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Evaporation temperature refers to the saturation temperature at which the refrigerant changes from liquid to gaseous in the evaporator, that is, the temperature at which the refrigerant begins to boil and absorbs heat at a specific pressure. In refrigeration systems, evaporation temperature is a very critical parameter because it has a direct impact on refrigeration efficiency and capacity:
The significance of the evaporation temperature
- Determines the cooling effect: the lower the evaporation temperature, the greater the ability of the refrigerant to absorb heat from the medium being cooled, and the better the cooling effect.
- Cooling capacity to energy efficiency ratio: The evaporation temperature directly affects the cooling capacity of the refrigeration system, because the refrigeration process is achieved by the evaporation and heat absorption of the refrigerant. However, while the evaporation temperature is too low to provide a lower cooling temperature, it may also lead to a decrease in compressor efficiency, which in turn affects the COP (energy efficiency ratio) of the entire system and increases energy consumption.
The relationship between evaporation temperature and evaporation pressure
Ideally, the evaporation temperature of the refrigerant is proportional to the corresponding saturation vapor pressure, which can be determined from the enthalpy diagram of the refrigerant. For most refrigerants, as the evaporation pressure increases, so does the evaporation temperature, and conversely, when the evaporation pressure decreases, so does the evaporation temperature. The two can be converted by looking up a table or using special software, e.g. in the case of R134a refrigerant, the temperature value at a given pressure can be found based on its saturated vapor pressure-temperature graph.
Effect of evaporation temperature on refrigeration capacity
An increase in evaporation temperature leads to an increase in cooling capacity. Because the refrigerant with a higher evaporation temperature can absorb more heat per unit mass under the same conditions, the cooling capacity increases. However, this does not mean that higher evaporation temperatures are better, as specific cooling needs need to be met in practice, and the operating condition of the compressor and the overall energy efficiency of the system are taken into account.
Reasons for low evaporation temperature of air conditioner:
Insufficient refrigerant: Refrigerant leakage or insufficient initial charge results in insufficient refrigerant in the evaporator to fully evaporate and absorb heat.
System blockages: Blockages such as dirty filter driers, clogged capillaries or expansion valves restrict the flow of refrigerant and reduce the amount of refrigerant entering the evaporator.
Poor heat transfer of evaporator: The heat exchange area of the evaporator is too small or the scale is serious, which reduces the heat exchange efficiency between the refrigerant and air or water.
Improper compressor matching: too large compressor causes too much liquid supply to the evaporator, and the refrigerant does not have time to completely evaporate and flows back to the compressor, resulting in a drop in evaporation temperature.
Other reasons: such as uneven distribution of refrigerant due to the failure of the indoor air supply system, or the failure of the control system leading to the wrong setting of the evaporation temperature.
5. Reasons for the high evaporation temperature of the air conditioner:
- Expansion valve opening too large: The expansion valve controls the flow and pressure of refrigerant into the evaporator. If the expansion valve is opened too wide, the refrigerant enters the evaporator too quickly, resulting in too much refrigerant in the evaporator, and some of the refrigerant may not be completely evaporated before returning to the compressor, thereby reducing the effective evaporation area and increasing the evaporation temperature.
- High condensing pressure: The condensing pressure increases when the condenser dissipates heat poorly, for example when the condenser surface is dirty, the cooling fan speed is not fast enough, or the temperature of the condenser's cooling medium (such as cooling water or outdoor air) is too high. Higher condensing pressures can cause the refrigerant entering the evaporator to be undercooled, resulting in higher evaporation temperatures.
- Over-charging of refrigerant: Over-charging of refrigerant will increase the density of refrigerant in the evaporator, increase the number of refrigerants per unit volume, and it is not easy to evaporate completely, thereby increasing the evaporation temperature.
- Reduced heat transfer efficiency of the evaporator: Fouling, clogging or contamination inside or outside the evaporator will affect the heat exchange efficiency between the refrigerant and the outside air, and even if there is enough refrigerant, it will not evaporate effectively, resulting in an increase in evaporation temperature.
- System load is too low: When the cooling load required by the air conditioning system is lower than the design load, the actual evaporation capacity of the evaporator is insufficient, which may cause the evaporation temperature to rise.
- Failure of control components: For example, the temperature sensor and controller are faulty, resulting in the expansion valve not being able to accurately adjust the opening according to the system load, which may cause the evaporation temperature control to be abnormal.
- Excessive ambient temperature: The high ambient temperature makes the refrigeration system need a higher evaporation temperature to achieve the same cooling effect, especially when the condensation part of the outdoor unit is difficult to dissipate heat, which indirectly increases the evaporation temperature.