Maximizing Screw Compressor Performance
A brief overview of compressor technologies will highlight the key characteristics of each type (figure 1).
- Centrifugal compressors typically are sized for large-capacity (1,000 cfm and higher) chemical and process industry systems often driven by gas turbines, steam or electric motors. They are most efficient at higher suction pressures and therefore are used for chilling water in air-conditioning applications.
- Rotary vane compressors commonly were used 30 years ago in combination with reciprocating high-stage compressors. Today, they are limited to booster applications and are not suitable for high compression ratios. Few rotary vane compressors are sold for new applications.
- Reciprocating compressors were once the compressor of choice for industrial refrigeration plants. Their use in new installations has been significantly reduced due to the acceptance of the screw compressor in this market. Reciprocating compressors are first-cost effective and suitable for low and high compression ratios; however, higher maintenance costs, a smaller cfm range (25 to 1,500 cfm), and the need to use multiple machines has led to the reduction in use of these compressors.
- Screw compressors, which use a slide valve for precise suction control, are available as twin- and single-screw designs. The twin-screw is widely used today and will be the focus of this article.
Because fewer screw compressors are re-quired to operate a system than when using reciprocating compressors, systems utilizing screw compressors have lower capital costs. Today's screw compressor capacities range from 70 to over 7,000 cfm and are used in industries such as warehousing, food processes, petrochemical, pharmaceutical, oil refining and natural gas processing.
Screw compressors of-fer advantages over other types of compressors; however, several factors need to be considered when selecting a screw compressor that will lead to efficient refrigeration operation. These items include compressor performance with regard to capacity and efficiency over the range of expected operating suction and discharge pressures; oil cooling methods; and compressor volume index (Vi).
Screw Compressor Performance vs. Volume IndexBy design, screw compressors are positive-displacement machines. A screw compressor traps a fixed volume of gas on the suction side and increases its pressure by reducing the internal volume of the compression chamber, thereby raising its pressure at the discharge side. The ratio of the volume of gas in the compressor cavity when the suction port closes to the volume of gas in the compressor cavity when the discharge port opens is referred to as the compressor's volume ratio or volume index. This volume index (Vi) determines the internal pressure ratio of the compressor.
Suction pressure is a function of the refrigeration load and the amount of compression available. The resulting suction pressure and the refrigerant type determine the suction gas volume, and the discharge volume is a function of the compressor Vi. Therefore, the discharge pressure at the compressor discharge port is a function of the suction pressure, refrigerant type and compressor Vi.
The system discharge pressure is a function of the condenser size and outside ambient conditions. This means the compressor discharge pressure may not match the system discharge pressure. If the compressor Vi does not match the current system conditions, either under-compression or over-compression will occur. In either case, the compressor will continue to operate and the same volume of gas will be moved, but power is wasted.
Fixed vs. Variable ViScrew compressors can be fixed Vi or variable Vi machines. Both types of compressors have advantages and disadvantages, but the main objective of either is to match the volume index of the compressor as closely as possible to system conditions.
The majority of screw compressors in operation have a fixed Vi. Fixed-volume-index compressors offer the advantages of lower initial costs due to fewer oil lines and control solenoids, fewer moving parts to reduce maintenance costs and greater reliability. The main disadvantage of a fixed Vi compressor is its inability to change with the system's fluctuating operating conditions, resulting in periods of performance inefficiencies.
Fixed-volume-index compressors have a limited number of increments, making compressor selection critical for efficient operation. Some typical screw compressor Vi ratings are 2.6, 3.6 and 5.0. A low Vi (2.6) typically is used on booster applications while a high Vi (5.0) is used on high- or single-stage applications (figure 2).
Variable-volume-index compressors have internal slide stops used to change the Vi. Some machines are capable of automatically varying the volume index continuously during operation while others are adjusted manually. As a system's condensing pressure changes, an automatically variable Vi compressor essentially repositions the discharge port, reducing the possibility of over- or under-compression. Variable-volume-index machines are desirable in applications with a large range of suction conditions or with a large variance in discharge pressure.
Now one might ask, if a variable-volume-index compressor is designed to continuously match the system's operating conditions, why wouldn't this type of compressor always be selected? Most automatically variable Vi compressors only operate with the correct Vi while at 100 percent capacity. Unloading of the slide valve to maintain a constant suction pressure causes changes to the internal Vi of the compressor that often leads to a non-optimum Vi condition. In some cases, the horsepower actually can increase as the machine unloads. Therefore, if a variable Vi compressor runs unloaded, it may use more horsepower than a fixed Vi compressor that is unloaded.
In addition, Vi adjustment is only as good as the calibration of the slide stop positioning, suction-pressure transducer and discharge-pressure transducer. If any of these are out of calibration, the controller may set the machine at an improper Vi position. Finally, as noted before, with a variable Vi compressor, there are more moving parts and controls, resulting in a higher first cost and maintenance.
Compressor Performance vs. Suction/Discharge PressureOperating a screw compressor at a higher suction pressure, at a lower discharge pressure, or both, always can save energy.
What happens when suction pressure is raised? The compressor uses more power, saving overall energy use. How is this possible? Higher suction pressure means there is more mass flow, thus the compressor capacity increases. The capacity increase (tons refrigeration, or TR) offsets the power increase (brake horsepower, or BHP), resulting in better BHP/TR (table 1).
At the higher suction pressure of -15oF (-26oC) saturated suction temperature (SST), the increased capacity more than offsets the increase in power. The compressor will either unload using less BHP, or fewer compressors will need to operate compared to the number of compressors needed at -20oF (-29oC) saturated suction temperature.
What happens when discharge pressure is lower? The compressor uses less power and gains capacity. The compression ratio (discharge pressure to suction pressure) is smaller and the compressor uses less BHP. The lower discharge pressure produces a lower condensed liquid refrigerant temperature, resulting in less flash gas to cool the refrigerant, less non-useful gas vapor to be compressed and more space in the compressor for useful gas vapor (table 2).
As when suction pressure is raised, lowering the discharge pressure (SDT) results in better BHP/TR.
Compressor Performance vs. Oil Cooling MethodsThe type of oil cooling selected also affects compressor efficiency. Liquid-injection or internal oil cooling is accomplished by injecting high-pressure liquid refrigerant into the side port of the compressor to absorb heat. The liquid is metered by a thermostatic expansion valve that regulates based on discharge temperature, typically maintaining between 110 to 140oF (43 to 60oC) discharge temperature. Some liquid refrigerant will flash off, absorbing the heat of compression, which in turn helps cool the oil.
Liquid-injection oil cooling is easy and inexpensive to install with minimal risk for complicated operation (figure 3). Compressor package costs also are reduced with this type of oil cooling because no heat exchanger or mixing valve is required to regulate oil temperature. However, there is a performance penalty associated with this method of oil cooling: It results in reduced compressor capacity. Flash gas must be compressed, adding horsepower; while the raised pressures inside the rotors cause greater leakage, reducing capacity.