If it is not too much to ask, would you care to elaborate some on these calculations?
/ Dice
Sure. Two equations you need:
First - 1W = 3.412BTU/hr.
Second - CFM = BTU/hr. / (Tout-Tin * 1.085) (temps in Fahrenheit)
So, a 500 watt server is 1,706BTU/hr
CFM = 1706 / (85-75 * 1.085) = 157.24CFM for a 10-degree rise
CFM = 1706 / (100-75 * 1.085) = 62.89CFM for a 25-degree rise
The important thing to remember is this basically models things assuming perfect airflow. Temperature rise can be unlimited (well, until the structure catches fire, but I digress) - ASSUMING all of the devices are ingesting the intake air and the airflow isn't being "short-circuited", allowing warm air to make it back to the intake. Think of it like so:
Intake air --> Heat generation --> Exhaust fan --> Discharge
Things go to crap when this happens:
Intake air --> Heat generation --> Exhaust fan --> Most of the discharge air
|<- Hot air <-|
The secret is making sure that your airflow isn't short-circuited. In my case, I have a fairly small, 3x4x8 closet. The servers exhaust to the rear, and the exhaust fan is located toward the back of the closet, so air should travel up the back wall and out. Now, this house is still getting remodeled, so I've only tried this out with a limited heat load. I'm hoping, in the next month or two, to have everything up and running so I know for sure.
I'm also not accounting for static pressure here. Most of these exhaust fans have a huge loss of flow when they are presented with any substantial flow restriction. You need to make sure plenty of intake air is available, with minimal restriction.
Finally, this doesn't account for heat loss through any other method. You will lose some heat (not much, most likely) through the walls, etc.
If my plan doesn't work out, the next step will be building a "frame" on the front of the rack (only 25U tall) that seals to the front of the rack and to the louvered closet door (when closed), ensuring that the servers all ingest air through the door, and no hot air gets reingested.
If you're working with a small space, you also need to ensure your exhaust fans will run on alternative power (UPS, generator, whatever) for at least as long as it takes the servers to shut down. If not, things are going to get *hot*, very quickly. If you can seal the intakes to prevent hot air from getting reingested, it won't be as bad... but then you have to deal with heat soak, etc. Suffice it to say, you better keep your exhaust fans running until everything's off.
If there are any civil engineer types in here (I'm but a lowly computer engineer), they're probably cringing at my math (I have taken a few shortcuts and overlooked some more esoteric concerns), but this is at least a decent guideline.