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I have a fairly large server farm running in an warehouse.  I'm looking for someone to help me design a good air flow solution.

If you're an HVAC specialist, drop me a PM.  I could use your help!

Not an expert by any means, but I would love to see pics of your setup. And, if you are near by me (probably not), I could sell some used datacenter A/C units. (4 stage dual compressor 10 ton Liebert CRACs)

I'm in the Seattle area.  Where are you located?  I'll try to get some pics tomorrow.  Right now I have about five racks, each of which could easily be moved for optimal airflow.

Heh, Ohio, like I said "probably not" ;D
If it is all miners, that is more heat than an average server rack, but if it has "normal" servers as well, then it is easier to cool (in general). It sounds like you already have it running, and you just want to optimize it a bit, is that right?

Yup, Ohio isn't down the street ;)

It's almost all miners.  They are open air rigs with two to a shelf x 5 shelves in a rack.  Here's a picture of one being assembled as an example:

https://i.imgur.com/PBS7zl.jpg

The space itself is fairly square - 25' by about 30'.  Because the outside air is usually quite cool (in the upper 40s here now) I don't have A/C.  I just have a single 2300 CFM exhaust fan and a 16" fresh air supply inlet on the other side of the unit.  By my calculations I'm creating just over 200,000 btu once all of my new rigs are up and running.  So my current thinking is to put in a 24" or 36" exhaust fan for up to 11,000 CFM (like this one: http://www.jdmfg.com/index.cfm?event=pageview&contentPieceID=6977)  and then add a couple more intake vents.

Then I need to figure out how to arrange the racks.  I'm thinking of something like below, but I am worried that the airflow won't work this neatly.  Mostly I just want to avoid having rigs sucking in hot air from another rig/rack.

https://i.imgur.com/jQ2sll.png

One thing I do know is that you are going to need a slight positive pressure with your airflow. Right now, you have only an exhaust fan, which creates negative airflow, which can result in hot spots.

You must balance the intake and exhaust so that the intake CFM is slightly higher than the exhaust CFM (but not by too much). This allows for airflow to be better distributed around the room. A small-scale example of this is a standard computer case.

Another thing to consider is possibly to put the racks at a slight angle, maybe 30 degrees and offset so that the exhaust from the rigs closest to the intakes don't cook the adjacent rigs on the same shelf.

Seems the last row (nearest to the exhaust side) will be cooked easily by the previous rows. You'll need to put them at an angle like what rjk said, but for different shelves too.
edit: maybe put them angled towards the exhaust like 20-30 degrees? that'll make sure the exhaust gases from the cards converge towards the middle, and get pushed by the fans in middle.

I've thought about the positive pressure. Is there any way to measure that?  I can try to take a guess but there are leaks of air from the roll up door and other places, so I can't be too precise.

Are you thinking like in a fish bone pattern?  It would seem to me that you would want it to open up wider near the exhaust end to disperse the heat.  If you had it get narrower, then it seems like it would make a funnel of really hot air

Yep, kind of like in a fishbone pattern especially when there's only one exhaust channel. The main reason why I'm suggesting this is because the heat from the first row would get blown over the second, and so on if you arranged them without angling. Hence, the cards in each row will have different temps and giving you a much bigger headache when trying to find a stable oc configuration.

I had two rows of my cards like below (sorry for the failed ascii art  :P)

(open window) (cards on motherboard 1) (cards on MB 2)
|                             |------|              |-------|
|<--exhaust direction  |------|              |-------|
|                             |------|              |-------|

and the cards on MB1 went regularly higher by 7-8*c until I shifted both mb's in the fishbone pattern.

Yeah, I could definitely see how the way you had it set up initially would let hot air from one rig go into another rig.

I'll give the fish bone pattern a try.  Hopefully it will work well because moving those racks is a HUGE pain  :P

Yeah fishbone was what I was thinking of, and your in-row fans are also good. It's somewhat difficult to measure the pressure without decent tools, but the balancing doesn't have to be extremely precise. If we assume your exhaust fan is 2300CFM, then the 2 intake fans could be 1500CFM each, they don't have to be exactly 1150 or 1200.

If you use a fishbone pattern, it might be necessary to have the racks closer to the exhaust spread out farther in a V pattern, if you have problems with heating up. A little experimentation is in order, I think, so you might want to get some movers' dollies and stick the racks on them. ;)

When you open the door a crack standing outside you will feel the air blow out through the gap. If you want to measure it use a magnehelic gauge.

I would be blowing air up through the racks from underneath. Have the intake low and the exhaust on the ceiling.

Considering in the number of fans you have it may be more cost effective power-wise to install a small split system. 4-5 standard fans prob equal a 1.5kw split system.