Primary return air system, in the case that the return air can be recycled, the return air is mixed with fresh air, and then treated, and sent to the clean room. This is the more common form of the system.

Secondary return air system, in the case that the return air can be recycled, the first part of the return air and fresh air mixed, after treatment and then mixed with the remaining return air, after treatment, into the clean room. This form of system is often used in clean rooms with high grade and low process heat. The utilization of secondary return air saves part of reheating heat and part of cooling capacity.

All fresh air system, also known as DC system. After the fresh air is treated, it is sent into the clean room and then discharged directly into the atmosphere without return air. This form of system is used in situations where the return air cannot be recycled, such as animal houses, biosafety clean rooms, some drug production workshops or certain processes in pharmaceutical manufacturing plants. Because the full fresh air system directly processes the outdoor fresh air to the indoor air supply state, the return air is not recycled, so it is a system form with high energy consumption. From the point of view of energy saving, when the whole fresh air system is used, it can be considered to add an energy recovery device in the system.

Air supply system + indoor air conditioning unit, can meet the specific room on the temperature and humidity of the more stringent requirements. Especially suitable for reconstruction of clean rooms or small area clean rooms with special requirements.