subcooling of the refrigerant liquid is essential.

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Pre-insulated piping

The concept of pre-insulated pipes aims to transport the conveyed media unchanged in the thermal properties. The origin of the structural design lies in the technology of district heating pipelines, whose objective is the transport of heat over long distances. The use of pre-insulated pipes above ground changes the requirements for the product. The climate (wind, rain, snow etc.), installation instructions and technical specifications in the field of plant operation and maintenance are significant influencing factors.

The optimum combination of metallic outer sheath, high-strength and energy-efficient foam insulation and a carrier pipe, which is matched to the properties of the transported product, ensure safe operation, continuous material flow, protection against condensation and a long service life.

The pipe sections are usually delivered in 6m or 12m lengths and are ready to install for on-site assembly. The high mechanical stability of the insulating composite allows the installation of pipe supports on the outer jacket, cold and thermal bridges can thus be safely prevented, an essential factor in the consideration of operating costs.

 

Thermal properties of pre-insulated pipes

The thermal insulation of pre-insulated pipes consists of a foam structure with self-contained cells. From this material, the pre-insulated pipes get one, two or more layers of insulation. Critical to the design is the operating temperature, the fire resistance, the use in hazardous areas and mechanical influences on the pipe.

The effectiveness of the thermal insulation depends on the foam density. A high foam density of the insulation creates a frictional connection between the medium pipe and the outer jacket, but reduces the insulating effect. In return, a lower foam density increases the thermal insulation effect of the thermal insulation, the mechanical strength is only insignificantly changed. Optimal coordination of these influencing factors is the challenge in creating a specific product solution.

The product concept is supported by the modular principle, the design is determined in close accordance with the requirements of the customer and the company.

The outer sheath is manufactured as a spiral folded sheath with an inner fold, diffusion-proof against moisture or dust and provided with a smooth outer surface.

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