On substances that deplete the ozone layer
Options for existing chiller equipment
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- 9.4.1 Positive displacement chillers
- 9.4.2 Centrifugal chillers
- 9.4.3 Non-vapor compression chiller replacements – absorption
- 9.5 Alternatives for high ambient conditions
Options for existing chiller equipmentWhen CFC and HCFC refrigerants are phased out (and in several countries that phased out HFCs), the functions performed by chillers employing those refrigerants have to be supported in one of the following ways: Retain/Contain: continued operation with stocked and/or reclaimed inventories in conjunction with containment procedures and equipment modifications to reduce emissions. Retrofit: modification to allow operation with alternative refrigerants (HFCs where permitted) depending on applicable regulations. Replace: early retirement/replacement with new chillers (preferably having higher efficiency which reduces energy-related climate impact) using allowed refrigerants or not-in-kind alternatives, The retrofit options depend on the specific refrigerant for which the chiller was originally designed. When any retrofit is performed, it is recommended that the machinery room be upgraded to the requirements of the latest edition of safety standards such as ASHRAE 15 (ASHRAE, 2013), and EN 378 (EN, 2008) or international standards such as ISO 5149 (ISO, 2014). It is also recommended that the manufacturers of the equipment be consulted in any retrofit program. 9.4.1 Positive displacement chillersA positive displacement compressor inherently can be applied to handle a number of different refrigerants and pressure ratios in a chiller if its motor has adequate power, the compressor, tubing, heat exchangers, and other components can meet pressure codes and regulations with the refrigerants, and the system materials and lubricant are compatible with the refrigerants. Despite this flexibility, there remain a number of issues in retrofitting positive displacement chillers to operate with new refrigerants. These issues were discussed in the 2010 RTOC report. 9.4.2 Centrifugal chillersCentrifugal compressors by nature must be designed specifically for a particular refrigerant and a particular set of operating conditions for the refrigerant cycle in which they are used. Direct refrigerant substitution in centrifugal chillers can be made only in cases where the properties of the substitute refrigerant are nearly the same as those of the refrigerant for which the equipment was designed, or when the impeller speed and/or impeller geometry can be changed easily. In the past this has been accomplished by gear changes in open drive chillers and with variable speed drives in both open and hermetic compressor chillers. The compressor surge margin must be checked using the properties of the substitute refrigerant. 9.4.3 Non-vapor compression chiller replacements – absorptionThe most common absorption chiller is the water-lithium bromide solution type, both direct and indirect fired versions. When comparing the size of a mechanical vapour compression chiller with an absorption chiller of similar capacity, the absorption chiller is about 25 to 50 % bulkier. This factor may be crucial when retrofitting existing systems because of the size of access ways and corridors. Another important factor is the cooling tower requirement of an absorption chiller. Because absorption chillers require cooling for their condenser as well as their absorbers where the mixing of the refrigerant and absorbent occurs, resulting in the emission of heat of mixing and heat of solution that need to be removed, the cooling tower of an absorption system can be 30 to 60 % larger than that of a mechanical vapour compression system of a similar capacity. On the other hand, the replacement of a mechanical vapour compression system with an absorption system saves a portion of the electric consumption of the building. This reduces the capacity requirements for the transformer, electric switchboards, and electric conduits. Provision has to be made for suitable gas piping and train when a natural gas fired system is used. Absorption systems do not require anti-vibration mounting since they have few moving parts. 9.5 Alternatives for high ambient conditionsChiller rating points commonly involve ambient air temperatures of 35oC (95oF). However, in very warm climates there is interest in knowing how chillers will perform when air temperatures reach 47oC or even 52oC, and also, how the design of chillers can be modified to improve high ambient operation. This section focuses on air cooled chillers. The design of water cooled chillers for high ambient conditions involves the same approaches as for air cooled chillers but is less challenging because condensing temperatures are lower for a given ambient temperature. Air-cooled chillers with positive displacement compressors (rotary, reciprocating, scroll, screw) designed for an ambient of 35oC will operate at high ambient conditions but with reductions in capacity and COP. An illustrative example is given for actual 3 ton residential air conditioners in the May 2010 TEAP XX/8 Task Force Report. For HCFC-22, capacity was reduced by 15% and COP was reduced by 35% when the ambient temperature increased from 35oC to 52oC. For R-410A the reductions were 18% and 40 % respectively. Design changes that can improve chiller performance at high ambient temperatures include applying a compressor with proper capacity and compression ratio for the ambient condition, adjusting the compressor built-in volume ratio for scroll and screw compressors, increasing condenser size, and providing a properly-sized expansion device. Features such as economizers are more effective in high ambient conditions than for lower condensing temperatures. HFC-134a has less performance degradation than R-410A, and HFC-32 (an A2L refrigerant) has even less performance degradation. These refrigerants are not interchangeable because of their differing capacities, pressure levels, and lubricant requirements. Chiller design must be tailored for the specific refrigerant selected. Chillers with centrifugal compressors require a different approach. Typically, additional compressor stages may be required. If condensers cannot be water-cooled because water is scarce or expensive, dry coolers are used. Air-cooled condensers with tubes and fins or microchannel heat exchangers are not commonly used with centrifugal compressors because they are sensitive to the increased pressure drop on the refrigerant side of these types of condensers as compared to dry coolers. The refrigerants used in centrifugal chillers for high ambient conditions typically are HFC-134a and HCFC-123. HFC-32, an A2L refrigerant, is being considered as a lower-GWP alternative to R-410A or HFC-134a for high ambient applications. R-444B has been shown to perform well at high ambient conditions (Schultz 2014). It is too early to know whether or when other lower GWP refrigerants will be available as alternatives to R-410A, HFC-134a, and HCFC-123 for chillers operating in high ambient conditions.
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