Controlling room pressure in a closed loop

[skuntz]

Here's the setup. I have a closed loop (recirculation) air system. I am blowing air through several feet of ductwork into a sealed room. The air is returned to the suction of the blower but not until after it passes through a set of filters. I need to maintain the pressure inside the room at not less than -0.1 inches w.g., but not more than 0 inches (gauge).

In general then the question is how do I know the pressure at any point in a closed loop and how can I control it?

 

[remp]

You need to introduce outsdie air into the system to pressurize the room. You will need a pressure sensor in the duct and a varible speed drive on the fan
Air leaking out needs replacement air. Due to pressure differential you get leakage. Your leakage might be minimal but it will exist.
If you have a single room and a single air handling unit it is easy.
Adjust the outsdie air damper until you get the differential room pressure you want across the door. You will alwasy require a minimum o/a in accordacne with building codes ASHRAER 62 and local authority requirements etc... If the min. o/a give too much pressure diff. you need a relief stratogy, like a fan or relief air damper.

Record the static pressure in the supply air duct and set this pressure as a set point that is achieved by the supply air fan variable speed drive. With this set up if the filter get dirty as they will over time the supply fan will ramp up slightly bringing in the same amount of ousdie air to cater for the leakage and hence keep the required diff. pressure.

 

[Zesti]

skuntz, if you need _negative_ pressure in the room then you need to loose some air on the supply side of your fan.

You suck 100% from the room, with x% having leaked in from the surroundings and 100-x% being circulated.
You supply 100-x% to the room and loose x% on the supply side.



PS: if "not more than 0 inches" is the upper limit then just leave the door open: 0 inches exactly without any trouble...

 

[mauricestoker]

You might have trouble running a VFD against filters and neg dP with just a 0.1 IWG range, so I'd assume you could go more negative.

Place a Dwyer across the wall, run the increase exhaust until the Dwyer gives the reading you want. Run a standard dP sensor from the exhaust (after the filters) to the VFD input, pretty much reverse of standard controls. If supply is allowed to vary, have the VFD run directly off a photohelic placed across the wall.