Why is the central air conditioning cooling water set to 37°C for outlet and 32°C for inlet? What are the effects of too high or too low a temperature?

In a water-cooled central air-conditioning system, the chiller dissipates heat from the condenser to the outside. The discharged condensation heat is carried by the cooling water to the cooling tower. After the heat is dissipated by the cooling tower, the water temperature drops from 37°C to 32°C and then returns to the condenser of the chiller. This cycle repeats, and the cooling water system circulates to dissipate heat.

In my country, the cooling water temperature is generally set according to the standard working conditions of the cooling tower. The outlet water temperature of the chiller enters the cooling tower at 37°C, cools down to 32°C through the cooling tower, and then returns to the inlet water temperature of the chiller.
The reason for this setting is based on the heat exchange requirements of the cooling water at both ends of the chiller condenser and the cooling tower, while taking into account the operating efficiency of the chiller and the effective heat dissipation of the cooling tower.
1. Heat exchange on the condenser side
In the condenser of the chiller, the high-temperature and high-pressure refrigerant vapor condenses into liquid, and the released condensation heat is exchanged to the cooling water through the heat exchange tube.
In order to ensure that the condensation heat in the condenser can be smoothly transferred to the cooling water, the condensation temperature of the refrigerant in the condenser must be higher than the cooling water temperature.
Usually, when the chiller is operating normally, the condensation temperature is about 40°C. At this time, the cooling water inlet temperature is 32°C, and the outlet temperature after heat exchange is 37°C, which can ensure the smooth progress of the condensation heat dissipation process.
2. Heat exchange on the cooling tower side
The cooling and heat dissipation of cooling water in the cooling tower is divided into contact heat dissipation and evaporation heat dissipation.
Contact heat dissipation transfers sensible heat to the ambient air based on the temperature difference between the cooling water temperature and the outdoor air temperature (dry bulb temperature).
Evaporative heat dissipation transfers latent heat to the ambient air based on the temperature difference between the cooling water temperature and the outdoor air wet bulb temperature.
According to the outdoor design parameters of summer air conditioning in my country, the maximum dry bulb temperature of outdoor air is about 35°C, and the maximum wet bulb temperature is about 28°C.
Therefore, setting the inlet water temperature of the cooling tower to 37°C can ensure that in most cases, the inlet water temperature of the cooling tower is higher than the dry bulb temperature of the outdoor air. At this time, there is both contact heat dissipation and evaporative heat dissipation, so that the cooling tower can dissipate heat efficiently.
The setting of the cooling tower outlet water temperature of 32°C is, on the one hand, the requirement of the chiller to ensure the cooling water flow rate according to the temperature difference of 5°C for cooling water, and on the other hand, it is also higher than the wet bulb temperature of the outdoor air, which can be guaranteed by evaporative heat dissipation.

3. The cooling water temperature is too high
When the cooling water temperature is too high, it is beneficial to the cooling tower heat dissipation, but it is not good for the operation and heat exchange efficiency of the chiller.
When the cooling water temperature is too high, the condensing temperature and pressure of the chiller increase, and the compression ratio becomes larger, which increases the burden on the compressor and the power consumption, thereby reducing the cooling efficiency of the chiller. In severe cases, it will cause high-pressure protection and shutdown.
For centrifugal chillers, it belongs to speed compression. When the condensing pressure increases and the pressure ratio increases, the surge protection mechanism may be triggered.
When the cooling water temperature is too high, the high temperature working environment accelerates the scaling of equipment and pipelines. For heat exchangers made of copper tubes, scaling will hinder their effective heat exchange and further reduce the cooling efficiency of the system.
4. The cooling water temperature is too low
When the cooling water temperature decreases, the condensing temperature and pressure decrease accordingly, and the cooling efficiency of the chiller is usually improved. However, when the cooling water temperature is too low, it will affect the safe and stable operation of the unit.
When the cooling water temperature is too low, the condensing pressure drops, and the pressure difference between the evaporator decreases, which may cause insufficient refrigerant flow, thereby triggering the low-pressure protection of the unit and affecting the normal operation of the system.
For units that use refrigerant to cool the motor, the pressure difference between the condenser and the evaporator decreases, which will also reduce the cooling effect and increase the risk of overheating of the motor, thereby causing the motor protection mechanism to start.
For the lubricating oil system of the compressor, the reduction in condensing pressure also reduces the oil pressure difference, which will hinder the effective circulation and distribution of the lubricating oil, and may trigger the oil shortage alarm of the unit, affecting the normal operation of the system.