Computer Room HVAC System Design

It works to your advantage when electronic racks in a computer lab are arranged in a fashion where they take air from one end and discharge heated air through the other side--you get a hot aisle cold aisle arrangement. You provide cold (conditioned) air in the cold aisle. The cooling fans of the equipment suck up that air, air gets heated by electrical load dissipated as heat by the equipment, gets discharged from the other end, and ends up in the hot aisle. You should have your return registers in the hot aisle to gather this air and take it back to the air conditioning unit. With this hot aisle cold aisle arrangement you not only resolve the hot-spot issue (hot air does not get mixed with the cold air while traveling back to the air conditioning unit), but you also get better performance out of the air conditioning equipment (as you increase the coil entering air temperature--providing a higher delta T for better heat transfer). A desirable computer room air conditioning system would have bottom supply (through the floor) or top supply (from the ceiling) in the cold aisle, and top return in the hot aisle.


I am currently designing a cooling and ventilation system for a server lab. The lab has six existing Compu-Aire units (CAC-2034). The room has ducted supply but free (non-ducted) return to the units--units have return air grilles in their bottom front panels. Since the owner is interested in having a hot aisle cold aisle arrangement I was thinking of making return air hard connection to each unit. Compu-Aire Engineering folks in Whittier told me the only place to connect return air to those existing units would be in the back of the units (connection to front would block the front control panel, and there is not enough room in the side to take all that return air). They said they can supply accessories needed to convert the free return air units to ducted return units (a solid panel to go in the front, and back panel with filters and flange for return air connection)--the conversion can be done in the field, by the mechanical contractor. The problem is, I don't have room to move the units out of their niches where they are presently butting the wall. So, I am designing for return air ducts that would bring the air from hot aisles and leave it in front of the unit without making a hard connection to the unit. Although this return air scheme would work without a fan, I want to be sure about the air movement through that route and am specifying inline axial fans for these return ducts (my fear being that if there is too much empty space between the racks the air would find the path of least resistance through the cold aisles to come back to the units). Since overhead space is tight I am not combining return air ducts through lateral connections. It would be many return ducts (probably ten) bringing hot air to the units.

Do you have any comments? I would love to hear from you.