Analysis of basic refrigeration parameters such as condensation, evaporation, and superheat.

1. Condensation temperature

The condensing temperature of a compressor system is the temperature at which the refrigerant undergoes a phase change during condensation inside the condenser. The saturated vapor pressure of the refrigerant at this temperature is the condensing pressure. The condensing temperature is a core operating parameter of the refrigeration cycle. In actual refrigeration systems, the fluctuation range of other design parameters is generally small; therefore, the condensing temperature is particularly critical, directly determining the cooling effect, operational safety and stability of the equipment, and also affecting the overall energy consumption level.

2. Evaporation temperature

Evaporation temperature is the temperature at which the refrigerant boils and evaporates in the evaporator. It corresponds one-to-one with the evaporation pressure and is an indispensable key parameter of the refrigeration system. Under ideal operating conditions, the evaporation temperature is equal to the refrigeration temperature; however, during actual operation, the refrigerant evaporation temperature is usually more than 5°C lower than the actual refrigeration temperature.

Adjusting the evaporation temperature essentially involves controlling the evaporation pressure, which means adjusting the pressure reading on the low-pressure gauge. The low-pressure can be changed by adjusting the opening of the thermostatic expansion valve (throttle valve): increasing the valve opening raises both the evaporation temperature and the low-pressure, thus increasing the cooling capacity; conversely, decreasing the valve opening lowers both the evaporation temperature and the low-pressure, resulting in a corresponding decrease in cooling capacity.

3. Inhalation temperature

Suction temperature refers to the temperature of the refrigerant when it enters the compressor, and it is always higher than the evaporation temperature under normal operating conditions. The evaporation temperature is the refrigerant saturation temperature, while the refrigerant entering the compressor has been completely vaporized and is in a superheated state. The suction temperature is the temperature of the superheated gas, and the difference between the suction temperature and the evaporation temperature is the suction superheat.

4. Subcooling

Subcooling is the difference between the saturated liquid temperature of the refrigerant at the condensing pressure and the actual liquid temperature of the refrigerant at the condenser outlet. In engineering practice, because the pressure drop in the condenser piping is much smaller than that of the evaporator, the discharge pressure is usually approximated as the condensing pressure. The difference between the saturated temperature and the liquid temperature at the condenser outlet calculated using this method is the actual subcooling. The suitable subcooling for air-cooled condensers is generally 3–5°C. During stable operation of the refrigeration system, a reasonable subcooling must be maintained at the condenser outlet.

5. Exhaust superheat

Exhaust superheat is a key indicator used to measure the difference between the medium temperature at the compressor exhaust pipe and condenser inlet and the refrigerant saturation temperature at the current condensing pressure. It directly reflects the extent to which the actual refrigerant temperature exceeds the corresponding pressure saturation temperature.

6. Inhalation superheat

Suction superheat refers to the difference between the compressor's suction temperature and its evaporation temperature. Properly controlling suction superheat is a core requirement for the stable operation of a refrigeration system. If there is absolutely no suction superheat, the return gas can easily carry liquid refrigerant, causing wet stroke and liquid slugging in the compressor, resulting in equipment damage. Maintaining a moderate suction superheat ensures that the liquid refrigerant completely evaporates and enters the compressor as dry vapor. However, excessive suction superheat has drawbacks, as it increases the compressor's discharge temperature and discharge superheat, worsening equipment operating conditions and shortening its lifespan. Therefore, suction superheat must be controlled within a reasonable range.

7. Factors affecting evaporation temperature

Evaporation temperature changes are influenced by multiple factors. In addition to being directly regulated by the expansion valve (throttle valve), it is also closely related to the heat load of the object being cooled, the heat transfer area of the evaporator, and the gas delivery rate of the compressor. If any of these conditions changes, the evaporation pressure and evaporation temperature of the refrigeration system will change accordingly.

8. The effect of heat load on evaporation temperature

Heat load specifically refers to the heat released by the object being cooled. Under the premise that the equipment operating conditions remain unchanged, if the heat load increases, the evaporation temperature and low-pressure will rise simultaneously, and the suction superheat will also increase accordingly.

9. The effect of changes in heat transfer area on evaporation temperature

The heat transfer area here mainly refers to the effective evaporation heat exchange area of the evaporator. Conventional equipment has a fixed evaporator design area, but in actual operation, problems such as insufficient refrigerant supply and oil accumulation in the evaporator can reduce the effective heat exchange area. The impact of increasing or decreasing the evaporator heat exchange area on the evaporation temperature follows a pattern largely consistent with the impact of changes in heat load.

An increase in the effective heat exchange area of the evaporator leads to a higher evaporation temperature; a decrease in the heat exchange area leads to a lower evaporation temperature. In actual operation and maintenance, the dynamic balance between the heat exchange area and the cooling capacity can be maintained by adjusting the compressor energy, adjusting the expansion valve opening, and simultaneously cleaning the evaporator to remove oil, thus ensuring temperature stability.

10. Relationship between evaporation pressure and evaporation temperature

The lower the evaporation pressure (system low pressure), the lower the corresponding evaporation temperature. With a constant refrigerant flow rate, the lower the evaporation temperature, the greater the temperature difference between the refrigerant and the heat exchange medium, and the stronger the heat absorption capacity of the equipment. However, within the normal pressure range, a continuous decrease in evaporation pressure can actually lead to a reduction in the overall cooling capacity of the system.