Maintaining dP of a clean room with FFU's 2

[Boiler1]

Hi All,

If the Fan Filter Units (FFU's) are ceiling mounted in the clean room application and the conditioned (make up) air is introduced in the void ( plenum) above, how the differential pressure between the airlock and the room is maintained.
The FFU's operate as full re-circulation and ramping them up or down will not affect the dP.
The only way the dP can be maintained is the for make up air to be actually ducted into the rooms rather than introduced in the void above.

Is the above correct or am I missing something?

Have seen applications with different density of FFU's in the ceilings when walking from room to room, not clear why unless different clean room classes are to be maintained.

Regards

 

[BronYrAur]

I am confused why you are saying that introducing makeup air into the plenum will not allow for pressurization? Why wouldn't it work?

 

[Boiler1]

The plenum is common for the room and it's air lock and I'm not sure if it's possible to push more make up air towards the air lock in that situation. That is even if you ramp up the air lock FFU's they are only gonna pull out at one end what they introduce at another.
I suppose, what I'm trying to avoid is having to HEPA filter the make up air if introduced directly into the room and the air lock. Basically trying to introduce the make up air upstream of FFU's and maintain the dP having the common void above

Regards

 

[SAK9]

Boiler1,

You can cascade air out of the clean room into the airlock(through an in-wall gravity damper) and control its exit out of the airlock through another in-wall gravity damper.This gives you enough control over the air lock pressure.

 

[GT-EGR]

As long as your FFUs aren’t directly ducted into and out of the room, and as long as the plenum is also constructed of the right materials, you are essentially combining your outside air directly into your return airstream - which is what will create your pressure. If the plenum is open to the room with open returns, the room sees the increased pressure that the plenum sees. The FFUs can be off and pressure will still be maintained.

As far as the airlock vs the clean space, they would need their own separate branch supplying the proper quantity of outside air (or exhaust air) needed to create the pressure against your leakage rate. You can’t just dump all the air in one space - the correct amount needs to be balanced to each pressure zone.

As far as the FFU quantity, different clean room classes, or even clean rooms of the same class all can have much higher/lower air change requirements. So if one room of the same size needs 20 air changes and one needs 200, you’d need 10x as many FFUs to meet that increased air change rate.

 

[GT-EGR]

As far as the gravity damper suggestion that doesn’t give you any active control, and without directing pressurization air separate for each compartment you won’t get a differential and the damper will just stay closed.

If you are trying to keep adjacent spaces at different pressures usually you are trying to minimize the openings between the two so that it takes less outside air just to plug up the cracks.

 

[Boiler1]

Thanks GT-EGR. Great Info!

 

[SAK9]

Agree that gravity damper based system does not give you active pressure control.You need active pressure control only if you need to maintain and validate pressure differentials such as in the pharma industry or in a biolab.The OP's setup seems to be for some sort of semiconductor industry where you do not need an active pressure control system.