Fan Sizing for Room Pressurization

[BronYrAur]

Just need a sanity check. When sizing an air handler for a room that is to be pressurized, I must add that pressure to the fan external static, correct? In other words, let's say that my duct and other external losses amount to 3"W.C. Let's also say that I want to pressurize the room to 0.25"W.C. (pretty high, but just as an example). When selecting the AHU, I will need to tell the manufacturer that I need so many CFM @ 3.25"W.C. external static - correct?

 

[briand2]

It's the "so many CFM" that you really need to think about; you need to calculate (amongst other things) how much air will be leaking out of the pressurised room at 0.25" Aq.

Brian

 

[ChasBean1]

Yes, you're right BronYrAur, although it's usually negligible compared to the other losses in the duct serving the room...

 

[quark]

Yes. That is correct and if you really want 0.25" wc (this is generally high) it is prudent to include in fan ESP.

I have a personalized opinion here. Most of the times, we tend to be redundant in duct design. Ex. rounding up to nearest even number, high roughness factor, future requirement and FOS. In this cases, you are already running your fan to the right of the design point and 0.25", if not considered initially, will help you to get the operating point to the left a bit.

After all, it is your own judgement.

 

[BronYrAur]

Thanks everyone. I just used 0.25" as an example to drive home the point. I have never had the need to pressurize anywhere close to that.

 

[Jabba007]

I am currently designing some stairwell pressurization systems. I plan to use relief dampers to control the upper limit of pressurization. Ie.. when the stair tower reaches .25" the relief damper opens and limits it to that upper set point. For my application, this is to stay within the allowable pressure range dictated by IBC, which is .15" to .35". I will also have to consider stack effect in the tower as well during both design summer and design winter conditions.

Jabba