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Subcooling in refrigeration. Analysis of cases of abnormal hypothermia. Other methods of charging refrigeration systems

In this article, we will talk about the most accurate way to refuel air conditioners.

You can fill any freon. Refuel - only one-component freons (for example: R-22) or isotropic (conditionally isotropic, for example: R-410) mixtures

When diagnosing cooling and air conditioning systems, the processes occurring inside the condenser are hidden from the service engineer, and often it is from them that one can understand why the efficiency of the system as a whole has fallen.

Let's look at them briefly:

  1. Superheated refrigerant vapors pass from the compressor to the condenser
  2. Under the action of the air flow, the temperature of freon decreases to the condensation temperature
  3. Until the last freon molecule passes into the liquid phase, the temperature remains the same throughout the entire section of the pipeline where the condensation process takes place.
  4. Under the action of the cooling air flow, the temperature of the refrigerant decreases from the condensation temperature to the temperature of the cooled liquid freon
Freon pressure is the same inside the condenser.
Knowing the pressure, according to the special tables of the freon manufacturer, it is possible to determine the condensation temperature under current conditions. The difference between the condensation temperature and the temperature of the cooled freon at the outlet of the condenser - the subcooling temperature - is usually a known value (checked with the system manufacturer) and the range of these values ​​\u200b\u200bfor this system is fixed (for example: 10-12 ° C).

If the subcooling value is below the range specified by the manufacturer, then the freon does not have time to cool in the condenser - it is not enough and refueling is required. The lack of freon reduces the efficiency of the system and increases the load on it.

If the subcooling value is above the range - there is too much freon, it is necessary to drain a part until the optimum value is reached. An excess of freon increases the load on the system and reduces its service life.

Refueling by subcooling without using:

  1. We connect the manometric manifold and the cylinder with freon to the system.
  2. We install a thermometer / temperature sensor on the line high pressure.
  3. We start the system.
  4. Using the pressure gauge on the high pressure line (liquid line), we measure the pressure, calculate the condensation temperature for this freon.
  5. Using a thermometer, we control the temperature of the supercooled freon at the outlet of the condenser (it should be in the range of the sum of the condensation temperature and the supercooling temperature).
  6. If the freon temperature exceeds the allowable (supercooling temperature is below the required range) - there is not enough freon, slowly add it to the system until the desired temperature is reached
  7. If the freon temperature is below the allowable temperature (supercooling temperature is above the range) - freon is in excess, some must be slowly bled off until the desired temperature is reached.
Using this process simplifies many times (the connection diagram in the drawings is in the instruction manual):
  1. We reset the device to zero, put it into hypothermia mode, set the type of freon.
  2. We connect the gauge manifold and the cylinder with freon to the system, and the high pressure hose (liquid) is connected through the T-shaped tee supplied with the device.
  3. We install the temperature sensor SH-36N on the high pressure line.
  4. We turn on the system, the subcooling value will be displayed on the screen, we compare it with the required range and, depending on whether the displayed value is higher or lower, we slowly bleed or add freon.
This method of refueling is more accurate than refueling by volume or by weight, as there are no intermediate calculations, which are sometimes approximate.

Alexey Matveev,
technical specialist of the Raskhodka company

The heat balance of a surface condenser has the following expression:

G to ( h to -h to 1)=W(t 2v -t 1v)from to, (17.1)

where h to- enthalpy of steam entering the condenser, kJ/kg; h to 1 =c to t to- enthalpy of condensate; from to\u003d 4.19 kJ / (kg × 0 С) - heat capacity of water; W– cooling water consumption, kg/s; t 1v, t 2v- the temperature of the cooling water at the inlet and outlet of the condenser. Condensed steam consumption G k, kg/s and enthalpy h to known from the calculation of the steam turbine. The temperature of the condensate at the outlet of the condenser is assumed to be equal to the saturation temperature of the steam t p corresponding to its pressure r to taking into account subcooling of condensate D t to: t k \u003d t p - D t to.

Condensate subcooling(the difference between the saturation temperature of the steam at the pressure in the condenser neck and the temperature of the condensate in the suction pipe of the condensate pump) is a consequence of the decrease in the partial pressure and temperature of the saturated steam due to the presence of air and the vapor resistance of the condenser (Fig. 17.3).

Fig.17.3. Change in the parameters of the steam-air mixture in the condenser: a - change in the partial pressure of steam p p and pressure in the condenser p k; b - change in steam temperature t p and relative air content ε

Applying Dalton's law to the steam-air medium moving in the condenser, we have: p k \u003d p p + p in, where r p and r in are the partial pressures of vapor and air in the mixture. Dependence of partial steam pressure on condenser pressure and relative air content e=G in / G k looks like:

(17.2)

When entering the condenser, the relative air content is low and r p » r k. As the steam condenses, the value e rises and the partial pressure of the vapor falls. In the lower part, the partial air pressure is most significant, because. it increases due to the increase in air density and the value e. This leads to a decrease in the temperature of the steam and condensate. In addition, there is a vapor resistance of the capacitor, determined by the difference

D p k \u003d p k - p k´ .(17.3)

Usually D r to\u003d 270-410 Pa (determined empirically).

As a rule, wet steam enters the condenser, the condensation temperature of which is uniquely determined by the partial vapor pressure: a lower partial vapor pressure corresponds to a lower saturation temperature. Figure 17.3, b shows graphs of changes in steam temperature t p and relative air content ε in the condenser. Thus, as the steam-air mixture moves to the place of suction and steam condensation, the temperature of the steam in the condenser decreases, since the partial pressure of saturated steam decreases. This is due to the presence of air and an increase in its relative content in the vapor-air mixture, as well as the presence of vapor resistance of the condenser and a decrease in the total pressure of the vapor-air mixture.



Under such conditions, supercooling of the condensate is formed Dt to =t p -t to, which leads to the loss of heat with cooling water and the need for additional heating of the condensate in the regenerative system of the turbine plant. In addition, it is accompanied by an increase in the amount of oxygen dissolved in the condensate, which causes corrosion of the pipe system of the regenerative heating of the boiler feed water.

Subcooling can reach 2-3 0 C. The means of combating it is the installation of air coolers in the condenser tube bundle, from which the vapor-air mixture is sucked off into ejector installations. In modern vocational schools, hypothermia is allowed no more than 1 0 C. Rules technical operation strictly prescribe the allowable suction of air into the turbine plant, which must be less than 1%. For example, for turbines with power N E=300 MW air suction should be no more than 30 kg / h, and N E\u003d 800 MW - no more than 60 kg / h. Modern condensers with minimal vapor resistance and rational arrangement of the tube bundle, in the nominal mode of operation of the turbine plant, have practically no subcooling.

air conditioner

Charging the air conditioner with freon can be carried out in several ways, each of them has its own advantages, disadvantages and accuracy.

The choice of method for refilling air conditioners depends on the level of professionalism of the master, the required accuracy and the tools used.

It is also necessary to remember that not all refrigerants can be recharged, but only single-component (R22) or conditionally isotropic (R410a).

Multicomponent freons consist of a mixture of gases with different physical properties, which, when leaked, evaporate unevenly and even with a small leak, their composition changes, so systems using such refrigerants must be completely recharged.

Filling the air conditioner with freon by mass

Each air conditioner is charged at the factory with a certain amount of refrigerant, the mass of which is indicated in the documentation for the air conditioner (also indicated on the nameplate), there is also information on the amount of freon that must be added additionally for each meter of the freon route (usually 5-15 gr.)

When refueling by this method, it is necessary to completely free the refrigeration circuit from the remaining freon (into a cylinder or bleed into the atmosphere, this does not harm the environment at all - read about this in the article on the effect of freon on climate) and vacuum it. Then fill the system with the specified amount of refrigerant by weight or using the filling cylinder.

The advantages of this method in high precision and sufficient simplicity of the process of refueling the air conditioner. The disadvantages include the need to evacuate freon and evacuate the circuit, and the filling cylinder, moreover, has a limited volume of 2 or 4 kilograms and large dimensions, which allows it to be used mainly in stationary conditions.

Filling the air conditioner with freon for hypothermia

The subcooling temperature is the difference between the freon condensation temperature determined by the table or pressure gauge scale (determined by the pressure read from the pressure gauge connected to the high pressure line directly on the scale or according to the table) and the temperature at the outlet of the condenser. The subcooling temperature should usually be in the range of 10-12 0 C (the exact value is specified by the manufacturers)

The subcooling value below these values ​​​​indicates a lack of freon - it does not have time to cool enough. In this case, it must be refueled

If the subcooling is above the specified range, then there is an excess of freon in the system and it must be drained before reaching optimal values hypothermia.

You can fill this way using special devices, which immediately determine the amount of subcooling and condensation pressure, or it is possible with the help of separate devices - a manometric manifold and a thermometer.

The advantages of this method include sufficient filling accuracy. But the accuracy of this method is affected by the contamination of the heat exchanger, therefore, before refueling with this method, it is necessary to clean (wash) the condenser of the outdoor unit.

Charging the air conditioner with refrigerant overheating

Superheat is the difference between the evaporation temperature of the refrigerant determined by the saturation pressure in the refrigeration circuit and the temperature after the evaporator. It is practically determined by measuring the pressure at the suction valve of the air conditioner and the temperature of the suction pipe at a distance of 15-20 cm from the compressor.

Overheating is usually in the range of 5-7 0 C (the exact value is indicated by the manufacturer)

A decrease in overheating indicates an excess of freon - it must be drained.

Hypothermia above normal indicates a lack refrigerant system must be filled until the required superheat value is reached.

This method is quite accurate and can be greatly simplified using special instruments.

Other methods of charging refrigeration systems

If the system has a viewing window, then by the presence of bubbles one can judge the lack of freon. In this case, the refrigeration circuit is filled until the flow of bubbles disappears, this should be done in portions, after each wait for the pressure to stabilize and the absence of bubbles.

It is also possible to fill by pressure, while achieving the condensation and evaporation temperatures specified by the manufacturer. The accuracy of this method depends on the cleanliness of the condenser and evaporator.

Improving the efficiency of refrigeration

installations due to refrigerant subcooling

FGOU VPO "Baltic State Academy of the Fishing Fleet",

Russia, *****@***ru

Reducing the consumption of electrical energy is a very important aspect of life in connection with the current energy situation in the country and in the world. Reducing the energy consumption of refrigeration plants can be achieved by increasing the cooling capacity refrigeration units. The latter can be carried out using various types of subcoolers. Thus, considered different kinds subcoolers and designed the most efficient.

cooling capacity, subcooling, regenerative heat exchanger, subcooler, shell-to-tube boiling, intra-tube boiling

By subcooling the liquid refrigerant before throttling, a significant increase in the efficiency of the refrigeration plant can be achieved. Subcooling of the refrigerant can be achieved by installing a subcooler. The subcooler for liquid refrigerant flowing from the condenser at condensing pressure to the control valve is designed to cool it below the condensing temperature. Exist various ways subcooling: by boiling a liquid refrigerant at intermediate pressure, by means of a vaporous agent leaving the evaporator, and by means of water. Subcooling the liquid refrigerant makes it possible to increase the cooling capacity of the refrigeration plant.

One of the types of heat exchangers designed to supercool liquid refrigerants are regenerative heat exchangers. In devices of this type, subcooling of the refrigerant is achieved due to the vaporous agent leaving the evaporator.


In regenerative heat exchangers, heat exchange occurs between the liquid refrigerant coming from the receiver to the control valve and the vaporous agent leaving the evaporator. Regenerative heat exchangers are used to perform one or more of the following functions:

1) increasing the thermodynamic efficiency of the refrigeration cycle;

2) subcooling of the liquid refrigerant to prevent vaporization in front of the control valve;

3) evaporation a small amount liquid carried away from the evaporator. Sometimes, when using flooded type evaporators, an oil-rich layer of liquid is deliberately diverted into the suction line to ensure oil return. In these cases, regenerative heat exchangers serve to evaporate the liquid refrigerant from the solution.

On fig. 1 shows a diagram of the installation of the RT.

Fig.1. Installation diagram of a regenerative heat exchanger

Fig. 1. The scheme of installation of the regenerative heat exchanger

The simplest form of a heat exchanger is obtained by metallic contact (welding, soldering) between liquid and steam pipes to provide countercurrent. Both pipelines are covered with insulation as a whole. For maximum performance, the liquid line must be located below the suction line, since the liquid in the suction line can flow along the bottom generatrix.

The most widespread in the domestic industry and abroad are shell-and-coil and shell-and-tube regenerative heat exchangers. In small refrigeration machines Oh manufactured by foreign companies, coil heat exchangers of a simplified design are sometimes used, in which the liquid tube is wound onto the suction tube. The Dunham-Busk company (USA) to improve heat transfer, the liquid coil wound on the suction line is filled with aluminum alloy. The suction line is equipped with internal smooth longitudinal ribs, which provide good heat transfer to steam with minimal hydraulic resistance. These heat exchangers are designed for installations with a cooling capacity of less than 14 kW.

For installations of medium and large productivity, shell-and-coil regenerative heat exchangers are widely used. In devices of this type, a liquid coil (or several parallel coils) wound around the displacer is placed in cylindrical vessel. Steam passes in the annular space between the displacer and the casing, while providing a more complete steam washing of the surface of the liquid coil. The coil is made from smooth, and more often from finned pipes on the outside.

When using heat exchangers of the "pipe in pipe" type (as a rule, for small refrigeration machines), special attention is paid to the intensification of heat transfer in the apparatus. For this purpose, either finned tubes are used, or various inserts (wire, tape, etc.) are used in the vapor region or in the vapor and liquid regions (Fig. 2).

Fig.2. Heat exchanger regenerative type "pipe in pipe"

Fig. 2. Regenerative heat exchanger type “pipe in pipe”

Subcooling by boiling liquid refrigerant at an intermediate pressure can be carried out in intermediate vessels and economizers.

In low-temperature two-stage compression refrigeration units, the operation of the intermediate vessel installed between the compressors of the first and second stages largely determines the thermodynamic perfection and efficiency of the operation of the entire refrigeration unit. The intermediate vessel performs the following functions:

1) “knocking down” the overheating of the steam after the first stage compressor, which leads to a decrease in the work expended by the high pressure stage;


2) cooling the liquid refrigerant before it enters the control valve to a temperature close to or equal to the saturation temperature at intermediate pressure, which reduces losses in the control valve;

3) partial separation of oil.

Depending on the type of the intermediate vessel (coiled or coilless), a scheme with one or two-stage throttling of the liquid refrigerant is carried out. In pumpless systems, serpentine intermediate vessels are preferred, in which the liquid is under condensing pressure, providing liquid refrigerant to the evaporative system of multi-storey refrigerators.

The presence of the coil also excludes additional oiling of the liquid in the intermediate vessel.

In pump-circulation systems, where the liquid supply to the evaporation system is provided by the pressure of the pump, coilless intermediate vessels can be used. The current use of efficient oil separators in the schemes of refrigeration units (washing or cyclone on the discharge side, hydrocyclones in the evaporation system) also makes it possible to use coilless intermediate vessels - devices that are more efficient and simpler in design.

Water subcooling can be achieved in counterflow subcoolers.

On fig. 3 shows a two-pipe counterflow subcooler. It consists of one or two sections assembled from double pipes connected in series (pipe in pipe). The inner pipes are connected with cast-iron rolls, the outer pipes are welded. The liquid working substance flows in the annular space in countercurrent to the cooling water moving through the inner pipes. Pipes - steel seamless. The outlet temperature of the working substance from the apparatus is usually 2-3 °C higher than the temperature of the incoming cooling water.

pipe in pipe"), each of which is supplied with liquid refrigerant through the distributor, and the refrigerant from the linear receiver enters the annular space, the main disadvantage is the limited service life due to the rapid failure of the distributor. The intermediate vessel, in turn, can be to be used only for cooling systems running on ammonia .

Rice. 4. Sketch of a liquid freon subcooler with boiling in the annulus

Fig. 4. The sketch of supercooler with boiling of liquid Freon in intertubes space

The most suitable device is a liquid freon subcooler with boiling in the annulus. A diagram of such a subcooler is shown in fig. 4.

Structurally, it is a shell-and-tube heat exchanger, in the annular space of which the refrigerant boils, the refrigerant from the linear receiver enters the pipes, is supercooled and then fed to the evaporator. The main disadvantage of such a supercooler is the foaming of liquid freon due to the formation of an oil film on its surface, which leads to the need for a special device for removing oil.

Thus, a design was developed in which it is proposed to supply a supercooled liquid refrigerant from a linear receiver into the annular space, and to ensure (by preliminary throttling) the boiling of the refrigerant in the pipes. This technical solution is illustrated in Fig. 5.

Rice. 5. Sketch of a liquid freon subcooler with boiling inside the pipes

Fig. 5. The sketch of supercooler with boiling of liquid Freon inside pipes

This scheme of the device makes it possible to simplify the design of the subcooler, excluding from it a device for removing oil from the surface of liquid freon.

The proposed liquid freon subcooler (economizer) is a housing containing a package of heat exchange pipes with internal finning, as well as a pipe for the inlet of the cooled refrigerant, a pipe for the outlet of the cooled refrigerant, pipes for the inlet of the throttled refrigerant, a pipe for the outlet of the vaporous refrigerant.

The recommended design makes it possible to avoid foaming of liquid freon, increase reliability and provide more intensive subcooling of the liquid refrigerant, which, in turn, leads to an increase in the cooling capacity of the refrigeration unit.

LIST OF USED LITERATURE SOURCES

1. Zelikovsky on heat exchangers of small refrigeration machines. - M.: Food industry, 19s.

2. Ion cold production. - Kaliningrad: Prince. publishing house, 19s.

3. Danilova refrigeration units. - M.: Agropromizdat, 19s.

IMPROVING THE EFFICIENCY OF REFRIGERATING PLANTS DUE SUPERCOOLING OF REFRIGERANT

N. V. Lubimov, Y. N. Slastichin, N. M. Ivanova

Supercooling of liquid Freon in front of the evaporator allows to increase refrigerating capacity of a refrigerating machinery. For this purpose we can use regenerative heat exchangers and supercoolers. But more effective is the supercooler with boiling of liquid Freon inside pipes.

frigerating capacity, supercooling, supercooler

Undercharging and recharging the system with refrigerant

According to statistics, the main reason for the abnormal operation of air conditioners and the failure of compressors is improper refueling. refrigeration circuit coolant. The lack of refrigerant in the circuit may be due to accidental leaks. At the same time, excessive refueling, as a rule, is the result of erroneous actions of personnel caused by their insufficient qualifications. For systems that use a thermostatic expansion valve (TXV) as a throttling device, subcooling is the best indicator of a normal refrigerant charge. Weak subcooling indicates that the charge is insufficient, strong indicates an excess of refrigerant. Charging can be considered normal when the liquid subcooling temperature at the condenser outlet is maintained within 10-12 degrees Celsius with the air temperature at the evaporator inlet close to the nominal operating conditions.

The subcooling temperature Tp is defined as the difference:
Tp \u003d Tk - Tf
Tk is the condensation temperature read from the HP manometer.
Tf - temperature of freon (pipe) at the outlet of the condenser.

1. Lack of refrigerant. Symptoms.

The lack of freon will be felt in every element of the circuit, but this deficiency is especially felt in the evaporator, condenser and liquid line. As a result of an insufficient amount of liquid, the evaporator is poorly filled with freon and the cooling capacity is low. Since there is not enough liquid in the evaporator, the amount of steam produced there drops dramatically. Since the volumetric efficiency of the compressor exceeds the amount of steam coming from the evaporator, the pressure in it drops abnormally. A drop in evaporation pressure leads to a decrease in the evaporation temperature. The evaporating temperature can drop to below zero, resulting in freezing of the inlet pipe and the evaporator, and the overheating of the steam will be very significant.

Superheat temperature T superheat is defined as the difference:
T overheating = T f.i. – T suction.
T f.i. - the temperature of the freon (pipe) at the outlet of the evaporator.
T suction - suction temperature read from the LP manometer.
Normal overheating is 4-7 degrees Celsius.

With a significant lack of freon, overheating can reach 12–14 ° C and, accordingly, the temperature at the compressor inlet will also increase. And since the cooling of the electric motors of hermetic compressors is carried out with the help of suction vapors, in this case the compressor will overheat abnormally and may fail. Due to the increase in the temperature of the vapors in the suction line, the temperature of the vapor in the discharge line will also be increased. Since there will be a shortage of refrigerant in the circuit, it will also be insufficient in the subcooling zone.

    Thus, the main signs of a lack of freon:
  • Low cooling capacity
  • Low evaporation pressure
  • High superheat
  • Insufficient hypothermia (less than 10 degrees Celsius)

It should be noted that in installations with capillary tubes as a throttling device, subcooling cannot be considered as a determining factor for assessing the correct amount of refrigerant charge.

2. Overfilling. Symptoms.

In systems with an expansion valve as a throttling device, liquid cannot enter the evaporator, so the excess refrigerant is in the condenser. abnormal high level liquid in the condenser reduces the heat exchange surface, the cooling of the gas entering the condenser deteriorates, which leads to an increase in the temperature of saturated vapors and an increase in the condensation pressure. On the other hand, the liquid at the bottom of the condenser stays in contact with the outside air much longer, and this leads to an increase in the subcooling zone. Since the condensing pressure is increased and the liquid leaving the condenser is perfectly cooled, the subcooling measured at the condenser outlet will be high. Because of high blood pressure condensation, there is a reduction in the mass flow through the compressor and a drop in cooling capacity. As a result, the evaporation pressure will also increase. Due to the fact that excessive charging leads to a decrease in vapor mass flow, the cooling of the electric motor of the compressor will deteriorate. Moreover, due to the increased condensing pressure, the electric motor current of the compressor increases. Deterioration of cooling and an increase in current consumption leads to overheating of the electric motor and, ultimately, to failure of the compressor.

    Outcome. The main signs of refrigerant recharging:
  • Decreased cooling capacity
  • Evaporation pressure increased
  • Increased condensing pressure
  • Increased hypothermia (more than 7 ° C)

In systems with capillary tubes as a throttling device, excess refrigerant can enter the compressor, causing water hammer and eventually compressor failure.