adding floor fans to a ac

May 19, 2009 5:35 PM

Can a 3ton ac intake be incresed by adding floor fans to the intake?

We have a 3ton ac for one of our server rooms that really needs more cooling/ more heat taken out of the room. One of my coworkers believes that by strapping two 6500 BTU floor fans to the hot air return of the ac the 3 ton unit will in essence be supercharged and will pull even more hot air out of the room. This does not make sense to me, and if anything I believe it will slow the return down, as the fans will cover the entire opening of the plenum. I will fully admit that I don't know much about thermodynamics. But I do know that even if it does work, strapping two 6500 BTU fans to a 3 ton unit won't be doing much.

posted by brent_h to technology (6 comments total)

Having dealt with the same situation for quite a long while at work, I am going to suggest that the only way fans will help the situation at all is if you arrange them to blow cool air through your server cabinets/racks as a temporary measure.

The room will not get any cooler by doing so, but you may be able to get the servers closer to room temperature (e.g. the servers themselves will be cooler), which is all that matters. We have a bunch of them attached to our server cabinets with zip ties, which looks ridiculous, but works, until we can get more cooling installed.

I am confused by the reference to "BTU" for a fan, though. Since a fan doesn't provide any cooling at all, only air flow, it doesn't make sense to rate them in terms of heat removal (e.g. BTU/hour). Usually fans would be rated in cubic feet per minute (CFM), is that what these are? Or are they not "fans" but portable cooling units?
posted by FishBike at 5:42 PM on May 19

I probably should have phrased the entire question as CFM as what I interested in is how much air the ac can pull out of the room on return with the fans help vs without it. These are just floor fans and not portable cooling units.
posted by brent_h at 6:04 PM on May 19

Computer room air conditioners usually have relatively high airflow rates. For a 3-ton unit, something around 2000 CFM would be typical--the one we have does 1800 CFM. So it might seem a pair of 6500 CFM fans would boost the airflow a lot. But they won't because they only move 6500 CFM when there aren't restrictions on the airflow. These floor fans produce essentially zero flow if there's any back-pressure at all. For example we couldn't get them to blow even 1 CFM through our perforated metal cabinet doors and had to take the doors right off.

The air conditioner will be quite restrictive with its filters and cooling coils. It has likely a 3/4 or 1 horsepower blower motor inside running a fairly high pressure blower, not a propeller style fan like these floor fans. So I predict the floor fans will have essentially no effect on the airflow through the AC unit.

But let's assume for a moment that the fans do boost the airflow. This still won't significantly increase the cooling capacity of the AC unit. More air will go through it, but the temperature drop will be reduced--you'll get more air out, but it'll be warmer. As long as the air flow is sufficient already (which it surely is as long as the filters and coils aren't clogged with dirt) there is hardly anything to be gained from further increases in airflow. You will also have the additional heat from the fan motors themselves to get rid of.

I realize there isn't a lot of HVAC theory or math in my answer. This is just the gut feel from a guy who has been dealing with exactly this problem for way longer than I should have (grin) and who has tried this exact solution just in case it would help. It made no difference, but blowing more air across the servers with extra fans helped quite alot.
posted by FishBike at 6:36 PM on May 19

a 3 ton unit is something like 10,000 watts. Are your servers really putting out that much heat? Or, what is the problem you are trying to solve- if the A/C unit isn't pulling out the right amount of heat, in theory, the room would keep getting hotter until you got to some kind of equilibrium. Probably well over 100 degrees. If it's not wildly uncomfortable in the room and your servers aren't hot to the touch, you are probably fine.

No, it probably won't work, BTW. Unless you have an airflow issue inside the room.

Frankly, I would check into the health of the AC system. Make sure its coils are clean and its filters are changed and the freon charge is correct.
posted by gjc at 8:00 PM on May 19

if the A/C unit isn't pulling out the right amount of heat, in theory, the room would keep getting hotter until you got to some kind of equilibrium. Probably well over 100 degrees.

I once wondered what would happen if the heat load in our server room were to slightly exceed the rated capacity of our air conditioner. Would it cause the room to slowly heat up indefinitely (or at least until stuff started burning out/tripping breakers)?

It turns out it doesn't quite work that way. The rated capacity of the air conditioner is based on certain assumptions, including the temperature difference between the heat source (room) and heat sink (water or air cooling the condenser). As the room heats up, the capacity of the air conditioner increases slightly too. So it's not quite the thermal runaway situation one might expect. But it's a pretty steep slope--I can reliably predict that each new server we add to our room will raise the temperature another 1 deg. C.

So yes the room does heat up until it reaches an equilibrium, but that isn't necessarily at a super-high temperature. We have of course stopped adding new equipment to our room, not wanting to continue this experiment until we do find a point where the temperature runs away.

Still, it's a good idea to have the unit checked, because if it isn't working up to full capacity, the same effect will be seen, just at a heat load that should normally be within its capacity. However, our computer room has a bit more than 10,000 watts heat load, and a bit more than 3 tons cooling capacity, and it's too warm. So it's all adding up to "not enough cooling capacity".
posted by FishBike at 10:44 AM on May 20

Oops, I just realized the asker didn't say anything about heat load, the 10,000 watts figure came from gjc's answer. So to the original asker of this question -- if you know your electrical load for your server room, and it's significantly less than 10,000 watts, you're probably not getting your 3 tons of cooling.
posted by FishBike at 10:45 AM on May 20