Why the air-conditioning supplyreturn water temperature is 7℃-12℃

Why the air-conditioning supplyreturn water temperature is 7℃-12℃

Reason analysis of 7 degree water supply:

　　 1. Starting from the refrigeration unit, the refrigerant evaporates in the evaporator and absorbs heat to cool the secondary side water. If the evaporation temperature is too low, it will cause frost on the surface of the evaporator and affect the heat transfer efficiency. Therefore, the evaporation temperature of the refrigerant is kept above 0 degrees. There is generally a temperature difference of 3 to 5 degrees on both sides of the evaporator, and a safety margin of 2 degrees is left. Therefore, the outlet water temperature should be set to 5 to 7 degrees. However, considering increasing the evaporating temperature can improve the efficiency of the host, the air-conditioning water supply is generally designed to be 7 degrees .

　　2. Starting from thermal comfort and health, it is required to fully control the indoor temperature and humidity. In summer, the human body comfort zone is 25 degrees, the relative humidity is 60%, and the dew point temperature is 16.6 degrees. The task of the air conditioner to remove heat and moisture can be regarded as extracting heat from the environment of 25 degrees, and extracting moisture from the environment at a dew point temperature of 16.6 degrees. At present, the heat removal and moisture removal of the air conditioning method is to cool and condense and dehumidify the air through an air cooler, and then send the cooled and dry air into the room to achieve the purpose of heat removal and moisture removal.

If the air supply of the air conditioner only needs to meet the requirements of indoor heat rejection, the temperature of the cold source should be lower than the dry bulb temperature of the indoor air (25 degrees). Considering the difference between the heat transfer temperature and the delivery temperature of the medium, the temperature of the cold source only needs 15 To 18 degrees. If the air supply of the air conditioner needs to meet the indoor dehumidification requirements, due to the condensation dehumidification method, the temperature of the cold source needs to be lower than the indoor air dew point temperature 16.6 degrees, considering the 5 degrees heat transfer temperature difference and the 5 degrees medium transport temperature difference to achieve a 16.6 degrees dew point The temperature requires a cold source temperature of 6.6 degrees, which is why the existing air conditioning system uses 5 to 7 degrees of chilled water.

　　3. In other words, for conventional air conditioning systems, the key to humidity control is to ensure a sufficiently low chilled water supply temperature. If the summer design temperature and humidity are analyzed according to DB24°C/RH50%, the corresponding dew point temperature is 12.9°C. That is to say, when the conventional self-taken fresh air and primary return air treatment are adopted, the air needs to be cooled to about 12.5°C (machine dew point) to ensure the indoor relative humidity requirements; if the fresh air centralized treatment or fresh air pre-treatment is adopted For the treatment plan, at least the air needs to be cooled to below 12°C. To achieve such a treatment effect, 7°C is the low z* standard for the chilled water supply temperature. In other words, during the entire cooling season, it is necessary to ensure that the temperature of the air-conditioning chilled water supply is not higher than 7°C.

　　 12 degree backwater analysis:

　　 The return water temperature is set to 12 degrees, taking into account the characteristics of the fan coil and other terminal equipment. When the water supply temperature is 7 degrees and the return water temperature is set to 12 degrees, that is, when the temperature difference between the supply and return water is 5 degrees, the energy consumption of the water pump and the system and the heat exchange efficiency of the terminal equipment reach the best economic balance point of z*. When the water supply temperature is guaranteed to be 7 degrees, if the return water temperature is increased, which is equivalent to increasing the temperature difference between the supply and return water, then for a specific fan coil, the water flow should be reduced under the same heat exchange; The decrease of the flow rate, that is, the decrease of the water flow rate, will lead to the decrease of the heat transfer coefficient of the coil. For the fan coil, to ensure that the heat exchange is unchanged, it is necessary to increase the area of the surface cooler.

　　 Assuming that the air volume of the fan coil is constant, the average water temperature is the same but the water temperature difference is different, for example, when the temperature of the chilled water inlet and outlet changes from 7℃/12℃ to 6℃/13℃, the cooling capacity of the fan coil decreases by 12%. If reducing the return water temperature is equivalent to narrowing the temperature difference, the water flow rate should be increased under the condition of constant heat exchange, which will increase the energy consumption of the water pump and increase the initial investment of the system.

　　 As the temperature difference between the supply and return water increases, while the cooling capacity of the fan coil decreases, the dehumidification capacity of the fan coil decreases significantly, which can easily lead to the unsatisfactory dehumidification requirements of the air-conditioned room. On the other hand, appropriately reducing the temperature of the chilled water supply can partially offset the adverse effects caused by the increase in the temperature difference of the chilled water supply.

　　 Therefore, if the large temperature difference design can truly achieve the purpose of energy saving during operation, for the fan coil unit, certain measures must be taken to compensate for the large temperature difference. Such as lowering the chilled water temperature, increasing the number of coils, installing spoilers inside the coils to enhance heat transfer, etc.