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Process technology equipment and systems chapter 5 & 6, Compressor, Turbines & Motors
117 Compressors O BJECTIVES After studying this chapter, the student will be able to: Explain the principles of compression. • Describe how centrifugal compressors operate. • Describe how axial flow compressors operate. • Identify and describe centrifugal and positive displacement compressors. • Identify the basic components of a rotary screw compressor. • Describe the operation and basic components of sliding vane compressors. • Explain how lobe compressors operate. • Describe how liquid ring compressors operate. • Explain the scientific principles associated with reciprocating compressors. • Identify the basic components of a compressor system. • Start up and shut down a positive displacement compressor. • Start up and shut down a dynamic compressor. • 117 Chapter 5 ● Compressors 118 Key Terms Aftercooler—a heat-exchange device designed to remove excess heat from the discharge side of a multistage compressor. Centrifugal compressor—uses centrifugal force to accelerate gas and convert energy to pressure. Compression ratio—the ratio of discharge pressure (psia) to suction pressure (psia). Multistage compressors use a compression ratio in the 3 to 4 range, with the same approximate compres- sion ratio in each stage. For example, if the desired discharge pressure is 1,500 psia, a 4-stage compressor with a 3.2 compression in each stage might be used. The pressure at the discharge of each stage would be: 1 st 5 47 psia, 2 nd 5150 psia, 3 rd 5 480, 4 th 5 1,536 psia. Demister—a cyclone-type device used to swirl and remove moisture from a gas. Desiccant dryer—used to remove moisture from compressor gases as they are passed over a chemical desiccant, which adsorbs the water. Diaphragm compressor—utilizes a hydraulically pulsed diaphragm that moves or flexes to posi- tively displace gases. Double-acting compressor—a reciprocating compressor that compresses gas on both sides of the piston. Dryer—removes moisture from gas. Intercooler—a heat exchange device designed to cool compressed gas between the stages of a multistage compressor. Lobe compressor—a rotary compressor that contains kidney bean–shaped impellers. Oil separator—removes oil from compressed gases. Receiver—a compressed-gas storage tank. Stage—each cylinder in a compressor; specifically, the area where gas is compressed. Thermal shock—a form of stress resulting in metal fatigue when large temperature differences exist between a piece of equipment and the fluid in it. Compressor Applications and Classification The compression of gases and vapors in the process industry is very impor- tant. Compressors are used in a variety of applications. In a modern plastics facility, compressors are used to transfer granular powders and small plastic pellets from place to place. In natural gas plants, compressors are used to establish feed gas process pressures. Compressors also provide clean, dry air for instruments and control devices. In a refinery or chemical plant, com- pressors are used to compress gases such as light hydrocarbons, nitrogen, Compressor Applications and Classi cation 119 hydrogen, carbon dioxide, and chlorine. These gases are sent to headers, from which they are distributed to a variety of applications. There are three basic designs for compressors (Figure 5.1): dynamic, posi- tive displacement, and thermal. Dynamic compressors include centrifugal (radial flow) and axial (straight-line) flow compressors. Positive displace- ment compressors include rotary and reciprocating compressors. Dynamic compressors accelerate airflow by drawing air in axially and spinning it outward (centrifugal compressors) or in a straight line (axial flow com- pressors). Positive displacement compressors compress gas into a smaller volume and discharge it at higher pressures. Thermal compressors use ejectors to direct high-velocity gas or steam into the process stream, en- training the gas, and then converting the velocity into pressure in a diffuser assembly. This chapter focuses primarily on dynamic and positive displace- ment compressors. A compressor is part of a much larger system. The system’s resistance to flow typically dictates compressor performance. Minor problems are occa- sionally experienced with compressor systems. These troubles are usually the result of dirt, adjustment problems, liquid in the system, or inexperi- ence in operating the system. Experienced technicians can quickly fix the problem by making the proper adjustment, cleaning the equipment, replac- ing a minor part, or removing an adverse condition. Figure 5.1 Compressor Family Tree Centrifugal-Radial Flow Positive Displacement Rotary Axial Flow Rotary Screw Lobe Compressor Helical Lobe Straight Lobe Integral Gear Fixed Stator Vanes VariableStatorVanes Multistage Liquid Ring Sliding Vane Piston Compressor Diaphragm Reciprocating Dynamic Thermal Balanced/Opposed Single/Multistage Single/Multistage Horizon/Vert. Split Ejectors Single/Multistage Scroll Chapter 5 ● Compressors 120 The principles of compression are: Gases and vapors are compressible. • Compression decreases volume. • Compression moves gas molecules close together. • Compressed gases will resume their original shape when • released. Compressed gases produce heat because of molecular friction. • The smaller the volume, the higher the pressure. • Force • 4 Area 5 Pressure. Gas volume varies with temperature and pressure. • Liquids and solids are not compressible (except under • tremendous pressures). Dynamic Compressors Dynamic compressors are classified as either centrifugal or axial flow. Both types operate by changing the velocity of gas and converting energy to pressure. Centrifugal Compressors During operation, gas enters a centrifugal compressor at the suction in- let and is accelerated radially by moving impellers (Figures 5.2 and 5.3). Centrifugal compressors have one moving element, the driveshaft and im- peller. In a centrifugal compressor, the impeller discharges into a circular, narrow chamber called the diffuser. This narrow opening completely sur- rounds the impellers. As back-pressure builds in the impeller, gas velocity is accelerated through the diffuser assembly and into a circular volute. As high-velocity gas moves through the diffuser and into the volute, kinetic Figure 5.2 Multistage Centrifugal Compressor Diffuser Impeller Casing Discharge Port Suction Port Shaft Stage # 1 Stage # 2 Stage # 3 Dynamic Compressors 121 energy is converted into pressure as gas speed slows in the ever-widening volute before exiting the discharge port. Because compressor performance is linked to the compressibility of the gas it is moving, centrifugal compressors are more sensitive to density and fluid characteristics than are reciprocating compressors. Most centrifugal compressors are designed to operate at speeds in excess of 3,000 RPM. Recent advances in technology have resulted in the development of a cen- trifugal compressor that runs at speeds in excess of 40,000 RPM. Centrifugal compressors can be single-stage or multistage. Single-stage compressors (Figure 5.4) compress the gas once, whereas multistage compressors deliver the discharge of one stage to the suction of another stage. Single-stage centrifugal compressors are designed for high gas flow rates and low discharge pressures; multistage compressors are designed Figure 5.3 Centrifugal Compressor Diffuser Plates Diffuser Impeller Packing Packing Gland Casing Drive Shaft Diffuser Passage Discharge Suction Port Figure 5.4 Single-Stage Centrifugal Compressor Chapter 5 ● Compressors 122 for high gas flow rates and high discharge pressures. Centrifugal compres- sors are also used for transferring wet product gases that typically damage positive displacement compressors. Compression ratio is defined as the ratio of discharge pressure (psia) to suction pressure (psia). Frequently, the desired discharge pressure is very high, over 100 times that of the inlet pressure. When a gas is compressed, the temperature of the gas increases. If a gas was compressed in one stage to a pressure 100 times that of the inlet pressure, the gas temperature would be extremely high. Multistage compressors, with cooling between stages, are used to develop high pressures to allow for the heat of compression. The compression ratio normally runs in the 3 to 4 range, with the same ap- proximate compression ratio in each stage. For example, if the desired dis- charge pressure is 1,500 psia, a 4-stage compressor with a 3.2 compression in each stage might be used. The pressure at the discharge of each stage would be: 1 st 5 47 psia, 2 nd 5150.5 psia, 3 rd 5 481.7, 4 th 5 1,541.4 psia. The simple calculation used to calculate the pressure increase on each stage is: Stage One 14.7 psia 3 3.2 5 47.04 psia Stage Two 47.04 psia 3 3.2 5 150.528 psia Stage Three 150.528 psia 3 3.2 5 481.689 psia Stage Four 481.689 psia 3 3.2 5 1541.407 psia The basic components of a centrifugal compressor are shown in Figure 5.2. The part of the impeller vane that comes into contact with gas first is called the suction vane tip. The part of the impeller vane that comes into contact with the gas last is called the discharge vane. The driver is an electric motor or turbine. The basic types of impellers used on centrifugal compressors are the open backward-bladed impeller, open radial-bladed impeller, and closed back- ward-bladed impeller. Figure 5.5 illustrates various impeller designs. Centrifugal compressors are considered to be the workhorses of the chemical-processing industry. They are chosen more often than other types for new installations because they have a very low initial installation cost, low operation and maintenance cost, simple new piping installations, inter- changeable drivers, large volume capacity per unit of plot area, and long service life. In addition, they can deliver much higher flow rates than posi- tive displacement compressors. Axial Flow Compressors In the industrial environment, axial compressors are the compressor of choice for jobs where the highest flows and pressures are required. Un- like centrifugal compressors, axial compressors do not use centrifugal force to increase gas velocity. An axial flow compressor is composed of a rotor Dynamic Compressors 123 that has rows of fanlike blades (Figure 5.6). Airflow is moved axially along the shaft. Rotating blades attached to a shaft push gases over stationary blades called stators. The stators are mounted on or attached to the casing. As the rotating blades increase the gas velocity, the stator blades slow it down. As the gas slows, kinetic energy is released in the form of pressure. Gas velocity increases as it moves from stage to stage until it reaches the discharge scroll. Multistage axial compressors can generate very high flow rates and discharge pressures. As a general rule, an axial compressor requires twice as many stages as a centrifugal compressor to perform the same operation; however, axials are 8% to 10% more efficient. Axial compressors are limited to approxi- mately 16 stages because of temperature and equipment stress. Axial flow compressors are often used in series flow with centrifugal compressors because they are capable of operating at greater capacities. The primary application of axial compressors involves the transfer of clean gases such as air. The internal components of an axial flow compressor are extremely sensitive to corrosion, pitting, and deposits. The stator blades in an axial compressor can be fixed, individually adjust- able, or continually variable. Individually adjustable stator blades can be adjusted from outside the casing. Continually variable blades are adjusted by a drive ring linked to a driveshaft that is automatically actuated by a power cylinder. Figure 5.5 Impeller Types Semi-Open Closed Straight Vane Straight Vane Curved Backward Curved Forward Chapter 5 ● Compressors 124 In contrast to a centrifugal compressor, axial compressors accelerate and compress gas in a horizontal, straight-through motion, without the turbu- lent changes in direction characterized by centrifugal compressors. Pound for pound, axial compressors are lighter, more efficient, and smaller than centrifugals. A 23,000-hp axial produces as efficiently as a 25,500-hp cen- trifugal. Even so, axial flow compressors are not as common as reciprocat- ing and centrifugal compressors. One main use of axial compressors is in gas turbine applications. Blowers and Fans Blowers and fans are simple devices typically classified as compressors. The two basic designs are axial flow and centrifugal flow. Most blowers and fans are single-stage devices designed to perform a specific function. Single-stage, centrifugal blowers are used for low-pressure air systems, refrigeration units, leaf blowing, ventilation systems, or laboratory hoods. Fans can be used to direct airflow into or out of industrial equipment such as cooling towers, flares, boilers, furnaces, HVAC (heating, ventilating, and air conditioning) systems, or air-cooled heat exchangers, or they can be used for ventilation of confined spaces. Figure 5.6 Axial Flow Compressor Suction Line Discharge Line Stator Blades Shaft Rotor Blades Seals Bearings Inlet Guide Vanes First Stage Second Stage Bearings Seals Discharge Port Suction Port Stator Blades Casing Drum Rotor Blades Shaft Inlet Guide Vanes Cover Motor Side View Top View Rotary Compressors 125 Fans can be classified as centrifugal, propeller, tube-axial, or vane-axial. Centrifugal fans are designed to move gases over a wide range of condi- tions. Propeller fans consist of a propeller and a motor mounted on a ring. This fan is primarily designed to operate over a wide range of volumes at low pressures and to move air from one enclosed area into another. Tube- axial fans are mounted directly in the pipe cylinder and are designed to move air or gas at medium pressures. The vane-axial fan resembles the tube-axial fan. The motor and fan are mounted directly in the tube. A se- ries of vanes help direct flow over a wide range of volumes and pressures. Each of these four fans can be direct drive or belt driven. Positive-Displacement Compressors Positive-displacement compressors operate by trapping a specific amount of gas and forcing it into a smaller volume. They are classified as either rotary or reciprocating. Rotary compressors are further classified as rotary screw, sliding vane, lobe, or liquid ring. Reciprocating compressors are classified as piston or diaphragm. Positive-displacement compressors remove a set volume of gas for every rotation or stroke of the primary transfer elements. In process systems where fluid density and suction pressures vary, positive displacement de- vices provide steady service. Rotary compressors can deliver pressures between 100 and 130 psia. Reciprocating compressor discharge pres- sures that range from 0 to 30,000 psig. Rotary Compressors Rotary compressors take their name from the rotating motion of the trans- fer element. A good case could be made that centrifugal compressors are rotary. Centrifugal compressors do rotate, but they do not positively dis- place or compress the gas. In contrast, the rotating elements of a rotary compressor displace a fixed volume of fluid inside a durable casing on each rotation. Rotary Screw Compressors The rotary screw compressor is commonly used in industry. This device closely resembles the lobe compressor and operates with two helical ro- tors that rotate toward each other, causing the teeth to mesh (Figure 5.7). As the left rotor turns clockwise, the right rotor rotates counterclockwise, forcing gas to become trapped in the central cavity. Rotary screw compres- sors are designed with an inlet suction line and an outlet discharge port. The two rotors are attached to a driveshaft, timing gears, and a driver that provides the energy to operate. Chapter 5 ● Compressors 126 Flow enters the device and is moved axially toward the discharge port. The majority of compression takes place very close to the compressor outlet. The moving elements of the rotary screw compressor do not touch each other or the inner wall. A set of timing gears allows the power rotor to turn the alternate rotor. Because of this design, the rotating elements do not require lubrication, making them a perfect choice for dry gas service. Be- cause of the small tolerances that exist between the moving elements, some internal slip occurs during operation. Rotary screw compressors operate at speeds between 1,750 and 3,600 RPM and have capacity ratings above 12,000 cfm (cubic feet per minute) on the inlet volume and discharge pressures between 3 and 20 psig. Some rotary screw units can operate between 60 and 100 psig. Another feature associ- ated with the rotary screw compressor is its ability to be used as a vacuum device. This system is designed to handle 500 to 10,000 cfm on the suction side and to pull a vacuum between 5 and 25 inches of mercury. Sliding Vane Compressors The sliding vane compressor uses a slightly off-center rotor with sliding vanes to compress gases. The major components of a sliding vane compressor are shown in Figure 5.8. The gas inlet port is positioned so that gas flows into the vanes when they are fully extended and form the largest pocket. As the vanes turn toward the discharge port, the gases are compressed. The body of the compressor is fabricated from cast iron or steel. A set of cooling water jackets is fabricated into the initial design and tested for tight- ness. The rotor and shaft are made of high-strength alloy steel. The rotor is precision made with slots around the entire rotor. The sliding vanes are composed of asbestos-phenolic resin, metal, or high-temperature, durable metal. Sliding vane compressors require lubrication between the vane and contact surface. Lubricating oil is injected into the suction side of the com- pressor. This procedure helps prevent internal slip and provides a positive seal. Sliding compressors are typically nonpulsing systems. As gas enters the sliding vane compressor, it is captured in vanes and swept around the casing, filling the chamber. As the vanes rotate toward Figure 5.7 Rotary Screw Compressor Helical Rotors Timing Gears Base Driver . psia 3 3.2 5 47.04 psia Stage Two 47.04 psia 3 3.2 5 150 .52 8 psia Stage Three 150 .52 8 psia 3 3.2 5 481 .68 9 psia Stage Four 481 .68 9 psia 3 3.2 5 154 1.407 psia. stage would be: 1 st 5 47 psia, 2 nd 51 50 psia, 3 rd 5 480, 4 th 5 1 ,53 6 psia. Demister—a cyclone-type device used to swirl and remove moisture from a gas.