Cooling Media Physics

The table below computes the energy to move sufficient air, water and refrigerant around a system to cool 12,000 BTU. As can be seen, air requires ten times the energy to move the same amount of heat as refrigerant. In practice, as air is rarely well controlled, an air mover is required in the server as well as in the computer room air handler (CRAH) so the effective power requirement is 20 times that of refrigerant. Refrigerant is normally piped around the system so only one mover is required.

Load in BTU
12000
12000
12000
Air (std day)
Water @ 50F
R134a @ 95F
Units
Density
0.06
62.45
72.94
lb/cu-ft
Specific Heat
0.24
1.00
72.00
BTU/lb

States

gas-gas
liquid-liquid
liquid-gas 30%
Delta T
18
9
0
deg F
Flow rate wt.
46.20
22.27
3.97
lb/min/ton
Flow rate Vol
739.69
0.36
0.13
CFM/ ton
Static press
0.04
35.00
20.00
PSI
Power Required
87.00
40.63
8.27
Watts
Fan/ Pump Efficiency
30%
30%
30%
Total W
290
135
28
Watts
% of load
8.3%
3.9%
0.8%

Note that refrigerant uses a partial phase change to capture the heat from the hot components, presenting the same coolant temperature across the whole of the cold plate. Air and water on the other hand, get progressively warmer. Downstream components are thus in danger of becoming overheated unless the entering fluid is cooled further.

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