return vs relief fan

[russmech]

About a year or so ago, I read (and didn't finish) a good article of the relative merrits of return vs relief fans in HVAC systems. I now wish to read the article again but can't remember which magazine I read it in (I've since moved and had to clean out my magaizine collection. Does anyone remember reading that article that could tell me where I might have read it (or have a similar source of information)? Getting old is hell. Thank you.

 

[ChasBean1]

Russmech, we can pick apart the particular issue you have in mind in this forum, which is always fun. A return fan needs to return the air, a relief fan needs to exhaust it... We are all about fun here.

 

[rworthen]

I remember reading two I think in the last 3 years in the ASHRAE journal but I don't recall the exact issues. Usually the argument made is that return fans make controlling the minimum outside air difficult especially with VAV systems. As I recall the argument goes like this: in the mixed air section, you must have a negative pressure condition inorder to draw outside air in. The return fan is upstream of the mixed air section so all the pressure developed by this fan must be dropped across the mixed air damper. With an exhaust fan on the other hand, you are assured of a negative pressure condition in the mixed air section.

 

[ChasBean1]

If you find out what the article is, please post it. I haven't the faintest idea how the two could possibly be used interchageably, especially with economizer control.

 

[russmech]

I found it. Its ASHRAE Journal's September 2000 issue. The article is "Comparing Economizer Relief Systems" by Steven T. Taylor, P.E. ASHRAE members can download it from ASHRAE's website (so I've been told). Steven Taylor is a principal of Taylor Engineering in Alameda, Ca. I would think he would be willing to send or email a copy of the article to anyone interested. It's pretty well written and has other good references at the end of the article for minimum ventilation control for VAV systems (a subject that is closely related to this one). Thanks for everones participation in the Forum.

 

[Damperman]

russmech beat me to it but he is absoutely correct. The Taylor article is one of the best available on return vs relief fans. Speaking of minimum outide air . . .the April 2002 ASHRAE Journal has a pretty good article by Larry Felker. And finally, check out the Kele Controls web site for VAV articles by Gil Avery.

 

[ChasBean1]

Economizer control can be awful w/the relief fan setup. I just looked at Taylor's article and he does note that as a disadvantage and has a good description why. This is an excellent article overall, although a couple of statements I'd question, such as a return fan needing to be close to its associated supply, potential balancing issues with relief fan system not noted, etc.