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Comparison of Ethylene Glycol and Propylene Glycol as Heat-Transfer Fluids, Study notes of Biochemistry

This document compares the use of ethylene glycol and propylene glycol as heat-transfer fluids, discussing their thermal conductivities, flow increases necessary to achieve the same heat transfer as pure water, and their respective advantages and disadvantages. It also covers water quality, freeze protection, burst protection, corrosion, and system monitoring.

Typology: Study notes

2021/2022

Uploaded on 09/27/2022

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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
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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

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

Water Quality: High quality water will help maintain system efficiency and prolong

glycol fluid life. Recommended water characteristics include:

 Less than 50 ppm calcium (as CaCO 3 ),

 Less than 50 ppm magnesium (as CaCO 3 ),

 Less than 100 ppm total hardness (as CaCO 3 ),

 Less than 25 ppm chloride (as CaCO 3 ), and

 Less than 25 ppm sulfate (as CaCO 3 ).

Freeze Protection Versus Burst Protection: Water volume expands by 9% when

frozen. Glycols depress water’s freezing point providing protection to temperatures as

low as -70 F to -100 F.

Freeze protection prevents ice crystal formation at the lowest temperature expected in the

coolant circuit. This type of protection is necessary for year-round pumping. Continuous

pumping will also prevent freezing but is costly and risky because of possible power

failures.

Burst protection requires less glycol and allows some freezing to turn the coolant into a

slush that is not easily pumped, but will not cause the pipe to burst. This method is used

in closed circuits that are not operated in cold weather.

Corrosion: All glycols produce acids in the presence of air (oxidants). The acids can

reduce pH and cause corrosion. When the system pH drops below 7, rust will form on

any ferrous metal, and nonferrous metals start to corrode. For HVAC applications,

glycols are formulated with passivating and buffering corrosion inhibitors to counteract

acids formed by the oxidation of glycols.

System Monitoring: Glycols can typically be expected to last 12 years or longer,

providing corrosion inhibitor strength is maintained. Inhibitor analysis is usually offered

as a free service by glycol manufacturers.

Glycol fluid pH can be a good barometer for the condition of the glycol. Although the

pH is primarily a function of the corrosion inhibitor and, therefore, will vary from

product to product, a few rules of thumb are helpful in determining what constitutes

proper pH.

Most concentrated inhibited glycols have a pH in the range of 9.0 to 9.5. A pH reading

below 8.0 indicates that a significant portion of the inhibitor has been depleted and that

more inhibitor needs to be added.

When the pH falls below 7.0, most manufacturers recommend replacing the fluid. A pH

value of less than 7.0 indicates that oxidation of the glycol has occurred. The system

should then be drained and flushed before severe damage occurs.

ETHYLENE GLYCOL

Typical Physical Properties of Aqueous Solutions

Volume % Weight % Freeze Point Burst Protection Boiling Point

PROPYLENE GLYCOL

Typical Physical Properties of Aqueous Solutions Volume % Weight % Freeze Point Burst Protection Boiling Point

  • 0 0 32.0 0.0 32.0 0.0 212.0 100. F C F C F C
  • 10 11.1 24.2 -4.3 20.0 -5.0 212.6 100.
  • 20 22.0 14.9 -9.5 5.0 -15.0 215.1 101.
  • 25 27.3 9.3 -12.6 -5.0 -20.0 216.7 102.
  • 26 28.4 8.1 -13.3 -10.0 -20.0 217.0 102.
  • 27 29.5 6.9 -13.9 -10.0 -20.0 217.3 102.
  • 28 30.5 5.7 -14.6 -10.0 -25.0 217.6 103.
  • 29 31.6 4.4 -15.4 -15.0 -25.0 217.9 103.
  • 30 32.6 3.0 16.1 -15.0 -25.0 218.2 103.
  • 31 33.7 1.6 -16.9 -20.0 -25.0 218.5 103.
  • 32 34.7 0.2 -17.7 -20.0 -25.0 218.9 103.
  • 33 35.8 -1.2 -18.5 -20.0 -30.0 219.2 103.
  • 34 36.8 -2.8 -19.3 -25.0 -30.0 219.5 104.
  • 35 37.8 -4.3 -20.2 -30.0 -30.0 219.8 104.
  • 36 38.9 -6.0 -21.1 -35.0 -35.0 220.1 104.
  • 37 39.9 -7.6 -22.0 -40.0 -40.0 220.4 104.
  • 38 40.9 -9.4 -23.0 -45.0 -40.0 220.8 104.
  • 39 42.0 -11.2 -24.0 -55.0 -45.0 221.1 105.
  • 40 43.0 -13.1 -25.0 -65.0 -55.0 221.4 105.
  • 41 44.0 -15.0 -26.1 -75.0 -60.0 221.7 105.
  • 42 45.0 -17.0 -27.2 -90.0 -65.0 222.1 105.
  • 43 46.1 -19.1 -28.4 -100.0 -75.0 222.4 105.
  • 44 47.1 -21.3 -29.6 <-100 <-75 222.7 105.
  • 45 48.1 -23.5 -30.9 <-100 <-75 223.1 106.
  • 46 49.1 -25.9 -32.2 <-100 <-75 223.5 106.
  • 47 50.1 -28.3 -33.5 <-100 <-75 223.9 106.
  • 48 51.1 -30.8 -34.9 <-100 <-75 224.2 106.
  • 49 52.1 -33.5 -36.4 <-100 <-75 224.6 106.
  • 50 53.1 -36.2 -37.9 <-100 <-75 225.1 107.
  • 51 54.1 -39.1 -39.5 <-100 <-75 225.5 107.
  • 52 55.1 -42.0 -41.1 <-100 <-75 226.0 107.
  • 53 56.1 -45.1 -42.8 <-100 <-75 226.4 107.
  • 54 57.1 -48.3 -44.6 <-100 <-75 226.9 108.
  • 55 58.1 -51.6 -46.5 <-100 <-75 227.4 108.
  • 56 59.1 -55.1 -48.4 <-100 <-75 228.0 108.
  • 57 60.1 -58.7 -50.4 <-100 <-75 228.6 109.
  • 58 61.0 -62.4 -52.4 <-100 <-75 229.2 109.
  • 59 62.0 -66.3 -54.6 <-100 <-75 229.8 109.
  • 60 63.0 -70.3 -56.8 <-100 <-75 230.5 110.
  • 61 64.0 <-70 <-60 <-100 <-75 231.2 110.
  • 62 64.9 <-70 <-60 <-100 <-75 232.0 110.
  • 63 65.9 <-70 <-60 <-100 <-75 232.8 111.
  • 64 66.9 <-70 <-60 <-100 <-75 233.6 111.
  • 65 67.8 <-70 <-60 <-100 <-75 234.5 112.
  • 70 72.6 NA NA NA NA 239.9 115.
  • 80 82.0 NA NA NA NA 256.4 124.
  • 90 91.1 NA NA NA NA 284.0 139.
  • 100 100.0 -12.3 -24.6 NA NA 327.7 164.
  • 0 0.0 32.0 0.0 32.0 0.0 212.0 100. F C F C F C
  • 10 10.4 28.4 -2.0 20.0 -7.
  • 16 16.5 22.3 -5.4 10.0 -12.
  • 20 20.6 20.0 -6.6 214.0 100.
  • 21 21.6 19.0 -7.
  • 22 22.6 18.0 -7.7 0 -
  • 23 23.6 17.0 -8.
  • 24 24.5 16.0 -8.8 215 100.
  • 25 25.5 15.0 -9.
  • 26 26.5 14.0 -9.
  • 27 27.4 13.0 -10.
  • 28 28.4 12.0 -11.
  • 29 29.4 11.0 -11.6 -10 -23 216 101.
  • 30 30.3 9.0 -12.
  • 31 31.3 8.0 -13.
  • 32 32.3 7.0 -13.
  • 33 33.3 5.0 -14.
  • 34 34.3 4.0 -15.4 217 101.
  • 35 35.3 2.0 -16.5 -20 -
  • 36 36.2 1.0 -17.
  • 37 37.2 -1.0 -18.
  • 38 38.2 -3.0 -19.3 218 102.
  • 39 39.2 -4.0 -19.8 -30 -
  • 40 40.2 -6.0 -20.
  • 41 41.2 -8.0 -22.0 -40 -40 219 102.
  • 42 42.2 -10.0 -23.1 -50 -
  • 43 43.2 -12.0 -24.
  • 44 44.1 -14.0 -25.3 -60 -51 220 103.
  • 45 45.1 -16.0 -26.
  • 46 46.1 -18.0 -27.
  • 47 47.1 -20.0 -28.6 221 103.
  • 48 48.0 -22.0 -29.
  • 49 49.0 -25.0 -31.
  • 50 50.0 -27 -32.5 222 104.
  • 51 51.0 -29 -33.
  • 52 52.0 -32 -35.2 223 105.
  • 53 53.0 -34 -36.
  • 54 54.0 -36 -37.4 224 105.
  • 55 55.0 -39 -39.
  • 56 56.0 -41 -40.2 225 106.
  • 57 57.0 -44 -41.
  • 58 58.0 -47 -43.5 226 106.
  • 59 59.0 -50 -45.
  • 60 60.0 -53 -46.8 227 107.
  • 62 62.1 NA NA 228 107. 61 61.0 NA NA
  • 64 64.1 NA NA 229 108. 63 63.1 NA NA
  • 65 65.1 NA NA 230 108.
  • 100 100 -60 -51