Why Does the Refrigeration System Definitely Need Subcooling? What Is the Significance of Subcooling for the Refrigeration System?
2025-03-11
I. Definition of Subcooling:
Definition of subcooling degree: The difference between the saturated liquid temperature corresponding to the condensation pressure of the condenser and the actual temperature of the liquid at the outlet of the condenser.
Representation of the subcooling degree on the pressure-enthalpy diagram:
In engineering, the exhaust pressure is generally approximated as the condensation pressure. The difference between the saturated liquid temperature corresponding to the exhaust pressure and the temperature of the liquid at the outlet of the condenser is regarded as the subcooling degree. The reason for this approximation is that the pressure drop of the condenser is relatively small compared with that of the evaporator. The difference between the exhaust pressure and the true condensation pressure is small, and the error brought by this approximation can be ignored.
II. Why Does the Refrigeration System Definitely Need Subcooling?
Simply put, if there is no subcooling in the refrigeration system, the refrigerant will "flash" in the liquid pipe, reducing the refrigeration effect;
For an air-cooled condenser, a subcooling degree of 3 - 5°C is more appropriate.
When the refrigeration system is in normal circulation, there is generally a certain subcooling degree at the outlet of the condenser.
If there is no subcooling, the liquid in the two-phase refrigerant will "flash" when there is a slight pressure loss in the "liquid pipe". The saturated liquid will inevitably evaporate due to the decrease in pressure. The evaporation of the liquid will absorb the surrounding heat, and the remaining liquid will cool down accordingly and reach the saturated temperature at the corresponding pressure again. In this way, the two-phase refrigerant will flash, reach saturation, and move forward until it reaches the inlet of the evaporator. Finally, the dryness of the two-phase refrigerant reaching the evaporator will be much larger than the designed dryness, and the liquid phase composition will decrease, failing to meet the evaporation capacity of the evaporator, and of course, the refrigeration effect will be reduced.
In addition, only with a certain subcooling degree is it possible to draw a credible pressure-enthalpy diagram. For a basic single-stage compression refrigeration system, there are four points on the pressure-enthalpy diagram, which are:
① Suction port of the compressor — Superheated vapor;
② Discharge port of the compressor — Superheated vapor;
③ Outlet of the condenser — Subcooled liquid;
④ Inlet of the evaporator — Two-phase refrigerant.
Points ①, ② and ③ are all in a single-phase state. According to the pressure and temperature values, the corresponding enthalpy values can be calculated.
According to the pressure and enthalpy values, the position can be determined in the pressure-enthalpy diagram;
The inlet of the evaporator is a two-phase refrigerant. Even if the pressure and temperature values are known, it is difficult to calculate the enthalpy value.
However, the surface area of the throttling device is generally small, and the heat dissipation can be ignored. The refrigerant passing through the throttling device can be approximated as an isenthalpic pressure reduction process. Therefore, the enthalpy value at point ③ can be approximately equal to the enthalpy value at the outlet of the condenser.
If the outlet of the condenser is a two-phase or saturated liquid, it is difficult to obtain the enthalpy value. If two points are missing among the four state points, the pressure-enthalpy diagram of the system cannot be drawn.
III. Significance of the Subcooling Degree of the Refrigeration System:
Why can't the corresponding enthalpy value be calculated according to the pressure and temperature in the two-phase state?
Because in the two-phase region, the pressure and temperature are in one-to-one correspondence, and a saturated pressure corresponds to a saturated temperature. The pressure line and the temperature line are a coincident horizontal line in the two-phase region.
Due to measurement errors, it is difficult to make the measured pressure value and temperature value correspond one-to-one:
① If the temperature value is lower than the saturated temperature corresponding to the pressure, it indicates a subcooled liquid;
② If the temperature value is higher than the saturated temperature corresponding to the pressure, it indicates a superheated gas.
This contradicts the previous condition that the refrigerant is in a two-phase state.
Then can it be said that the refrigerant is at the saturated liquid or on the saturated line corresponding to the measured pressure value? And the enthalpy value is replaced by the enthalpy value of its saturated liquid or saturated gas.
This is also not possible because if the saturated temperature differs by 0.1 degree up or down, the corresponding enthalpy value will differ by 100 - 200kJ/kg.
It is difficult to determine whether the actual position of the two-phase point on the pressure-enthalpy diagram is on the saturated liquid line, on the saturated gas line, or at a certain position on the horizontal line in between.
Therefore, there must be sufficient subcooling or superheating degrees to calculate the corresponding enthalpy value.
IV. Methods for Achieving Subcooling:
Definition of subcooling degree: The difference between the saturated liquid temperature corresponding to the condensation pressure of the condenser and the actual temperature of the liquid at the outlet of the condenser.
Representation of the subcooling degree on the pressure-enthalpy diagram:
In engineering, the exhaust pressure is generally approximated as the condensation pressure. The difference between the saturated liquid temperature corresponding to the exhaust pressure and the temperature of the liquid at the outlet of the condenser is regarded as the subcooling degree. The reason for this approximation is that the pressure drop of the condenser is relatively small compared with that of the evaporator. The difference between the exhaust pressure and the true condensation pressure is small, and the error brought by this approximation can be ignored.
II. Why Does the Refrigeration System Definitely Need Subcooling?
Simply put, if there is no subcooling in the refrigeration system, the refrigerant will "flash" in the liquid pipe, reducing the refrigeration effect;
For an air-cooled condenser, a subcooling degree of 3 - 5°C is more appropriate.
When the refrigeration system is in normal circulation, there is generally a certain subcooling degree at the outlet of the condenser.
If there is no subcooling, the liquid in the two-phase refrigerant will "flash" when there is a slight pressure loss in the "liquid pipe". The saturated liquid will inevitably evaporate due to the decrease in pressure. The evaporation of the liquid will absorb the surrounding heat, and the remaining liquid will cool down accordingly and reach the saturated temperature at the corresponding pressure again. In this way, the two-phase refrigerant will flash, reach saturation, and move forward until it reaches the inlet of the evaporator. Finally, the dryness of the two-phase refrigerant reaching the evaporator will be much larger than the designed dryness, and the liquid phase composition will decrease, failing to meet the evaporation capacity of the evaporator, and of course, the refrigeration effect will be reduced.
In addition, only with a certain subcooling degree is it possible to draw a credible pressure-enthalpy diagram. For a basic single-stage compression refrigeration system, there are four points on the pressure-enthalpy diagram, which are:
① Suction port of the compressor — Superheated vapor;
② Discharge port of the compressor — Superheated vapor;
③ Outlet of the condenser — Subcooled liquid;
④ Inlet of the evaporator — Two-phase refrigerant.
Points ①, ② and ③ are all in a single-phase state. According to the pressure and temperature values, the corresponding enthalpy values can be calculated.
According to the pressure and enthalpy values, the position can be determined in the pressure-enthalpy diagram;
The inlet of the evaporator is a two-phase refrigerant. Even if the pressure and temperature values are known, it is difficult to calculate the enthalpy value.
However, the surface area of the throttling device is generally small, and the heat dissipation can be ignored. The refrigerant passing through the throttling device can be approximated as an isenthalpic pressure reduction process. Therefore, the enthalpy value at point ③ can be approximately equal to the enthalpy value at the outlet of the condenser.
If the outlet of the condenser is a two-phase or saturated liquid, it is difficult to obtain the enthalpy value. If two points are missing among the four state points, the pressure-enthalpy diagram of the system cannot be drawn.
III. Significance of the Subcooling Degree of the Refrigeration System:
Why can't the corresponding enthalpy value be calculated according to the pressure and temperature in the two-phase state?
Because in the two-phase region, the pressure and temperature are in one-to-one correspondence, and a saturated pressure corresponds to a saturated temperature. The pressure line and the temperature line are a coincident horizontal line in the two-phase region.
Due to measurement errors, it is difficult to make the measured pressure value and temperature value correspond one-to-one:
① If the temperature value is lower than the saturated temperature corresponding to the pressure, it indicates a subcooled liquid;
② If the temperature value is higher than the saturated temperature corresponding to the pressure, it indicates a superheated gas.
This contradicts the previous condition that the refrigerant is in a two-phase state.
Then can it be said that the refrigerant is at the saturated liquid or on the saturated line corresponding to the measured pressure value? And the enthalpy value is replaced by the enthalpy value of its saturated liquid or saturated gas.
This is also not possible because if the saturated temperature differs by 0.1 degree up or down, the corresponding enthalpy value will differ by 100 - 200kJ/kg.
It is difficult to determine whether the actual position of the two-phase point on the pressure-enthalpy diagram is on the saturated liquid line, on the saturated gas line, or at a certain position on the horizontal line in between.
Therefore, there must be sufficient subcooling or superheating degrees to calculate the corresponding enthalpy value.
IV. Methods for Achieving Subcooling:
- Install a subcooler after the condenser;
- When designing and selecting the type, appropriately increase the area of the condenser;
- Set a regenerator in the refrigeration system. The SGHX in the regenerative cycle system is a regenerator.
The principle is: Use the low temperature of the suction temperature to condense the liquid of the refrigerant to achieve the purpose of subcooling. At the same time, it can achieve the superheating of the suction pipe. Although it is ineffective superheating, it can at least prevent the liquid refrigerant from entering the compressor and causing liquid hammer of the compressor.
There are mainly the following methods for this kind of regenerator:
1: Copper tube regenerator:
Internal structure: Use the low temperature of the suction temperature to condense the liquid of the refrigerant to achieve the purpose of subcooling. At the same time, it can achieve the superheating of the suction pipe and prevent the liquid refrigerant from entering the compressor and causing liquid hammer of the compressor.
- Tank-type regenerator:
The internal structure is the same as that of the copper tube regenerator; - There is also a common method, that is, welding the suction pipe and the liquid pipe together, and using the contact heat transfer between the two to achieve the purpose of subcooling.
V. How to Calculate the Size of the Subcooler?
Having said so much, how can we calculate the magnitude of the subcooling heat transfer?
Take the rated cooling capacity of 12.5KW as an example:
Calculate according to the standard working conditions of the air conditioner:
The calculated mass flow rate is 385.98kg/h;
We calculate with a subcooling degree of 5°C:
The calculated subcooling enthalpy difference = 267.4324 - 260.6443 = 6.79kj/kg;
The calculated subcooling capacity = 6.79 * 385.98 * 1000 / 3600 = 728W;
As long as Refprop is embedded in excel, its function is really powerful; it can perform the thermodynamic calculation of the entire refrigeration system, and the subcooling calculation is just a small example.
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