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In the operation system of water chillers, whether they are air-cooled or water-cooled, to ensure the stable operation and safety of the evaporator and compressor, they are all equipped with low evaporation pressure protection or low pressure protection mechanisms. The following will start from the basic working principle of the evaporator and deeply analyze the possible root causes of this fault.
The working principle of the evaporator
The evaporator of a chiller is the core component of the refrigeration cycle. Its working principle is based on the heat absorption effect of the vaporization of liquid refrigerant, and it cools the chilled water through heat exchange. The following is a detailed description of its working principle:
Refrigerant evaporation heat absorption: The low-temperature and low-pressure liquid refrigerant enters the evaporator through a throttling device (such as an expansion valve) and flows through the heat exchange tubes (such as purple copper tubes) inside the evaporator. At this point, the refrigerant absorbs the heat from the chilled water outside the tube and undergoes a phase change and vaporization, transforming from a liquid state to a low-temperature and low-pressure gaseous state. This process requires continuous absorption of heat (latent heat of vaporization) to lower the temperature of the chilled water.
Chilled water circulation cooling: Chilled water (usually at 7℃) flows through the side of the evaporator shell and exchanges heat with the refrigerant that has vaporized inside the tubes. After releasing heat, the temperature of the chilled water rises. Then, it is sent to the fan coil unit or terminal equipment by the chilled water pump, absorbs the heat from the room or equipment, and returns to the evaporator, forming a closed cycle.
Refrigerant gas reflux compression: The low-temperature and low-pressure refrigerant gas after vaporization is sucked into the compressor, compressed into high-temperature and high-pressure gas, and then enters the condenser to release heat, completing the refrigeration cycle.
As one of the core components of a chiller, the evaporator works on the principle that the refrigerant absorbs the heat of the chilled water inside the evaporator to achieve the cooling effect. When the evaporation pressure is too low, it will trigger a series of chain reactions, affecting the normal operation of the entire system. Therefore, setting up low-pressure protection is of vital importance.
Analysis of possible causes of the fault
Abnormal flow of chilled water: When the flow rate of chilled water decreases, the flow velocity of water in the pipe will also decrease, and the water flow state will change from turbulent flow to laminar flow. Under turbulent conditions, fluid molecules move vigorously and can exchange heat with the copper tube wall more effectively. Under laminar flow conditions, fluid molecules flow relatively orderly, significantly reducing the convective heat transfer efficiency. This leads to a deterioration in the heat transfer effect of the evaporator and subsequently triggers low-pressure protection.
Copper pipe scaling and water quality issues: If scale forms inside the copper pipe and the water quality is not clean, the heat exchange thermal resistance between the water and the copper pipe will increase significantly. The increase in thermal resistance hinders the heat transfer process, reduces the comprehensive heat transfer coefficient, and affects the normal heat exchange of the evaporator. In addition, there are also cases where the outlet water temperature is abnormal. If the temperature sensor malfunctions, the temperature detected by the unit will be higher than the actual temperature, causing the unit to operate under continuous load. When the actual temperature is already lower than the designed temperature, the evaporation pressure will also decrease accordingly, triggering a low-pressure alarm.
Insufficient refrigerant charge: Refrigerant is the key medium for the chiller to achieve the refrigeration cycle. If the refrigerant charge is too low, the amount of refrigerant participating in heat exchange in the evaporator will decrease, and it cannot fully absorb the heat of the chilled water, resulting in a drop in evaporation pressure and thus triggering low-pressure protection.
Blockage of the throttle valve or refrigerant dryer: The throttle valve plays a role in regulating the flow of refrigerant in the refrigeration system, while the refrigerant dryer is responsible for filtering and drying the refrigerant. Once the throttling valve or the refrigerant dryer gets clogged, the flow of the refrigerant will be hindered, the flow rate will decrease, resulting in insufficient refrigerant supply in the evaporator, reduced evaporation pressure and causing a malfunction.
The outlet water temperature of the condenser cooling water is too low: The flow of the refrigerant depends on the pressure difference maintained between the condenser and the evaporator. In winter or transitional seasons when refrigeration is still required, if the outlet water temperature of the cooling water in the condenser is too low, it indicates a decrease in condensing pressure, which in turn leads to a small pressure difference between the condenser and the evaporator. There is not enough power to drive the refrigerant to flow into the evaporator, resulting in insufficient refrigerant in the evaporator, a decrease in evaporation pressure, and eventually triggering the low-pressure protection.
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