Adjustment of TrV with external equalization. Thermostatic valves

V. Shishov, Chief Engineer Farmina company
(see magazine "Refrigeration" No. 8, 2005)

The expansion valve is the most common type of control device for refrigeration units. The performance range of these expansion valves in air conditioning mode ranges from 0.15 kW to 1300 kW. Exists a large number of various types mechanical expansion valves: dismountable and sealed, with fixed and adjustable overheating, etc. Modern expansion valves are distinguished by the following features: collapsible, modular design , facilitating service maintenance

; replaceable valve assemblies operating on any of the known refrigerants (HFC, HCFC, CFC); the presence of a MOP function that protects the electric motors of low-temperature compressors when the system reaches the operating mode (limiting the maximum operating boiling pressure). For evaporators installed in small cabinets or cooling counters, expansion valves with internal alignment are used. Externally equalized expansion valves are recommended for use with evaporator hydraulic resistances >=0.2 bar for air conditioning systems, >=0.14 bar for medium temperature modes and >=0.07 bar for low temperature modes. Therefore, in low temperature systems and for air coolers with distribution device

("spider") externally aligned expansion valves should almost always be used. In the case of external pressure compensation, the compensation pipeline cuts into the suction line immediately after the capsule (temperature-sensitive cylinder). With external equalization, the evaporator and expansion valve work more efficiently: the evaporator is better filled with liquid refrigerant, and vapor over the membrane practically does not condense in the expansion valve.

It is recommended to mount the capsule on the horizontal part of the suction line, as close as possible to the evaporator, in the area of ​​the first third of the pipe circumference. In order to monitor the appearance of liquid refrigerant in the pipe (wet stroke), the capsule is installed differently: on pipes with a diameter of 12÷16 mm - the capsule is installed for 1 hour, 18÷22 mm - for 2 hours, 25÷35 mm - for 3 hours. The capsule cannot be installed inside the line, as the presence of oil will complicate its operation. If there is a danger of hot air coming into contact with the capsule, it must be thermally insulated.. TRVs are sold with factory settings. Static overheating (start of valve opening) for T2/TE2 – 4K.

When the control screw is rotated clockwise, the superheat increases, when rotated counterclockwise, it decreases. At a boiling point of 0°C for expansion valves brand T2/TE2 full turn The screw changes the overheating temperature by about 4K, starting with TE5 - 0.5K.

To avoid overfilling the evaporator with liquid, rotate the adjusting screw clockwise, increasing the superheat until the pressure fluctuations stop. Then gradually rotate the screw to the left until vibrations begin.

After this, turn the screw to the right approximately 1 turn (for T2/TE2).

With this setting, there are no pressure fluctuations and the evaporator operates in optimal mode. Superheat variations within the range of ±0.5 K are allowed. If the evaporator is not sufficiently filled with boiling refrigerant, overheating is reduced by turning the adjusting screw counterclockwise until pressure fluctuations begin. After this, turn the screw to the right one turn (for expansion valve type T2/TE2). If

throughput The expansion valve is too large or small and the setting mode cannot be found, then you need to replace the expansion valve or change the valve assembly. When choosing a thermostatic expansion valve, it is also necessary to ensure that its capacity corresponds to the performance of the cooling device (), since only in this case it is possible to ensure absolutely stable operation of the adjustable

refrigeration unit

As soon as static superheat Δt 3 is reached, the expansion valve begins to open and, when fully opened, provides its rated performance. In this case, the overheating increases by the amount of overheating of the open expansion valve Δt. The sum of the static overheating Δt 3 and the overheating of the open expansion valve Δt is the operating overheating Δt mon. Manufacturers of expansion valves set the value of static superheat, as a rule, in the range from 3 to 5 K. It can be changed in one direction or another by rotating the adjusting screw and pressing or releasing the spring. This operation leads to an equidistant shift of the operating characteristic of the expansion valve to the left or right, as a result of which it becomes possible to ensure stable control of the installation by positioning the operating characteristic of the expansion valve in such a way that it intersects the characteristic of the cooling device exactly at the operating point of the nominal cooling capacity. For those operating at very small temperature differences, it is necessary to provide a heat exchanger, which, by supercooling the liquid refrigerant, allows for increased superheating.

Completed upon departure from the manufacturer's factory TRV setting Fits most installations. If there is a need for additional adjustment, then you need to use the adjusting screw (see Fig. 2). When the screw rotates to the right (clockwise), the superheat increases; when rotated to the left (counterclockwise), the superheat decreases.

For T2/TU2 brand TRVs, a full turn of the screw changes the superheating temperature by approximately 4° at a temperature of 0°C.

Starting with a TE5 expansion valve, a full turn of the screw gives an overheating temperature of about 0.5 K at a boiling point of 0°C.

Starting with TKE3 brand TEV, a full turn of the screw gives a change in superheat of approximately 3 ° at a boiling point of 0 ° C.

Rice. 2. Adjusting the expansion valve using the adjusting screw. The following adjustment method is recommended. Additionally, at the outlet of the pipeline from the cooling device, in addition to the pressure gauge (5), an electronic thermometer (3) is installed, the sensor (6) of which is attached to the thermal cylinder (4) of the expansion valve, as shown in Fig. 3.

Rice. 3. Diagram of the expansion valve adjustment method:
1 - thermostatic valve with internal equalization; 2 - cooling device;
3 - electronic thermometer; 4 - thermal cylinder; 5 - pressure gauge;
6 - primary sensor of the electronic thermometer. To ensure stability of the expansion valve adjustment over time, it is necessary to carry out it at a temperature in the cooled volume close to the temperature at which the compressor is turned off. It is not allowed to adjust the expansion valve (adjustment) when high temperature in a refrigerated volume.

The recommended adjustment is to set the expansion valve to the limit mode at which pulsations begin. To ensure this at a constant value of superheat Δt per = t v.p -t 0, it is necessary to slowly open the expansion valve until pulsations begin. In this case, the readings of the pressure gauge P v.p and thermometer t v.p should not change. When the expansion valve is subsequently opened, pulsations in the readings of the pressure gauge P VP and thermometer T VP may begin. From this moment you need to start closing the expansion valve until the pulsations stop (about half a turn of the control screw).

Rice. 4. The sequence of adjusting the expansion valve
to nominal mode. To avoid overfilling the evaporator with liquid, proceed as follows. By rotating the adjusting screw to the right (clockwise), increase the superheat until the pressure fluctuations stop. Then gradually turn the screw to the left until the oscillation starts, then turn the screw to the right by about 1 turn (for T2/TE2 and TKE by ¼ turn). With this setting, there are no pressure fluctuations and the evaporator operates in nominal mode. Changes in superheat within the range of ±0.5°C are not considered as fluctuations.

If the evaporator is overheating, this may be due to insufficient liquid supply. You can reduce overheating by rotating the adjusting screw to the left (counterclockwise), gradually reaching the point of pressure fluctuations. After this, turn the screw to the right one turn (for TEV types T2/TE and TKE by ¼ turn). With this setting, pressure fluctuations stop and the evaporator operates in nominal mode. Changes in superheat within the range of ±0.5°C are not considered as fluctuations.

If the expansion valve is adjusted to the minimum possible superheat required for the normal operation of a given refrigeration unit, the filling of the cooling device with liquid refrigerant will be achieved at the nominal level, and the pulsations in the superheat value of the refrigerant vapor will stop. During the process of adjusting the expansion valve, the condensation pressure must remain relatively stable and close in value (P k ~ P k.n) under nominal operating conditions, since the cooling capacity of the expansion valve depends on them.

When adjusting, the following complications are possible:

1. It is not possible to achieve pulsations by adjustment.

This means that when completely open TRV, its performance is lower than that of the cooling device. This is due to the following reasons: either the flow area (f) of the expansion valve is small, or there is not enough refrigerant in the installation and an insufficient amount of liquid refrigerant from the expansion valve enters the inlet.

2. It is not possible to eliminate pulsations after they occur.

This means that the performance of the expansion valve is higher than the capacity of the cooling device. This is due to the fact that either the flow area (f) of the expansion valve is too large, or the cooling device does not have enough liquid refrigerant.

Adjustment of the expansion valve is not possible when the superheat reaches greater value(this occurs when the expansion valve is practically closed, the pressure is low, and the total temperature difference between the air temperature at the inlet to the cooling device t b1 and the boiling point of the refrigerant t 0 is large). This means that less vapor is generated in the cooling device than the compressor is capable of sucking in, i.e. The cooling capacity of the cooling unit is insufficient.

Therefore, if it is not possible to find a setting mode that eliminates pressure pulsations, it is necessary to replace the expansion valve, or replace the seats with holes (cartridges), if the design of the expansion valve provides for a set of replaceable cartridges. In this case, in order to reduce consumption, you need to replace the expansion valve or change the cartridge with the hole. If the superheat in the evaporator is too high, the throughput of the expansion valve is small. Then, to increase consumption, you also need to change the cartridge. Danfoss TE brand expansion valves are supplied with a set of replaceable cartridges. TKE brand TRVs have a fixed seat hole.

The throttle (or nozzle) opening of many expansion valves is made in the form of a replaceable liner, which makes it possible to provide a new value for its performance by simply replacing this element. Thermal control (power, control) path of the expansion valve, i.e. a complex consisting of the upper part of the expansion valve (the supra-membrane cavity that forms the thermoregulatory element) and a thermal cylinder is also sometimes replaceable, which allows you to select best option filling the thermal cylinder (steam, liquid or adsorption filling), most suitable for the specific operating conditions of this installation.

Rice. 5. Replacement of the replaceable expansion valve insert and replaceable cartridges.

ROUTINE MAINTENANCE AND PROCEDURE FOR SETTING THE TRV

1. During operation, the tightness of the valve and its connections on the pipeline should be periodically checked. Leakage may occur as a result of loosening threaded connections and shrinkage of gaskets.

To restore the tightness of the valve connection points, tighten the nuts securing the flanges and the equalizing line.

If a leak is identified at the point where the fitting is screwed together with the body, restoring the tightness can be achieved by tightening the fitting.

A leak in the seal of the adjustment unit is eliminated by tightening the nut using a special wrench included in the delivery kit.

Leaks at the junction of the valve head with the body should only be repaired in a workshop.

Weight work should only be carried out using wrenches. The use of impact objects is not permitted.

The tightness test must be carried out in compliance with the “Safety Rules for Freon Refrigeration Units”.

2. If during operation part of the cooling device does not freeze, and the suction pressure quickly decreases after turning on the refrigeration unit, then this indicates incorrect setting TRV (its small opening).

To ensure normal operation of the refrigeration unit, it is not recommended to change the factory settings of the valves. It should be remembered that the expansion valve, by regulating the degree of filling of the cooling device with refrigerant, only indirectly affects the temperature in the refrigeration chambers. If it is necessary to change the temperature in the refrigeration chambers, this should be achieved by changing the settings of specially designed relays and temperature controllers. Temperature regulation by changing the setting of the expansion valve, i.e. by changing the value of superheat when the valve opens, leads to a decrease in the efficiency of the installation, as well as to premature failure of the unit.

If, nevertheless, it becomes necessary to adjust the overheating of the beginning of the valve opening, change the setting by slowly turning the adjusting screw with a delay every half turn to normalize the operating mode of the installation.

When choosing a thermostatic expansion valve, it is also necessary to ensure that its throughput capacity matches the performance of the cooling device (evaporator), since only in this case is it possible to ensure absolutely stable operation of the controlled refrigeration unit. For this purpose, minimal overheating should be provided over the entire range of possible performance of the cooling device. As can be seen from the figure, regulation can be stable only if the point of intersection of the operating characteristic curves of the cooling device and the operating characteristic of the expansion valve corresponds to the operating point of the installation's cooling capacity.

As soon as static superheat Δt3 is reached, the expansion valve begins to open and, when fully open, provides its rated performance. In this case, the overheating increases by the amount of overheating of the open expansion valve Δtpo. The sum of the static overheating Δt3 and the overheating of the open expansion valve Δtpo constitutes the operating overheating Δtпн. Manufacturers of expansion valves set the value of static superheat, as a rule, in the range from 3 to 5 K. It can be changed in one direction or another by rotating the adjusting screw and pressing or releasing the spring. This operation leads to an equidistant shift of the operating characteristic of the expansion valve to the left or right, as a result of which it becomes possible to ensure stable control of the installation by positioning the operating characteristic of the expansion valve in such a way that it intersects the characteristic of the cooling device exactly at the operating point of the nominal cooling capacity. For cooling devices operating at very small temperature differences, it is necessary to provide a heat exchanger, which, by supercooling the liquid refrigerant, allows for increased superheating.

The setting of the expansion valve when shipped from the factory corresponds to most settings. If there is a need for additional adjustment, then you need to use the adjusting screw (see Fig. 2). When the screw rotates to the right (clockwise), the superheat increases; when rotated to the left (counterclockwise), the superheat decreases.

For T2/TU2 brand TRVs, a full turn of the screw changes the superheating temperature by approximately 4° at a boiling point of 0°C.

Starting with a TE5 expansion valve, a full turn of the screw gives an overheating temperature of about 0.5 K at a boiling point of 0 °C.

Starting with TKE3 brand TEV, a full turn of the screw gives a change in superheat of approximately 3 ° at a boiling point of 0 ° C.

The following adjustment method is recommended. Additionally, at the outlet of the pipeline from the cooling device, in addition to the pressure gauge (5), an electronic thermometer (3) is installed, the sensor (6) of which is attached to the thermal cylinder (4) of the expansion valve, as shown in Fig. 3.

To ensure stability of the expansion valve adjustment over time, it is necessary to carry out it at a temperature in the cooled volume close to the temperature at which the compressor is turned off. It is not allowed to adjust the expansion valve (adjustment) at high temperatures in the cooled volume.

The recommended adjustment is to set the expansion valve to the limit mode at which pulsations begin. To ensure this at a constant value of superheat Δtper = tv.p -t0, it is necessary to slowly open the expansion valve until pulsations begin. In this case, the readings of the pressure gauge Рв.п and thermometer tв.п should not change. When the expansion valve is subsequently opened, pulsations in the readings of the pressure gauge Рв.п and thermometer tв.п may begin. From this moment you need to start closing the expansion valve until the pulsations stop (about half a turn of the control screw).

To avoid overfilling the evaporator with liquid, proceed as follows. By rotating the adjusting screw to the right (clockwise), increase the superheat until the pressure fluctuations stop. Then gradually turn the screw to the left until the oscillation starts, then turn the screw to the right by about 1 turn (for T2/TE2 and TKE by 1/4 turn). With this setting, there are no pressure fluctuations and the evaporator operates in nominal mode. Changes in superheat within the range of ±0.5 °C are not considered as fluctuations.

If the evaporator is overheating, this may be due to insufficient liquid supply. You can reduce overheating by rotating the adjusting screw to the left (counterclockwise), gradually reaching the point of pressure fluctuations. After this, turn the screw to the right one turn (for expansion valves of type T2/TE and TKE by 1/4 turn). With this setting, pressure fluctuations stop and the evaporator operates in nominal mode. Changes in superheat within the range of ±0.5 °C are not considered as fluctuations.

If the expansion valve is adjusted to the minimum possible superheat required for the normal operation of a given refrigeration unit, the filling of the cooling device with liquid refrigerant will be achieved at the nominal level, and the pulsations in the superheat value of the refrigerant vapor will stop. During the process of adjusting the expansion valve, the condensation pressure must remain relatively stable and close in value (Рк ~ Рк.н) under nominal operating conditions, since the cooling capacity of the expansion valve depends on them.

When adjusting, the following complications are possible:

1. It is not possible to achieve pulsations by adjustment.

This means that when the expansion valve is fully open, its performance is lower than that of the cooling device. This is due to the following reasons: either the flow area (f) of the expansion valve is small, or there is not enough refrigerant in the installation and an insufficient amount of liquid refrigerant from the condenser is supplied to the expansion valve input.

2. It is not possible to eliminate pulsations after they occur.

This means that the performance of the expansion valve is higher than the capacity of the cooling device. This is due to the fact that either the flow area (f) of the expansion valve is too large, or the cooling device does not have enough liquid refrigerant.

Adjustment of the expansion valve is impossible when the superheat reaches a higher value (this occurs when the expansion valve is practically closed, the evaporation pressure is low, and the total temperature difference between the air temperature at the inlet to the cooling device tb1 and the boiling point of the refrigerant t0 is large). This means that less vapor is generated in the cooling device than the compressor is capable of sucking in, i.e. The cooling capacity of the cooling unit is insufficient.

Therefore, if it is not possible to find a setting mode that eliminates pressure pulsations, it is necessary to replace the expansion valve, or replace the seats with holes (cartridges), if the design of the expansion valve provides for a set of replaceable cartridges. In this case, in order to reduce consumption, you need to replace the expansion valve or change the cartridge with the hole. If the superheat in the evaporator is too high, the throughput of the expansion valve is small. Then, to increase consumption, you also need to change the cartridge. Danfoss TE brand expansion valves are supplied with a set of replaceable cartridges. TKE brand TRVs have a fixed seat hole.

The throttle (or nozzle) opening of many expansion valves is made in the form of a replaceable liner, which makes it possible to provide a new value for its performance by simply replacing this element. Thermal control (power, control) path of the expansion valve, i.e. a complex consisting of the upper part of the expansion valve (a supra-membrane cavity that forms a temperature-regulating element), a capillary tube and a thermal cylinder, is also sometimes replaceable, which allows you to select the best option for filling the thermal cylinder (steam, liquid or adsorption filling), most suitable for the specific operating conditions of a given installation .

Routine maintenance of the expansion valve

1. During operation, the tightness of the valve and its connections on the pipeline should be periodically checked. Loss of tightness can occur as a result of loosening of threaded connections and shrinkage of gaskets.

To restore the tightness of the valve connection points, tighten the nuts securing the flanges and the equalizing line.

If a leak is identified at the point where the fitting is screwed together with the body, restoring the tightness can be achieved by tightening the fitting.

A leak in the seal of the adjustment unit is eliminated by tightening the nut using a special wrench included in the delivery kit.

Leaks at the junction of the valve head with the body should only be repaired in a workshop.

Weight work should only be carried out using wrenches. The use of impact objects is not permitted.

The tightness test must be carried out in compliance with the “Safety Rules for Freon Refrigeration Units”.

2. If during operation a part of the cooling device does not freeze, and the suction pressure quickly decreases after turning on the refrigeration unit, then this indicates an incorrect setting of the expansion valve (its opening is small).

To ensure normal operation of the refrigeration unit, it is not recommended to change the factory settings of the valves. It should be remembered that the expansion valve, by regulating the degree of filling of the cooling device with refrigerant, only indirectly affects the temperature in the refrigeration chambers. If it is necessary to change the temperature in the refrigeration chambers, this should be achieved by changing the settings of specially designed relays and temperature controllers. Temperature regulation by changing the setting of the expansion valve, i.e. by changing the value of superheat when the valve opens, leads to a decrease in the efficiency of the installation, as well as to premature failure of the unit.

If, nevertheless, it becomes necessary to adjust the overheating of the beginning of the valve opening, change the setting by slowly turning the adjusting screw with a delay every half turn to normalize the operating mode of the installation.

3. Disassembly of the valve not related to valve adjustment is not permitted.

Types of evaporators

Evaporator is one of the elements refrigeration machine, in which the working substance boils due to heat supplied from a low temperature source. The vapor formed during the boiling of the refrigerant is sucked out from evaporator compressor to carry out further processes in the refrigeration machine cycle. Depending on the underlying principle, evaporators are divided into a number of groups according to the nature of the cooled source:

1. evaporators for cooling liquid coolants;
2. evaporators for air cooling;
3. evaporators for cooling solid media;
4. evaporators-condensers.

depending on conditions circulation of cooled liquid:

1. With closed system circulation of cooled liquid (shell and tube and shell and coil);
2. with an open level of cooled liquid (vertical pipe, panel).

by the nature of filling with the working substance:

1. flooded;
2. non-flooded (irrigation, shell-and-tube with boiling in pipes, coil with top liquid supply).

Evaporators can be divided into other groups (depending on the surface on which the working substance boils; according to the nature of the movement of the working substance, etc.). They are used as an intermediate coolant liquid in evaporators. pickles(aqueous solutions of salts NaCl, CaCl2), water, alcohol, water solution ethylene glycol, etc.

With increasing brine concentration, the temperature at which solidification begins (crystallization) first drops, then becomes equal to the temperature of the cryohydrate point and then increases. Ends crystallization process regardless of concentration at cryohydrate temperature. As ice or salt crystals fall out, with a decrease in the temperature of the brine, the remaining liquid phase will either increase its concentration (left curve) or decrease (right curve) to the state of a eutectic solution corresponding to the concentration of the cryohydrate point. For NaCl solution cryohydrate temperature equal to -21.2°C, and the concentration is 28.9%; for CaC12 solution - -55°C and 42.5%, respectively

Technique for regulating expansion valves

When choosing a thermostatic expansion valve, it is also necessary to ensure that its throughput capacity matches the performance of the cooling device (evaporator), since only in this case is it possible to ensure absolutely stable operation of the controlled refrigeration unit. For this purpose, minimal overheating should be provided over the entire range of possible performance of the cooling device. As can be seen from Fig. 1, regulation can be stable only if the point of intersection of the operating characteristic curves of the cooling device and the operating characteristic of the expansion valve corresponds to the operating point of the installation's cooling capacity.

refrigeration unit

As soon as static superheat Δt 3 is reached, the expansion valve begins to open and, when fully opened, provides its rated performance. In this case, the overheating increases by the amount of overheating of the open expansion valve Δt. The sum of the static overheating Δt 3 and the overheating of the open expansion valve Δt is the operating overheating Δt mon. Manufacturers of expansion valves set the value of static superheat, as a rule, in the range from 3 to 5 K. It can be changed in one direction or another by rotating the adjusting screw and pressing or releasing the spring. This operation leads to an equidistant shift of the operating characteristic of the expansion valve to the left or right, as a result of which it becomes possible to ensure stable control of the installation by positioning the operating characteristic of the expansion valve in such a way that it intersects the characteristic of the cooling device exactly at the operating point of the nominal cooling capacity. For cooling devices operating at very small temperature differences, it is necessary to provide a heat exchanger, which, by supercooling the liquid refrigerant, allows for increased superheating.

Completed upon departure from the manufacturer's factory TRV setting Fits most installations. If there is a need for additional adjustment, then you need to use the adjusting screw (see Fig. 2). When the screw rotates to the right (clockwise), the superheat increases; when rotated to the left (counterclockwise), the superheat decreases.

For T2/TU2 brand TRVs, a full turn of the screw changes the superheating temperature by approximately 4° at a boiling point of 0°C.

Starting with a TE5 expansion valve, a full turn of the screw gives an overheating temperature of about 0.5 K at a boiling point of 0°C.

Starting with TKE3 brand TEV, a full turn of the screw gives a change in superheat of approximately 3 ° at a boiling point of 0 ° C.

Rice. 2. Adjusting the expansion valve using the adjusting screw. The following adjustment method is recommended. Additionally, at the outlet of the pipeline from the cooling device, in addition to the pressure gauge (5), an electronic thermometer (3) is installed, the sensor (6) of which is attached to the thermal cylinder (4) of the expansion valve, as shown in Fig. 3.

Rice. 3. Diagram of the expansion valve adjustment method:
1 - thermostatic valve with internal equalization; 2 - cooling device;
3 - electronic thermometer; 4 - thermal cylinder; 5 - pressure gauge;
6 - primary sensor of the electronic thermometer. To ensure stability of the expansion valve adjustment over time, it is necessary to carry out it at a temperature in the cooled volume close to the temperature at which the compressor is turned off. It is not allowed to adjust the expansion valve (adjustment) at high temperatures in the cooled volume.

The recommended adjustment is to set the expansion valve to the limit mode at which pulsations begin. To ensure this at a constant value of superheat Δt per = t v.p -t 0, it is necessary to slowly open the expansion valve until pulsations begin. In this case, the readings of the pressure gauge P v.p and thermometer t v.p should not change. When the expansion valve is subsequently opened, pulsations in the readings of the pressure gauge P VP and thermometer T VP may begin. From this moment you need to start closing the expansion valve until the pulsations stop (about half a turn of the control screw).

Rice. 4. The sequence of adjusting the expansion valve
to nominal mode. To avoid overfilling the evaporator with liquid, proceed as follows. By rotating the adjusting screw to the right (clockwise), increase the superheat until the pressure fluctuations stop. Then gradually turn the screw to the left until the oscillation starts, then turn the screw to the right by about 1 turn (for T2/TE2 and TKE by ¼ turn). With this setting, there are no pressure fluctuations and the evaporator operates in nominal mode. Changes in superheat within the range of ±0.5°C are not considered as fluctuations.

If the evaporator is overheating, this may be due to insufficient liquid supply. You can reduce overheating by rotating the adjusting screw to the left (counterclockwise), gradually reaching the point of pressure fluctuations. After this, turn the screw to the right one turn (for TEV types T2/TE and TKE by ¼ turn). With this setting, pressure fluctuations stop and the evaporator operates in nominal mode. Changes in superheat within the range of ±0.5°C are not considered as fluctuations.

If the expansion valve is adjusted to the minimum possible superheat required for the normal operation of a given refrigeration unit, the filling of the cooling device with liquid refrigerant will be achieved at the nominal level, and the pulsations in the superheat value of the refrigerant vapor will stop. During the process of adjusting the expansion valve, the condensation pressure must remain relatively stable and close in value (P k ~ P k.n) under nominal operating conditions, since the cooling capacity of the expansion valve depends on them.

When adjusting, the following complications are possible:

1. It is not possible to achieve pulsations by adjustment.

This means that when the expansion valve is fully open, its performance is lower than that of the cooling device. This is due to the following reasons: either the flow area (f) of the expansion valve is small, or there is not enough refrigerant in the installation and an insufficient amount of liquid refrigerant from the condenser is supplied to the expansion valve input.

2. It is not possible to eliminate pulsations after they occur.

This means that the performance of the expansion valve is higher than the capacity of the cooling device. This is due to the fact that either the flow area (f) of the expansion valve is too large, or the cooling device does not have enough liquid refrigerant.

Adjustment of the expansion valve is impossible when the superheat reaches a higher value (this occurs when the expansion valve is practically closed, the evaporation pressure is low, and the total temperature difference between the air temperature at the inlet to the cooling device t b1 and the boiling point of the refrigerant t 0 is large). This means that less vapor is generated in the cooling device than the compressor can absorb, i.e. the cooling capacity of the cooling device is insufficient.

Therefore, if it is not possible to find a setting mode that eliminates pressure pulsations, it is necessary to replace the expansion valve, or replace the seats with holes (cartridges), if the design of the expansion valve provides for a set of replaceable cartridges. In this case, in order to reduce consumption, you need to replace the expansion valve or change the cartridge with the hole. If the superheat in the evaporator is too high, the throughput of the expansion valve is small. Then, to increase consumption, you also need to change the cartridge. Danfoss TE brand expansion valves are supplied with a set of replaceable cartridges. TKE brand TRVs have a fixed seat hole.

The throttle (or nozzle) opening of many expansion valves is made in the form of a replaceable liner, which makes it possible to provide a new value for its performance by simply replacing this element. The thermoregulating (power, control) path of the expansion valve, i.e. a complex consisting of the upper part of the expansion valve (the supra-membrane cavity that forms the temperature-regulating element), a capillary tube and a thermal cylinder, is also sometimes replaceable, which allows you to select the best option for filling the thermal cylinder (steam, liquid or adsorption filling), most suitable for the specific operating conditions of a given installation.

Rice. 5. Replacement of the replaceable expansion valve insert and replaceable cartridges.

Temperature difference

The temperature difference across the evaporator is calculated as follows:

ΔT=Ta1-Ta2,

Where ΔTa is in the range from 2 to 8 K (for tubular-fin evaporators with forced air flow).

In other words, when normal operation In a refrigeration unit, the air passing through the evaporator must be cooled not lower than 2 K and not higher than 8 K.

Rice. 2 – Scheme and temperature parameters of air cooling on the air cooler:

Ta1 And Ta2– air temperature at the inlet and outlet of the air cooler;

• FF– refrigerant temperature;
• L– equivalent length of the evaporator;
• That– boiling point of the refrigerant in the evaporator.

Lamellar heat exchangers

We devote a series of articles to the design and application of finned heat exchangers used in central air conditioners (AHU) and air handling units. Too often, some heat exchanger manufacturers provide incorrect data, which gives designers reason to expect unrealistic results. In this article we will reveal problems from the air side: the impact on the performance of the lamella profile (fins) and pressure loss. Let us also lightly touch on the problem of the influence of condensation in the case of a cooling battery.

Introduction

Let us consider the case of forced airflow of a heat exchange battery. In this case, the speed of air movement lies in the range from 1 to 5 m/sec. The air flow between the lamellas is not laminar, even though it may seem so in some cases. Laminar flow cannot be stable here, since an intense vortex is created behind each tube. Even if a laminar air flow manages to bypass this vortex, it will “bump into” the next tube (see Fig. 2). So we will consider an ordinary turbulent flow, for which equations like 1/a = C*v^n are most often used, where a is the air heat transfer coefficient, v is the air speed, C and n are the corresponding coefficients. A similar equation applies to the pressure loss on the air side delta p = D*v^m The coefficient n varies according to the data given in the literature, from 0.41. up to 0.9, coefficient m varies between 1.3 and 2, more often 1.8.

The most accurate method for determining all coefficients is the American standard ARI 410. It is based on measuring the performance of several heat exchangers with different numbers of rows, with different steps fins. Hot and cold water(both dry and wet cooling). More than 30 laboratory tests are typically performed for one type of heat exchanger geometry and profile. The measurement results fall on a straight line in a logarithmic diagram (see Fig. 3). The accuracy of the method is ±5% of the total productivity for any combination of rows, contours, and finning steps.

conclusions

Obviously there are no miracles in technology. Higher productivity is paid for by a larger surface area or greater pressure losses.

Manufacturers usually indicate air resistance lower than the real one. We advise you to show healthy pessimism and test at least one heat exchanger when determining the qualifications of the supplier. The same applies to checking the performance of the heat exchanger.

The reliability of a heat exchanger manufacturer can also be preliminarily checked by asking about the calculation methodology used in its calculation program. Biggest difference between the real and the specified calculated value can be for coolers, in the case when the calculation did not take into account the drops of condensate formed on the surface of the lamella.

For coolers operating in conditions of condensation formation, the use of a hydrophilic coating reduces the loss of air pressure and increases the value of its speed at which a drop eliminator should be used.

In water cooling units - chillers - produced by the Xiron-Holod Company, only proven heat exchangers/condensers are used.

Thermostatic valves

Thermostatic valves(TRV) are the most common regulators for supplying refrigerant to evaporators. Adjusting the expansion valve superheat set point significantly affects the cooling capacity of the equipment.

If the expansion valve is adjusted to high superheat or its thermal bulb is not installed correctly, then this is the cause of low suction pressure. If the overheating setting of the expansion valve is set incorrectly, then perform the following operations:

• measure the temperature in the suction pipeline at the place where the thermal cylinder is attached;
• determine the pressure in the suction pipeline at the place where the thermal cylinder is attached.

If the expansion valve has an external equalization line, then the pressure gauge installed on it directly and accurately displays the detected pressure. For expansion valves with internal equalization, the pressure is determined using a pressure gauge at the compressor suction valve. Then the calculated pressure drop in the suction pipe between the thermostatic expansion valve and the compressor suction valve is added to this value. The sum of the pressure gauge and the calculated pressure drop is approximately equal to the pressure in the pipeline at the location of the thermal cylinder.

The unit operates with increased load when its performance is insufficient or the cold consumption has increased. The only solution This problem can be solved by replacing the unit with another more productive one. A significant thermal load on the evaporator occurs at high fan speeds, resulting in increased suction pressure. You can reduce the fan speed and at the same time change the difference between the temperature of the air flow passing through the evaporator and the boiling point of the refrigerant. The recommended temperature difference is usually 11°C for air conditioning and 6 - 9°C for refrigeration. Industrial shock freezing of fruits from the company “Xiron-holod”

Frame, crankcase, crankcase

The basic requirements that the frame, crankcase and crankcase designs must satisfy are: strength And rigidity. The latter determines the accuracy and preservation relative position axes of the compressor movement mechanism during operation. Frames, crankcases and crankcases absorb the forces generated during compressor operation and transmit the reaction from the compressor to the foundation. torque, unbalanced forces and moments of inertia of moving masses, as well as compressor weight.

Crosshead compressor frames are under atmospheric pressure. Openings, hatches and openings in frames are sealed with lightweight covers and casings. Horizontal opposed compressors predominantly use multi-bearing box-section frames, creating a lightweight and rigid structure.

The crankcases and crankcases of crosshead compressors are under pressure from the intake refrigerant vapors. This pressure during compressor operation does not exceed 0.6 MPa for most refrigerants. However, during long periods of parking of the machine, the pressure in the crankcase may increase to a value determined by the ambient temperature. Therefore, checking the strength and tightness of crankcases and related housing parts compressors, operating in automatic mode, are carried out according to the same standards as for housing parts on the discharge side.

Frames, crankcases and crankcases are usually made of cast iron, sometimes welded from steel sheet. In small compressors transport vehicles Aluminum alloys are used to reduce weight. Cast parts in most cases for preservation correct position axes and planes have to be subjected to aging (artificial or natural), and welded ones - annealing. These same primary requirements- the exact relative position of axes and planes is also required for machining. In addition, the sealing planes (surfaces) of crankcases and crankcases must provide the possibility of assembly with counter parts, ensuring tightness. Permissible deviations in the seating dimensions of frames, crankcases and other compressor parts, as well as the microgeometry of the main seating surfaces are given in specialized literature.

Refrigerants - basic definitions, brief historical overview, notation

Refrigerant (refrigerant) is the working fluid of the refrigeration machine, changing its state of aggregation in different parts of the refrigeration circuit. During the transition from liquid to gaseous state, which occurs in the evaporator, the refrigerant takes away heat from environment due to the endothermic nature of the evaporation process, thereby producing cold. Then the selected heat is removed from the refrigeration machine as a result of subsequent condensation of the refrigerant in the condenser and transferred to another medium, and the process of transition of the refrigerant from a gaseous state to a liquid is exothermic in nature.

So that a substance can perform functions refrigerant, it is necessary, first of all, that at atmospheric pressure its boiling point is as low as possible, the volumes of vapors formed during evaporation are insignificant, and the condensation pressure is not too high and is easily achievable. Besides, refrigerant must be non-aggressive towards structural materials and oils, as less toxic as possible, non-flammable and explosion-proof. Finally, it is desirable that, under the conditions in which the most common refrigeration networks are found, its specific enthalpy is significant. In other words, it is impossible to find a substance that simultaneously satisfies all these requirements.

The first refrigerant was water, since since 1755 it served “to obtain frigories (negative calories)” in laboratory installation, which was created by William Gullen. Later, in 1834, the American Jacob Perkins made compression machine , which worked on diethyl ether, and in 1844, also by the American John Gorrie - a machine with compression and expansion of air. But the French were not in debt and in 1859 FerdinandCarre built ammonia absorption chiller, and four years later Charles launched a methyl ether compressor. Until the end of the 19th century. two more new refrigerants were used: carbon dioxide(C02) and sulfur dioxide (S02), in addition, one of the already mentioned refrigerants - ammonia - is used not only in adsorption refrigeration machines, but also in compression ones (Linde).

These last three refrigerants, namely ammonia (R717), carbon dioxide (R744) and sulfur dioxide (R764), remained the most common until 1930. But after introduction in 1930 in the USA new category refrigerants: chlorofluorocarbons, well known by the abbreviation CFC, all previously mentioned refrigerants, with the exception of ammonia, almost completely disappeared. However, starting in 1980, scientists began to sound alarming signals, drawing public attention to harmful effects CFC on the environment. Therefore, manufacturers have begun to develop refrigerants that are less harmful to the future of the planet, some of which have already appeared on the market. These refrigerants replacing the CFC group mainly belong to two categories chemical compounds: chlorofluorocarbons, or HCFCs, and hydrofluorocarbons, or HFCs.

Although the number of widely used refrigerants has been significantly reduced, their range remains quite large. To facilitate their designation, a system of alphanumeric indices was introduced. This system is established for all chemical compounds whose composition does not always exactly match the CFC, HCFC or HFC categories we described above. They are designated by the letter R followed by three numbers, i.e. Rcdu, where:

• c (hundreds) is equal to the number of carbon atoms reduced by one;
• d (tens) is equal to the number of hydrogen atoms increased by one;
• and (units) is equal to the number of fluorine atoms.

For determining chemical formula compounds complement its composition chlorine so that the total number of monovalent atoms, i.e., hydrogen, fluorine and chlorine atoms taken together, is 4 for methane derivatives, 6 for ethane derivatives, 8 for propane derivatives, etc. According to the "Textbook of Refrigeration Engineering" Pohlmann 1998

Gas compression - basic concepts

IN production processes significant quantities of gases and their mixtures are processed at pressures other than atmospheric; in addition, gases are also used for auxiliary purposes (for squeezing, mixing and spraying various substances). All these processes are carried out when compressed or rarefaction of gases. Compression or rarefaction of a gas (change in volume) is accompanied by a change in its pressure and temperature.

Adiabatic, isothermal and polytropic compression and rarefaction. As is known from thermodynamics, a change in the state of a gas with changing volume and pressure can occur in three ways: isothermal, adiabatic and polytropic. The change in gas pressure during compression largely depends on whether, during compression, heat exchange occurs between the compressed gas and the surrounding external environment. In practice, such heat exchange is inevitable, and in many cases even necessary, for which artificial cooling of the compressed gas is used.

Theoretically, two limiting cases can be imagined gas compression, and all real gas compression processes will be intermediate between them.

In the first case, all the heat released during gas compression, is completely removed outside, and the process of changing the state of the gas, i.e., changing its volume and pressure, occurs at one constant temperature; such a process is called isothermal. In the second case, on the contrary, all the heat released during gas compression, remains completely inside the gas, increasing its temperature, while there is no heat loss to the environment; such a process is called adiabatic.

In fact gas compression does not proceed isothermally or adiabatically, but in each particular case it only approaches one of these processes. Such a real process of gas compression, in which simultaneously with a change in volume and pressure there is also a change in temperature and heat removal to the outside, is called polytropic.

Heat

Heat is the energy obtained as a result of a change in temperature. Heat transmitted from a warmer body to a colder one. Heat- this is the temperature component of energy transfer during the operation of machine systems. Considering that work is the transfer of energy by a force that moves a mass over a certain distance, heat is transferred from one body to another due to temperature differences. Heat transfers internal kinetic energy from molecules of a warmer body to a colder one. This energy transfer reduces the level kinetic energy molecules of a warmer body, which produces a corresponding decrease in its temperature. Simultaneously kinetic energy equalizes and the temperature of the colder body increases.

The transfer of energy that affects body temperature is called heat transfer. Although heat transfer occurs primarily in response to temperature differences, it is also caused by friction force- these are interactions that occur at the molecular level as the movement of bodies relative to each other. Such interactions change the level kinetic energy molecules between two surfaces. As a result of stretching the rubber band, the molecules move past each other, changing their speed and the temperature of the entire body. This change thermal energy- conversion process, where part of the work done to move the body is stored in the form of energy. This section describes different kinds thermal energy and heat transfer between bodies.

Requirements

Western Australian regulations require milk to be cooled to 4°C or below within 3.5 hours of milking. However, to obtain a high quality product, it should be cooled to below 4°C as quickly as possible. In addition, it is very important to store milk at a temperature below 4°C between milking operations.

Cooling systems

Systems with direct cooling(direct evaporation of refrigerant) and thermal storage systems are the main systems used on dairy farms to cool dairy products. Direct refrigeration systems include a refrigeration unit that supplies a cooling refrigerant that removes heat from the milk stored in the bulk tank.

Systems with heat accumulation They use a refrigeration unit that cools the refrigeration medium, which is stored in a heat storage tank. The refrigeration medium is then used to cool the milk using a heat exchanger before the milk enters the bulk tank. Typically, milk enters the bulk tank at a temperature below 4 o C.

Rice. 1. Typical compartment for milk storage. Shown is a refrigerated tank (milk cooler), transfer pump, built-in cleaning filter and plate cooler.

Pre-cooling

Pre-cooling with a plate cooler helps reduce bacterial growth and also reduces the load on the refrigeration unit. This significantly reduces cooling costs. To check the effectiveness of pre-cooling:

·measure the temperature at the water inlet;

Measure the temperature at the milk outlet. Ideally, the difference between the specified inlet and outlet temperatures should be 3°C or less. In practice, inefficiency is often due to the following reasons:

· inadequacy of the number of plates for of this stream milk;

· inadequacy of the water flow rate (the water flow rate should be 2.5 - 3 times greater than the maximum milk flow rate);

· incorrect installation - manufacturers recommend installing single-pass plate coolers in such a way that milk is supplied from below. To prevent sediment from depositing between the plates, it should be filtered before passing through the plate cooler. Plate coolers must be washed with filters installed!

Temperature between milkings

Regulating milk temperature between milkings is very important to maintain quality during tank storage. Cooling should “turn on” before the temperature reaches 4°C.

Education

All new milker(s) or farm employees must receive full training in the operation of the milk cooling and storage system before they are assigned responsibility for the operation of the unit.

Department Agriculture- Western AustraliaFarmnoteNo 36/99 Authors: J.R.M. (Ian) and Bell S. J. Gallagher

Boiling

Boiling is a process of intense evaporation that occurs when the vapor pressure of a liquid is equal to the ambient pressure. To raise the vapor pressure to this level, a large amount of energy must be transferred to the liquid. The energy increases the temperature of the liquid to its saturation temperature. As the temperature of a liquid increases, the kinetic energy and the number of molecules separated from the liquid increase, and the vapor pressure at the surface of the liquid also increases. Molecules with high energy at the surface of the liquid break bonds and overcome the force of attraction. When rising above the surface of the liquid, their mass increases, and the pressure acting on the surface of the liquid increases increases steam pressure. Once the vapor pressure equals the surrounding atmospheric pressure, the atmosphere can no longer prevent the molecules from separating from the liquid, and boiling. For example, to increase the vapor pressure of water room temperature to atmospheric temperature, the liquid temperature must be raised to 100°C. This increase in temperature supplies kinetic energy, which separates enough molecules from the surface of the liquid that their total mass causes a vapor pressure of 101.3 kPa. Boiling occurs when the temperature and vapor pressure of a liquid intersect with saturation temperature.

For process boiling Additional energy must be continuously supplied to maintain steam pressure. Energy produces the necessary transformation potential energy molecules to change the phase of latent heat.

Boiling is characterized by significant movement and the rapid appearance of vapor bubbles throughout the entire volume of liquid. Bubbles expand and rise to the surface of the liquid as a result of the difference in density between the liquid and the vapor. The bubbles collapse at the surface and release steam into the environment. When vapor bubbles near the surface of a liquid break, tiny droplets of liquid are released into the atmosphere. It is these drops of water that make steam visible over the vessel.

Condensation

Condensation is a process involving latent heat, as a result of which steam passes into liquid phase. This occurs whenever saturated steam is exposed to a temperature below saturation temperature. Because saturated steam exists at the boiling point of a liquid, it is at the lowest temperature possible to retain the properties of vapor. Consequently, a minimal decrease in temperature causes steam condense. When saturated steam cools, its molecules are subject to the forces of the liquid molecular structure and return to the liquid state.

If condensation occurs in a closed vessel, density (kg/m3) and vapor pressure decrease. As a result, there is a corresponding decrease in the saturation temperature of the liquid. To maintain the condensation process under such conditions, the temperature of the liquid must continuously decrease to match the decrease in saturation pressure. Conversely, if steam is continuously supplied to a vessel to replace the mass of steam that condenses and is removed from the vessel, its density, pressure and saturation temperature will be constant. This way condensation used in systems machine cooling and continues until the heat is completely removed from the steam.

Evaporation

Evaporation is a subtle thermodynamic process caused by the slow transfer of heat to a fluid from the environment. Process evaporation produces rapid changes in the volume or mass of a liquid. Evaporation occurs as a result of absorption by liquid molecules thermal energy from the environment due to a small temperature difference. This increase in energy correspondingly increases the kinetic energy of the fluid. When kinetic energy is transferred through collisions, some molecules near the surface reach speeds that are much higher than average speed neighboring molecules. When some high-energy molecules approach the surface of a liquid, they break bonds, overcome the force of gravity and pass into the atmosphere as vapor molecules.

Vaporization Evaporation occurs if the vapor pressure above the liquid is lower than the saturation pressure, which corresponds to the temperature of the liquid. In other words, evaporation occurs when the vapor pressure and temperature lines of a liquid intersect at the saturation temperature line at a point below atmospheric pressure. These terms and conditions are at saturation temperature lines below the horizontal line of vapor pressure, which corresponds to the temperature of the liquid.

Volume of evaporated liquid continuously decreases as molecules separate from the surface and enter the surrounding atmosphere. After separation, some vapor molecules collide with others in the atmosphere, transferring some of their kinetic energy. When the reduction in energy reduces the speed of the vapor molecules below the level of separation from the liquid, they flow back and thus restore some of the lost volume. When the number of molecules leaving a liquid is equal to the number falling back, a state of equilibrium. Once this condition occurs, the volume of the liquid will remain unchanged until changes in vapor pressure or temperature produce corresponding changes in the rate of evaporation.