Heat Transfer Fluids

It is prudent at this stage to briefly consider the problems that can be expe­rienced in either refrigeration or heat recovery systems when water treatment is required to prevent freezing. The antifreeze treatment of pure water may be achieved by various means, typical ones being various brines, ethylene glycol, and propylene glycol.

In the treating of water by any of these methods, it must be remembered that due to property changes, they can cause problems on both the heat transfer characteristics and fluid flow characteristics compared with pure water.

Many proprietary trade heat transfer fluids are in common use. Depending on operating temperatures, typical characteristics are

Temperature

(°C)

Solution

Solution by weight

Density {kg nr3}

Specific heat

(kjkg-‘oc-1)

Thermal

Conductivity

Wml0C

Viscosity (pa s x 10-3)

Freezing

Point

(°C)

Boiling

Point

<°C)

-1

Sodium chloride

12

1092.6

3.6

0.485

2.2

-8.5

102.0

Calcium chloride

12

1109.0

3.47

0.57

2.4

-7.2

101.0

Methanol water

15

985.3

4.187

0.485

3.2

-10.3

86.0

Ethylene water

20

977.2

4.35

0.47

5.3

-11.1

87.2

Ethylene glycol

25

1036.5

3.85

0.52

3.7

-10.7

102.8

Propylene glycol

30

1033.3

3.93

0.45

8.0

-10.6

102.2

-10

Sodium chloride

21

1166.3

3.35

0.43

4.2

-17.2

102.2

Calcium chloride

20

1198.3

3.0

0.54

4.8

-17.2

101.1

Methanol water

22

967.6

4.06

0.45

5.3

-15.5

83.3

Ethylene water

25

977.2

4.27

0.43

8.3

-15.5

86.1

Ethylene glycol

35

1057.3

3.60

0.48

6.8

-17.8

103.9

Propylene glycol

40

1046.0

3.73

0.41

20.0

-4.2

103.3

Water at

999.87

4.2174

0.569

1.792

0

100

(0°C)

• Dynamic viscosity of 0.9 x 10"3 to 1.2 x 10 3 Pa s

• Flash points from 108 to 263 °C

The influences these factors have on fluid flow in the tubes are as follows:

• What may be turbulent flow in the heat exchanger for water will reduce to transitional or laminar flow for the heat transfer fluid, reducing the coefficient of heat transfer to a value 70% or more of that for water.

• Viscosity changes make conventional water pipe sizing tables useless; these must be upgraded by the application of appropriate correction factors.

• Density changes.

• Specific heat capacity changes.

• Thermal conductivity changes.

Table 4.15 gives typical properties of some fluids. For other fluids, contact the manufacturer for exact data. With heat transfer oils, care must be taken that chemical changes such as carbonization do not take place.

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