The model of the filter supply plenum with velocity vectors has appeared in several trade journals in recent years. To my knowledge it has never received criticism similar to that offered by Mr. Charlie Middleton (CleanRooms, June 2000, p. 6).

The plenum represents a portion of a recirculating system in which there would likely be two air movers. The primary mover would be the recirculation fans and the second would be the makeup unit(s). In such a system the highest relative pressure would be at the outlet of the recirculation fans and the lowest pressure at the inlet of those fans. The reference pressure is strictly arbitrary and is established by the makeup unit. To say that the system is a “pusher” or a “puller” has no meaning in such a system. My experience in computer modeling of airflow and mapping operating systems with appropriate instrumentation suggests that the representation on page 16 of the May 2000 issue of CleanRooms is excellent.

Further, the software used to calculate the flow is widely employed giving accurate results, and it is obvious that the user was experienced in setting the boundary conditions of the model.

David L. Brewer
TH Inc.
(Contamination Control Consultant Specializing in CFD)
West Jordan, UT