Terminal HEPA Filtration- 'Top side' or 'Room side' serviceable?

INTRODUCTION

Terminal HEPA filtration is prescribed for air supply to cleanrooms classified as class 350 or greater. (AS 1386.3-1989, section 4.2) In some cases, large laminar flow modules are used to produce an ultra-clean zone, but in general a number of terminal HEPA filtration modules are used.

The terminal HEPA filtration modules available can be classified into two categories – top-side serviceable and room-side serviceable. Selection of the correct type of module is an important design consideration.

Top-side serviceable modules must be replaced from the ceiling cavity, while room side serviceable modules have HEPA filters that can be changed from within the cleanroom itself. Top-side replaceable units require a ceiling frame (or a ceiling “T-rail” system) to provide a sealing surface and ensure good cosmetic appearance.

FINANCIAL CONSIDERATIONS

The initial cost of a top-side replaceable module complete with ceiling frame is typically 80% to 90% of a comparable room-side serviceable module. Without a ceiling frame, the cost is typically 60% to 70% of the room-side serviceable alternative.

However, the replacement module cost is typically 35% to 100% more!

So the expected HEPA filter lifetime must be taken into account to determine the lowest lifetime cost. With good prefiltration (F6 or better) and a well-designed air supply, the HEPA filter should last 4 to 5 years.

ACCESS CONSIDERATIONS

If top-side modules are to be installed, there must be sufficient access for a technician to disconnect and remove a loaded or damaged module and install the replacement. If access is a problem, room-side serviceable modules should be specified.

CONTAMINATION CONSIDERATIONS

Room contamination may or may not be an issue depending on the application.

When a top-side serviceable module is replaced, the room is exposed to uncontrolled contamination from the ceiling cavity. When a room-side serviceable module is replaced, the room is exposed to contamination from the ducting. However, the ducting should be protected from gross contamination by the prefiltration, and there should be minimal air movement if the supply fan is turned off. So the contamination introduced into the room must be less with a room-side serviceable system.