Room Pressure and volume change

[djack18]

I have a client that needs to maintain a negative pressure in a production room. Our question is: keeping air flow rates constant(Qin and Qout), will a decrease in the room volume cause an increase in the room pressure? And is there a formula that addresses this situation. I do not have exact numbers at the moment. Thanks,

 

[Drazen]

Clients normally don't set exact figures for negative pressure, but only specify that they need it.

Consider that building tightness directly affects the issue, so exact selection that takes into account only ventilation system does not help.

Normally, you should, for instance, specify exhaust fan with 5% more flow than supply fan.

There are some processes that does require exact control of negative pressure. Than pressure sensor-regulated fan is needed.

The issue of room volume change I do not completely understand.

If there are some partitions occasionally used, you will have to know which volume combinations may exist, so to select adequate fans that cover range, as pressure situation will certainly change.

 

[imok2]

A smoke test can help determine whether a room is under negative pressure A tube containing smoke is held near the bottom of the negative pressure room door, about 2 inches in front of the door. The smoke tube is held parallel to the door, and a small amount of smoke is then generated by gently squeezing the bulb. Care is taken to release the smoke from the tube slowly to ensure the velocity of the smoke from the tube does not overpower the air velocity. If the room is at negative pressure, the smoke will travel under the door and into the room. If the room is not a negative pressure, the smoke will be blown outward or will stay stationary.

 

[willard3]

PV=nrt answers the question of whether the room pressure changes as the room volume changes......is this what you are asking?

You should get a Mechanical Engineer involved in this and not try to guess.

 

[mauricestoker]

Decreasing the room volume would increase the air changes per hour (ACH), but might not change the differential pressure. It would allow the supply volume to be cut back at the damper and increase the differential volume flow, which will increase the differntial pressure. Once the required ACH are met on the exhaust side, you can adjust the differential pressure by placing a magnahelic across the perimeter, and adjsting the supply damper until you get the desired IWG difference. I'd also recommned considering high quality door hardware with closer.

 

[KiwiMace]

Maurice is right, unless your volume changer is a piston the size of a wall and changing rapidly, which I don't think it is.

I am thinking you are talking about setting the room up empty, then adding some large machine in there. This will not affect your balance, and therefore your pressure, unless it is appreciably obstructing the airflow or somehow makes or closes leaks.

 

[DRWeig]

Consider controlling to a differential pressure transmitter. It'll go whacko when the door is opened and closed, but is employed often for such rooms. 0.05" WC is a ballpark setpoint.

Good on ya,

Goober Dave