The chiller operates under standard working conditions. The return water temperature of the condenser is 30°C and the outlet temperature is 35°C. The lower the cooling tower water temperature of the chiller, the lower the condensation pressure of the chiller. Therefore, the cooling water of the chiller must be reduced within a certain range. Inlet water temperature can improve the performance coefficient of the chiller, but it can also cause other problems.
a. Effects of low condenser outlet water temperature
1. Impact on the main flow path of the refrigeration cycle
If the outlet water temperature of the condenser is low, it means that the condensation saturation temperature is low and the condensation pressure is low. We know that the pressure difference between the condenser and the evaporator is equal to the saturation pressure of the condenser - the saturation pressure of the evaporator. Under the working conditions of the evaporator Under the same condition, the saturation pressure of the condenser decreases, and the pressure difference naturally decreases. At this time, there may be too little refrigerant flowing to the evaporator, causing a low pressure alarm of the unit.
2. Impact on motor cooling circuit
Most brands of chillers use closed motors and use refrigerant for cooling. As shown in the figure below, a small part of the refrigerant from the condenser is injected into the stator winding of the motor, and then returns to the evaporator after absorbing heat. Therefore, The flow resistance of the refrigerant also needs to be overcome by the pressure difference between the condenser and the evaporator. If the outlet water temperature of the condenser is too low, resulting in too low condensation pressure, it is likely to trigger the generator overheating protection.
3. Impact on system oil circuit
At present, most compressors of chillers use sliding bearings, which require continuous lubrication with lubricating oil. If the unit works under low condenser water temperature conditions for a long time, the unit is likely to be short of oil and alarm.
4. Impact on centrifugal chillers
If the cold water temperature is too low, the condensation pressure will also be too low, which can easily cause the centrifugal chiller to surge.
b. Effects of excessively high condenser outlet water temperature
1. Affect the operating efficiency of the whole machine;
The lower the condenser outlet water temperature (or saturation temperature), the higher the overall operating efficiency of the system. Therefore, if the outlet water temperature is too high, it will affect the operating efficiency of the entire machine;
2. Surge or over-current protection
The higher the condenser outlet water temperature, which means the higher the condensation saturation pressure. For the centrifuge, the operating pressure ratio will also increase, and surge protection may be triggered at this time; in addition, due to the increase in condensation pressure, The working conditions have also become worse. If the user load is larger at this time, it may also lead to larger operating power or current.
3. The condenser is more prone to scaling
Under high temperature conditions, copper tubes are more likely to scale, thus affecting heat transfer performance.
4. High voltage protection
The higher the condenser outlet water temperature, the higher the condenser pressure; we know that the condenser is a container and has a certain safety design range, so the unit will set a safety protection value to protect the condenser. Once this safety margin is exceeded value, the unit will alarm and shut down.
Generally speaking, the main cause of the increase in condenser temperature is the problem of cooling water. There are usually four situations:
1. The cooling water inlet pipe and outlet pipe are installed in the wrong position
The normal installation position is generally that the water inlet pipe is low and the water outlet pipe is high, or "low in and high". If the water inlet pipe is in a high position, it is impossible for all the cooling water to circle and fill the condenser, and the heat transfer area will become smaller and the cooling efficiency will be reduced. The agent vapor cannot be effectively condensed, causing the surface temperature of the condenser to rise.
2. The quality of cooling water is too poor
This causes scaling on the inner wall of the cooling water pipe in the condenser and increases the thermal resistance, which affects the heat exchange between the refrigerant and the cooling water and reduces the heat transfer effect. This kind of failure often occurs in chillers that have been used for a long time and are not cleaned regularly. The solution is to remove scale.
3. Insufficient cooling water volume and insufficient water pressure.
Water-cooled condensers rely on cooling water to take away the latent heat released when the refrigerant vapor condenses. Therefore, the cooling water pressure is insufficient and the flow rate cannot meet the rated requirements. The heat dissipation capacity will be limited, eventually causing the outer surface temperature of the condenser to rise. .
4. The cooling water temperature is too high, higher than the rated operating temperature.
The higher the cooling water temperature, the smaller the refrigerant cooling temperature difference, the smaller the heat transfer, the refrigerant cannot be effectively cooled, and the condenser surface temperature will increase.
