CHAPTER 3: SECONDARY SYSTEMS IN INDUSTRIAL REFRIGERATION INTRODUCTION Together with HCFC22 that is now being phased out, ammonia (NH3 also known as R717) is the most common refrigerant in industrial applications, while carbon dioxide (CO2 also known as R744) is gaining in low-temperature, cascaded systems where it primarily replaces ammonia (TEAP, 2010). In new cold storage facilities in particular, the use of natural refrigerants is already an established market in a number of world regions, in the case of ammonia-only systems. The use of ammonia and other HFC-free solutions is not only driven as a means to pre–empt future use restrictions on fluorinated gases, but as an efficient tool to achieve good energy efficiency performance and emissions savings. Also in the near future technical development of alternatives in industrial refrigeration is expected to emphasise NH3 and CO2 (TEAP, 2010). However, with facilities around the world being subject to increasingly stringent safety regulations, owners of large industrial plants and equipment manufacturers start seeking refrigeration solutions that enable them to continue using or start implementing ammonia systems, while ensuring compliance with rigorous (and sometimes restrictive) safety regulations that concern installations that serve public areas or in regions with stringent policies.
CHARGE REDUCTION THROUGH INDIRECT SYSTEMS & CO2 As a result, various technology solutions leading to a significant ammonia charge reduction have gained in popularity with industrial end-users. In cold store applications, indirect systems have been traditionally implemented by coupling the ammonia system (or the HFC system, though ammonia is the most common refrigerant in industrial refrigeration systems) with a waterbased brine/glycol system, whereby only brine/glycol is circulated in the temperature–controlled area of a facility, with the reduced ammonia charge confirmed in the machinery room area where the appropriate safety devices must be installed. Indirect systems’ most pronounced benefit is, besides the required substantial reduction of refrigerant charge, also lower risk of leakage and today they are a preferred solution in several applications on some markets (UNEP, 2010). Indirect system options have been applied for example in supermarket installations in Europe as of 1995, as well as by US commercial chains as of 2006 as a means to significantly reducing refrigeant charges (50 to 85%) and achieve a much better refrigerant containment (RTOC, 2010). Another means to achieving significant charge reduction – up to 90% compared to an ammonia–only system
– is the use of CO2 in combination with NH3. Carbon dioxide has been used for industrial systems in the United Kingdom, France, Germany, the Netherlands, Switzerland, Australia, Japan and the United States of America (Pearson A B). Here, CO2 is used either: • As the low stage refrigerant in cascade installations with ammonia, or • As secondary coolant whereby the carbon dioxide evaporates and condenses at nominally the same pressure Systems using CO2 as a “volatile secondary” refrigerant are cost effective thanks to greatly reduced pumping and pipework costs where brine or glycol would have been the alternative (Pearson A B). The use of the two natural working fluids combines the advantages of ammonia and CO2 , as the latter has good properties, in particular at low temperature, but is not a substitution for ammonia. As a summary, arrangements where ammonia is used as the primary refrigerant and CO2 has been adopted either as the secondary refrigerant and/or in the low temperature part of a cascade system gives several advantages including low capital cost and high efficiency. It is probably the most suitable approach for an “environmentally friendly” solution in situations where a direct ammonia system is not feasible (IIR, 2008).
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