Danfoss thermostatic valve operating principle. Routine maintenance and procedure for setting up the TV

Currently available a large number of documents and technical instructions developers, which describe in detail the design of expansion valves, their operation, and the technology for their selection and installation.

Most documents indicate that the expansion valves are configured at the manufacturer's factory and, as a rule, do not require additional adjustment. At the same time, the question arises: how to adjust the expansion valve if for some reason there is a need for additional adjustment? We recommend the following method. In addition to the commonly used pressure gauges, you need to install an electronic thermometer, the sensor of which should be mounted on the thermal expansion valve (see Fig. 8.4).

To maintain the stability of the setting over time, it is necessary to make it at a temperature in the refrigerated volume close to the compressor cut-out temperature (a setting that ensures stability at a temperature of 25 °C may lead to pulsation at a temperature of 20 °C). It is not allowed to adjust the expansion valve when high temperature in a refrigerated volume!

The recommended setting technology is to first bring the expansion valve to the limit mode, at which pulsations begin. To do this, at a constant value of superheat (the readings of the thermometer and LP pressure gauge do not change), you need to slowly open the expansion valve until pulsations begin. If overheating pulsations appear (pulsations in the readings of the thermometer and pressure gauge), you need to close the expansion valve until the pulsations stop.

Attention! Never turn the adjusting screw more than one turn (the limiting mode leading to pulsation can occur when the screw is rotated 1/4 or even 1/8 turn).

After each change of setting (turning the adjusting screw), you should wait at least 15 minutes (in the future this will allow you to save time on setting). When the installation enters pulsating mode, it is enough to slightly close the expansion valve (for example, half a turn). In this case, the expansion valve will be set to the minimum possible superheat, which is provided by this installation, the filling of the evaporator with liquid refrigerant will be optimal, and the pulsations will stop.

Note: during adjustment, the condensing pressure should remain relatively stable, but its value should be as close as possible to the nominal operating conditions, since the performance of the expansion valve depends on it.

unevaporated liquid particles (though it is unknown how long it will work in this mode, which can lead to very serious malfunctions).

Two difficulties may arise during setup:

1. You are unable to achieve pulsations. This means that the expansion valve, even when fully open, has a performance lower than that of the evaporator. IN general case this can happen for the following reasons: either the flow area of ​​the expansion valve is too small, or there is not enough refrigerant in the installation, or not enough liquid is supplied to the inlet of the expansion valve.
2. You are unable to eliminate pulsations once they occur. This means that the expansion valve, even when completely closed, maintains a performance higher than throughput evaporator. In general, this is due to the fact that either the flow area of ​​the expansion valve is too large, or the evaporator does not have enough performance.

The adjustment stops when the superheat reaches too high a value (this occurs when the expansion valve is practically closed, the evaporation pressure is abnormally low, and the total temperature difference is too large). This means that the evaporator produces less vapor than the compressor can absorb, that is, the evaporator capacity is insufficient.

NOTE: Anomalies that can cause the above problems when adjusting the expansion valve (expansion valve too small or too large, poor liquid make-up, lack of refrigerant in the circuit, lack of evaporator capacity) will be analyzed in more detail when each of these faults is examined in detail. Here we will formulate the main conclusion from this section: setting up the expansion valve can be a labor-intensive and time-consuming process, so do not begin the setup procedure without being absolutely sure of a deep understanding of our recommendations. In all cases, when you begin adjusting the expansion valve, be sure, as a precaution, to note the initial setting (the initial position of the adjustment screw) and accurately count the number of turns of the adjustment screw you have made (fine adjustment can be achieved by turning the screw as little as 1/8 turn ).

All refrigeration units are equipped with thermostatic valves (TEV), with the help of which the amount of refrigerant supplied to the evaporators of the refrigeration equipment is adjusted. Thermostatic valve danfoss- one of best devices of our time, which is produced by the famous Danish concern of the same name.

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The principle of operation and the task that the thermostatic valve performs is to provide the evaporator required quantity refrigerant volume, determined by the thermal load on the unit at a given time. For example, thermostatic air conditioner maintains the output superheated vapor within certain limits.

Kinds

Respectively functional purpose, trv danfoss divided into the following types:

Thermal control valves ETS

Functional purpose: supplying coolant to the evaporators of refrigeration equipment and air conditioners. Thanks to the complete balance between valve and body, coolant flows in both directions. The valve closes very tightly.

Advantages:

• operates regardless of the direction of movement of the working medium in the unit, which is ensured by a balancing device.
• Various drive models make it possible to completely shift the valve in 2625 - 3810 steps within 8.4-12.7 seconds.
• If there is a direct current drive, the valve is moved at a speed of 150 steps per 1 second.

Thermostatic motorized valves AKV

With their help, the refrigerant is injected into the evaporators. The adjustment is carried out using the pulse width method. This means that the width of the pulses sent by the unit controller determines the degree of opening.

Advantages:

• thanks to the collapsible design, the valve assembly (nozzle) for the Danfoss AKV valve can be changed;
• the valve does not require adjustment during operation;
• The device is universal in design, since it is a solenoid and thermostatic valve at the same time.

Thermostatic valve T2 and TE2

To fill “dry” (unflooded) evaporators designed for low power, thermostatic valves T2 and TE2 are used. The rated cooling capacity of such units ranges from 380 W to 9,100 W with R404A/R507. They are used in conventional refrigeration units, heat pumps, air coolers, chillers, transport refrigerators, and ice generators.

They differ:

• large operating temperature range;
• the presence of a replaceable valve assembly;
• ease of setting the required performance;
• ease of storage.

Important! If necessary, you can supply a special adapter designed for both flanging and soldering.

The TE2 type includes the danfoss tx2 thermostatic valve, designed for automatic adjustment of the flow of refrigerants with the presence of fluorine in the evaporators of cooling units.

Designed to operate at pressures up to 28 atm at temperatures from -40 degrees to +10 degrees.

Thermostatic valve RNT

Controls the flow of liquid refrigerants into the evaporators of units. With its help, “dry” evaporators are filled, in which the thermal load on them is directly proportional to the overheating of the refrigerants. The device operates in a thermal range of -40 – + 50⁰С at a permissible operating pressure of 28 bar (for PHT 85 and PHT 125), and 20 bar (for PHT 300).

TRV TU/TC

The performance of TU/TC valves depends on:

• pressure created by the thermal cylinder filler;
• pressure during the boiling of the refrigerant;
• degree of spring tension.

Therefore, the adjustment of such thermostatic valves comes down to constantly maintaining a balance between the pressure level in the cylinder, which is formed on one side of the membrane, and the total pressure between the spring tension and boiling acting on the other side.

Such settings are used in normal refrigeration equipment, heat pumps, air conditioners, coolers, etc.

They differ:

• lightness and strength;
• the presence of bimetallic fittings, which facilitates convenient and safe soldering;
• using capillary tubes, which contributes to a long service life.

The overheating of this type of Danfoss valve can be adjusted using an adjusting screw.

Important! It is possible to mount with any MOP (maximum operating pressure) values, which helps protect the compressor from high blood pressure when boiling.

Thermostatic valve (TRV) TGE

This series is characterized by the presence of non-replaceable valve units. Such valves have been developed for commercial purposes: they are used in high-performance air conditioning systems.

The devices are capable of passing liquid refrigerant into “dry” type evaporators, in which the thermal load on the evaporator is directly proportional to the superheat of the refrigerants.

The advantages of the unit include:

• operation in conditions of high humidity, which is very important when equipping heat pumps;
• balancing of the valve assembly in any direction of flow of the working medium (valves of the TGE 20 and TGE 40 series);
• speed and ease of installation;
• there is a minimal risk of possible leaks, since all welds are laser.

Thermostatic valve danfoss tgel-35 refers to direct-flow sealed products of the TGE modification, which have a built-in valve assembly (R410 refrigerant) with static superheating 4K. Functions optimally at temperatures from -40 degrees. up to +10 degrees. and pressures not exceeding 46 atmospheres.

TRV TE5 – TE55

With the help of units, the supply of refrigerant to medium-sized refrigeration equipment is regulated. The valves are designed to fill “dry” (unflooded) evaporators with refrigerant, the flow of which can be judged by the degree of superheat when leaving the evaporator.

Thanks to the presence of a replaceable valve assembly, it is ensured:

• ease of installation;
• the best option to support a specific performance;
• the presence of channels that balance pressure.

Can be used in the temperature range from -60⁰С to +11⁰С!

Replacing the expansion valve

If refrigeration equipment functions intermittently, then you first need to find out the cause of the possible breakdown.

For example, when there is no supply of hot or cold air from the air conditioner, then one of the reasons for it bad work The air filter may be clogged.

To resume normal operation, the filter and other accessories should be cleaned and, as far as possible, prevent dirt and dust from entering them.

If the valve, for example, cannot equalize the pressure in the circuits, then it is best to replace it. By the way, this technological process Troubleshooting such as replacing the valve is a simple procedure that you can do yourself.

In addition, we offer a list of the most common breakdowns of refrigeration equipment when it is necessary to replace the device:

• productivity is too low;
• the presence of pressure pulsation, which is expressed by high productivity;
• very high pressure is formed at the suction;
• liquid refrigerant flows from the thermal cylinder or there is a leak;
• the compressor is constantly overfilled with liquid, which is caused by the valve’s throughput being too large;
• the unit is permanently closed;
• the valve does not respond to any method of influence;
• observation of constant fluctuations in temperature and pressure in the system.

TRV valve 2

Thermostatic valve tn 2 r 134 is a fairly precise unit with which the supply of refrigerants is regulated, depending on the intensity of their boiling in the evaporators. Flow regulation is carried out by the presence of specific temperature indicators and pressure of the vapor-type refrigerant at the exit from the evaporator.

Thermostatic valves of models TRV 2 type tes 2 s external alignment usually made of brass and designed to operate in systems with an optimum pressure of 34 bar. They can easily withstand external influences and have a long service life.

Solenoid

The Danfoss solenoid valve is quite popular among similar devices. Without solenoid valves, it is impossible to imagine the full functioning of refrigeration units, air conditioners, gas supplies, etc.

The main components of a Danfoss solenoid valve are a coil and a core (piston or disk), which are housed in a plastic or metal housing. With the help of the Danfoss valve core, the flow of working media is adjusted or the passage of working substances is blocked.

When setting up a solenoid-type valve, you must take into account the direction of refrigerant flow, which is indicated by arrows on the housings, otherwise the unit will not function.

If it is necessary to install a valve before the expansion valve, they must be very close to each other. This placement eliminates the possibility of water hammer occurring during possible openings.

There are two types of unit adjustment: electronic control of Danfoss valves and mechanical.

The second type can be divided into 2 modifications:

• devices in which valve assemblies can be changed;
• devices with non-replaceable valve assemblies.

Products whose design provides for the presence of replaceable valve units include expansion-type devices equipped with automation designed to regulate the supply of refrigerant containing chlorine and fluorine.

The danfoss r410a thermostatic valve belongs to angle devices, both with external equalization and without external equalizer, which can be purchased complete with a nozzle (analogous to a valve assembly). Correct selection nozzles for Danfoss expansion valves determine the further functioning of the entire unit.

The Danfoss 068u4261 thermostatic valve (TRV) is characterized by a standard factory setting of static superheat of 5 K.

The rated power when operating the danfoss tcbe 068u4504 valve is possible at temperatures:

• evaporation – te = + 5 °C;
• condensation – tc = + 32 °C;
• refrigerant liquids – tl = + 28 °C, at a maximum operating pressure of up to 45.5 bar.

The danfoss tex 5 067b3250 thermostatic valve regulates the flow of refrigerant containing fluorine in the evaporators of cooling structures.

Trv danfoss tes 5:

• characterized by a wide selection of models;
• has a large amplitude of performance;
• equipped with a capillary tube, replaceable power supplies, valve units and thermal cylinders;
• used in refrigeration equipment with pressure up to 28 atm.

The danfoss tes2 and tex2 danfoss expansion valves, which are designed to operate in the temperature range from -40⁰С to +10⁰С, are very popular. Among the mechanical analogues of the TES2 angle type, the Danfoss r404a tes 2 2-40 c +10 c without mor with external alignment is in demand.

Features a 3/8” flange inlet connection. Designed for efficient operation at pressures up to 34 atmospheres.

The danfoss tdez 8 068h5169 thermostatic valve is usually equipped with a 150 cm capillary tube with a 3/8 inch inlet fitting and is designed to operate in temperature conditions from +10⁰С to -25⁰С.

Ball

Danfoss ball valves are embedded into systems by soldering or using a threaded connection.

Important! Most often, ball-type valves are installed during adjustment or repair of a pipeline, the shutoff of which is carried out manually.

You can buy Danfoss ball-type expansion valves in the online store, where specialists will not only help with advice, but also carry out a professional selection of Danfoss expansion valves for specific application conditions. The price of danfoss valve depends on the unit model, supplier, exchange rate and other factors.

Types of evaporators

Evaporator is one of the elements of a 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.

Rice. 1. Regulator and evaporator performance curves for the case of regulating the refrigerant supply to the evaporator using a expansion valve.

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 TRV brand T2/TU2 full turn screw changes the superheat 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 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 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 not possible when the superheat reaches greater 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 thermoregulatory (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), a capillary tube 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.

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 from 1755 it served “to obtain frigories (negative calories)” in a laboratory installation 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 absorption refrigeration machine on ammonia, 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 the growth of bacteria and also reduces the load on 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 a given milk flow;

· 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.