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Compressors

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1- Centrifugal Compressors

2- Axial Compressors

3- Screw Compressors

4- Scroll Compressors

5- Reciprocating Compressors

6- Compressor Valves

Introduction

The compression of gases and vapors in the process industry is very important. 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, compressors are used to compress gases such as light hydrocarbons, nitrogen, hydrogen, carbon dioxide, and chlorine. These gases are sent to headers, from which they are distributed to a variety of applications.

Key Terms

Aftercooler—a heat-exchange device designed to remove excess heat from the discharge side of a multistage compressor.

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 compressionratio in each stage.

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.

Enthalpy— is a measurement of energy in a thermodynamic system. It is the thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume.

Entropy— a measure of the unavailable energy in a closed thermodynamic system that is also usually considered to be a measure of the system’s disorder, that is a property of the system’s state, and that varies directly with any reversible change in heat in the system and inversely with the temperature of the system; broadly :  the degree of disorder or uncertainty in a system.

Classification

There are three basic designs for compressors: dynamic, positive displacement, and thermal. Dynamic compressors include centrifugal (radial flow) and axial (straight-line) flow compressors. Both types operate by changing the velocity of gas and converting energy to pressure. Positive displacement compressors include rotary and reciprocating compressors. they operate by trapping a specific amount of gas and forcing it into a smaller volume. Dynamic compressors accelerate airflow by drawing air in axially and spinning it outward (centrifugal compressors) or in a straight line (axial flow compressors). 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, entraining the gas, and then converting the velocity into pressure in a diffuser assembly.

1. Centrifugal Compressors

gas enters a centrifugal compressor at the suction inlet and is accelerated radially by moving impellers. Centrifugal compressors have one moving element, the driveshaft and impeller. In a centrifugal compressor, the impeller discharges into a circular, narrow chamber called the diffuser. This narrow opening completely surrounds 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 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.

Centrifugal compressors can be single-stage or multistage. Single-stage compressors 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 for high gas flow rates and high discharge pressures.

2. Axial compressors

Axial compressors are the compressor of choice for jobs where the highest flows and pressures are required. Unlike centrifugal compressors, axial compressors do not use centrifugal force to increase gas velocity. An axial flow compressor is composed of a rotor that has rows of fanlike blades. 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.

The stator blades in an axial compressor can be fixed, individually adjustable, 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.

In contrast to a centrifugal compressor, axial compressors accelerate and compress gas in a horizontal, straight-through motion, without the turbulent changes in direction characterized by centrifugal compressors.

3. Rotary Screw Compressors

The rotary screw compressor is commonly used in industry. This device closely resembles the lobe compressor and operates with two helical rotors that rotate toward each other, causing the teeth to mesh. As the left rotor turns clockwise, the right rotor rotates counterclockwise, forcing gas to become trapped in the central cavity. Rotary screw compressors 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.

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. Because of the small tolerances that exist between the moving elements, some internal slip occurs during operation.

4. Lobe Compressors

Lobe compressors are characterized by the two kidney bean–shaped impellers used to trap and transfer gases. The close clearances between the casing and impellers are maintained by a set of timing gears. During operation, the two impellers move in opposite directions on parallelmounted shafts as the lobes sweep across the suction port. The parallel shafts are composed of a driveshaft and an idler shaft. The driveshaft forces the idler shaft to turn through the gears. The gears and bearings are located on the outside of the compressor. Compressed gases are released to the discharge line.

The internal lobes on a rotary lobe compressor are designed not to touch. A few thousandths of an inch clearing exists between the casing and lobes. The design clearances on the internal lobes of a lobe compressor allow some slip. The slip is aggravated at high discharge pressure when low-density gases are being pushed. Process slip is constant only when system pressure is constant.

Lobe compressors are designed to have constant-volume discharge pressures and constant-speed drivers. Lobe compressors do not use discharge or suction valves because they are not designed to operate at a specific pressure. Discharge pressures are determined by the system’s process pressure.

Lobe compressors can be used in wet and dry gas service. The rotation of the lobes may be up or down; that is, the discharge port can be at the top or at the bottom of the unit. In dry service, the upward rotation is preferred. In wet service, the downward rotation is recommended so any condensed liquids can escape. Lobe compressors can be used as compressors or vacuum pumps.

5. Liquid Ring Compressors

A very unusual compressor design is the liquid ring compressor. It combines the centrifugal action of the liquid with a positive displacement, rotary action. A liquid ring compressor has one moving transfer element and a casing that is filled with makeup water or seal liquid. As the rotor turns, the fluid is centrifugally forced to the outer wall of the elliptical casing. An air pocket is formed in the center of the casing. As the liquid ring compressor rotates, a small percentage of the liquid escapes out the discharge port. Makeup water or seal liquid is admitted into the compressor during operation. The liquid medium helps cool the compressed gases. The off-center position of the rotor creates an offset in the air pocket. Located on the rotor are suction and discharge ports. The inlet ports are much larger than the discharge ports. As the vanes turn, gases are compressed in the volute-shaped air pocket.

6. Sliding Vane Compressors

The sliding vane compressor uses a slightly off-center rotor with sliding vanes to compress gases. 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 tightness. 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 compressor. 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 the discharge, the vane length shortens because of the rotor’s eccentric position with the shaft, and volume is decreased. As volume decreases, pressure increases until maximum compression is achieved. At this point, the gas is discharged out of the compressor. This type of compressor does not use suction or discharge valves because it is designed to discharge against system pressure.

7. Scroll Compressors

A scroll compressor has two interleaved spiral vanes designed to compress fluids into ever decreasing volumes. Scroll compressors run quietly and smoothly at lower volumes, trapping fluid between the scrolls. In most cases, one scroll is fixed and one orbits eccentrically without rotating.

8. Reciprocating Piston Compressors

Their distinctive back-and-forth motion characterizes reciprocating compressors. During operation, reciprocating compressors perform best with clean gases. Entrained water, dirt, and impurities will cause excessive wear on the piston and cylinder. Reciprocating compressors are selected when low flow rates and high discharge pressures are required.

There are several advantages of using a reciprocating piston compressor. They have a flexible pressure range and overall capacity, low power cost, and high efficiency rating. They can handle density and gas composition changes, and small volumes and can deliver high pressures.

Reciprocating piston compressors work by trapping and compressing specific amounts of gas between a piston and the cylinder wall. The back-andforth motion incorporated by a reciprocating compressor pulls gas in on the suction or intake stroke and discharges it on the other. Spring-loaded suction and discharge valves work automatically as the piston moves up and down in the cylinder chamber. Reciprocating piston compressor design varies from model to model. These variations usually occur in the total number of cylinders and in the arrangement of the suction and discharge lines. Most piston compressors have one to four cylinders. Each cylinder has its own piston, rings, and automatic valves. Common crankshafts can be shared with multiple connecting rods. The same cylinder can be equipped with multiple suction and discharge valves in double-acting compressors.

9. Diaphragm Compressors

Diaphragm compressors utilize a hydraulically pulsed diaphragm that moves or flexes to positively displace gases. This type of compressor is closely related to a reciprocating compressor. This type of compressor is a combination of several systems; a gas compression system and a hydraulic system. Gas compression occurs when a flexible metal diaphragm or membrane hydraulically flexes. In this type of an operation only the membrane and the compression chamber come into contact with the gas. For this reason the diaphragm compressor is ideal for applications that involve explosive and toxic gases. Membranes are designed to be durable and tough and able to withstand high temperatures and a variety of conditions. Diaphragm compressors can generate very high pressures and are used to compress hydrogen, hydrogen chloride, carbon monoxide, compressed natural gas. Diaphragm compressors come in one, two, three, or more stages. Each stage requires the use of one diaphragm.

10. Ejectors

An ejector works by converting the pressure energy of a motive fluid into kinetic energy (velocity) as it flows through a relatively small converging — diverging nozzle. The increased velocity of the motive fluid causes a corresponding reduction in pressure creating suction in the mixing chamber, into which the process fluid is drawn. The process fluid mixes with and becomes entrained in the motive fluid stream. The mixed fluid then passes through the converging — diverging diffuser, where the velocity is converted back into pressure energy. The resultant discharge pressure is higher than the suction pressure of the ejector.

Historically, ejectors have been primarily used to generate vacuum. Ejectors can be staged in series to achieve deep vacuum levels. They can use many different types of motive fluid. Air and steam are the most common. To avoid contamination and other problems, it is important to choose a motive fluid compatible with the process fluid. Ejectors are one of the few compressor types that are relatively immune to liquid carryover in the suction gas.

An ejector is not as efficient as most types of mechanical compressors but has the advantage of simplicity and no moving parts. This helps make ejectors very reliable with minimal maintenance costs.

Basics

For a compression process, the enthalpy change is the best way of evaluating the work of compression. Years ago the capability of easily generating P-H diagrams for natural gases did not exist. The result was that many ways of estimating the enthalpy change were developed. They were used as a crutch and not because they were the best way to evaluate compression horsepower requirements.

Today the engineer does have available, in many cases, the capability to generate that part of the P-H diagram required for compression purposes. This is done using equations of state on a computer. This still would be the best way to evaluate the compression horsepower. The other equations are used only if access to a good equation of state is not available.

There are two ways in which the thermodynamic calculations for compression can be carried out — by assuming:

  1. isentropic reversible path — a process during which there is no heat added to or removed from the system and the entropy remains constant.
  2. polytropic reversible path — a process in which changes in gas characteristics during compression are considered.

The amount of work required is dependent upon the polytropic curve involved and increases with increasing values of n. The path requiring the least amount of input work is n = 1, which is equivalent to isothermal compression, a process during which there is no change in temperature. For isentropic compression, n = k = ratio of specific heat at constant pressure to that at constant volume.

If a P-H diagram is available (as for propane refrigeration systems), the work of compression would always be evaluated by the enthalpy change of the gas in going from suction to discharge conditions.

 

It is usually impractical to build sufficient heat-transfer equipment into the design of most compressors to carry away the bulk of the heat of compression. Most machines tend to operate along a polytropic path which approaches the isentropic. Most compressor calculations are therefore based on an efficiency applied to account for true behavior.

System

A compressor is part of a much larger system. The system’s resistance to flow typically dictates compressor performance. Minor problems are occasionally experienced with compressor systems. These troubles are usually the result of dirt, adjustment problems, liquid in the system, or inexperience in operating the system. Two conditions associated with centrifugal compressors are surge (pumping) and stone-wall (choked flow).

“Surge”: at some point on the compressor’s operating curve there exists a condition of minimum flow/maximum head where the developed head is insufficient to overcome the system resistance. This is the surge point. When the compressor reaches this point, the gas in the discharge piping back-flows into the compressor. Without discharge flow, discharge pressure drops until it is within the compressor’s capability, only to repeat the cycle. The repeated pressure oscillations at the surge point should be avoided since it can be detrimental to the compressor. Surging can cause the compressor to overheat to the point the maximum allowable temperature of the unit is exceeded. Also, surging can cause damage to the thrust bearing due to the rotor shifting back and forth from the active to the inactive side.

“Stonewall” or choked flow: occurs when sonic velocity is reached at any point in the compressor. When this point is reached for a given gas, the flow through the compressor cannot be increased further without internal modifications.

 

Multi-Stage Compression

The simplest compressor designs feature a single cylinder/chamber arrangement. While straightforward, this setup is limited in its efficiency and capacity for delivering high volumes of pressurized gas. That’s where multi-stage compressors come in. By increasing the number of stages, these machines work more effectively.  Multi-stage compressors feature a series of stages. Between each compression stage, the compressed gas passes through a heat exchanger, where it is cooled. Cooling the gas reduces the amount of work necessary to compress it further. In a two-stage compressor, gas is then forced into an additional chamber where it is pressurized to the required extent. In a three-stage compressor, an additional cycle of compression and cooling occurs before this.

P&ID Symbols

Resources

References

1-ENGINEERING DATA BOOK by Gas Processors Suppliers Association
2-Process Technology – Equipment and Systems by Charles E. Thomas


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