Detailed Explanation of the Significance of Temperature and Pressure Parameters Related to Chiller Units

2025-05-07

Evaporation Pressure and Evaporation Temperature
During the operation of the chiller unit, the evaporation temperature and evaporation pressure are closely related to the heat brought into the evaporator by the chilled water. When the heat load is large, the return water temperature of the chilled water in the evaporator rises, causing the evaporation temperature of the evaporator to increase, and the corresponding evaporation pressure also rises. Conversely, when the heat load decreases, the return water temperature of the chilled water drops, and both the evaporation temperature and evaporation pressure decrease. In actual operation, when the heat load of the air-conditioned room decreases, the return water temperature of the chilled water drops, and both the evaporation temperature and evaporation pressure decrease accordingly.
According to the provisions of China's JB/T3355—1998 standard, the rated operating condition of the chiller unit is that the outlet temperature of the chilled water is 7°C, and the return water temperature of the cooling water is 30°C. Other corresponding parameters are that the return water temperature of the chilled water is 12°C, and the outlet temperature of the cooling water is 35°C.
According to the national standard GB/T18403.1—2001, the rated operating condition of the chiller unit is that the inlet and outlet water temperatures of the chilled water are 12°C/7°C, and the inlet and outlet water temperatures of the cooling water are 30°C/35°C. Therefore, the operating condition of the chiller unit when leaving the factory is that the inlet and outlet water temperatures of the chilled water are 12°C/7°C, and the inlet and outlet water temperatures of the cooling water are 30°C/35°C.
During operation, under the condition of meeting the requirements of air conditioning use, the outlet temperature of the chilled water should be increased as much as possible. Generally, the evaporation temperature is 2°C to 4°C lower than the outlet temperature of the chilled water. The evaporation temperature is usually controlled within the range of 3°C to 5°C. An excessively high evaporation temperature often makes it difficult to achieve the required air conditioning effect, while an excessively low evaporation temperature not only increases the energy consumption of the unit but also easily causes the evaporation pipeline to freeze and crack.
Condensation Pressure and Condensation Temperature
In the chiller unit, the pressure indicated by the high-pressure gauge is called the condensation pressure, and the temperature corresponding to this pressure is called the condensation temperature. The level of the condensation temperature, with the evaporation temperature remaining unchanged, is decisive for the power consumption of the unit. As the condensation temperature rises, the power consumption increases. In addition, an increase in the condensation pressure of the centrifugal refrigeration unit will cause the main engine to surge. Conversely, as the condensation temperature decreases, the power consumption decreases accordingly.
Therefore, during the operation of the chiller unit, attention should be paid to ensuring that the indicators such as the temperature, quantity, and quality of the cooling water are within the qualified range. When air exists in the condenser, the temperature difference between the condensation temperature and the outlet of the cooling water increases, while the temperature difference between the inlet and outlet of the cooling water decreases instead. At this time, the heat transfer effect of the condenser is not good, and the outside of the condenser feels hot to the touch. In addition, the scaling and silt on the water side of the condenser tubes also play a considerable role in affecting heat transfer.
Pressure and Temperature of Chilled Water
The chiller units for air conditioning generally operate under the conditions specified in the standard operating conditions, that is, the return water temperature of the chilled water is 12°C, the supply water temperature is 7°C, and the temperature difference is 5°C.
The chilled water flow rate in the evaporator is inversely proportional to the temperature difference between the supply and return water, that is, the larger the chilled water flow rate, the smaller the temperature difference; conversely, the smaller the flow rate, the larger the temperature difference. Therefore, the operating condition of the chiller unit stipulates that the temperature difference between the supply and return water of the chilled water is 5°C, which actually specifies the chilled water flow rate of the unit. The control of this chilled water flow rate is manifested as controlling the pressure drop of the chilled water passing through the evaporator.
Under the standard operating conditions, the pressure drop between the supply and return water of the chilled water on the evaporator is set to 0.5kgf/cm². The method of setting the pressure drop is to adjust the opening degree of the outlet valve of the chilled water pump and the opening degree of the supply and return water valves of the evaporator.
Pressure and Temperature of Cooling Water
The chiller unit operates under the standard operating conditions, with the return water temperature of the condenser being 30°C and the outlet temperature being 35°C. For the chiller unit in operation, the environmental conditions, load, and refrigeration capacity have all become fixed values. At this time, the condensing heat load is undoubtedly also a fixed value. The standard stipulates that the temperature difference between the inlet and outlet water is 5°C, and the cooling water flow rate must also be a certain value. Moreover, this flow rate is inversely proportional to the temperature difference between the inlet and outlet water. Therefore, when the chiller unit operates under the standard operating conditions, it is only necessary to specify the temperature difference between the inlet and outlet water of the cooling water. This flow rate is usually controlled by the pressure drop of the cooling water entering and leaving the condenser.
Under the standard operating conditions, the pressure drop of the outlet water of the condenser is set to about 0.75kgf/cm². The method of setting the pressure drop is also to adjust the opening degree of the outlet valve of the cooling water pump and the opening degree of the inlet and outlet pipe valves of the condenser.
In order to reduce the power consumption of the chiller unit, the temperature of the condenser should be reduced as much as possible. There are two measures that can be taken: one is to reduce the return water temperature of the condenser, and the other is to increase the flow rate of the cooling water.
For the centrifugal chiller unit, either too high or too low condensation pressure will cause surging. When the centrifugal chiller unit encounters such a situation, it should be noted that the difference between the condensation pressure and the evaporation pressure should not be too small and should meet the requirements for preventing surging; otherwise, surging will occur. In the autumn when the temperature is relatively low, it is more advantageous to operate the reciprocating chiller unit because the condensation pressure is relatively low at this time, and the power consumption is greatly reduced.
Suction Temperature of the Compressor
The suction temperature of the compressor refers to the temperature of the refrigerant gas in the suction cavity of the compressor; for the centrifugal compressor, it should be the temperature of the refrigerant gas on the suction guide vanes. The level of the suction temperature not only affects the level of the discharge temperature but also has an important impact on the volumetric refrigeration capacity of the compressor. When the suction temperature of the compressor is high, the discharge temperature is also high, and the specific volume of the refrigerant when it is inhaled is large. At this time, the unit volumetric refrigeration capacity of the compressor is small, which is not what we want. Conversely, when the suction temperature of the compressor is low, its unit volumetric refrigeration capacity is large.
However, a low suction temperature of the compressor may cause the liquid refrigerant to be inhaled by the compressor, causing the reciprocating compressor to experience "liquid hammer". For the centrifugal compressor, due to the excessively low suction temperature resulting in an excessively low suction pressure of the compressor, surging may occur. Therefore, the suction superheat of the compressor should be specified.
Discharge Temperature of the Compressor
The discharge temperature is much higher than the condensation temperature. The direct influencing factor of the discharge temperature is the suction temperature of the compressor, and the two are in a direct proportional relationship. If the suction and discharge valve plates of the reciprocating compressor are not tight or broken, causing leakage (internal leakage), the discharge temperature will rise significantly. In the centrifugal refrigeration unit, if air enters the refrigeration system, both the suction temperature and the discharge temperature will increase.
Intermediate Pressure and Temperature of the Chiller Unit
The intermediate throttling and gas-supplementing device is called a power saver. The pressure inside the power saver is the intermediate pressure of the unit, and the corresponding refrigerant temperature is the intermediate temperature. The principle for determining the intermediate pressure is to minimize the total power consumption of the low-pressure and high-pressure stage compressors of the two-stage centrifugal refrigeration compressor as much as possible and maximize the refrigeration capacity of the cycle.
Oil Pressure Difference, Oil Temperature and Oil Level Height
The lubricating oil system is an essential part of the normal operation of the unit, which provides lubrication and cooling conditions for the moving parts of the unit. Judging from the composition characteristics of the lubricating oil systems of various units, except that the reciprocating unit stores the lubricating oil in the compressor crankshaft and attaches it to the refrigeration system, both the centrifugal and screw units have independent lubricating oil systems, with their own oil storage containers and oil coolers specifically used to reduce the oil temperature. Therefore, the oil pressure difference, oil temperature, and oil level height of the lubricating oil are the three essential elements for ensuring the lubrication and cooling of the moving parts of the unit under normal working conditions.
The function of the oil pressure difference: It enables the lubricating oil to flow in the pipeline of the oil system under the drive of the oil pump and overcome the flow resistance when it is transported to each working part. Without sufficient oil pressure difference, it is impossible to ensure that the system has enough lubricating and cooling oil volume and the power required to drive the energy regulation device.
The control range of the oil pressure difference:
For the reciprocating unit: 1.5~2.5kgf/cm²
For the twin-screw unit: 1.5~12.5kgf/cm²
For the centrifugal unit: 1.5~2.5kgf/cm².
Oil temperature: That is, the temperature of the lubricating oil during the operation of the unit. The level of the oil temperature has an important impact on the viscosity of the lubricating oil. If the oil temperature is too low, the viscosity of the oil will increase, the fluidity will decrease, and it is not easy to form a uniform oil film, failing to achieve the expected lubrication effect; at the same time, it will also cause the flow speed of the oil to decrease, reducing the amount of lubricating oil and increasing the power consumption of the oil pump.
Oil level height: It is the height of the lubricating oil in the storage container. Each unit's oil reservoir is equipped with an oil level display device. Generally, it is specified that the oil level height in the oil storage container should be within 5mm above and below the central horizontal line of the sight glass. The purpose of specifying the oil level height is to ensure that when the oil pump is working, the oil circulation has enough supply to maintain a continuous circulating working state. An excessively low oil level is likely to cause the oil pump to lose oil, and even lead to operational failures or damage accidents of the unit. Therefore, when the oil level is too low, the lubricating oil of the same brand should be timely replenished into the lubricating system until the oil level height in the oil tank reaches the specified height of the sight glass.
Operating Current and Voltage of the Unit
The rated power supply voltage required for general units is: 380V, three-phase, 50Hz, and the instability rate of the average phase voltage of the power supply is less than 2%. The operating voltage of all motors should be within the range of ±5% of the voltage specified on the compressor nameplate.