REFRIGERATION
ACHIEVING ULTRA-LOW TEMPERATURES TWO-STAGE RECIPROCATING COMPRESSORS ARE DESIGNED FOR AN EXTENSIVE APPLICATION RANGE
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ttention in the refrigeration industry has recently turned to ultra-low temperature refrigeration and the challenges it can present. According to ASHRAE, ultra-low temperature refrigeration includes refrigeration temperatures between -50°C (-58°F) and -100°C (-148°F). Some applications include the conditioning and processing of petrochemicals, pharmaceutical products, and other specialized processes. Single-stage vapour compression systems can produce evaporating temperatures down to approximately -43°C (-45°F). Suction pressure and discharge temperature are two limiting factors that prevent the use of these systems at lower temperatures. At lower temperatures, the saturation pressure may be below atmospheric pressure. In the event of a leak, contaminants will be drawn into the system. It is extremely important that the system is always leak-free to prevent air and moisture ingress. Operation in a vacuum is also dangerous for the motor in a semi-hermetic compressor. Doing so will usually result in permanent motor damage. The approximate upper limit for compression ratio in a singlestage compressor is 20:1. This corresponds to operating conditions of -43°C (-45°F) saturated suction temperature SST and approximately 120°F (49°C) saturated discharge temperature SDT using R404A refrigerant. As the compression ratio increases, the mass flow rate decreases. There are two reasons for this. First, the volume of each kilogram or pound of refrigerant returning to the compressor increases at lower pressures. Second, high-pressure vapour remaining above the pistons after compression will re-expand, preventing suction gas from entering the cylinder until the vapour re-expands to a point where the cylinder pressure is slightly below suction line pressure. At higher compression ratios the discharge temperature increases primarily due to the re-expansion of vapour and the higher heat
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of compression. If the discharge temperature gets too high, this will result in lubricant failure, compressor overheating and accelerated wear. When it comes to achieving temperatures below the limits of a standard single-stage refrigeration cycle, two or more additional compression stages are required.
TWO-STAGE OPTIONS One approach to supporting two-stage compression is to use a single compressor that dedicates some cylinders to the low stage and dedicates others for the high stage. Although this type of compressor is sometimes referred to as an internally compounded compressor, it is typically referred to as a two-stage compressor. When using a two-stage compressor, the refrigerant type must be the same in both stages. This presents a challenge with two-stage compression processes that involve a single compressor. To provide refrigeration in the ultra-low temperature range, the evaporator and suction line will operate in a vacuum. For example, based on an SDT of 43.3°C (110°F), a two-stage compressor may be approved for operation down to 62.2°C (-80°F) SST. However, at this SST, the suction pressure is only 6.06 psia or 12.3 in. Hg, which of course is in a vacuum.
Phil is the Ontario sales manager for Bitzer Canada Inc. and provides training and technical support for Bitzer’s clientele. He can be contacted at: pboudreau@bitzer.ca.
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