cleanroom pressures 5

[balancer2]

we are monitoring cleanroom pressures 24/7. the fill rooms are maintaining 0.10 in.wc. with the doors closed. when the doors are "propped open", the room pressures equalize and send a signal of 0.000 to -0.007 back to the front end. the customer believes the room is becoming negative. i've tried to explain the sensitivty of the sensors and the fact that small eddies and drafts into the rooms will cause the signal to sometimes read negative. thye insist the FDA will not except that explanation. we are dealing with constant volume systems "at this time". this is our #1 customer and we want to resolve this issue. any experience in this matter or suggestions will be appreciated.

 

[IRstuff]

What basis do they have for claiming that the cleanroom should be maintaining correct pressure with the doors propped open?

Is the other room also pressurized?

TTFN

Eng-Tips Policies FAQ731-376

 

[friartuck]

Balancer 2

There are a couple of things that you need to clarify. What grade is the clean room.

Basic class 100,000 systems for example can sometimes have floating room pressures since the process is none critical.

When you get to class 1000 for instance, you will need a lobby which has the doors controlled so that only one can be opened at a time.(maintaining pressure)

Yes the room pressure will fall when you open the door, but you can enable a delay in the alarm for say 5 secs. This stops nuisance alarms.

We did a clean room with lobbys etc. and even then we got alarms.

People will try to disable the alarms, jam the doors open etc.

0.1 inch WG. is also quite a high pressure. It must be difficult to open the doors.

We work on between 10 to 15Pa max 20Pa and when the pressures are at 25Pa (0.1" WG) you need a bit of muscle to open the doors.

Good luck



Friar Tuck of Sherwood

 

[lilliput1]

Try moving the location of the sensor probes. Either within 12" above the floor or at the ceiling or combination of both.

 

[MintJulep]

What is your goal?

Making the room not go negative, or making the reading not go negative?

 

[KenRad]

It seems that you have one of two choices if you are really losing room pressure. 1. This is a cleanroom, so you could convince the customer that cleanroom doors should never be "propped" open if the room is to stay clean. 2. If the customer insists that the room must be able to maintain a certain positive with the doors propped, then you need to look at room pressure control. A motorized outside air damper controlled from the room static sensor would accomplish this.

Since you are already trying to keep a very high pressure, I would not suggest going any higher to compensate for the door opening. I believe variable outside air flow is the way to go.

---KenRad

 

[lilliput1]

The room is definetely positive because it is pressurized. However ddy currents at the doorway has been found to occur. Air current can be flowing in different direction at the top from that at the bottom. Thus the placement of the sensors may help. CDC is aware of this and recommends sensors for isolation rooms be withi 12" above the floor.

I assume that the filing is under a laminar flow, Class 100 or better condition that would protect the product even if eddy currents do draw some less clean air in.

Is there an air lock or lobby as refered to by frisrtuck?

Do the same test without propping the door but operating the door normally.

 

[danw2]

1) Do you have to make a bi-polar (negative & positive around zero) measurement? What happens if you measure (or only look at) zero & above?

2) It wasn't a clean room, it was furnace positive pressure, but the drafts across the DP's low-side (reference leg) impulse tube temporarily going more positive than the high side caused negative excursions. We put a sintered filter in front of the impulse tubing to dampen the effect of the drafts which eliminated the negative excursions.

Dan