Requirements for reciprocating compressors in refrigeration systems

Concerns in design and selection of compressors for chillers include performance, efficiency, reliability, maintenance, lifecycle costs, and environmental impact.

By Amin Almasi, rotating equipment consultant

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A chiller is a machine that removes heat from a liquid via its refrigeration system most likely a vapor-compression package. This cold liquid can then be circulated through a heat exchanger to cool air or equipment as required. Concerns in design and selection of compressors for chillers include performance, efficiency, reliability, maintenance, lifecycle costs, and environmental impact. Reciprocating compressors have been widely used in chiller packages.

The requirements for chiller reciprocating compressor’s performance, efficiency, and reliability become higher due to energy, operational, and environmental concerns. For some reciprocating compressor designs in chiller refrigeration applications, it seems that the compressor performance approaches its practical limit. The further performance and reliability improvement and economical component designs need more focus on critical topics such as cylinder valve design, pulsation control, unloading systems, compressor control, flow resistance throughout the compressor package, compression process, heat transfer within compression system, and temperature pattern in the compressor components.

There are other factors affecting the performance and reliability of a chillier refrigeration reciprocating compressor. Cylinder valve dynamics, valve losses, and reliability or life duration of valve components are important. For the compression process, re-expansion volume, heat transfer, and blow-by between the piston and cylinder wall are the most significant factors. The gas flow resistance, friction loss, pulsation, and vibration should also be considered for the whole refrigeration compressor package.

Cost-effective and reliable operation of reciprocating compressors requires a balance between pulsation control, efficiency, component reliability, and performance. With high-speed units, acoustic excitation frequencies overlap the acoustic and mechanical response frequencies of the chiller passages and piping system.

The compression cylinders provide confinement for the gas during compression. A piston is driven in a reciprocating action to compress the gas. Arrangements may be of single- or dual-acting design. In the dual-acting design, compression occurs on both sides of the piston during both the advancing and retreating stroke. Gas pressure is sealed and wear of expensive compressor components is minimized through the use of piston rings and rider bands, respectively.

The gas is drawn into the cylinder by the suction valve, compressed, contained, and then released by discharge valves. Cylinder valves are actually sophisticated, specially designed check valves that operate automatically by differential pressures. Depending on system design, cylinders may have one or multiple suction and discharge valves.

Selection and sizing

Today, most reciprocating compressor manufacturers provide powerful sizing software to aid engineers, chiller packagers, and end users for the compressor sizing and selection. There are key areas to consider when using such a software tool. Without proper care and attention to details, these sizing tools may generate misleading and undesirable sizing suggestions.

In the compressor sizing and selection, the estimation of the number of stages, required power, and inter-stage pressures are important. Each reciprocating compressor manufacturer has a finite number of frames and a pre-designed family of standard cylinders that fit those frames. Thus, there are a limited number of combinations of frame and cylinders. At least one of the possible combinations is the best solution that a particular manufacturer can provide for chiller package.

Using the calculated estimates for the number of stages and required power, the number of cases — selected frame and cylinders — to review can easily be reduced down to about 10 to 80 combinations. Usually the minimum number of cylinder per stage should be considered. One cylinder per stage is preferable if proper operation is possible. Obviously this isn’t possible for some cases — for example, for some large refrigeration compressors, it might be preferred to select two cylinders per stage since two-cylinder balanced-opposed is better than a single cylinder machine for vibration and dynamic aspects. There are other cases that mandate two cylinders per stage. Single-stage two-cylinder models and two-stage four-cylinder models have been commonly used in different industries including refrigeration systems. In those cases, identical cylinders should be used for each stage. With such a number of cases —  say 10 to 100 combinations — to check, the best solution can be determined within minutes or hours. It is a common requirement to avoid using tandem cylinders. The next step is to determine the size of the cylinders per stage. The driver, nearly always an electric motor, can have an impact on sizing, since it really determines available load and speed.

Cylinders are often sized toward achieving balanced compression ratios across all stages. However, an alternate approach used by some manufacturers is to refine sizing the cylinders and compressor unit based on maintaining balanced discharge temperatures across all stages.

Cylinder efficiencies are important selection factors but often neglected by many parties. Cylinder efficiencies are related to gas passages and valve port designs. They determine how high losses are because of cylinder passageways and details of cylinder valves. Cylinder efficiencies tend to impact power requirements more than flow rates.

Stroke and rod diameters are determined from the frame being considered. Maximum speed (rpm) is usually related to the frame considered. Some margins should be considered for the maximum speed of each frame.

Unsafe combinations should be dropped from consideration. Examples are high rod loads, high inter-stage pressures, high inter-stage, temperatures, and overload. Furthermore, if a combination is technically safe, but reasonably close to problem areas, that combination should also be dropped unless it is the only one left.

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