FEATURES OF THE SCREW COMPRESSOR
The rotary screw compressor is a positive displacement machine that operates without the need for suction or discharge valves. It has the ability to vary suction volume internally while reducing part load power consumption. Screw compressors provide a much wider operating range and lower maintenance costs than conventional reciprocating gas compressors. Screw compressors are also much smaller and create much lower vibration levels than reciprocating compressors.
Screw Gas Compressors Offer Reduced Maintenance
The only significant moving parts in a screw gas compressor are the male and female rotors. There are no valves, pistons, rings, or connecting rods that require regular maintenance. With the elimination of the pistons, rings and valves, annual maintenance costs are also reduced on screw gas compressors. It is not uncommon to operate screw compressors for several years without ever performing any significant maintenance repairs. When comparing screw gas compressors and reciprocating gas compressors, it is important to consider maintenance costs into the overall project cost.
Screw Compressor Turn Down for Gas Compression Applications
Screw compressors offer turn down capabilities up to 90% of full load with very good part load power requirements. This turn down capability occurs within the machine and is independent of engine speed or bypass. This makes the machine an attractive alternative for gas compression applications where flow rates and operating conditions are not constant. The capacity control can typically be handled manually or automatically within the machine to meet the exact demands of the overall system. Controlling the flow rate of natural gas through the machine significantly reduces the need for an external discharge to suction bypass valve. This bypass system is the most common means of automatic capacity control on reciprocating compressors. Using an external bypass valve or a suction pressure control valve consumes much more power than reducing the flow rate of gas within the compressor. The ability to adjust capacity within the machine is comparable to varying the stroke length and suction volume on a reciprocating machine. However, the screw compressor can vary capacity automatically where the reciprocating machine must have the variable volume pockets adjusted manually. The turn down capability using variable volume pockets is much less than the screw machine as well.
Screw Compressors Offer High Compression Ratios
Screw compressors can operate from roughly 2 to 20 ratios of compression on a single stage while maintaining high volumetric efficiencies. These efficiencies are achieved by injecting large quantities of lube oil into the machine during the compression process. Oil is typically injected into the compressor at an approximate rate of 10-20 USGPM per 100 HP, which significantly reduces the discharge temperature of the process gas. The oil is discharged with the process gas and will be removed later from the gas stream. The oil also serves as the driving force for the nondriven rotor. Where dry screw compressors rely on timing gears, the oil flooded screw compressor uses a film of lube oil between the drive and non-drive rotors.
Reciprocating compressors can operate at much higher pressures than screw compressors, but are typically limited to roughly 4 ratios of compression per stage. Multi-staging and intercooling these compressors is required to prevent poor volumetric efficiencies, rod loading and excessive discharge temperatures. In applications where the system compression ratios exceed four, multi-stage reciprocating machines will offer better adiabatic efficiencies, or power requirements, than the single stage screw compressors. In this case the user must weigh the advantages of the screw compressor to the power savings of the reciprocating compressor.
Screw Compressors Accommodate Wide Operating Ranges
The screw compressor itself can operate over a very wide range with little or no changes required to the machine. This makes it very well suited for natural gas compression applications where flow rates and operating conditions are often changing. Over time, as field conditions change, many reciprocating compressors encounter rod loading or temperature problems and require costly retrofits. This often entails changing cylinders, changing cooler sections and often restaging the machine. In contrast, screw compressors are designed to operate over the entire range with no changes to the compressor itself.
Screw Compressors Enable Smaller Package Sizes
Rotary screw gas compressors provide high capacities with minimal installation space compared to reciprocating compressors. Based on a full speed design of 3,600 rpm, a large screw gas compressor can provide over 50 MMSCFD of gas based on a 100 psig suction pressure. The physical size of the compressor is much smaller than a comparable reciprocating gas compressor.
Lower Vibration with Screw Gas Compressors
With only two major moving parts operating in a circular motion, screw compressors create much lower vibration levels than reciprocating compressors. Although the slide valve assembly also moves to control capacity, it happens at such a slow rate that we do not consider it a maintenance concern. With lower vibration levels, screw compressors do not require the same type of foundation as the reciprocating compressors, which can result in lower installation costs.
In general, screw compressors for gas compression are considered to provide very high reliability, resulting in lower maintenance costs and reduced down time compared to reciprocating compressors.
SCREW COMPRESSOR BASIC OPERATING PRINCIPLES
A rotary screw compressor is very simple in design. Some of the major components include one set of male and female helically grooved rotors, a set of axial and radial bearings and a slide valve, all encased in a common housing. Figure 2 represents a typical cutaway of a rotary screw gas compressor. The slide valve cannot be seen in this picture. It is a “V” shaped device located along the bottom of the machine between the male and female rotors. We will look at the slide valve in detail later in this paper.
