Compressors

1. Introduction

Compressors have played a major role in setting our standard of living and they have contributed significantly to the industrial revolution. Early compressors like the bellows (used to stoke a fire or the water organ use to make music) marked the beginning of a series of compression tools. Without compression techniques we could not have efficiently stabilized crude oil (by removing its trapped gasses) or separated the various components of gas mixtures or transported large quantities of gas cross country via gas pipelines. Today, compressors are a part of our everyday existence. Compressors exist in almost every business and household as vacuum cleaners and heating, refrigeration & air conditioning blowers.

Air or gas compressors may define as: "Compressors are machines designed for compressing air or gas from an initial intake pressure to a higher discharge pressure (by reducing gas volume)."

1.1 History

The history of compressors is as varied as are the different types of compressors. Somewhere in ancient times the bellows was developed to increase flow into a furnace in order to stoke or increase furnace heat. This was necessary to smelt ores of copper, tin, lead and iron. This led the way to numerous other inventions of tools and weapons.

One of the earliest recorded uses of compressed gas (air) days back to 3rd century B.C. This early use of compressed air was the “water organ.” The invention of the “water organ” is commonly credited to Ctesibius of Alexandria1. The concept was further improved by Hero of Alexandria (also noted for describing the principles of expanding steam to convert steam power to shaft power).

Ctesibius also developed the positive displacement cylinder and piston to move water.

In the 1850sPhilander and Francis Roots devised what has come to be known as the Roots blower.

In 1808 John Dumball envisioned a multi-stage axial compressor.        Unfortunately his idea consisted only of moving blades without stationary airfoils to turn the flow into each succeeding stage. Not until 1872 did Dr. Franz Stolze combine the ideas of John Barber and John Dumball to develop the first axial compressor driven by an axial turbine.

1.2 Function of a Compressor

A compressor increases the pressure of a gas. This means that reduces the volume of the gas and increases its density without turning that gas into a liquid. Compressors can do this in a number of ways. However, the commonality between all compressors is the fact that they all use some sort of fuel, such as gasoline or electricity, to power whatever compression method they use. Also, because the compressor increases the pressure on the gas, it increases the temperature of the gas. Many other types of compressors are used for various chemicals and fuels that require compression.

An air compressor is a versatile device used for supplying compressed air and/or power into a specific space. It makes a vital mechanical device for the homeowners (refrigerators and air conditioners), jet engines, commercial businesses, refining industries, manufacturing industries, and automotive industries. In fact, air compressor has been used in the industries for more than 100 years.

1.3 Working Of compressor

In an air compressor, there are two major parts - a compressing system and a power source. The compressing mechanism can be a piston, rotating impeller, or vane depending upon which type of compressor you are referring to. As for the power, it is supplied by an electric motor or other energy sources. The compressing mechanism, as the name suggests, helps in compressing atmospheric air by using energy from the power source.

The basic working principle of an air compressor is to compress atmospheric air, which is then used as per the requirements. In the process, atmospheric air is drawn in through an intake valve; more and more air is pulled inside a limited space mechanically by means of piston, impeller, or vane. Since the amount of pulled atmospheric air is increased in the receiver or storage tank, volume is reduced and pressure is raised automatically. In simpler terms, free or atmospheric air is compressed after reducing its volume and at the same time, increasing its pressure. 

There is a pressure setting knob that can be manipulated as per the demands of the operator. When pressure in the receiver or tank increases to the maximum level, the pressure switch is shut off and intake of air in the compressor is stopped. Contrary to this, when the compressed air is used, the pressure inside the compressor falls. As a consequence, the pressure drops to a low setting, and the pressure switch is turned on, thus allowing atmospheric air to enter the unit. This way, the cycle of taking air inside the unit and removing compressed air continues in an air compressor. 

1.4 Compressors Vs Pumps

         Compressors are similar to pumps, both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas. Liquids are relatively incompressible - a liquid, the volume of a liquid does not change with pressure and temperature. While some can be compressed, the main action of a pump is to pressurize and transport liquids. If a gas is used in a pump or liquid in compressor as a fluid, it is very dangerous to their parts i.e. impellers etc.

           

 Another difference between a compressor and a pump is their parts i.e. volute in pumps is analogous to diffuser in compressor.

1.5 Compression theory

Compressors are mechanical devices used to increase the pressure of air, gas or vapor and in the process move it from one location to another. The inlet or suction pressure can range from low sub-atmospheric pressure levels to any pressure level compatible with piping and vessel strength limits. The ratio of absolute discharge pressure to absolute suction pressure is the compressor pressure. Stage compression is limited to the mechanical capabilities of the compressor and, generally approaches a CR of 4. To achieve high pressures multiple stages must be employed.

Compression theory is primarily defined by            the Ideal Gas Laws and the First & Second Laws of Thermodynamics. As originally conceived the Ideal Gas Law is based on the behavior of pure substances and takes the following form:

                         PV = RT                                             (1.1)

Where,

             P = Absolute Pressure

             V = Specific Volume

             R = Gas Constant

             T = Absolute Temperature

This equation is based on the laws of Charles, Boyle, Gay-Lussac and Avogadro (see Appendix B2 Glossary of Terms). Note all properties should be defined in the same measuring system.

The ideal gas law can be manipulated to obtain several useful relationships, like

PQ = WRT                                                                    (1.2)

Where,

Q = Volumetric Flow Rate

W = Mass Flow Rate

By using compressibility factor, the ideal gas equation becomes

 PV = ZRT                      (1-3)

and

             PQ = ZWRT                 (1-4)

          

  Compressor performance is generally shown as pressure ratio plotted against flow.

A pure substance is one that has a homogeneous and constant chemical composition throughout all phases (solid, liquid and gas). For most compressor applications a mixture of gases may be considered a pure substance as long as there is no change of phase. The significance of introducing this concept is that the state of a simple compressible pure substance is defined by two independent properties.

Partial pressure

The total pressure is equal to the sum of the partial pressures

 P = P1 + P2 + P3 + ...                         (1-5)

This relationship is defined by Dalton’s Law (see Appendix B2). If the total pressure of the mixture is known than the partial pressure can be calculated from the mole fraction.

horse power calculations

The brake horse power (BHP) require to drive the compressor can be determined by calculating the gas horsepower (GHP) and then correcting for mechanical losses.

              Hd × Wg

 GHP = ———————                              (1-6)

             60 × 33,000 × Ep

Where

BHP = Brake horse power

Hd = Head (adiabatic) – ft-lb/lb

Wg = Weight flow of the gas – lbs/hr

Ep = Adiabatic efficiency

and

            SCFD × MW

Wg = ———————                                               (1-7)

            24 × 379.5

If capacity is available GHP can be calculated directly.

The brake horsepower is

BHP = GHP x (1 + % Mechanical Losses)        (1-8)

1.7 Efficiency of a compressor

A compressor system may consist of a compressor, a motor, a controller and other devices such as a water pump for the purpose of water injection. Therefore, a system efficiency of a compressor system, ῃ _sys, can be defined as a series product of compressor overall efficiency, ῃ_overall, motor efficiency, ῃ _motor, controller efficiency, ῃ _controller, and her efficiency of an auxiliary device, ῃ _auxiliary, as:

 1.9

Where,

(1.10)

And generally described as,

                                                                      

 (1.11)

1.8 capacity and Speed of a compressor

The capacity of an air compressor is determined by the amount of free air (at sea level) that it can compress to a specified pressure, usually 100 psi per minute, under the conditions of 68°F and a relative humidity of 38 percent. This capacity is expressed in cubic feet per minute (CFM) and is usually included in the nomenclature of the compressor.

The number of pneumatic tools that can be operated at one time from an air compressor depends on the air requirements of each tool; for example, a 55-pound class rock drill requires 95 CFM of air at 80 psi. A 210-cfm compressor can supply air to operate two of the drills, because their combined requirements are 190 CFM.

However, if a third such drill is added to the compressor, the combined demand is 285 CFM, and this condition overloads the compressor and the tools and results in serious wear.

1.10 compressor Lubrication

The lubricant has four key responsibilities in every lubricated component application, including reducing friction and wear, removing heat, removing contaminants, and preventing corrosion.

In some types, Positive displacement compressors require some lubricants for their performance, classified as oil based or non oil based.

Dynamic compressors do not require a lubricant within the compression chamber and can consequently deliver oil-free air or gas, which is desirable for many refining and process gas applications requiring large volumes of hot gas to supply production needs. Dynamic compressors employ both element and hydrodynamic journal and thrust bearings in their designs. The choice is heavily influenced by compressor size and application.

The lubricant selection for centrifugal and axial flow compressors is relatively simple. The components, high speed gears and plain bearings, element bearings and seals operate at speeds that create hydrodynamic and electro-hydro dynamic conditions, as long as the oil supply is maintained. Accordingly, oil fortification is predominantly for corrosion, heat and oxidation resistance and only to a slight degree for physical surface protection. Small compressors may be equipped with grease-lubricated element bearings.

1.11 Compressor drivers

There are many types of equipment, often referred to as prime movers, which can be used to drive a compressor:

·         Steam turbines and gas turbines

·         Natural gas engines, gasoline engines and diesel engines

·         Electric motors

·         Hydraulic power systems

1.12 APPLICATIONS OF COMPRESSORS

Gas compressors are used in various applications where either higher gas pressures or lower volumes of gas are needed:

·         In pipeline transport of purified natural gas to move the gas from the production site to the consumer.

·         In petroleum refineries, natural gas processing plants, petrochemical and chemical plants, and similar large industrial plants for compressing intermediate and end product gases.

·         In refrigeration and air conditioning systems

·         In gas turbine systems to compress the intake combustion air

·         In storing purified or manufactured gases in small volume, high pressure cylinders for medical, welding and other uses.

·         In many various industrial, manufacturing and building processes to power all types of pneumatic tools.

·         In pressurized aircraft to provide a breathable atmosphere of higher than ambient pressure.

·         In some types of jet engines (such as turbojets and turbofans) to provide the air required for combustion of the engine fuel. The power to drive the combustion air compressor comes from the jet's own turbines.

·         In SCUBA diving, hyperbaric oxygen therapy and other life support devices to store breathing gas in a small volume such as in diving cylinders.

·         In submarines, to store air for later use in displacing water from buoyancy chambers, for adjustment of depth.

·         In turbochargers and superchargers to increase the performance of internal combustion engines by increasing mass flow.

·         In rail and heavy road transport to provide compressed air for operation of rail vehicle brakes or road vehicle brakes.

TYPES OF COMPRESSORS

2. Types of Compressors

Operating conditions have a significant impact on compressor selection and compressor performance. The influences of pressure, temperature, molecular weight, specific heat ratio, compression ratio, speed, vane position, volume bottles, loaders and un-loaders, etc.          are the conditions that impact compressor capacity and therefore the compressor selection. They also impact the compressor efficiency.

There are many different designs that enable this work to be done. Each design has strengths and weaknesses that make it suitable for its respective application. Compressors may be classified according to the following output or discharge pressures: 

·         High pressure—greater than 2,000 kPa, gauge/290 PSIG

·         Intermediate pressure equal to 800–2,000 kPa, gauge/116–290 PSIG

·         Low pressure—equal to 100-800 kPa, gauge/14.5–116 PSI

The two basic classifications of compressors are

I)                    Positive displacement           II)      dynamic compressors

Positive displacement compressors are constant volume, variable energy (head) machines that are not affected by gas characteristics.

 Dynamic compressors are variable volume, constant energy (head) machines that are significantly affected by gas characteristics. Dynamic compressors, such as turbines and axial flow compressors, produce large volumes of relatively low pressure gas. The graph (2.1) is between pressure ratio and flow rate (CFM).

Graph (2.1) Pressure ratio vs flow rate

The type of compressor that will be used for a specific application therefore depends on the flow rate and pressure required and the characteristic of the gas to be compressed.

Chart 2-1 Types of Compressors

These types are further specified by:

·         the number of compression stages

·         cooling method (air, water, oil)

·         drive method (motor, engine, steam, other)

·      lubrication (oil, Oil-Free where Oil Free means no lubricating oil contacts the compressed air)

·         packaged or custom-built

In general, dynamic compressors are the first choice since their maintenance requirements are the lowest. The next choice are rotary type positive displacement compressors since they do not contain valves and are gas pulsation free. The last choices are reciprocating compressors since they are the highest maintenance compressor type and produce gas pulsations. However, the final selection depends upon the application requirements as discussed below.

2.1 Positive displacement compressors

Positive displacement compressors are used for low flow and/or low molecular weight (hydrogen mixture) applications. The various types are presented below.

2.1-1 Rotary Blowers Lobe

A rotary lobe compressor consists of identically synchronized rotors. The rotors are synchronized through use of an external, oil-lubricated, timing gear, which positively prevents rotor contact and which minimizes meshing rotor clearance to optimize efficiency. This feature also allows the compressor to be oil free in the gas path. The rotors of the two-lobe compressor each have two lobes. When the rotor rotates, gas is trapped between the rotor lobes and the compressor casing.

A typical two-lobe blower operating sequence is shown in Figure (2.1). Note that the upper rotor or lobe is turning clockwise while the lower lobe is turning counterclockwise.

Position #1: gas enters the lower lobe cavity from the left as compressed gas is being discharged from this cavity to the right and simultaneously gas is being compressed in the upper lobe cavity.

Position #2: the upper lobe cavity is about to discharge it’s compressed gas into the discharge line.

Position #3: gas enters the upper lobe cavity from the left as compressed gas is being discharged from this cavity to the right and simultaneously gas is being compressed in the lower lobe cavity.

Position #4: the lower lobe cavity is about to discharge it’s compressed gas into the discharge line.

Figure (2.1) a typical two-lobe blower operating sequence

The rotating rotor forces the gas from the gas inlet port, along the casing, to the gas discharge port. Discharge begins as the edge of the leading lobe passes the edge of the discharge port. The trailing lobe pushes the entrapped gas into the discharge port, which compresses the gas against the backpressure of the system. Rotary lobe compressors are usually supplied with noise enclosures or silencers to reduce their characteristic high noise level. A schematic of a two-lobe rotary compressor is shown in Figure (2.2).

Figure (2.2) Two Lobe Rotary Compressor

Applications

Rotary Blower Lobes type compressors are used in Conveying for (powder, polyethylene).

Precautions

Most problems can be avoided by first checking that the:

·         Driver operates properly before connecting it to the blower;

·         Blower turns freely before connecting it to the driver and process piping

·         Oil type is correct and

·         Oil reservoir is filled to the proper level

2.1-2 ROTARY VANE COMPRESSOR

A sliding-vane rotary compressor uses a series of vanes that slide freely in longitudinal slots that are cut into the rotor. Centrifugal force causes the vanes to move outward against the casing wall. The chamber that is formed between the rotor, between any two vanes, and the casing is referred to as a cell. As the rotor turns, an individual vane passes the inlet port to form a cell between itself and the vane that precedes it. As an individual vane rotates toward the end of the inlet port, the volume of the cell increases. The increase in the cell volume draws a partial vacuum in the cell.

Figure (2.3) sliding vane rotary compressor

 The vacuum draws the gas in through the inlet port. When a vane passes the inlet port, the cell is closed, and the gas is trapped between the two vanes, the rotor and the casing. As rotation continue toward the discharge port, the volume of the cell decreases. The vanes ride against the casing and slide back into the rotor. The decrease in volume increases the gas pressure. The high pressure gas is discharged out of the compressor through the gas discharge port. Sliding-vane rotary compressors are characterized by a high noise level that results from the vane motion. A schematic of a sliding vane rotary compressor is shown in Figure (2.3) & (2.4).

Application

Rotary vane air compressors are used in Air blowers (low volume). Also use as gas turbine starters.

Efficiency

Rotary vane compressors can have mechanical efficiencies of about 90%.

2.1-3 Rotary screw Compressor

A rotary screw compressor is a type of gas compressor which uses a rotary type positive displacement mechanism. They are commonly used to replace piston compressors where large volumes of high pressure air are needed, either for large industrial applications or to operate high-power air tools such as jack-hammers.

The gas compression process of a rotary screw is a continuous sweeping motion, so there is very little pulsation or surging of flow, as occurs with piston compressors.

Working

The single-stage design consists of a pair of rotors that mesh in a one-piece, dual-bore cylinder. The male rotor usually consists of four helical threads that are spaced 90 degrees apart. The female rotor usually consists of six helical grooves that are spaced 60 degrees apart.

The rotor speed ratio is inversely proportional to the thread-groove ratio. In the four-thread, six-groove, screw compressor, when the male rotor rotates at 1800 rpm, the female rotor rotates at 1200 rpm. The male rotor is usually the driven rotor, and the female rotor is usually driven by the male rotor.

A film of foil is normally injected between the rotors to provide a seal between the rotors and to prevent metal-to-metal contact. An oil-mist eliminator, installed immediately downstream of the compressor, is required for plant and instrument air service. However, designs are available that do not require lubrication.

dry screw-type compressors


            Screw compressors that do not require lubrication are commonly referred to as "dry screw-type compressors".

The inlet port is located at the drive-shaft end of the cylinder. The discharge port is located at the opposite end of the cylinder. Compression begins as the rotors enmesh at the inlet port. Gas is drawn into the cavity between the male rotor threads and female rotor grooves. As rotation continues, the rotor threads pass the edges of the inlet ports and trap the gas in a cell that is formed by the rotor cavities and the cylinder wall.

Further rotation causes the male rotor thread to roll into the female rotor groove and to decrease the volume of the cell. The decrease in the volume increases the cell pressure. Oil is normally injected after the cell is closed to the inlet port.

The oil seals the clearances between the threads and the grooves, and it absorbs the heat of compression. Compression continues until the rotor threads pass the edge of the discharge port and release the compressed gas and oil mixture. A typical single stage screw compressor is shown in Figure (2.6).

                               Figure (2.6) single stage screw compressor

Applications

Rotary screw compressor is used to plant and instrument air, low flow process (off gas, recycle, Sulfur blowers).

2.1-4 scroll compressor

A scroll compressor (also known as a scroll vacuum pump) uses two interleaved spiral-like vanes to compress gases. The vane geometry may be involute, Archimedean spiral, or hybrid curves. The scroll compressor concept was first developed in the early 1900s.

A scroll is an involute spiral which, when matched with a mating spiral scroll form as shown in Figure (2.7), generates a series of crescent-shaped gas pockets between the two scroll elements.

Scroll compressors work by moving one spiral element inside another stationary spiral to create a series of gas pockets that become smaller and increase the pressure of the gas. The largest openings are at the outside of the scroll where the gas enters. As these gas pockets are closed off by the moving spiral, the pockets move towards the center of the spirals and become smaller and smaller. This increases the pressure of the gas until it reaches the center of the spiral and is discharged through a port near the center of the scroll. The entire process is continuous.

The moving scroll orbits in an eccentric path within the stationary (fixed) scroll as it creates the series of gas pockets. During compression, several pockets are being compressed simultaneously, resulting in a very smooth process. Maintaining an even number of gas pockets on opposite sides reduces any vibration inside the compressor. The two scrolls (one colored blue and the other colored pink) can be seen in the right center of the photo in Figure (2.7).

Scroll compressors are very widely used, for example, in air conditioning systems. They operate more smoothly, quietly, and reliably than other types of compressors in the lower volume range.

2.1-5 Rotary liquid ring

Liquid ring rotary compressors consist of a round, multi-blade rotor that revolves in an elliptical casing. The elliptical casing is partially filled with a liquid, which is usually water. As the rotor turns, the blades form a series of buckets. As the rotor turns, the buckets carry the liquid around with the rotor. Because the liquid follows the contour of the casing, the liquid alternately leaves and returns to the space between the blades.

                             Figure (2.9) a liquid ring rotary compressor

The space between the blades serves as a rotor chamber. The gas inlet and discharge ports are located at the inner diameter of the rotor chamber. As the liquid leaves the rotor chamber, gas is drawn into the rotor chamber through the inlet ports. As the rotor continues to rotate, the liquid returns to the rotor chamber and decreases the volume in the chamber. As the volume decreases, the gas pressure increases. As the rotor chamber passes the discharge port, the compressed gas is discharged into a gas/liquid separator and then to the process. A typical liquid ring rotary compressor is shown in Figure [2.9].

Applications

Crude unit vacuum, various saturated gas applications.

2.1-6 Reciprocating Compressors

A reciprocating compressor or piston compressor is a positive-displacement compressor that uses pistons driven by a crankshaft to deliver gases at high pressure.

The intake gas enters the suction manifold, then flows into the compression cylinder where it gets compressed by a piston driven in a reciprocating motion via a crankshaft, and is then discharged. Applications include oil refineries, gas pipelines, chemical plants, natural gas processing plants and refrigeration plants. One specialty application is the blowing of plastic bottles made of Polyethylene Terephthalate (PET).

The basic components of a reciprocating compressor are a crankshaft, crossheads, piston rod packing, cylinders, pistons, suction valves, and discharge valves. Figure (2.11) is an illustration of a three-stage reciprocating compressor. Note that the third stage piston and cylinder are mounted on top of the second stage piston and cylinder. A prime mover (not shown) rotates the crankshaft. The crankshaft converts the rotary motion of the prime mover into reciprocating motion of the pistons.

The compression cycle of a reciprocating compressor consists of two strokes of the piston. Figure (2.10) show the working of these cycles: The suction stroke and the compression stroke.

suction stroke

The suction stroke begins when the piston moves away from the inlet port of the cylinder. The gas in the space between the piston and the inlet port expands rapidly until the pressure decreases below the pressure on the opposite side of the suction valve. The pressure difference across the suction valve causes the suction valve to open and admit gas into the cylinder. The gas flows into the cylinder until the piston reaches the end of its stroke.

Figure (2.10) Suction Strokes and Compression strokes

compression stroke

The compression stroke starts when the piston starts its return movement. When the pressure in the cylinder increases above the pressure on the opposite side of the suction valve, the suction valve closes to trap the gas inside the cylinder. As the piston continues to move toward the end of the cylinder, the volume of the cylinder decreases and the pressure of the gas increased.

When the pressure inside the cylinder reaches the design pressure of the stage, the discharge valve opens and discharges the contents of the cylinder to the suction of the second stage. The second stage takes suction on the discharge of the first stage, further compresses the gas and discharges to the third stage. The third stage takes suction on the discharge of the second stage and compresses the gas to the final discharge pressure.

Figure (2.11) Reciprocating Compressors

Application

 Plant and instrument air off gas (low flow) recycle (low flow) H2 make-up, gas reinjection (low flow)

2.1-7 Diaphragm compressors

A diaphragm compressor (also known as a membrane compressor) is a variant of the conventional reciprocating compressor. The compression of gas occurs by the movement of a flexible membrane, instead of an intake element. The back and forth movement of the membrane is driven by a rod and a crankshaft mechanism. Only the membrane and the compressor box come in touch with the gas being compressed.

Diaphragm compressors are used for hydrogen and compressed natural gas (CNG) as well as in a number of other applications.

2.2 Dynamic compressors

Two types of dynamic compressors are in use today: they are the axial compressor and the centrifugal compressor. The axial compressor is used primarily for medium and high horsepower applications, while the centrifugal compressor is utilized in low horsepower applications.

Both  the  axial  and  centrifugal  compressors  are  limited  in  their range of operation by what is commonly called stall (or surge) and stone  wall.  The stall / surge  phenomena  occurs  at  certain  conditions  of flow, pressure ratio, and speed (rpm), which result in the individual compressor airfoils going into stall similar to that experienced by an airplane wing at a high angle of attack. The stall margin is the area between the steady state operating line and the compressor stall line. Stone wall occurs at high flows and low pressure, while it is difficult to detect. Stone wall is manifested by increasing gas temperature.

There are two common design directions of dynamic compressor production. Centrifugal compressors are most common for industrial and process applications.

Dynamic compressors are used wherever possible result of their low maintenance requirements. The single stage integral gear centrifugal compressor allows the use of a dynamic compressor in many applications where positive displacement compressors have previously been used. The two types of dynamic compressors are:

I)                    Centrifugal Compressors

II)                   Axial Compressors

2.2-1 Centrifugal compressors

Centrifugal compressors, sometimes termed radial compressors, are a sub-class of dynamic axisymmetric work-absorbing turbo-machinery. Centrifugal compressors are a common design found in industrial process and manufacturing applications. These machines provide oil-free air in large quantities and relatively low pressures. This design uses an impeller to accelerate the gas as it enters the compressor chamber. Some centrifugal compressors incorporate a drive gear coupled to smaller diameter-driven gears coupled to the fan shafts. The drive gear accelerates the rotational speeds of the shaft mounted fan, which increases the energy potential of the gas.

Some centrifugal compressors have fans mounted on a common shaft direct coupled to a drive motor. In this instance, the gas energy potential is increased as the gas passes across each of the side-by-side mounted fans.

Working

In each type, the gas enters the compressor and is accelerated by the impellor or fan blade, turning at very high rotational speed. The high-speed gas is routed from one chamber or stage to the next in sequence where the next impellor adds more energy to the gas stream. After the last compression stage, the gas contacts a diffuser, a funnel-shaped channel at the outlet side of the compressor. As the gas flow enters this pressure, density and velocity are known.

Centrifugal compressors typically have three lobes or stages, but designs may support between two and six stages. The impellers operate volute, the gas flow-speed decreases, and the gas pressure increases as streaming gas accumulates. The kinetic energy from the streaming air is converted into pressure. The behavior of the gas is predictable if the fluid at speeds ranging from a few thousand up to 60,000 rpm. The machines process gas through multiple sequential stages to deliver flows approaching 18,000cfm. These machines are intended to operate continuously. The common components for a centrifugal compressor that require a lubricant include the driver (electric motor, process turbine), a coupling, the gear sets and the bearings supporting the lobe shafts. Centrifugal compressors are divided into:

Centrifugal single stage (low ratio)

These types are known as single stage overhung compressors since the impeller is outboard of the radial bearings the cases are radially split.

Centrifugal single stage integral gear

This type of compressor is an in line type (similar to a pump) usually driven by motor through an integrally mounted gear box (not shown). These compressors are used for low flow high energy (head) applications. These compressors operate at high speeds, 8,000 - 34,000 rpm and are limited to approximately 400 horsepower.

Centrifugal multi-stage 


horizontal split

The casing is divided into upper and lower halves along the horizontal centerline of the compressor. The horizontal split casing allows access to the internal components of the compressor without disturbing the rotor to casing clearances or bearing alignment. If possible, piping nozzles should be mounted on the lower half of the compressor casing to allow disassembly of the compressor without removal of the process piping.

Centrifugal multi-stage with side loads

This type of compressor is used exclusively for refrigeration services. The only difference from the centrifugal multi-stage horizontal split compressor is that gas is induced or removed from the compressor via side load nozzles. This type of compressor can be either horizontally or radially split.

Centrifugal multi-stage (barrel)

The compressor casing is constructed as a complete cylinder with one end of the compressor removable to allow access to the internal components. Multi-stage, radially-slit centrifugal compressors are commonly called barrel compressors. Barrel compressors are used for the same types of applications as the multi-stage, horizontally-split centrifugal compressors.

Because of the barrel design, however, barrel compressors are normally selected for higher pressure applications or certain low mol gas compositions (hydrogen gas mixtures).

Centrifugal multi-stage integral gear

Integrally-geared, centrifugal compressors have a low speed (bull) gear that drives two or more high-speed gears (pinions). Impellers are mounted at one end or both ends of each pinion.

2.2-2 Axial flow compressors

A compressor in which the fluid enters and leaves in the axial direction is called axial flow compressor.

Axial flow machines are common to aviation gas turbines and are used selectively in specialized industrial and process applications. Axial flow machines produce high gas flows.

Axial flow compressors have a design and operation that resembles the jet engine but without the fuel combustion step. Axial flow compressors are used to supply high air flows into gas turbines for aircraft and for some industrial applications. Wind tunnel operators find axial flow compressors to be most useful given their extraordinarily high air flow requirements.

Axial compressors are rotating, airfoil-based compressors in which the working fluid principally flows parallel to the axis of rotation. This is in contrast with other rotating compressors such as centrifugal, axi- centrifugal and mixed-flow compressors where the air may enter axially but will have a significant radial component on exit.

Construction

Axial compressors consist of rotating and stationary components. A shaft drives a central drum, retained by bearings, which has a number of annular airfoil rows attached. Axial flow designs are characterized by a rotor with a set of curved fanlike blades and a stator. The stator may or may not also have a set of curved blades. The gas enters the compressor body and is spun from the center of the rotor in an outward direction with the rotor blades turning at extremely high speeds. These rotate between a similar numbers of stationary airfoil rows attached to a stationary tubular casing. The rows alternate between the rotating airs foils (rotors) and stationary airfoils (stators), with the rotors imparting energy into the fluid, and the stators converting the increased rotational kinetic energy into static pressure through diffusion.  A pair of rotating and stationary airfoils is called a stage. The cross-sectional area between rotor drum and casing is reduced in the flow direction to maintain axial velocity as the fluid is compressed. Machines capable of delivering 1,000,000 CFM or more have been built.

Working

The gas exits the surface of each fan blade in a radial and tangential direction into the stator blades or housing and is fed into the next stage. The gas is directed into the center of the rotor for the next stage and is pushed along, and the sequence is repeated multiple times. Each set of rotor and stator blades represents a compression stage (see Figure 3). 

A compressor in which the fluid enters and leaves in the axial direction is called axial flow compressor. So, the centrifugal component in the energy equation does not come into play. Here the compression is fully based on diffusing action of the passages. The main parts include a stationary (stator) part and a moving (rotor) part. The diffusing action in stator converts absolute kinetic head of the fluid into rise in pressure. The relative kinetic head in the energy equation is a term that exists only because of the rotation of the rotor.