MadSci Network: Computer Science

Re: How do I calculate how much heat computer components generate.

Date: Sat Dec 29 15:39:26 2001
Posted By: Joseph Weeks, President
Area of science: Computer Science
ID: 1008749947.Cs

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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