Ozone depletion effects

The refrigerants that have been widely used in air conditioning and other applications throughout the world since the nineteen thirties, comprise molecular combinations of chlorine, fluorine, carbon and hydrogen. Reference to the footnotes in Table 9.6 verifies this. There is a significant turnover in the quantity of refrigerant used for topping-up purposes in plant, as leaks occur and maintenance and replacement work is carried out. Butler (1991) has suggested that this amounts to 80 per cent of the total usage of refrigerant for air conditioning purposes.

Fully halogenated refrigerants are termed chlorofluorocarbons (CFC). They are chemically very stable and exist for a long time in the atmosphere, after leaking from a refrigeration plant. In due course they rise into the stratosphere and, at a height of between six and thirty miles above the surface of the earth, the molecules break down in the presence of solar

Refrigerant

ODF

Proportion of chlorine by weight

Atmospheric half life in years

CFCs

Rll

1.0

77.4%

65

R12

1.0

58.6

146

R113

0.8

56.7

90

R114

0.8

41.5

185

R115

0.4

22.9

380

HCFCs

R22

0.05

41.0

20

R142b

0.06

35.2

13

R141b

0.1(?)

60.6

10

R125

0.02

26.0

4

R123

0.02

46.3

2

HFCs

R152a

0

0

1

R134a

0

0

6

Reproduced by courtesy of McQuay Chillers, a Snyder General brand.

Radiation, losing an atom of chlorine. Two reactions then take place. First, the chlorine combines with a molecule of ozone to form chlorine monoxide and oxygen:

Cl + 03 -> CIO + 02

Secondly, the molecules of chlorine monoxide then combine with oxygen to form oxygen molecules and chlorine molecules.

2C10 + 02 -» 2C1 + 202

The chlorine molecules are once again free to combine with ozone and the process repeats up to 10 000 times, some CFCs having an atmospheric half life of the order of 100 years, according to AAF Ltd. (1990). The net result appears to be a steady depletion of the ozone content of the atmosphere at high altitudes. Since ozone is the agent in the upper atmosphere that prevents the entry of excessive, dangerous, ultraviolet radiation the consequences of ozone depletion are obvious and serious.

An ozone depletion factor (ODF) has been calculated for each refrigerant, in relation to a value of 1.0, assigned to R11, which is the worst offender because it contains the largest proportion of chlorine by weight.

If one of the chlorine atoms in the molecule of a CFC refrigerant is replaced by a hydrogen atom the stability of the molecule is much reduced and its ozone depleting effect becomes smaller. This has encouraged the use of such refrigerants, which are termed hydrochlorofluorocarbons (HCFC), as an interim measure, before the use of CFC refrigerants can be fully phased out. An example of an HCFC refrigerant is R22, which has been extensively used for many years in reciprocating compressors, but is no longer acceptable.

Refrigerants with no chlorine present have ODF values of zero. Such refrigerants are termed hydrofluorocarbons (HFC).

Of the refrigerants listed, R134a offers promise, being similar in many respects to R12 but with no ozone depletion effect.

Posted in Air Conditioning Engineering


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