Screw compressors have been used on air and various refrigeration and process applications for a great number of years. In recent years the machines have become very popular in the natural gas industry in booster and gas gathering applications. Declining field pressures in the USA and Canada are forcing the industry to look at more flexible alternatives to the conventional reciprocating compressor.
This article will discuss specific applications where screw compressors are used and the advantages of the rotary screw to conventional reciprocating machines. We will look at some specific features of the screw compressor, which make it the machine of choice for many applications requiring high reliability, low maintenance costs and a very wide overall operating range.
We will look at a graphic model of a typical process flow diagram and review the components required to make up a rotary screw compressor package. We will also take a detailed look at the internals of the machine to better understand the overall operation, capacity control system, and the associated power savings that go with it.
It is extremely important to optimize adiabatic efficiencies in order to provide reduced power costs. On rotary screw compressors this is done using a feature called Vi, or internal volume ratio.
The Vi can be changed on different machines using a couple of different methods. We will compare both these methods and their associated advantages.
Project economics always play an important role in any equipment selection. With the proper flexibility built into the initial package, these units can be used on numerous applications.
Oil flooded rotary screw compressors have been widely used on various air and refrigeration applications for over forty years. These machines did not make a significant presence in the natural gas industry until the early 1990’s. Until this time, reciprocating compressors had been used almost exclusively for natural gas compression. As gas fields have matured and field pressures have dropped, screw compressors have become a very attractive alternative and supplement to reciprocating machines.
This article will discuss some of the applications and features of screw compressors, basic operating principles and the advantages of the rotary screw over conventional reciprocating compressors for the natural gas compression industry. We will look at the machine itself, as well as the overall compression system and the components required in a screw compressor package. The screw compressors we will focus on are the oil flooded, heavy duty process gas machines rather than the air derivative types. We will provide numerous illustrations to help better understand the screw machine.
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Figure 1 shows the basic geometry of the rotary screw compressor. View (A) is a simple representation of the actual rotors. We have labeled the male rotor lobe and the female rotor flute. As the rotors turn in an outward direction, the male flute will unmesh from the female flute forming an area for the gas to enter. The gas becomes trapped in the machine and compression occurs when the lobes of the rotors begin to mesh together again. The shaded area represents the pocket of gas that occurs within a specific flute. View (B) is a representation of the side view of the machine. The same flute is shaded for comparison. Here we see the suction port in the upper left corner and the discharge port in the lower right corner. The rotors will turn in an outward direction forcing the male and female
flutes to unmesh, allowing process gas to enter the top of the machine. The gas will travel around the outside of the rotors until it reaches the bottom where the compression actually occurs. Gas will be discharged in the lower right corner of the picture.
Screw Compressor Features:
The rotary screw compressor is designed for low pressure applications with inlet pressures ranging from vacuum pressure up to 100 psig and discharge pressures up to 350 psig. These pressure ranges are typical for most process style machines and can vary depending on manufacturer, frame size and operating speed. There are some screw machines available capable of operating at higher pressures by using cast steel casings but these are not yet commonly used in the natural gas industry due to capital cost and availability.
Screw compressors are commonly used in a variety of air, process gas, process refrigeration and natural gas applications, including individual wellhead boosters, low pressure gathering systems, low stage boosters to existing reciprocating machines, solution gas and flare gas compression. They have been used on sweet and sour gas as well as acid gas applications with H2S concentrations of 35% and CO2 concentrations of 65%. Although most natural gas applications are based on a specific gravity of 0.57 – 0.65, screw compressors can be used on very light gases such as hydrogen and very heavy mole weight gases where specific gravities exceed 2.0.
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The most common applications for screw compressors in natural gas service range in horsepower from roughly 90 to 1,500 and are available in both engine and electric drive. Screws were originally developed to operate with electric drive two pole motors at 3,550 rpm. As they have become more popular in the natural gas industry, engine drive applications have become much more common. On most of these applications, the screw is operating direct drive at 1800 rpm, or half the rated speed.
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. Screws provide a much wider operating range and lower maintenance costs than conventional reciprocating machines. The machines are much smaller and create much lower vibration levels than piston machines as well.
Reduced Maintenance
The only significant moving parts in a screw 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 machines. It is not uncommon to operate screws for several years without ever performing any significant maintenance repairs. When comparing screw compressors and reciprocating machines, it is important to consider maintenance costs into the overall project cost.
Turn Down
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 areas 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 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 machines. Using an external bypass valve or a suction pressure control valve consumes much more power than reducing the flow rate of gas within the machine. 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.
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 machine 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 non-driven rotor.
Where dry screw compressors rely on timing gears, the oil flooded screw uses a film of lube oil between the drive and non-drive rotors.
Reciprocating machines can operate at much higher pressures than screw machines, but are typically limited to roughly 4 ratios of compression per stage. Multi-staging and inter-cooling these machines 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 machine to the power savings of the reciprocating compressor.
Accommodates 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 to the natural gas industry where flow rates and operating conditions are often changing. Over time, as field conditions change, many reciprocating machines encounter rod loading or temperature problems and require costly retrofits. This often entails changing cylinders, changing cooler sections and often re-staging the machine. In contrast, screws are designed to operate over the entire range with no changes to the machineز
Smaller Package Sizes
Rotary screws provide high capacities with minimal installation space compared to piston machines. Based on a full speed design of 3,600 rpm, a large screw machine 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 piston type machine.
Lower Vibration
With only two major moving parts operating in a circular motion, screws create much lower vibration levels than reciprocating machines. 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 machines, which can result in lower installation costs.
In general, screw compressors are considered to provide very high reliability, resulting in lower maintenance costs and reduced down time compared to reciprocating machines.
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. the figure bellow represents a typical cutaway of a rotary screw compressor.
The slide valve can not be seen in this picture. It is a “V” shaped device located along the bottom of the machine between the male and female rotors.
The Compression Process
Most people are familiar with the basic operation of a reciprocating compressor. To help understand the operation of a screw compressor, we will compare the compression process to a piston type machine.
Lets think of the male rotor lobe as a piston and the female flute as a cylinder. On a reciprocating machine, as the piston begins to pull away from top dead center, the suction pressure overcomes the pressure inside the cylinder, forcing the suction valve open and allowing gas to enter.
We need to remember that a screw compressor does not have any suction or discharge valves. On a screw compressor, as the rotors begin to unmesh the male rotor lobe will roll out of the female rotor flute. The volume vacated by the male rotor will fill with suction gas. As the rotors continue to unmesh, the volume in each flute will increase. the figure 3 bellow shows a comparison of the beginning of the suction process on a reciprocating machine and a screw compressor. View (A) represents a single acting reciprocating cylinder. View (B) illustrates a top view from the suction end showing the rotors unmeshing and allowing the gas to enter. View (C) shows a side view of the screw compressor at the same point. We can only show the rotor closest to you as the other one is hidden. In order to keep the illustrations simple, we will only show one rotor flute in operation.
On a reciprocating machine, gas will continue to enter the cylinder until the piston reaches the end of the stroke, or bottom dead center. At this point the suction valve will close and the input volume of the cylinder is established. If we multiply the input volume of each cylinder by the number of cylinders and then by the compressor rpm, we will establish the compressor displacement.
On a screw compressor, gas will continue to enter each flute until the rotor lobes roll out of mesh with each other. As the rotors finish unmeshing with each other, the flute passes by the edge of the suction port, closing it off from the system. The point where the flute travels past the edge of the suction port is where the maximum volume of that flute occurs. This represents the suction volume of the flute. The suction volume is the volume of trapped gas within the flute at the end of the suction process. Multiplying the volume of input gas in the male and female flute by the number of lobes on the male rotor and then by the rotor rpm establishes the displacement of the screw compressor. Figure 4 shows a comparison of the reciprocating and screw machines at the end of the suction process. Once again, view (A) represents the reciprocating cylinder, view (B) and (C) are the screw compressor. The
volume of trapped gas in the cylinder is still at suction pressure because the compression process has not begun yet.
Once the suction process is completed and the input volume is established, the compression process can begin. Figure 5 shows the same comparison of the reciprocating and screw machines during the compression process. On the reciprocating machine, the piston begins to move upward away from bottom dead center, reducing the volume in the cylinder and causing an increase in pressure of the trapped gas. Here the screw compressor is not unlike the recip machine. As the rotors continue to rotate, they begin to mesh together along the bottom. Once again, lets think of the male rotor lobe as a piston and the female flute as a cylinder. As the rotors mesh together the male rotor lobe moves into the female flute and reduces the volume in the flute. View (B) now shows the bottom of the rotors to illustrate where the compression occurs. Keep in mind, we are still looking at the rotors from the suction end. The compression will continue as the gas moves toward the discharge port. View (C) shows the volume in the flute beginning to reduce through compression.
The compression process will continue in the reciprocating machine until the internal pressure within the cylinder exceeds the discharge pressure of the system. At this point the discharge valve will open and allow the trapped gas within the machine to escape. The discharge process of a screw compressor is very different than a reciprocating machine. There are no valves in the screw to allow discharge gas to escape from the flute. The location of the discharge port along the axis of the rotors is critical as it determines when the compression process is complete, and the discharge process begins.
If we look at the individual flute containing the trapped gas we see two rotor lobe tips, one on the discharge side of the flute and one on the backside of the flute. The tip on the discharge side is called the leading tip, as it will be the first one to reach the discharge port. The second tip is referred to as the trailing tip. Figure 6 shows the leading and trailing tip of the rotor lobes. As the leading tip of the rotor passes by the edge of the discharge port, the compression process is complete and the gas will be forced into the discharge line.
We saw how the suction volume of the compressor was established in Figure 4. The discharge volume of the machine is the volume of gas trapped in the flute just before the leading tip of the rotor enters the discharge port. Figure 6 shows a comparison of the reciprocating machine and the screw compressor at the beginning of the discharge process.
The discharge process will be completed on a reciprocating compressor when the piston reaches top dead center and the discharge valve closes again. There must be a small clearance between the top of the piston and the head to avoid piston damage. There will always be some inefficiency in the reciprocating machine due to this trapped pocket of gas left behind by the piston. The volume of gas that remains in the cylinder will re-expand during the next suction process. This will reduce the amount of gas entering the cylinder, causing a reduction in volumetric efficiency of the machine. On high compression ratio applications, this will cause a significant decrease in efficiency, resulting in reduced flow rates.
The discharge process on a screw compressor will continue until the male rotor lobe has completely rolled into the female flute, displacing all of the gas and lube oil remaining in the threads.
Unlike the reciprocating machine, there are no trapped pockets of gas remaining in the machine. As a result, the volumetric efficiency of a screw compressor remains very high as pressure differentials across the machine increase.
Figure 7 provides a comparison of the reciprocating and screw machines at the completion of the discharge process. In View (B) we are still looking at the bottom of the rotors, but we are now looking from the opposite end. All of the previous rotor pictures have been from the suction end of the machine. We are now looking from the bottom at the discharge end of the rotors.
Screw compressors have two discharge ports, referred to as the radial and the axial port. The radial port is the “V” shaped cut in the slide valve and the axial port is the butterfly shaped port machined in the end casing of the compressor between the bearing bores. The discharge process will start when the leading tip of the rotor opens to the radial cut out area in the slide valve as shown in Figure 6 above. The axial port will relieve the last bit of trapped gas and lube oil from the rotors as the male lobe completely fills the female flute, closing the threads as shown in Figure 7 above.
In Figure 8 we can see the radial discharge port machined in the slide valve. The view is from the discharge end of the compressor with the rotors removed from the casing. The two bore holes will normally form the casing around the rotors. In Figure 9 we can see the rotor housing of the machine with the discharge end casing attached. The top half of the rotor housing has been cut away to show the internals. As you can see, the ports are common to the discharge flange of the compressor. They are defined differently because they each serve different purposes.
References:
1. A Practical Guide to Compressor Technology 2nd ed – Heinz P. Bloch (Wiley, 2006).
2. Screw Compressors: a comparison of Applications and Features to Conventional Types of Machines – J. Trent Bruce / Toromont Process Systems Calgary, Alberta, Canada.