A cleanroom designer must consider many factors when specifying a new cleanroom. Air changes, temperature and humidity control, size of clean room, amount of human and machine activity and climate conditions to name a few. One of the critical characteristics of a clean room is positive air pressure. Adequate positive air pressure keeps “unwanted” air from leaking in, thereby compromising the cleanroom integrity. A cleanroom must be designed to maintain positive air pressure through out the typical traffic and external conditions that may occur through out the day. The biggest source of air leakage may be the cleanroom ceiling.
Most cleanrooms have hung ceiling grids with clean room compatible tile. These tiles have gaskets to reduce leakage, but even under ideal conditions, that leakage will never be zero. Therefore airflow design must include ceiling leakage so that incoming air will adequately compensate for this leakage. On a similar note, a designer must consider the quality of the ceiling tile as well as the installers.
How much air leaks through a cleanroom ceiling tile with a factory gasketed grid?
To find the answer to leakage rates for cleanroom ceiling tile, Gerbig Engineering built a 4 foot by 6 foot by 1 foot sealed box with three ceiling tiles resting on a typical 1/2 gasketed aluminum ceiling grid. The tiles are the typical vinyl covered gypsum with a weight of 1.07 lbs/sf. By using a compressed air source and a Dwyer mass flow meter and our eight point pressure transducer, we were able to plot flow into the box verses pressure. So far as we know, this is the only study of leakage rates in cleanroom ceilings ever conducted.
Our results are given below. However, readers should note that these numbers are for tile installed in a grid under ideal conditions. Our technicians were able to razor cut the gasket edges so that no corner leaks occurred. In actual practice razor cut edges to within 0.003 rarely occur. We suggest designers use two times the values in the graph for leakage values of tee grid ceilings.
The measurements were as follows
Air pressure Air Flow (Standard
(Inches on Water Column) Cubic Feet Per Minute
The values can be approximated by the equation P=.083 times Flow. This equation should be subject to the same design compensation suggested above (assume twice the leakage value)
Conclusion: By using the equation above, a designer has a better tool for compensation for ceiling tile leakage when designing the clean room.