Glycol Heat-Transfer Fluids
Ethylene Glycol versus
Propylene Glycol
Water is probably the most efficent heat-transfer fluid known. If it did not freeze, water
would be the ideal heat-transfer fluid for cooling applications. When freeze conditions
exist (<35 F), ethylene glycol and propylene glycol can be added to water to provide
freeze protection and burst protection. Both glycols have lower heat-transfer efficiencies
than water and are more dense, resulting in higher volumetric flowrates or heat-exchange
areas required to maintain the same temperature levels (see Tables 1 and 2). Higher
flowrates lead to higher pressure drops, energy consumption, and equipment wear. As a
result, it is important to accurately determine the minimal concentration of glycol needed
to do the job in order to maintain system efficiency.
Between the two, ethylene glycol (C
2
H
6
O
2
) is a better heat transfer fluid than propylene
glycol (C
3
H
8
O
2
). Propylene glycol is less toxic and is considered when toxicity is a
concern.
Table 1 - Ethylene Glycol Versus Propylene Glycol Thermal Conductivities
Temperature
(F)
Ethylene Glycol Thermal
Conductivity [Btu/(hrft^2)(F/ft)]
at 30% Volume
Propylene Glycol Thermal
Conductivity [Btu/(hr
ft^2)(F/ft)] at 30% Volume
10 0.238 0.235
20 0.243 0.239
30 0.247 0.243
40 0.251 0.247
50 0.255 0.251
60 0.259 0.254
70 0.263 0.258
80 0.266 0.261
Table 2 - Flow Increases Necessary to Achieve Same Heat Transfer as Pure Water
Percent Solution at 50F Ethylene Glycol Volume
Flow Increase vs. Water
Propylene Glycol Volume
Flow Increase vs. Water
0 1.00 1.00
10 1.020 1.008
20 1.050 1.014
30 1.090 1.043
40 1.140 1.075
50 1.210 1.132
