Always wondered what Data Centers and Swimming Pools have in common?

When asked to describe how the airflow should work best in a raised floor data center the best analogy I am able to come up with is a swimming pool. Bear with me…

In today’s Data Centers the rows of server racks are frequently arranged in a hot aisle cold aisle system, if not, they should. For non Data Center folks see at the end for a quick explanation what that is. Cold air comes from the bottom and likes to stay low. It’s like water in a pool. Incoming air needs to match or being slightly oversupplied to the air used through the servers. That’s like returnflow perforations in a pool. But to get the cold air all the way up to the edge of the racks the cold aisle cannot have big leaks. Biggest leaks are:

- the front and back entrance of the aisle
- voids of racks in the aisle due to building columns, etc.
- openings in the racks, unused U’s without blanking panels.
- voids in the racks between mounting rail and side panel

What is not a leak is the top, as long as air supply is matched and like in a pool with a little overflow. In fact that overflow can be used for closed loop control of air supply. One way to achieve balanced airflow is to fill aisles sequentially until the server air intake matches the supply capacity. Vent shutters can help to reduce flow where needed.

So if all voids are closed the air will fill the cold aisle like water in a swimming pool. As simple as that. Next time you walk into the Data Center just have that visual in mind.

There are simple ways to get these leaks closed, fire proof “meat locker drapery” for the ends of the aisle, or a simple door as an example… doesn’t have to be complicated, as long as it meets your fire code. And having an open top of the row helps you with that.

Using the rest of the building as the hot aisle has a nice savings effect too. In most parts of the world it’s rare that outside temperatures are hotter than the exhaust of the servers, therefore what used to be side load for the air-conditioner is now free cooling for most of the year.

Now voids in the racks between mounting rails and side panel are also a thermal short circuit in the rack, amplified by the doors. The fans in the servers will create with the perforation of the front door suction and the with the back door perforation pressure. So instead of all server air coming in and out the doors, the pressure differential will drive circulating air within the rack. Easy test, try to get your hand between the mounting rail and the side panel, if you can, you have a problem.

Why is this all important? Inside Air-conditioning equipment has simplistically three variables describing its ability of removing heat (assuming fixed flow of coolant or compressor capacity):

- effective length/surface of coils (typically fixed)
- air flow (can be adjusted to a certain degree)
- temperature differential

Another important factor determining the efficiency is the temperature the cooling coil has to operate, warmer is better.

Without separating the cold and hot air the temperature differential between hot and cold air is not really high, a lot of back mixing happens. As a consequence it isn’t uncommon that the factory rating of air-conditioning equipment isn’t attainable because the warm air isn’t warm enough for the unit to extract the amount of heat it was rated for. Sometimes only a fraction of the recommended rated temperature differential is achieved across the coils resulting in similar loss of capacity. Ironically the Servers generate a very significant temperature differential and would make the air-conditioning coils very effective, but because the hot and cold air back mix in the room, it’s lost.

If cold air is prevented from mixing with the hot air, automatically a higher coil temperature differential can be achieved. That’s it. It gets you more cooling rating into your Data Center (which you thought you already had and paid for!) and lowers your electricity bill. It’s impossible to give a universal number of improvements, but I wouldn’t be surprised to see several percent points, even low double digit dependent from where you start.

Hot Aisle Cold Aisle:
In the cold aisle all servers have their front facing the aisle. If you walk through the cold aisle looking left and right you see the front panels of the servers. That is where the servers pull in their cooling air, therefore that is where the cold air supply is routed in. Conversely the back end of the servers face the hot aisle, that is where the air exits the servers taking the heat out of them. That is where the hot air is collected to go back to the air-conditioner. Why is this done? If the Servers would be arranged front/back front/back, the hot air from one server would blow into the front of the next server.