|MadSci Network: Computer Science|
Traditionally air conditioning capacity has been measured in units of BTUs per hour or tons of air conditioning. I think you are on the right track; you start by adding up all of the watts of electricity that your equipment is consuming. Then all you need are a couple of conversion factors. Lets say that you have three servers, each consuming 300 watts of electricity, and a monitor producing 100 watts of heat. So the heat load from the electrical equipment is 1000 watts total. Now it is true that a few watts of power exits the room in the wires, but essentially all of the electric power that the equipment uses is converted to heat. If you didn't want to use the equipment nameplate rating for determining the heating load, you could use a clamp on amp meter to measure the electric current actually being used by your equipment (but it is probably not worth the effort, since the extra cooling capacity probably won't cost very much). The Handbook of Chemistry and Physics lists conversion factors; many calculators also have the ability to convert watts to BTUs. One watt of energy equals 3.412 BTUs per hour. So if you equipment is generating 1000 watts of heat, it is putting off 3412 BTUs of heat per hour. If a person works in the room, they will contribute 400 BTUs per hour to the cooling load. So 3412 BTUs/hr plus one person @ 400 BTUs equals about 3800 BTUs per hour. If this is an outside room, you may also have an additional heat load, particularly through a sunny window. If you tell the air conditioning company the size of the window and the direction that it faces, they can give you an estimate of the heat gain through the window. Small room air conditioners that fit in windows typically will have cooling capacities ranging from 5,000 to perhaps 10,000 BTUs per hour. These type of air conditioners are traditionally rated in terms of BTUs per hour. Generally there is not much of a price difference between a 5,000 to 7,000 BTU/hr unit. The advantage of not getting a unit too big is that a smaller unit will run longer at a time, removing more humidity and potentially being a bit more efficient. These small window units will not work in an interior office, since there is no place for them to discharge the heat that they have removed from the space; in an office with an outside window, they are an inexpensive solution to your problem. If, on the other hand, you have a larger heat load, the larger air conditioners are rated in units of tons of cooling. Larger air conditioners typically come in two parts; the condenser/compressor unit that goes outside, and the evaporator/fan unit that goes within the cooled space. One ton of cooling equals 12,000 BTUs per hour. A typical house might require a 2.5 to 3 ton air conditioner. Of course, heat units such as BTUs and tons are all related to water. A British Thermal Unit (BTU) is the amount of energy used to increase the temperature of one pound of water by one degree F (just as a calorie is the amount of energy needed to heat one gram of water one degree C). A ton of air conditioning is nominally the amount of heat removed by passing air over one ton of ice. Good luck with your air conditioning purchase. May your servers run cool.
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