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Halogen free (natural) refrigerants NH3 (Ammonia) as alternative refrigerant The refrigerant NH3 has been used for more than a century in industrial and larger refriger- ation plants. It has no ozone depletion po- tential and no direct global warming poten- tial. The efficiency is at least as good as that of R22, in some areas even more favour- able; the contribution to the indirect global warming effect is therefore small. In addition, its price is exceptionally low. Is it therefore an ideal refrigerant and an optimum substi- tute for R22 or an alternative for HFCs!? NH3 has indeed very positive features, which can be exploited quite well in large refrigeration systems and heat pumps. Unfortunately there are also negative as- pects, which restrict the wider use in the com- mercial area or require costly and some- times new technical developments. A disadvantage with NH3 is the high isen- tropic exponent (NH3 = 1.31 / R22 = 1.19 / R134a = 1.1), which results in a discharge temperature even significantly higher than that of R22. Single stage compression is therefore already subject to certain restric- tions below an evaporating temperature of around -10°C. The question of suitable lubricants is also not satisfactorily solved for smaller plants in some kinds of applications. The most com- monly used mineral oils and polyalpha-ole- fins are not soluble with the refrigerant. They must be separated with complex technology and seriously limit the use of "direct expan- sion evaporators" due to the deterioration in the heat transfer. Special demands are made on the thermal stability of the lubricants due to the high discharge gas temperatures. This is especi- ally valid when automatic operation is con- sidered where the oil is supposed to remain in the circuit for years without losing any of its stability. NH3 has an extraordinarily high enthalpy dif- ference and thus a very small circulating mass flow (approximately 13 to 15% com- pared to R22). This feature, which is favour- able for large plants, makes the control of the refrigerant injection more difficult with small capacities. Further to be considered is the corrosive action on copper containing materials; pipe lines must therefore be made of steel. This also hinders the development of motor wind- ings resistant to NH3 as basis for semi-her- metic constructions. Another difficulty arises from the electrical conductivity of the re- frigerant in case of higher moisture content. Additional characteristics include toxicity and flammability, which require special safety measures for the construction and operation of such systems. Resulting design and construction criteria Based on the present "state of technology", industrial NH3 systems demand a complete- ly different plant technology, compared to usual commercial systems. Due to the insolubility with the lubricating oil and the specific characteristics of the re- frigerant, high efficiency oil separators and flooded evaporators with gravity or pump circulation are usually employed. Because of the danger to the public and to the pro- duct to be cooled, the evaporator often can- not be installed directly at the cold space and the heat must be transported by a sec- ondary refrigerant circuit. Due to the thermal behaviour, two stage compressors or screw compressors with generously sized oil coolers must be used even at medium pressure ratios. Refrigerant lines, heat exchangers and fit- tings must be made of steel; larger size pipe lines must be examined by a certified inspector. In some cases, aluminium can also be used as a material. Depending upon the size of the plant and the refrigerant charge, corresponding safety measures and special machine rooms are required. The refrigeration compressor is usually of "open" design, the drive motor is a separate component. These measures significantly increase the expenditure for NH3 plants, especially for medium and smaller capacities. Efforts are therefore being made world-wide to develop simpler systems which can also be used in the commercial area. A part of the research programs is dealing with part soluble lubricants, with the aim of improving oil circulation in the system. Sim- plified methods for automatic return of non- soluble oils are also being examined as an alternative. BITZER is strongly involved in these projects and has a large number of operating compressors. The experi- ences up to now have revealed that systems with partly soluble oils are dif- ficult to manage. The moisture content in the system has an important influ- ence on the chemical stability of the circuit and the wear of the compressor. Besides, high refrigerant solution in the oil (wet operation, insufficient oil tem- perature) leads to strong wear on the bearings and other moving parts. This is due to the enormous volume change when NH3 evaporates in the lubricated areas. These research developments are being continued, with focus also on alternative solutions for non-soluble lubricants. Various equipment manufacturers have developed special evaporators, allowing significantly reduced refrigerant charge. There is a strong trend towards so-called "low charge" systems, i.a. with regard to safety requirements, which are also largely determined by the refrigerant charge. In addition to this, there are developments for the "sealing" of NH3 plants: compact liquid chillers (charge below 50 kg), in- stalled in a closed container and partly with an integrated water reservoir to absorb NH3 in case of a leak. This type of compact unit can be installed in areas which were pre- viously reserved for plants with refrigerants of safety group A1 due to safety requirements. 28PDF Image | REFRIGERANT REPORT 21
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