Condensing temperature and condensing pressure refer to the pressure corresponding to the refrigerant when it condenses from a gas to a liquid in the condenser. In a refrigeration system, the pressure inside the condenser is difficult to measure directly. However, in reality, the pressure drop of the refrigerant in the discharge pipe and the condenser is extremely small, so during design, commissioning, and maintenance, the discharge pressure is usually approximated as the condensing pressure. There is a one-to-one correspondence between condensing temperature and condensing pressure, representing the saturated temperature of the refrigerant during condensation. It should be noted that the condensing temperature is not the temperature of the cooling medium; there is a heat transfer temperature difference between the two. So, how is the condensing temperature determined? According to experience, the condensing temperature of the system can be estimated as the ambient temperature plus (10–15)°C. Taking an air conditioner with an ambient temperature of 35°C in the summer as an example, the condensing temperature at this time is about 35 + (10–15)°C = 45°C.
If the discharge pressure of the refrigeration system is lower than this pressure, it is necessary to consider whether the refrigerant charge is insufficient; if the discharge pressure is higher than this pressure, it is necessary to check whether the refrigerant charge is excessive, whether the heat exchange of the condenser is adequate, whether the expansion valve opening is too small, and other related conditions.
The evaporation temperature is the temperature at which the liquid refrigerant boils. Under a constant refrigerant flow, the lower the evaporation pressure, the lower the evaporation temperature. However, simply lowering the system's evaporation temperature will continuously reduce the cooling capacity of the refrigeration compressor, and the cooling rate may not necessarily increase. Moreover, the lower the evaporation temperature, the lower the system's coefficient of performance. How can we determine if the system's evaporation pressure is normal? According to experience, the evaporation temperature of an air conditioning system can be estimated by subtracting (10–15)°C from the ambient temperature. For example, for an air conditioner set at 22°C indoors in summer, with the indoor unit temperature at 22°C, the evaporation temperature is approximately 22 - (10–15)°C ≈ 12°C. The pressure corresponding to the evaporation temperature is the evaporation pressure, which can also be checked using the temperature-pressure chart provided on our public account. If the evaporation pressure of the refrigeration system is lower than this pressure, it is necessary to check whether there are issues with the heat exchange of the evaporator or if the expansion valve is too closed; if the evaporation pressure is higher than this pressure, considerations should include whether there is too much refrigerant or if the expansion valve is too open.
Suction temperature refers to the temperature of the refrigerant at the compressor suction valve or inlet. To ensure the safe operation of the compressor and prevent liquid slugging, the suction temperature is required to be slightly higher than the evaporator temperature, so that the refrigerant vapor becomes superheated gas. Whether the suction temperature is normal can directly reflect if the expansion valve opening is appropriate. Many people know the suction superheat and its optimal value is 5-7°C, but many beginners do not understand the actual significance of this parameter. In fact, knowing the suction superheat allows one to calculate the normal suction temperature of the system.
The calculation formula for suction temperature is: Suction Temperature = Suction Superheat + Evaporation Temperature. For example, if the evaporation temperature of a certain refrigeration system is 12℃ and the optimal suction superheat is 5-7℃, the system's optimal suction temperature at this time can be estimated to be about 17℃. We can also judge whether the suction temperature is normal by touch. If the suction pipe feels cooler than 17℃, it can be determined that the expansion valve opening is too large or the refrigerant charge is too much; if the suction temperature is higher than 17℃, it can be judged that the expansion valve opening is too small or the refrigerant charge is insufficient. If the suction pipe does not condense, it indicates that the suction temperature is too high, and the refrigerant charge or the expansion valve opening should be checked; if the suction pipe is frosted (iced), it indicates that the suction temperature is too low, and the refrigerant charge or the expansion valve opening should be checked for excess or too large, respectively.
Exhaust temperature is the temperature of the gas discharged after the compressor has done work, and it can be measured with a thermometer on the exhaust pipe. The exhaust temperature is proportional to the pressure ratio and the intake temperature; that is, the higher the pressure ratio and the higher the intake temperature, the higher the exhaust temperature. Many peers are relatively unfamiliar with the concept of 'exhaust superheat.' 'Exhaust superheat' refers to the difference between the temperature of the compressor's exhaust pipe (or the condenser inlet temperature) and the saturation temperature corresponding to the condenser pressure. In a normal refrigeration system, the exhaust superheat is usually 20–30°C. Based on this empirical value, one can determine whether the exhaust temperature is normal. For example, for an air conditioner with a condenser temperature of 45°C, its normal exhaust temperature is approximately exhaust superheat + condenser temperature = 25 + 45 = 70°C.
The operating current of the compressor is influenced by multiple factors, specifically: poor heat dissipation of the outdoor unit; abnormal external power supply voltage; air mixed into the refrigeration system; poor oil return, leading to excessive temperature of the casing/internal parts; excessive impurities or moisture in the system. In addition, the refrigerant and compression ratio also affect the current: the more refrigerant compressed by the compressor, the more difficult the compression, and the higher the current. The higher the compression ratio of the refrigerant, the more work the compressor performs, and the corresponding current will also increase. Therefore, the operating current of the compressor can largely reflect whether the refrigerant charge of the refrigeration system is normal. Generally, the current value is indicated on the nameplate of the refrigeration equipment.
