Book - School of Science and Technology

534 Refrigeration: water chillers and heat pumpsAmmonia (NH 3 ), while high in efficiency and low in cost, has not been thought suitablefor many air-conditioning applications due to its toxic nature and the serious resultswhich might attend a burst or leakin the system. However, ammonia is widely used in thefood refrigeration and cold storage industries on account of its excellent refrigerationproperties and is being considered increasingly for air-conditioning applications. To dateammonia has proven to be a very effective and safe refrigerant where plant has beendesigned, installed and maintained in accordance to relevant safety standards and codesof practice.Carbon dioxide (CO 2 ) is an excellent refrigerant from an environmental and safetypoint of view. The main barriers to its use is that it results in low energy efficiency and thatit operates at higher pressures (around 100 bar) and has a substantially higher volumetriccapacity than most other refrigerants, this means that equipment designs are unsuitable.Considerable development is taking place on CO 2 for small refrigeration systems includingcar air-conditioningSulphur dioxide (SO 2 ) and methyl chloride (CH 3 Cl) were used for small plants butalthough they have good thermodynamic characteristics, one has an unpleasant smell andthe other is toxic: neither is now used for application to air-conditioning.A range of synthetic refrigerants, halogenated hydrocarbons sometimes referred to asfreons, which are colourless, non-inflammable, non-corrodent to most metals and generallynon-toxic, are those now in common use. They may be categorised as falling withinone of three chemical forms:. CFCs (chlorofluorocarbons). These have a high ozone-depleting potential (ODP) contributingto the breakdown of the ozone layer, are banned by the Montreal Protocoland ceased manufacture in the European Community by January 1995. Examples areR11, R12 and R114.. HCFCs (hydrochlorofluorocarbons). These have limited ODP, are classified under theMontreal Protocol as transitional substances and are currently being phased out.Examples are R22, R123 and R124. R123 is available as a `retrofit' refrigerant forR11. In this context, `retrofit' means a fluid which may be substituted into an existingsystem but will require material changes to equipment.. HFCs (hydrofluorocarbons). These contain no chlorine and therefore have zero ODPand in consequence are not controlled by the Montreal Protocol. However, there hasbeen uncertainty about HFCs ever since the Kyoto Protocol. The UK government'sposition is that HFCs are not a sustainable technology in the long term and should onlybe used where other safe, technically feasible, cost effective and more environmentallyacceptable alternatives do not exist. In the meantime a range of hydrofluorocarbonshas been developed and is available commercially to replace the long established CFCand HCFC refrigerants. Some are blends of two or more HFCs and are designed toapproximately mimic the properties of an existing CFC or HCFC, or to counteractundesirable properties of a single HFC. Examples are R407C, R410A and R134a.R407C is a blend of HFC-32, HFC-125 and HFC-134a and is designed to be a closematch for R22. R410A is another HFC blend intended for use in applications thatformerly used R22 and is a better choice for smaller systems. R134a is a pure fluid andis a `drop-in' refrigerant for R12. Here, `drop-in' means a fluid which can be substituteddirectly, requiring replacement of some serviceable components only. R134a is emergingas the preferred refrigerant for larger air-conditioning applications.In addition to an ODP classification, refrigerants are also given ratings for globalwarming potential (GWP), an index providing a simple comparison with carbon dioxidewhich has an index rating of unity. The properties and values for the ODP and GWP