Pressurization of Room w/ HVAC System

[08HokieEng]

Does anyone have experience with pressurizing a room when there are also HVAC AHUs in the room? Our HVAC contractor says he plans on pressurizing the room by ducting in air from the outside in conjunction with air from the inside. We weren't sure if this was SOP as we figured air into the AHU would equal air out of the AHU, which would not induce any positive pressure.

Not sure if that's a better option than using a separate make-up air system with fan/filter that blows directly into the room.

Thanks to all.

 

[KiwiMace]

Yes it doesn't really matter what is going on inside the space, as long as you have a large enough quantity of air being delivered into the room which has a leakage path back out sufficiently tight enough to backpressure the room to the desired pressure.

If he plans on ducting air in from outside then he probably plans on at least attempting to pressurize it.

 

[BronYrAur]

I'm a little confused by your question, but the more outside air you bring in, the more positive you are relative to outside. You don't really care about the pressure difference between the room and the AHU, you just care about the difference between the room and outside, correct? If so, the more outside air you bring in, the more positive the room will be.

Having said that, the construction of the room is critical. You will never hold a descent pressure in a room that is not tightly sealed or has doors that are always open, etc.

 

[scha0786]

It all depends also on if there is exhaust in the room or return air. As long as you supply more then you are exhausting the space should be slight positive.

I would put a gravity relief damper to outside that modulates open/close based on space pressure (maintain 0.05+)

hope that helps

 

[bip1694]

Yes, it is normal to pressurization the room as long as you balance your air system to pressurize the space. A gravity PRD will do.

We do pressurize the space to avoid the outside air/gases enter into the room.

 

[urgross]

This would be fairly common practice as a fail safe for A/BSL-3 facilities, at least in DoD. Outside air is not used in that case, but air from the dedicated AHU. In case of loss of dP across the secondary containment, in particular for loss of containment exhaust, adjoining spaces, including mechanical room, have fast-acting barometeric dampers open. The bubble tight dampers for the secondary containment supply close resulting in adjoining space being relative positive compared to containment space.