Choosing the Right Refrigerant
For both new equipment and retrofit situations, the hydrofluorocarbon-based refrigerant R-507 has been shown to have suitable properties and provide low temperature performance when compared to its chlorofluorocarbon-based predecessor R-502. Hydro-fluorocarbon-based R-507 is composed of HFC-125 and HFC-143a. Because these two compounds do not contain chlorine in their structure, they do not contribute to ozone depletion. General properties of R-507 and R-502 are listed in table 1.
R-502 is composed of the hydrochlorofluorocarbon 22 (HCFC-22) and chlorofluorocarbon 115 (CFC-115). Because of its ozone-depletion potential, R-502 production was phased out at the end of 1995. Since then, HCFC-22 has been used as an interim fluid. Because of its ozone-depletion potential, HCFC-22 is scheduled for phaseout in the United States in 2010 for new equipment and in 2020 for servicing existing equipment. Performance penalties of lower efficiencies at lower temperatures and higher discharge temperature exist for R-22 when compared to R-502.
In finding a long-term replacement for R-502, it was desirable to find a fluid with no ozone-depletion potential that was of a low order of toxicity, nonflammable and compatible with commonly used piping, sealing devices and compressor materials. It also was important that the replacement be a close match to R-502 in capacity, pressure, efficiency, flow rate, discharge temperature and handling and servicing.
Regarding toxicity and flammability, both R-502 and R-507 are classified by American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) as Safety Group A1. This means the materials show no flame propagation at atmospheric pressure under test conditions and offer a low order of toxicity. R-507 generally is compatible with metals such as copper, steel, stainless steel, aluminum and brass. Alloys containing chrome, cobalt, nickel or molybdenum - often used in severe chemical environments - should be compatible under most circumstances; however, some testing and qualification may be necessary.
Put to the TestMaterials should be qualified for use by performing compatibility testing under the range of conditions that will be encountered during use. Elastomeric and plastic sealing devices should be evaluated for compatibility with the refrigerant and lubricant being used. Polyol ester lubricant is recommended for use with R-507. Elastomers such as ethylene-propylene copolymer and ethylene-propylene diene terpolymer tend to be compatible with both R-507 and polyol ester lubricant. Certain formulations of Neoprene, nitrile, hydrogenated nitrile and butyl rubber are compatible.
Elastomer compatibility data from the respective refrigerant and lubricant vendors is a good starting point prior to any qualification tests. Because of the extreme environments often encountered, a highly compatible sealing material such as polytetrafluoroethylene (PTFE) may be chosen. If more elastomer-like properties are required, a PTFE-encapsulated elastomer may serve well.
Ideally, proper materials selection will isolate refrigerant and lubricant from the process stream being cooled. However, the potential for introduction of refrigerant and lubricant must be taken into account, so the process stream reactivity with the refrigerant and lubricant must be evaluated.
In the case of R-507, it is well known that HFC refrigerants are more stable than most of their HCFC and CFC predecessors because the fluorine-carbon bond is harder to break than the chlorine-carbon bond. In spite of this, strong oxidizers and mineral acids may react with hydrofluorocarbons. Chemically active metals such as potassium, calcium, powdered aluminum, magnesium and zinc should be avoided.
One distinct difference between the CFCs and either HCFCs or HFCs is that the latter two classes of compounds contain hydrogen while the former does not. Mixing a stream of a hydrogen-containing compound with a stream containing a strong oxidizer such as oxygen or chorine - under the right conditions of temperature, pressure and concentration - can lead to a rapid exothermic reaction. A spark source with adequate energy discharging in a vapor-phase mixture of the oxidizer and any hydrogen-containing compound can ignite explosively. The energy required to generate ignition under such circumstances will depend upon a number of factors, including the amount of moisture present, the geometry of the vessel and the concentration of reagents and their individual flammability characteristics.
Under the same conditions of elevated temperature and pressure (concentration of oxidizer), hydrofluorocarbons such as R-507 require more energy to ignite than materials that are flammable at atmospheric conditions. Comparing a suitable hydrocarbon, ammonia and R-507, the hydrocarbon refrigerant requires the least amount of energy for ignition. Ammonia would require more energy for ignition than hydrocarbon but less energy than R-507. The lowest concentration of refrigerant needed for ignition, or lower explosive limit (LEL), is smallest for hydrocarbon, with increasing LELs for ammonia and R-507. As R-507 does not have explosive limits, it is nonflammable at any concentration in air at atmospheric pressure.
Indirect SystemsThe particular process streams and their reactivity with the refrigerants under consideration will dictate, at least in part, if a direct system can be used. If none of the refrigerants are compatible, it may be necessary to employ an indirect system so the refrigerant is isolated from the process stream.
An indirect system requires that the fluid in the secondary loop be compatible with the process stream. Other major factors in whether an indirect system can be used are system capacity required and plant layout.
Typically, large central systems are configured as indirect systems with secondary loops serving various loads throughout the plant or within a given section of a plant. Most often, direct systems will be used when the capacity requirements are relatively small, the number of different loads is small and the variation in loads is moderate. In any case, R-507 can be applied using scroll, screw or reciprocating compressors to address a range of system capacities. Because R-507 is based on hydrofluorocarbons, a polyol ester lubricant must be used. R-507 is not miscible with mineral oil or alkylbenzene.
R-507 heat transfer characteristics are about 30% higher than R-502, comparable to R-22 (figure 1). Superior heat exchanger performance can be realized with copper because enhanced surfaces are possible. This means compact, more cost-effective heat exchangers can be used. Compressor efficiency with R-507 at low temperature conditions is higher than with R-502. Energy efficiency with R-507 typically is 7 to 9% better than R-22.
For low temperature applications (-60 to -10°F [-51 to -23°C] evaporating temperature), the performance of R-507 has been shown to be comparable to that of R-502, allowing R-507 to be used in new equipment or retrofit in existing systems.
Converting Existing SystemsTo retrofit an existing R-502 system, the mineral oil or alkylbenzene compressor lubricant must be removed to less than a 5% residual and replaced with polyol ester lubricant. The R-507 charge is about 85% by weight of the R-502 charge due to the lower density of R-507.
Seals and gaskets used in the existing system should be identified so that compatibility with R-507 is known. Incompatible materials should be replaced, and any sealing materials that have been significantly affected by heat set, compression set or extraction should be replaced. The equipment manufacturer should be contacted for specific instructions on system components to be modified or replaced. Conversion guidelines detailing recommended procedures are available from the refrigerant manufacturers.
It is possible to convert existing systems running on R-22 or other interim refrigerants containing R-22 such as R-402A. Ammonia systems also can be converted to R-507. Unlike systems with miscible refrigerant/lubricant combinations such as R-502 with mineral oil, a conversion from ammonia to R-507 also must take into account that ammonia and mineral oil are immiscible.
Oil SeparatorsBecause ammonia and mineral oils are immiscible, systems rely on oil separators to capture and return the lubricant to the compressor. Mineral oil has a higher density than ammonia and settles to the bottom of the separator. This makes isolation and return of the mineral oil relatively simple.
Oil separators used with ammonia systems may employ manual or automatic oil return. Polyol ester lubricant is not acceptable for use when retrofitting from ammonia to R-507 due to incompatibility of ammonia and polyol ester lubricant. In this case, polyalkylene glycol (PAG) lubricant is used. Separation of PAG lubricant from R-507 cannot be accomplished using the oil separators of the ammonia system because the replacement refrigerant and lubricant are miscible.
In carrying out a conversion of an ammonia system to R-507, piping sizing should be evaluated to confirm that velocities with R-507 are high enough to ensure good oil return. If conversion from ammonia to R-507 is being evaluated, remember that the technical service staff of the R-507 manufacturer can help determine what changes are required for a specific system.