ASHRAE 51.5

[Josh2008]

Hi,

I'm working on calculating the net cfm into an electrical room to prevent the ingress of dust and contaminents. I have used an equation that has been referenced in a number of threads (that is said to come from ASHRAE 51.5 1999).

Threads

http://www.eng-tips.com/viewthread.cfm?qid=62681

http://www.eng-tips.com/viewthread.cfm?qid=218830

http://www.eng-tips.com/viewthread.cfm?qid=43928

Last link the derivation explained (bernoullis eq)

I have ANSI/ASHRAE 51-07 and I cannot find this equation or reference of the derivation of the equation.

Would somebody shead some more light on this.
What about standards for room pressurization? I'm not sure how many inches of water column I require to prevent the ingress of dust.

The room is 22000 cfeet (20x20x55)
I calculated approximately 1.9 sqfeet of openings (ASHRAE fundamentals 2001) which I believe is conservative.

Please see the attached excel sheet

When I derive Bernoillis EQ I get Q=A*2406*(deltaP)^1/2 using a square opening coefficient of 0.6 (which I don't know where this comes from either)

Thanks

 

[JLSeagull]

NFPA 496 Purged and Pressurized Enclosures for Electrical Equipment applies to pressurization to prevent ingress of combustible gas etc. The scope of NFPA 496 applies to electrical equipment, control rooms, buildings and analyzer rooms etc. classified as hazardous by the National Electrical Code Article 500 or Article 505. The NFPA 496 pressure is 0.10 inch of water. The spreadsheet use of 0.15 would seem conservative.