pressurization with exhaust louvers

[fcxdfm]

I am modifying an existing HVAC system serving a pump station in a waste water treatment plant. It has to be maintained at 0.1" WC positive pressure relative to the atmosphere. The room has existing exhaust louvers at exterior wall, and the client would like us to re-use them. Is it possible to maintain the sapce at 0.1" wc positive pressure with exhaust louvers, and supply fans?

Thanks for advise...

 

[willard3]

Fan sheaves could have been changed, fan could have been changed, controls could have been modified and etc.

Louver seal-edges deteriorate with time and may need replacement to be tight, but you won't know until you measure.

Call a balancing contractor and find out if you are 0.1" wc positive now with existing fan/louver/control equipment.

 

[fcxdfm]

Thanks, Willard3.

The existing room is not pressurized, and the existing louvers are for air intake. My design approach is to add new supply fans and convert the intake louvers (48"x24") to exhaust type. The pressure drop through the new louvers will be designed at 0.1" wc. Would like to doube check if my concept is right - Can the room be pressurized with big louver opening...

Thanks in advance for help.

 

[MintJulep]

There is a relationship between the flow through a restriction and the pressure drop across it.

So yes, it is possible to maintain positive pressure in a space with exhaust louvers. At least until the wind blows, or the door opens, or a seal around a wall penitration deteriorates, or....

 

[willard3]

You can also put in an oversized belt-driven fan and reduce the rpm until you have 0.1" WC Delta P.

As the seals age and caulking dries out, change the rpm of the fan until you have the correct delta P again.

As Mint Julep points out, all bets are off if you operating personnel aren't the standard fool and leave the door open, remove the door seals and etc.....nothing is fool-proof because there is no standard fool.

 

[fcxdfm]

Thanks, MintJulep and willard3.

Willards,
Good idea to provide the fan with VFD, which is controlled by the pressure sensor inside the space.. In this project, the space is relatively big though, over 200,000 cubic ft, (with two levels open to each other). May need to provide several sensors and get some average reading...Any good suggestion?

Thanks again...

 

[ChrisConley]

If the space is open, and the requirement for space pressure control isn't very tight they you could get away with maybe four sensors, one on each wall exposure.

Depending on which direction the wind was blowing you could ensure that the 'least' differential pressure in the space is 0.1".

One other concern would be estimating the volume of air required. Search the site and you'll find several discussions about the difficulty in predicting the amount of air required to pressurize a building.

* It isn't that you can't make something foolproof, the problem is that they keep making better fools....*

 

[KiwiMace]

If, as it sounds, there is too much louvre for the airflow rate you need, just blank-off some of the louvres. Start with the louvres that face the prevailing wind direction.

0.1" (or 25-30Pa in my money) across the louvre is a very typical design figure for what were originally inlet louvres. Unless there is some specific requirement for the 0.1" to be maintained, I wouldn't worry about VFD controls. In the middle of a ww plant after all. Use the louvre manu design chart for reverse use - Inlet louvres usually have special rainwater channels and don't flow like exhaust louvres.

If you are worried about switchgear corrosion, enclose it and separately ventilate.

 

[lzh007]

I think you should check under which pressure differential will the louver open, and the pressure drop @ the desired air flow. a 90 degree wall cap may reduce the wind effect.

 

[lilliput1]

How about using a gravity operated backdraft damper that you can adjust for the pressure drop required. Better to put more pressure drop to give you cushion, but check that doors don't get too hard to open.