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Requirements for reciprocating compressors in refrigeration systems

Aug. 19, 2014
Concerns in design and selection of compressors for chillers include performance, efficiency, reliability, maintenance, lifecycle costs, and environmental impact.

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.

Under-sizing a compressor is totally unacceptable. This should be seriously avoided. Over-sizing a compressor on load and power can lead to buying a driver with more power than required, as well as compressor frame and cylinders larger than needed. Over-sizing a compressor on flow might be acceptable as the use of unloading devices —  speed, suction valve unlodaers —often allow the unit to be unloaded to meet the desired flow rate.

Thus, during the initial sizing process, sizing software might tend to slightly oversize the unit in regard to achieving desired flow rates. Then, it should favor the frame and cylinder combinations that require the least amount of power to compress the slightly adjusted gas volumes.

Nowadays, reciprocating compressors using variable-speed drives offer a wide range of operating points. Regarding the variable-speed reciprocating compressors that should achieve several operation points, flow varies almost linearly with speed and clearance —  suction volumetric efficiency.

Variable-speed reciprocating compressors

Variable-speed operation is one of the best options for the capacity control of a reciprocating compressor. As noted, the capacity of a reciprocating compressor varies more or less linearly with speed. This offers an excellent step-less and highly efficient capacity control opportunity. Reciprocating compressors often have large load torque variations with every cycle; some can be up to 50% of the average torque. Such a reciprocating compressor is slamming current in and out in order to keep up with the transient torques. While using flywheels or large rotor inertia to damp out the load dynamics could be an option, alternative options should be investigated to achieve optimum purchase and operation costs. If the electric motor is being driven by a variable speed drive with sophisticated drive algorithms, for example, controllers can track the load torque variations, and then both the efficiency and transient stability problems can be solved together.

The other significant factor is the starting problem. The transient load torque is also present at starting, so the motor has to be able to accelerate through the load transients and be capable of starting when the compressor is sitting at the highest load.

High-speed compressors

There is a trend toward the use of high-speed reciprocating compressors in all applications, including refrigeration systems. These compressors are very sensitive to off-design operation. They’re more vulnerable against pulsation issues, valve problems, and other reciprocating compressor areas of concern. High-speed reciprocating compressors can experience diminished cylinder performance due to higher-than-expected valve losses and pulsation levels. These high pulsations consequently contribute to the higher-than-anticipated vibration levels found on a number of high-speed reciprocating compressors. Internal gas passage acoustics, cylinder-to-cylinder and cylinder-to-piping interactions, and nonlinear flow losses can have a significant effect on high-speed reciprocating compressors.

Capacity control

In general forms, the capacity control methods for reciprocating compressors are variable-speed drives, suction valve unloaders, step-less methods, clearance pockets, and bypass recycling. There are two main reasons why a reciprocating compressor capacity regulation is used. First, it’s used to adjust the suction flow to match the refrigeration demand; the second reason is to save energy. Compressor capacity-control methods are utilized to maintain a required compressed gas delivery under variable refrigeration conditions.

Unloaders and clearance pockets are special devices that control the capacity, or flow, carried by the compressor. Suction valve unloaders manipulate the suction valves’ action to allow the gas to recycle. Clearance pocket systems alter the cylinder head space, or clearance volume. They may be fixed or variable volume. Clearance pockets should usually be avoided, particularly the variable volume clearance pockets because they could offer some reliability and operational problems.

The suction valve unloader method is a preferred capacity control method on reciprocating compressors, if the speed variation is not possible or sufficient to achieve the required part-load ranges. The minimum speed because of some limitations in the compressor frame or VSD electric motor system might not be sufficient for the necessary reduction in the volume flow. The capacity control using suction valve unloading in its simplest form is carried out via an unloader system, which keeps the suction valve open and lets the gas return to the suction. In other words, suction valve unloaders are mechanisms that are held open or bypass suction valves at each end of double-acting cylinders. This provides complete unloading of one or both ends of the cylinder. For a single-cylinder (double-acting) compressor stage, suction valve unloaders can achieve three-step loading that provides nominal cylinder capacities of 0%, 50%, and 100%. Thus, considering two (double-acting) cylinders per stage, the capacity can be controlled theoretically in five stages 0%, 25%, 50%, 75%, and 100%.

Unloaders are usually pneumatically operated by instrument air and equipped with positioner indication. The unloader actuator should be sized to operate on minimum air pressure, as well. All lines to and from unloaders are fabricated from a proper grade of stainless steel —  usually stainless steel 316 — since any issues or problem in such a system can damage the whole compressor package.

Another option is the step-less capacity control using a sophisticated control system combined with the suction valve unloaders. The operation of the suction valve unloader, which acts against the closing of the valve components, can be adjusted via a sophisticated control system in a way to control the volume of gas that will be compressed in each cycle. In each compression cycle, the suction valve closes only at a specific point that the step-less system calculated; therefore the intake of gas is reduced to a desired flow. With this kind of controlled and precisely actuated suction valve unloader system, the specific work for the compression of the gas might be slightly increased; this results from the pressure difference between gas intake and gas delivery when the suction valve is held open. However, overall, such a step-less capacity control method can offer reasonable part-load efficiencies, whereas it adjusts the flow in the range of about 15% to 100% of the nominal flow in a step-less fashion.

In relatively reciprocating compressor installations, regulating the suction valve via hydraulically operated and electronically controlled actuators has been accepted and used as the step-less capacity control system. They are expensive and special systems which should be used in medium or relatively large machines where such a step-less fast response capacity control is required. The suction valve unloader will be actuated by a hydraulic cylinder which in turn is controlled via a solenoid valve. The control signal is generated in an electronic unit which receives its information from many different sensors. For instance, the system receives information about the actual position of the piston from a top dead center sensor and of the mass flow requirement from the control system. The advantages of this combined control system lies in the fast response and the freely programmable reaction of the movement of the suction valve unloaders. This system can coordinate the control of all regulated suction valves so that, also at part-load, the inter-stage pressures can be kept at the desired levels. This control can also be used for unloaded startup to reduce the load on the electric motor driver.

The clearance volumes, as the capacity control method, were popular many years ago. They are not popular any more. Particularly the variable volume clearance pockets have resulted in many operational and reliability issues and should be avoided. In such a system, the adjustable volume flow was controlled by the movement of a hydraulically positioned piston which altered the size of the clearance volume. Some actuator failures were reported for such a system. In other cases, the variable volume clearance pockets failed to operate because of weakness of such variable completed clearance pockets, whether automatic or manual versions, and the intermittent operation that can cause many operational problems and damages to them. The fixed volume clearance pockets might be used in some special compressors where one cylinder per stage is used and there is no opportunity to use variable-speed or other superior capacity control methods to achieve the required part-load operation. Such a capacity control method works by connecting the working chamber with an additional clearance pocket and thereby reducing the volume flow by reducing the volumetric efficiency. This type of control reduces the volume flow while reducing the energy input within acceptable limits.

Amin Almasi is a rotating equipment consultant in Australia. He is a chartered professional engineer of Engineers Australia (MIEAust CPEng – Mechanical) and IMechE (CEng MIMechE), and he holds a master’s degree in mechanical engineering. Almasi also is a registered professional engineer in Queensland (RPEQ). He specializes in rotating machines including centrifugal, screw, and reciprocating compressors, gas turbines, steam turbines, engines, pumps, offshore rotating machines, LNG units, condition monitoring, and reliability. Almasi is an active member of Engineers Australia, IMechE, ASME, and SPE. He has authored more than 100 papers and articles.

Bypass control, or recycle control, uses an external bypass around the compressor to recycle gas from the compressor discharge to the inlet. The take-off point for the bypass should preferably be the downstream of the discharge cooler so that cooled gas will be spilled back to the suction. Bypass control is inefficient. However, for transient, short-time or fine adjustment, it is preferred over other control methods because of its smoothness and simplicity. This method is commonly accompanied by the use of suction valve unloaders or fixed clearance pockets, which reduce compressor capacity in discrete predetermined steps.

In a multi-stage reciprocating compressor, a reduction in the flow of the first stage can cause a drop of all inter-stage pressures —  suction pressure to the next stage — and consequently it can lead to excessive high-pressure ratios and discharge temperatures in subsequent stages. Moreover, this pressure shifting may cause an overload in the last stage. The minimum capacity that can be obtained depends on the number of compression stages. On the other hand, when more stages are used and each stage has relatively low pressure ratios, a better capacity control could be performed, the more stages used for a given overall compression ratio, the wider the achievable control range.

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