The variable air volume air conditioner has a cold water coil. Hot water coils are also required for hot and cold single air duct systems and variable air volume systems that need to consider winter preheating. The selection and check calculation of the cooling and heating coils are generally carried out by the air conditioner manufacturer using computer programs. The designer only needs to provide the technical parameters of the selected air conditioner: the inlet and outlet air parameters of the coil; the inlet and outlet water temperatures; , water side resistance requirements and the arrangement position of the coil in the air conditioner.

(1) The inlet and outlet air parameters of the coil are the basis for the selection and check calculation of the coil, and are determined by the design calculation. Considering the design margin of the variable air volume air conditioning system and the terminal device, the air supply temperature margin of 0.5~1.0℃ should be considered on the basis of satisfying the design requirements for the outlet air temperature of the coil.

(2) The temperature of the inlet and outlet water of the coil is also called the water temperature difference of the coil. It is determined according to the form of cold and heat sources and the type of air conditioner. For variable air volume air conditioning systems, when considering the temperature difference between supply and return water, the following factors should also be noted:

1) The temperature difference between the inlet and outlet water of the fan coil unit is generally small;

2) The temperature difference between the inlet and outlet water of the fresh air conditioner is generally large;

3) When the air volume of the variable air volume air conditioning system is small, the temperature difference between the inlet and outlet water of the coil will also decrease.

In order to avoid the phenomenon of large flow and small temperature difference, for the cooling coil, especially for the system with large temperature difference, the water temperature difference of each air conditioner and fan coil unit should be comprehensively considered. It is not necessary for all coils to have the same temperature difference in the project, but it is necessary to ensure that the average temperature difference of the system matches the processing temperature difference of the chiller. According to the design experience, the temperature difference between the inlet and outlet water of the cooling coil of the variable air volume air conditioner should be greater than the design temperature difference of the water system by 0.5 to 1°C.

(3) Wind side and water side resistance of the coil. The air side resistance directly affects the energy consumption of the fan, and the wind speed on the coil surface is generally around 2.5m/s, not exceeding 2.75m/s. Coil contamination will seriously affect indoor air quality, and cleaning must be considered. In order to facilitate the cleaning of the coil, ASHRAE Standard 62 recommends that the air resistance of the coil should be less than 190Pa, that is, under a certain surface wind speed, the number of rows and fins of the coil is limited.

(4) The water-side resistance of the coil affects the determination of the pump head. For the normal temperature air-conditioning system, the water-side resistance of the coil is generally in the range of 30~50kPa.

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