Achieving Ultra-Low Temperatures
Two-Stage reciprocating compressors are designed for an extensive application range
By: Phil Boudreau
Attention 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 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.
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.
Of course, it is always best to maintain suction pressures that are above atmospheric pressure. This is a mandatory requirement with single-stage semi-hermetic compressors but not with two-stage semi-hermetic compressors. If a two-stage compressor will operate in a vacuum, it becomes even more imperative that the system is leak-free.
In a two-stage semi-hermetic compressor, the motor is cooled at an intermediate pressure. The intermediate pressure of some six-cylinder compressors is approximately equal to the square root of the absolute suction and discharge pressures after they are multiplied together. For example, with a 20 psia suction and 325 psia discharge, the interstage pressure will be approximately 3(20 x 325) or 80.6 psia. This corresponds to -2.4°C (27.7°F) with R404A. With smaller four-cylinder compressors however, the intermediate pressure will likely be even lower.
Due to the direct-staging inside a two-stage compressor, high suction pressures may overload the compressor during pulldown. The reed valves can be damaged with prolonged operation at high suction pressure. Therefore, two-stage compressors often require some form of suction pressure regulation.
To avoid operating in a vacuum at lower SSTs, it is necessary to split the compression process into two stages where each stage uses a different refrigerant type. In this case, the high-stage system will generally operate with R404A, R507A, R448A, R449A, R407A/F or other refrigerant that has similar pressure-temperature characteristics. The high stage will generally operate at conditions similar to a low-temperature freezer for food storage. The refrigerant used in the low stage, however, will be one with a higher saturation pressure value for a given temperature. The type of system described here is commonly known as a cascade system.
High pressure refrigerant in a single-stage compressor
One example of a refrigerant used in the low stage is R508B. Note that at 0 psig or 14.7 psia, the saturation temperature for R508B is -87.2°C (-124.9°F). Therefore, it is possible to achieve some very low SSTs using a refrigerant such as R508B.
However, before using a high-pressure refrigerant such as R508B in a single-stage compressor, the compressor manufacturer must be contacted for advice and guidance. This is to ensure the compressor will be suitable for the application. If the compressor manufacturer authorizes the use of the refrigerant, they may also provide other recommendations such as limiting the suction pressure during pulldown and during standstill, keeping the suction temperature above a certain limit, recommending a lubricant and control strategy, to name a few.
When the cascade system is not running, the pressure within the system will increase until the saturation temperature is equal to the average equipment ambient temperature. So, if the ambient temperature is 25°C (77°F), the pressure of R508B will increase significantly and will even exceed the critical pressure of the refrigerant. In this situation, the low stage needs to be designed to withstand these high pressures, or a fade-out vessel must be used. A fade-out vessel is simply a vapour storage tank sized to allow the volume of the system to increase when necessary. On start-up, there will need to be some control of the pull-down so the low stage compressor is not overloaded.
If the system holds a relatively large amount of refrigerant, it may even be necessary to use a small standby condensing unit to recondense the vapour in the low stage receiver as it forms. This effectively reduces the pressure inside the low stage. The condensing unit could simply be cycled based on the pressure of the refrigerant in the low stage.
A cascade heat exchanger, in the form of a plate type, tube-in tube or other type is used to transfer heat from the low-stage condenser to the high-stage evaporator. This counterflow heat exchanger must be sized to allow for pull-down of the low side and the metering device used to feed the evaporator side of the cascade heat exchanger must be able to maintain a very stable superheat.
During system start-up, it is important to get the high stage up and running first so a good heat sink is available for the low stage. Once the high stage suction pressure decreases to a low enough value, the low stage can then be energized. The low stage may also need to have some form of suction pressure regulation also. This is to prevent operation outside the compressor envelope, which could result in compressor overload.