In the air conditioning system with independent control of temperature and humidity, the role of the fresh fan is not only limited to bearing the fresh air humidity load, but also needs to deal with the indoor humidity load. Compared with the fresh fan in the conventional air conditioning system, the new fan used for independent control of temperature and humidity in the air conditioning system has higher requirements for dehumidification. According to the relevant literature, the moisture content of fresh air in an air-conditioning system with independent control of temperature and humidity usually needs to be treated to 80g kg dry air.
According to the above calculation, without considering the indoor sensible heat, the moisture content difference of the fresh fan in the temperature and humidity independent control air conditioning system is 1333g kg dry air with a processing enthalpy difference of 4259kj/kg。Compared with the new fan in the traditional air conditioning system, the difference in moisture content is increased by about 1067%, and the enthalpy difference of treatment increased by about 401%。However, the reality is that the new fan usually bears a part of the indoor sensible heat load, which puts forward higher requirements for the dehumidification capacity and enthalpy difference of the new fan.
Under the working conditions throughout the year, the temperature and humidity content of the fresh air vary greatly. Taking Guangzhou as an example, the enthalpy difference of fresh air treatment in July is about 1 of the enthalpy difference of fresh air treatment in April83 times, the difference in moisture content of fresh air treatment in July is about 1 of the difference in moisture content of fresh air treatment in April99 times. The application of direct expansion fresh air fan in the air conditioning system with independent control of temperature and humidity has been limited to a certain extent, mainly due to the adjustment of evaporation temperature, condensation temperature and compressor, resulting in a limited range of system capacity adjustment, relatively single working conditions, and it is difficult to adapt to the change of outdoor fresh air load throughout the year.
To sum up, in order to meet the requirements of dehumidification and cooling, adapt to the changes of fresh air throughout the year, and realize the efficient load unloading of fresh fans, the basic conditions that need to be met by direct expansion fresh fans are that they need to have better dehumidification ability and stronger ability to deal with enthalpy difference when used in air conditioning systems with independent control of temperature and humidity. For single-stage direct expansion fresh air fans, the use of frequency conversion control can increase the range of cooling capacity variation. However, at low loads, problems such as slower refrigerant flow rates and difficulty returning oil to the system can occur. In contrast, the two-stage or multi-stage direct expansion fresh air fan includes two or more refrigeration systems, and the unit has stronger cooling and dehumidification capabilities, which can meet the requirements of the temperature and humidity independent control air conditioning system in high humidity areas or places with large wet loads. By controlling the operation of different levels of refrigeration system, the cooling capacity of the unit can change in a wide range and has good stability.