Basic Knowledge of Refrigeration Maintenance and Debugging

I. Condensation Temperature
The condensation temperature of the compressor system refers to the temperature at which the refrigerant condenses in the condenser. The pressure of the refrigerant vapor corresponding to this temperature is the condensation pressure.
The condensation temperature is one of the main operating parameters in the refrigeration cycle. For an actual refrigeration device, since the variation range of other design parameters is relatively small, the condensation temperature can be said to be the most important operating parameter. It is directly related to the refrigeration effect, safety, reliability, and energy consumption level of the refrigeration device.
II. Evaporation Temperature
The evaporation temperature refers to the temperature at which the refrigerant evaporates and boils in the evaporator. It corresponds to the corresponding evaporation pressure, and the evaporation temperature is also an important parameter in the refrigeration system.
In an ideal state, the evaporation temperature is the refrigeration temperature. However, in actual operation, the evaporation temperature of the refrigerant is 3 to 5 degrees lower than the refrigeration temperature.
III. Suction Temperature
The suction temperature refers to the temperature at which the refrigerant enters the compressor, and it is generally higher than the evaporation temperature. Because the evaporation temperature is the saturation temperature of the refrigerant, and the suction temperature is the temperature of the superheated gas. At this time, the refrigerant becomes a superheated gas. The difference between the suction temperature and the evaporation temperature at this time is the suction superheat.
IV. Superheat
Definition of superheat: It refers to the temperature difference between the steam in the low-pressure side and the thermostatic bulb.
Measurement method of superheat: Measure the evaporation pressure as close as possible to the thermostatic bulb, convert the reading into temperature, and then subtract this temperature from the actual temperature measured at the thermostatic bulb.

The superheat should be between 5-8°C.
V. Subcooling
Definition of subcooling: It is the difference between the saturation liquid temperature corresponding to the condensation pressure of the condenser and the actual temperature of the liquid at the outlet of the condenser.
In engineering, the discharge pressure is generally approximated as the condensation pressure, and the difference between the saturation liquid temperature corresponding to the discharge pressure and the temperature of the liquid at the outlet of the condenser is taken as the subcooling. The reason for this approximation is that the pressure drop of the condenser is relatively small compared to that of the evaporator. For an air-cooled condenser, a subcooling of 3 to 5 degrees is more appropriate. When the refrigeration system is in normal circulation, there is generally a certain degree of subcooling at the outlet of the condenser.
VI.

Function of Suction Superheat
If there is no superheat during suction, there is a possibility of liquid return with the suction gas, and even wet compression and liquid hammer may occur, which will damage the compressor. In order to avoid this phenomenon, a certain suction superheat is required to ensure that only dry steam enters the compressor (due to the nature of the refrigerant, the existence of superheat indicates the complete evaporation of the liquid refrigerant).
However, too high a superheat also has disadvantages. A higher superheat will cause the discharge temperature of the compressor (discharge superheat) to rise, the operating conditions of the compressor to deteriorate, and the service life to decrease. Therefore, the suction superheat should be controlled within a certain range.
The expansion valve senses the temperature difference (this temperature difference is the suction superheat) between the return gas temperature and the actual evaporation pressure (corresponding to the saturation temperature) through the temperature sensing part placed on the return pipe of the compressor or the outlet of the evaporator, and adjusts the opening degree of the expansion valve based on the set superheat, which is equivalent to adjusting the liquid supply amount to the evaporator, and finally can control the suction superheat.
Now some models (such as variable-frequency multi-connected units) also have expansion valves specifically for controlling the condensation subcooling. When the subcooling is insufficient, the opening degree of the expansion valve of the subcooling circuit is increased, and the amount of liquid injection is increased to cool the refrigerant in the main circuit and improve the condensation effect.
The temperature at which the refrigerant evaporates in the evaporator has a great impact on the refrigeration efficiency. For every 1-degree decrease in it, the power needs to be increased by 4% to produce the same cooling capacity. Therefore, under the condition of permission, appropriately increasing the evaporation temperature is beneficial to improving the efficiency of the refrigeration system.
VII. Regulation of Evaporation Temperature
The regulation of the evaporation temperature is actually to control the evaporation pressure in practical operation, that is, to adjust the pressure value of the low-pressure pressure gauge. In operation, the opening degree of the thermal expansion valve (or throttle valve) is adjusted to control the level of the low-pressure. When the opening degree of the expansion valve is large, the evaporation temperature rises, the low-pressure also rises, and the refrigeration capacity will increase; if the opening degree of the expansion valve is small, the evaporation temperature decreases, the low-pressure also decreases, and the refrigeration capacity will decrease.
VIII. Factors Affecting the Evaporation Temperature
In the actual operation process of the refrigeration device, the change of the evaporation temperature is very complex. In addition to being directly controlled by the expansion valve (throttle valve), it is related to the heat load of the cooled object, the heat transfer area of the evaporator, and the capacity of the compressor. When one of these three conditions changes, the evaporation pressure and temperature of the refrigeration system will inevitably change accordingly. Therefore, in order to ensure that the evaporation temperature operates stably within the specified range, the operator needs to timely understand the change of the evaporation temperature and adjust the evaporation temperature appropriately and correctly according to the change law of the evaporation temperature.
IX. Influence of Heat Load on Evaporation Temperature
The heat load refers to the heat release of the cooled object. When the heat load increases and other conditions remain unchanged, the evaporation temperature will rise, the low-pressure will also rise, and the suction superheat will increase. In this case, only the expansion valve can be opened wider to increase the circulation amount of the refrigerant, and the expansion valve cannot be closed to lower the low-pressure just because the low-pressure rises. Doing so will make the suction superheat larger, the discharge temperature rise, and the operating conditions deteriorate. When adjusting the expansion valve, the adjustment amount each time should not be too large. After adjustment, it must operate for a certain period of time to reflect whether the heat load and the refrigeration capacity are balanced.
The influence of the change of the energy of the refrigeration compressor on the evaporation temperature: When the energy of the refrigeration compressor is increased, the suction volume of the compressor will increase accordingly. When other conditions remain unchanged, the high-pressure will rise, the low-pressure will decrease, and the evaporation temperature will also decrease. In order to continue to maintain the evaporation temperature required by the production process, the expansion valve needs to be opened wider to make the low-pressure rise to the specified range. After the refrigeration compressor operates with increased energy for a period of time, with the decrease of the temperature of the cooled object, the evaporation temperature and the low-pressure will gradually decrease (without any adjustment of the expansion valve), which is because the heat load decreases due to the decrease of the temperature of the cooled object. In this case, it should not be mistakenly thought that the pressure drop is due to insufficient liquid supply, so as to open the expansion valve wider to increase the liquid supply. Instead, the expansion valve should be closed to reduce the operation energy of the refrigeration compressor.

X. Influence of the Change of Heat Transfer Area on Evaporation Temperature
The heat transfer area mainly refers to the evaporation area of the evaporator, and the change of the heat transfer area mainly refers to the change in the size of the evaporation area. In a complete refrigeration device, the evaporation area is usually fixed. However, in actual operation, due to insufficient liquid supply or oil accumulation in the evaporator, the evaporation area is constantly changing. The influence of the increase or decrease of the evaporation area on the evaporation temperature is basically similar to the influence of the increase or decrease of the heat load on the evaporation temperature. When the evaporation area increases, the evaporation temperature will rise; when the evaporation area decreases, the evaporation temperature will decrease. In order to maintain the required temperature, the energy and the expansion valve should be adjusted, and the evaporator should be drained and cleaned to maintain the relative balance between the heat transfer area and the refrigeration capacity.
XI.

Relationship between Evaporation Pressure and Evaporation Temperature
The lower the evaporation pressure (low pressure), the lower the evaporation temperature.
The relationship between the evaporation temperature and the refrigeration capacity is that when the refrigerant flow rate is constant, the lower the evaporation temperature, the larger the temperature difference with the heat load (hot air), and the larger the refrigeration capacity. In other words, the lower the evaporation pressure, the larger the refrigeration capacity. And for the same mass of the same refrigerant, when it evaporates at different temperatures, its latent heat of evaporation is also different. The lower the evaporation temperature, the larger the latent heat of evaporation and the stronger the heat absorption capacity.
The influence of the change of the evaporation temperature on the refrigeration capacity, power, and COP of the compressor under the conditions of a condensation temperature of 40°C, a superheat of 10°C, a subcooling of 5°C, and other conditions remaining unchanged.