Long Term Natural Alternatives for Halogenated Refrigerants - Hydrocarbons

Hydrofluorocarbons (HFCs) are being used presently as medium term alternatives for environmentally damaging chlorofluorocarbon refrigerants (CFCs). However, higher global warming potentials (GWP) of HFCs are still a serious environmental concern though they do not contribute to stratospheric ozone depletion. V\4ren it comes to finding refrigerants that do not deplete the ozone layer and has minimal direct global warming potential, natural fluids stand out among the potential alternatives. Among natural refrigerants hydrocarbons (HC) is one category with no ozone depleting potential and with minimal GWP potentials. Besides, compared with CFCs and HFCs, hydrocarbon show better heat transfer performance, blend well with existing lubricants and require relatively lower amounts of system charge. However, flammability of HCs raise concerns over their use in refrigeration systems. Yet, HCs are gaining increased recognition in many areas of vapour compression refrigeration as a long term refrigerant. This paper discusses heat transfer and retrofit aspects, and issues in international standards for the use of flammable HCs in refrigeration and heat pump systems.


Introduction
By the end of the 20th centaury halogenated refrigerants have proved the undesirable implications they have on the environment. A more sensible approach to selection of environmentally benign alternative refrigerants would be to consider fluids that occur in nature. Historically, hydrocarbons (HCs) were among the first natural refrigerants to be used. Yet, their flammability associated safety issues paved the way for more inert haiogenated fluids including chlorofluorocarbons (CFCc) t1]. However, r.vith the disclosure of the potential environmental damages the continued use of halogenated refrigerant could cause, and having understood the damages already done, natural refrigerants are in the spot light of refrigerant rcscarch.
With this renewed interest on natural refrigerants, ammonia, carbon dioxide and hydrocarbons (HCs) stand prominently as potential candidates. Among these, HCs posses zero ozone depletion potentials (ODP) and extremely low giobal \ /arming potentials (GWP). Further, HCs in general show higher system efficiencies) reduced charge sizes, good miscibility with mineral oils (no need for synthetic oiis), lower compressor discharge temperatures, and better heat transfer within heat exchangers compared to many halogenated refrigerants that could be replaced with HCs [1].
The main advantages of HCs, compared to the other natural refrigerants, are that they are in many respects very similar to the haiogenated refrigerants the industry is already familiar with. For the use of HCs, no major changes in the refrigeration systems or components are needed. Additionaily it is known that mineral oil, which is wideiy used with halogenated refrigerant, show very good miscibiliiy with HCs and can be used as compressor lubricant [2]. With all these desirable characteristics and features for application of HCs in vapour compression refrigeration, the only practical factor against is their safety concerns arising from flammability. Under such conditions, installation practices and safety guide iines, handling procedures etc, covering the use of relatively large quantities of HCs in many potential applications, are required from a technical point of view t3] However, the obstacle of flammability that negatively affect the use of HCs in refrigeration systems is gradually dealt with in terms of regulations With all the environmentally friendlv features together with other desilable fluid characteristics (such as being non-tgxic, nonrestricted availabilitv etc.) HCs offer long term alternatives for many of the medium term halogenated refrigerants and refrigerant mixtures (i.e. those with higher degrees of undesirable environmental damages) being used at present. This paper presents an account on the use of hydrocarbons in vapour compression refrigeration systems; from domestic refrigerators to commercial heating/cooling systems. Also, tries to highlight the plesent regulatory aspects associated wiih the use of flammable hydrocarbon in refrigeration svstems that encourage the use of these fluids.

Hydrocarbon refrigerantsproperties and flammability
The hydrocarbons of interest and that suit refrigeration applications in general are ethane (R 170, CzFI6), propane (R 290, C:Hs), butane (R600, CqHro isobutane, R600a) and propylene (R 127A,CEHo). Isobutane (R600a) is the mosi frequently used hydrocarbon refrigerant [3]. In Europe it is the totallv dominating refrigerant in household refrigerators. In 2004,33% of the world production of domestic refrigerators and freezers used pure isobutane, or isobutane blends t4l I'ropane (R290) and propene (propylene, R1270) have also been used in air conditioners and in commercial refrigeration systems. In addition to single component pure isobutane, propane and propene blends of isobutane/propane and propane/ethane to match the vapour pressure curves of R12/R134a and R22/R407C, respectiveiy are also available presently. Butane (R600) has also been under discussion, but has not been used commercially to the extent of isobutane though its properties are quite similar to those of isobutane. Pentane and isopentane have been considered for use in centrifugal systems to replace R1l [5]. When using hydrocarbon refrigerants it is essential to use refrigerant grade products only.
Commercial grade hydrocarbons contain significant quantities of sulphur, water, and other impurities and could contribute to oil degradation, shorten the compressor iife and invaliclate any warranties. Aiso, unlike con-rmerciai liquid petroleum gases (LPG) hydrocarbon refi'igerants are not odourised.
Ancther problem with commercial LPG is that the composition of any specific hydrocarbon can be variable thus drastically changing the properties of the refrigerant from cylinder to cylinder Table 1  refrigeration systems are very small, perhaps less than 250g depending on the system concern. However, commercial and industrial refrigeration systems are more likely to have amounts of hydrocarbons that could pose real safety threat, for example when system charge exceeding 2.5kg.  c) In the case of secondary fluid based systems, due to any break of evaporator or condenser, the heat exchangers shall not allow the release of the refrigerant into the areas served by the secondary heat transfer fluid. d) A refrigerant detector shall function at a level not exceeding 20% of the lower flammability limit, which shall continue to activate at higher concentrations and automatically activate an alarm, start mechanical ventilation and stop the system when triggered.

Summary
Hydro carbons receive increased consideration in the refrigeration, air conditioning and heat pump industry as a category of natural, environmentally friendly refrigerants. Being natural fluids HCs has a number of positive aspects compared to halogenated refrigerants; common availability and no monopolies, no apparent refrigerant recycling, good availability etc., are just few to mention. However, the flammability of HCs in general put many potential users and uses (applications) off due to the safety risk associated with HCs.
In Europe there is a wide acceptance of HCs as refrigerant which is siowly crossing the boundaries to other counties due to legislations in place that globally restrict the use of HCFCs/HFCs. Good thermal performances, comfortable levels of behavior with common Iubricants, possibility of going for compact systems with less arnount of charge make HCs more attractive for smaller refrigeration units, however, that does not limit their use for large industrial applications, where blending of HCs could deliver specific process requirements. All in all, provided proper management and regulations in place to make sure sensible and safe use, HCs provide a class of energy efficient natural working fluids with a long term future in the industry. The key to mitigate the safety issues associated with the fiammability is to minimize the sysiem charge by way of having compact systems and to place the main system hardware outside whenever practically possible.