Thermodynamic calculations for a server room
Hi all. Thanks in advance for looking; this forum is a great resource! I operate several small server rooms and I'm looking for guidance as I try to perform a calculation that I haven't seen referenced anywhere (yet).
Right now I have three water-cooled heat pumps in this particular room which are 2.5 tons each. They're placed at the end of my server room, which is rectangular and approximately 25' long and 16' wide. The ceiling height is kind of hard to estimate. At its highest point it's about 11', the bottom of the steel beams are 10', and there are all kinds of impediments to airflow (racks, servers, poles for the ladder rack, the ladder racks themselves, fiber trays, steel electrical and fiber conduits, water pipes, etc). The 2.5 ton units are upflow and the nameplate says that the blower motors are 0.25HP (is that right??). The heat pumps are capable of getting down to 69 degrees right now, using a cooling cycle of approximately 5 minutes in duration. The thermostats are set to kick off for a mandatory 5 minutes before re-engaging. So when they cycle on again after the 5 minute timer, they're usually at 75 degrees, sometimes 76. Other vital information about this room - it has approximately 135 amps of AC power in use by the equipment, along with another 8 overhead fluorescent lights (they're the kind with two bulbs each, and look to be 4ft. long each). Under typical conditions, only one person is present in the room at any given time. Also maybe of potential interest is that the ambient temperature outside of the server room is typically 76 degrees.
What I'm trying to determine is how many more watts of heat I can add in the immediate proximity of the ACs (i..e within 4 or 5 feet) before they can no longer cool within reasonable parameters. When I say reasonable, I mean a reasonable duty cycle on the compressors, whatever that may be. Perhaps 20 minutes on, 5 minutes off? How much heat can I add and still reach perhaps 73 degrees at the thermostat after a 20 minute cooling cycle? What about 74 degrees? Or 75? What are these calculations called?
Let me preface this also by stating that I already have a handle on the basic watts to BTU/hr conversion, etc. As such, my calculations tell me that these ACs can remove 26.390KW worth of heat (90000 BTU/hr * 0.293 watts per BTU/hr) - I don't know how practical such a calculation is, or at what point i'll run out of cooling short of the 26.390KW figure. When it comes to the air-flow part, I know less than nothing, so any help is appreciated. Thanks again!