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I. Evaporation Pressure and Evaporation Temperature
The evaporation temperature, evaporation pressure, and the heat load on the cold water side are directly related: when the heat load increases, the temperature of the return water from the cold water system rises, and the evaporation temperature and evaporation pressure also increase accordingly; when the heat load decreases, the opposite occurs.
According to the national standard GB/T 18403.1—2001, the rated operating conditions of the chiller are: the inlet and outlet water temperatures of the chilled water are 12℃/7℃, and the inlet and outlet water temperatures of the cooling water are 30℃/35℃. The chiller is calibrated according to this operating condition at the time of manufacture.
During operation, while meeting the requirements of air conditioning use, it is advisable to increase the temperature of the cold water outlet as much as possible. The evaporation temperature is generally 2℃ to 4℃ lower than the outlet temperature of the cold water, and is usually controlled within the range of 3℃ to 5℃. If the evaporation temperature is too high, it is difficult to achieve the air conditioning effect; if it is too low, it will not only increase energy consumption but also pose a risk of freezing and cracking of the evaporation pipeline.
II. Condensation Pressure and Condensation Temperature
The pressure indicated by the high-pressure gauge is the condensation pressure, and the corresponding temperature is the condensation temperature. When the evaporation temperature remains constant, the condensation temperature plays a decisive role in the power consumption of the unit: an increase in the condensation temperature leads to an increase in power consumption, and excessive condensation pressure in centrifugal units can even cause surge; a decrease in the condensation temperature results in a reduction in power consumption.
During operation, it is necessary to ensure that the cooling water temperature, water volume, and water quality are within the qualified range. When air is present in the condenser, the temperature difference between the condensation temperature and the outlet temperature of the cooling water increases, while the temperature difference between the inlet and outlet of the cooling water decreases. The outer shell of the condenser becomes hot, and the heat transfer effect significantly deteriorates. In addition, the scaling and sludge on the water side of the condenser also cannot be ignored in terms of their impact on heat transfer.
There are two measures to reduce the condensation temperature: one is to lower the return water temperature of the cooling water, and the other is to increase the cooling water volume. For centrifugal units, excessively high or low condensation pressure can cause surge. Therefore, the difference between the condensation pressure and the evaporation pressure should not be too small and must meet the requirements for preventing surge. When the temperature is low in autumn, operating reciprocating units is more advantageous. At this time, the condensation pressure is low and the power consumption can be significantly reduced.
III. Cold Water Side Pressure and Temperature
When the air conditioning water chiller operates under standard conditions: the return water temperature is 12℃, the supply water temperature is 7℃, and the temperature difference is 5℃. The flow rate of the evaporator's cold water is inversely proportional to the temperature difference between the supply and return water. The greater the flow rate, the smaller the temperature difference, and vice versa. Specifying a 5℃ temperature difference essentially means specifying the water flow rate of the chiller. The control method is manifested as the pressure drop of the evaporator.
Under standard conditions, the pressure drop of the supply and return water of the evaporator is set at 0.5 kgf/cm. The setting method is to adjust the opening degree of the valve at the outlet of the cooling pump and the opening degree of the valves for the supply and return water of the evaporator.
IV. Cooling Water Side Pressure and Temperature
Under standard operating conditions, the return water temperature of the condenser is 30℃, the outlet water temperature is 35℃, and the temperature difference between the inlet and outlet water is 5℃. When the environmental conditions, load, and cooling capacity are all constant, the condensation heat load is also constant. The 5℃ temperature difference fixes the cooling water flow rate. This flow rate is also controlled by the pressure drop of the cooling water entering and leaving the condenser.
Under standard operating conditions, the set pressure drop of the cooling water entering and leaving the condenser is approximately 0.75 kgf/cm. The setting method is to adjust the opening degree of the outlet valve of the cooling pump and the opening degree of the inlet and outlet valves of the condenser.
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