Refrigerants

The comparative performance of refrigerants commonly used in the vapour compression cycle is given in Table 9.6, the values of which were all obtained by calculation, using the methods of this chapter. Actual operating conditions will therefore be somewhat different, owing to the effects of the factors mentioned in section 9.2.

In general, it has been assumed that the vapour enters the compressor in a saturated condition at 5°C. In refrigerants 113 and 114, however, where saturated suction gas would result in condensation during compression, enough superheat has been assumed to ensure saturated discharge gas. This superheat has not been counted as part of the refrigerating effect.

A condensing temperature of 40°C has been taken for all refrigerants.

The performances have been calculated on the assumption that compression is isentropic.

A number of factors have to be taken into consideration when the thermodynamic characteristics listed in the table are being evaluated. For example, if operating pressures are high, then the materials comprising the refrigeration system will be heavy and the equipment expensive. The refrigerant containers will also be heavy, and this will increase the cost of transport. Sub-atmospheric operating pressures, on the other hand, mean that any leakage will result in air entering the system.

Refrigerant

Number

Suction

Temp.

(°C)

Evaporating

Pressure

(bar)

Condensing

Pressure

(bar)

Com­

Pression

Ratio

Refrigerating effect (kJ kg"1)

Specific vol. of vapour (m3 kg-1)

Compressor displacement (1 s“1 kW“1)

Power in kW per kW refrigeration

% Carnot

Cycle

Efficiency

718

5

0.009

0.074

8.46

2370.0

147.0

62.0

0.1355

92.9

11

5

0.496

1.747

3.52

157.0

0.332

2.12

0.1395

90.2

111

5

5.160

15.55

3.01

1088.0

0.243

0.214

0.1456

86.4

114

12.7

1.062

3.373

3.18

106.2

0.122

1.14

0.1484

84.8

12

5

3.626

9.607

2.65

115.0

0.047

0.409

0.1502

83.8

113

10.4

0.188

0.783

4.16

129.5

0.652

5.03

0.1511

83.3

134a

5

3.451

10.032

2.91

145.0

0.0584

0.403

0.1516

83.0

22

5

5.838

15.34

2.63

157.8

0.040

0.255

0.1518

82.9

502

5

6.678

16.77

2.51

101.0

0.026

0.259

0.1631

77.1

9.11 Refrigerants 271

подпись: 9.11 refrigerants 271

718

Water, H20

11

Trichlorofluoromethane, CC13F

717

Ammonia, NH3

114

Dichlorotetrafluoroethane, CC1F2-CC1F2

12

Dichlorodifluoromethane, CC12F2

113

Trichlorotrifluoroethane, CC12FCC1F2

134a

Tetrafluoroethane, CF3CH2F

22

Chlorodifluoromethane, CHC1F2

502

Azeotropic mixture (48.8% R22, 51.2% R115) for low temperature work

Note:

An azeotrope is a mixture of refrigerants, the liquid and vapour phases of which, in equilibrium, have a constant boiling point.

In reciprocating compressors, the displacement (litre s-1 kW-1) should be low so that the required performance can be achieved with a small machine. For a centrifugal compressor, on the other hand, large displacements are desirable in order to permit the use of large gas passages. The reduced frictional resistance to such passages improves the compressor efficiency.

The power required to drive the compressor is obviously very important because it affects both the first cost and the running cost of the refrigeration plant.

Although not listed in the table, the critical and freezing temperatures of a substance have also to be noted when assessing its suitability for use as a refrigerant.

There are, of course, factors other than thermodynamic ones which have to be taken into consideration when choosing a refrigerant for a particular vapour compression cycle. These include the heat transfer characteristics, dielectric strength of the vapour, inflammability, toxicity, chemical reaction with metals, tendency to leak and leak detectability, behaviour when in contact with oil, availability and, of course, the cost.

The general concern about the ozone depletion and global warming effects of the refrigerants commonly in use has stimulated searching for alternative, more acceptable refrigerants. Among those considered are: well-established refrigerants such as ammonia (provided proper safety measures are adopted), hydrocarbons (also requiring safety precautions) and mixtures of two or more refrigerants, which introduce unusual effects and are incompatible with conventional mineral oils for lubrication of the compressor.

Butler (1998) discussed refrigerant mixtures with components that boil at different temperatures, giving changes in the boiling points (termed ‘glide’) as the mixture evaporates or condenses. ‘Bubble point’ is the temperature at which a refrigerant liquid just starts to evaporate and ‘dew point’ is the temperature at which the vapour starts to condense. With a single refrigerant these points coincide and an azeotropic mixture having liquid and vapour phases in equilibrium also has a constant boiling point. On the other hand, for a zeotropic mixture of refrigerants, the constituents have different boiling points and the difference is termed the glide value for the mixture. Glide may lead to differential frosting temperatures across an evaporator, and condensers and evaporators tend to be larger than for a single refrigerant. Changes in heat transfer characteristics and refrigerant handling difficulties are possible with glide. Note that a refrigerant and a lubricant is a zeotropic mixture requiring special consideration. Whereas hydrocarbon refrigerants are compatible with conventional mineral oils, HFC refrigerants (such as R134a) and mixtures of them are not and a polyester lubricant is required.

Posted in Air Conditioning Engineering


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