Closed Loop vs. Open Loop
Open Loop System
In the open loop system, the HEPA filtered air exits the room at the floor and returns to the fan-filter unit freely. The main advantage with this system is cost because there are no return air chases, ductwork, or plenum. Typically, air exits the room under the wall panels, but occasionally the air exits through wall grilles.
With open loop cleanrooms, room pressure is determined by the pressure loss of the air passing through the wall grilles or under the wall panels. For air exiting under the walls, Gerbig Engineering uses actual test data to determine the opening dimension under the wall.
The disadvantage in the open loop concept is the loss of humidity control. The humidity will be the same as the larger room (ambient area). Also, the open loop system can’t be used if the ambient area has no air conditioning.
Closed Loop System
In this system, the HEPA filtered air is contained within the cleanroom volume. Air is returned through the wall cavity or return air chases. The advantage in using this system is the humidity can be controlled to a different value than the larger space. Once air is returned to the chase or riser, there are two possible methods to completing the air circuit. One is to duct the air from the wall riser to the fan-filter unit and the other is to create a negative plenum by making a double ceiling. This cavity acts as a return air plenum. The choice between the two methods is mainly a matter of cost. With lots of filters in the ceiling, a double plenum system would cost less. This occurs with most ISO 7 (Class 10,000) cleanrooms and cleaner.