Cleanroom Design

[KenRad]

I've been given the task of expanding an existing class 10,000 cleanroom. The duct design is something I've not come across before, and was wondering if anyone is familiar with it.

The two air handlers serving this room are roof-mounted. The supply air discharges through openings in the supply air ductwork, into a ceiling plenum, where it is picked up by fan-powered HEPA filters in the ceiling grid. No ductwork connects the supply duct to the HEPA boxes.

The walls of the room are plenum walls, and they are open to the plenum above the ceiling. The return ductwork runs along the walls, with openings spaced evenly along the duct, facing toward the walls. As with the supply side, there is no direct connection between the return duct and the wall plenum.

The system has been in operation for about 14 years, and apparently has been working for the customer. Is this a common design for cleanroom ductwork? I'm used to either supply or return (or both) being directly connected to the ductwork. I imagine that some shortcircuiting could occur if your HEPA boxes weren't sized properly. Any thoughts?

---KenRad

 

[RossABQ]

I believe if you check, you'll see that one "supply air handler" is really a make-up air handler (MAH), and the other is a recirculating airhandler (RAH). (It could also be that both are RAH's with MUA capability).

The MAH makes up for exhaust loads and provides excess OSA for pressurization. The RAH is purely a sensible cooling unit. The area above the FFU's is essentially a mixing box. I would say it is much more common to have at least one side (supply or return) isolated from the other, but in the early days of cleanrooms a system as you are describing was also used, I believe because of ease of pressure control. If a tight temp spec isn't needed, I guess it will work OK, but not if you have a process that requires tight temp stability. You could connect the return wall chases to the RAH to improve it.

 

[lilliput1]

The (2) airhandlers are probably similar to provide about 60% capacity if one is down (because of reduced pressure drop w/ reduction in airflow, capacity w/ one off is not 50% but approximately 60%). Both need to provide outside air for ventilation & pressurization.
Now with clean rooms you need a high rate of air changes per hour (ACPH) through HEPA filters. Because of the high volume, only a small temperature rise is required. However you can not dehumidify the air unless you cool it down to about 55°F. Now, so as not to cool all the air and have to reheat it up, the (2) AHUs supply and return only the amount needed to cool and dehumidify the air using only part of the air and discharges it to the plenum. The fan powered HEPA filter (FPFU)unis however recirculates the high ACPH needed and mixes the return with the cooled and dehumidified air from the AHUs. Total up the airflows by the AHU(s) and that by the HEPA filter units to see what I mean. The plenum wall, I imagine it has low return grilles around the room, is used as a return air path for the FPFU as well as for the ducted return air back to the AHUs. The pressure in the room is (++) (more positive) than the (+) prssure in the plenum so room air is pushed up the plenum walls where it is drawn to the intake of the FPFUs. Both the space and the plenum needs to be airtight. This is a common approach in cleanrooms to minimize ducting the large volume of air. I recommend having separate plenums for the expansion but use the same approach and ACPH of the AHUs and the ACPH of the FPFUs but use variable speed fans to compensate for filter loading.

 

[quark]

I never saw this kind of set up but 15 years is beyond the scope of my experience. I suggest to go for a ducted system, instead.

Technically, this may be possible for a purely sensible load application. Still, if you have to maintain max. 22⁰C temperature in the space, say, you supply the air at 18⁰C. This is total recirculated air and we have to cool it down to 18⁰C in the next cycle by mixing with treated fresh air. If I consider lilliput's DP of 55F, then heat balance gives,

(x*22+y*12.77)/(x+y)= 18, by rearranging the terms you will get x/y = 5/4 or you have to supply about 45% of fresh air and leak it from controlled space for every cycle.

Secondly, it is prudent to filter the particulate matter by gradual filteration rather than loading the HEPA from the beginnig.

Third thing is that you should carefully size your ducts (both makeup air and return) so that they discharge the air into common plenum pressure.

The room pressure should be the sum of plenum pressure and the return duct losses, so for a fixed flowrate and plenum pressure, the room pressure is constant and slight variation in the room pressure (for example, door opening) may short cycle makeup air directly into the room bypassing HEPA.

If you have latent loads in the room, there will be condensation in the plenum over a period.

 

[lilliput1]

Quark,
The AHU return air portion return should be ducted direct back to the airhandler. The remaining return air would go through the plenum wall to the above ceiling plenum where it mixes with the supply air and drawn to the intake of the FFUs and discharged to the room. I don't think you would want the return air portion be ducted direct to the FFUs because this would not allow the mixing with the AHU primary air. The FFU fans do not have enough static pressure capacity to allow ducted return. The plenum is required to allow the mixing and to provide minimal atatic pressure loss. Have you ever seen a ducted FFU? How can you say in your 15 year exoerience you have never seen this plenum wall arrangement?
I don't know where you are comming from about preloading the HEPA filters. The variable speed I recommended is to ramp up the fan speed as the filter gets loaded thereby providing constant CFM airflow with filter condition changing from clean to dirty.
The supply air at the intake of the FFU should not be assumed. It should be calculated from the room loads (lights, equipment, people sensible, FFU fan heat). FFUs may be anywhere from 100 watts to 400 wats per unit depending on make. The room equipment load may or may not be sustantial depending on clean room usage. Some may want 68°F indoor design rather than 72°F.

The room envelope heat gain is a load to the return air to the FFU since it goes to the plenum. The mixing equation should account for this return air temperature rise to mix with the supply air at 55°F to arrive at the required supply air temperature at the intake of the FFUs.

The AHU supply is not 100% OA. That is why there is a return air duct. The OA required must be adequate for ventilation & pressurization. For pressurization of 0.05"wg assume 1.25 CFM OA/SF floor area and adjust as required during the commissioning. Better yet, Kenred should measure that on the existing AHUs and factor it with the ratio of new vs existing building area. Also Kenrad, see if you can find control sequence of operation of the existing. See if it has active pressurization controls and setback (reduced airflow at unoccupied times). Class 10,000 requires minimum 30 ACPH. You should make sure the supply air dewpoint is low enough to handle the loads. You would not want condensation to occur in the plenum or anywhere within the clean room.