Subcooling in refrigeration. Subcooling in air-cooled condensers: what is its norm? Subcooling refrigerant system
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 limited volume 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 from the table or pressure gauge scale (determined by the pressure read from the pressure gauge connected to the line high pressure directly on the scale or according to the table) and the temperature at the outlet of the condenser. The subcooling temperature should normally be between 10-12 0 C ( exact value manufacturers indicate)
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 until the optimum subcooling values are reached.
It is possible to fill in this way using special devices that immediately determine the amount of subcooling and condensation pressure, or you can use separate devices - a manometric manifold and a thermometer.
The advantages of this method include sufficient filling accuracy. But for accuracy this method the contamination of the heat exchanger affects, therefore, before refueling by 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 fishing fleet,
Russia, *****@***ru
Consumption reduction 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 units can be achieved by increasing the cooling capacity of 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. Exists 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 of a small amount of 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 metal contact (welding, soldering) between liquid and steam pipelines 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 ah, produced by foreign firms, coil heat exchangers of a simplified design are sometimes used, in which the liquid tube is wound on 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 a 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 tube-in-pipe heat exchangers (typically for small chillers) Special attention give 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 refrigeration units with two-stage compression, 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 to supply 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 .
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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 annulus, and to ensure (by preliminary throttling) the boiling of the refrigerant in the pipes. This technical solution is illustrated in Fig. five.
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 fins, 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
Variants of operation of the refrigeration unit: operation with normal superheat; with insufficient overheating; severe overheating.
Operation with normal superheat.
Refrigeration unit scheme
For example, the refrigerant is supplied at a pressure of 18 bar, the suction pressure is 3 bar. The temperature at which the refrigerant boils in the evaporator t 0 \u003d -10 ° C, at the outlet of the evaporator the temperature of the pipe with the refrigerant t t \u003d -3 ° C.
Useful overheating ∆t \u003d t t - t 0 \u003d -3 - (-10) \u003d 7. This normal work refrigeration unit with air heat exchanger. AT evaporator freon boils away completely in about 1/10 of the evaporator (closer to the end of the evaporator), turning into a gas. Further, the gas will be heated by room temperature.
Overheating is insufficient.
The outlet temperature will be, for example, not -3, but -6 ° С. Then the overheating is only 4 °C. The point where the liquid refrigerant stops boiling moves closer to the evaporator outlet. Thus, most of the evaporator is filled with liquid refrigerant. This can happen if the thermostatic expansion valve (TRV) supplies more freon to the evaporator.
The more freon will be in the evaporator, the more vapor will be formed, the higher the suction pressure will be and the boiling point of freon will increase (let's say not -10, but -5 ° C). The compressor will begin to fill with liquid freon, because the pressure has increased, the refrigerant flow has increased and the compressor does not have time to pump out all the vapors (if the compressor does not have additional capacity). With this operation, the cooling capacity will increase, but the compressor may fail.
Severe overheating.
If the performance of the expansion valve is less, then less freon will enter the evaporator and it will boil off earlier (the boiling point will move closer to the evaporator inlet). The entire expansion valve and pipes after it will freeze and become covered with ice, and 70 percent of the evaporator will not freeze at all. Freon vapor in the evaporator will heat up, and their temperature can reach the temperature in the room, hence ∆t ˃ 7. In this case, the cooling capacity of the system will decrease, the suction pressure will decrease, heated freon vapor can damage the compressor stator.
One of the biggest difficulties in the work of a repairman is that he cannot see the processes occurring inside the pipelines and in the refrigeration circuit. However, measuring the amount of subcooling can provide a relatively accurate picture of the behavior of the refrigerant within the circuit.
Note that most designers size air-cooled condensers to provide subcooling at the outlet of the condenser in the range of 4 to 7 K. Consider what happens in the condenser if the subcooling is outside this range.
A) Reduced subcooling (usually less than 4 K).
Rice. 2.6
On fig. 2.6 shows the difference in the state of the refrigerant inside the condenser during normal and abnormal subcooling. Temperature at points tw=tc=te=38°С = condensation temperature tk. Temperature measurement at point D gives the value td=35 °C, subcooling 3 K.
Explanation. When the refrigeration circuit is operating normally, the last vapor molecules condense at point C. Further, the liquid continues to cool and the pipeline along the entire length (zone C-D) is filled with liquid phase, which allows achieving a normal subcooling value (for example, 6 K).
In case of a lack of refrigerant in the condenser, zone C-D is not completely filled with liquid, there is only small plot this zone, completely occupied by the liquid (zone E-D), and its length is not enough to provide normal supercooling.
As a result, when measuring hypothermia at point D, you will definitely get its value below normal (in the example in Figure 2.6 - 3 K).
And the less refrigerant there is in the installation, the less its liquid phase will be at the outlet of the condenser and the less its degree of subcooling will be.
In the limit, with a significant shortage of refrigerant in the refrigeration circuit, at the outlet of the condenser there will be a vapor-liquid mixture, the temperature of which will be equal to the condensation temperature, that is, subcooling will be 0 K (see Figure 2.7).
Rice. 2.7
tv=td=tk=38°С. Subcooling value P/O = 38-38=0 K.
Thus, insufficient refrigerant charge always leads to a decrease in subcooling.
It follows that a competent repairman will not recklessly add refrigerant to an installation without making sure that there are no leaks and without making sure that the subcooling is abnormally low!
Note that as refrigerant is added to the circuit, the liquid level at the bottom of the condenser will rise, causing an increase in subcooling.
Let us now turn to the consideration of the opposite phenomenon, that is, too much hypothermia.
B) Increased hypothermia (usually more than 7 K).
Rice. 2.8
tv=te=tk= 38°С. td \u003d 29 ° C, therefore, subcooling P / O \u003d 38-29 \u003d 9 K.
Explanation. Above we have seen that the lack of refrigerant in the circuit leads to a decrease in subcooling. On the other hand, an excessive amount of refrigerant will accumulate at the bottom of the condenser.
In this case, the length of the condenser zone, completely filled with liquid, increases and can occupy the entire section E-D. The amount of liquid in contact with the cooling air increases and the amount of subcooling, therefore, also becomes larger (in the example in Fig. 2.8, P/O = 9 K).
In conclusion, we point out that measurements of the magnitude of subcooling are ideal for diagnosing the process of functioning of a classical refrigeration plant.
During a detailed analysis typical faults we will see how, in each specific case, to accurately interpret the data of these measurements.
Too low subcooling (less than 4 K) indicates a lack of refrigerant in the condenser. Increased subcooling (greater than 7 K) indicates an excess of refrigerant in the condenser.
2.4. THE EXERCISE
Choose from 4 air-cooled condenser designs shown in fig. 2.9 whichever you think is the best. Explain why?
Rice. 2.9
Due to gravity, liquid accumulates at the bottom of the condenser, so the vapor inlet to the condenser must always be at the top. Therefore, options 2 and 4 are at least a strange solution that will not work.
The difference between options 1 and 3 is mainly in the temperature of the air that blows over the supercooling zone. In the 1st variant, the air that provides subcooling enters the subcooling zone already heated, since it has passed through the condenser. The design of the 3rd option should be considered the most successful, since it implements heat exchange between the refrigerant and air according to the counterflow principle. This option has best performance heat transfer and plant design as a whole.
Think about it if you haven't already decided which direction of cooling air (or water) you want to go through the condenser.
- The influence of temperature and pressure on the state of refrigerants
- Subcooling in air-cooled condensers
- Analysis of cases of abnormal hypothermia
19.10.2015
The degree of subcooling of the liquid obtained at the outlet of the condenser is an important indicator that characterizes stable operation. refrigeration circuit. Subcooling is the temperature difference between a liquid and condensation at a given pressure.
Under normal atmospheric pressure, water condensation has a temperature index of 100 degrees Celsius. According to the laws of physics, water that is 20 degrees is considered supercooled by 80 degrees Celsius.
The subcooling at the outlet of the heat exchanger varies as the difference between the temperature of the liquid and the condensation. Based on Figure 2.5, the subcooling would be 6 K or 38-32.
In air-cooled condensers, the subcooling index should be between 4 and 7 K. If it has a different value, then this indicates unstable operation.
Interaction between condenser and fan: air temperature difference.
The blown air by the fan has an indicator of 25 degrees Celsius (Figure 2.3). It takes heat from freon, due to which its temperature changes to 31 degrees.
Figure 2.4 shows a more detailed change:
Tae - temperature mark of the air supplied to the condenser;
Tas is the air with the new condenser temperature after cooling;
Tk - readings from the pressure gauge on the condensation temperature;
Δθ is the difference in temperature indicators.
The temperature difference in an air-cooled condenser is calculated using the formula:
Δθ = (tas - tae), where K has limits of 5-10 K. On the graph, this value is 6 K.
The difference in temperature difference at point D, that is, at the outlet of the condenser, in this case is 7 K, since it is in the same limit. The temperature difference is 10-20 K, in the figure it is (tk-tae). Most often the value this indicator stops at 15K, but in this example it is 13K.
