Air and Gas Compressors

8/22/2019 Air and Gas Compressors

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P D H en g i n eer .c o m

CourseM-2019

Air and Gas Compressors

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Air and Gas Compressors (2 PDH)PDHengineer.comCourse No. M-2019

Introduction

Compressors are widely used in construction, power plants, process industry, assembly plant,refineries, air conditioning, and refrigeration, to mention some of the applications. Compressorsare power conversion machines, like pumps and electric motors. Compressed air systems arealternatives to hydraulic systems and electric operators in many applications. For these reasons,engineers, operations, and maintenance personnel should be aware of the applications andlimitations of various types of compressors.

The same basic principles apply to all gas and vapor compressors, as well as air compressors.

Diagrams and illustrations are taken from the following sources:Various issues ofPowermagazine, Marks Standard handbook for Mechanical Engineers, salesbrochures from Ingersol- Rand and Gardener-Denver, and Process Engineers guide to centrifugalcompressors, by Igor Karassik.

AIR COMPRESSORS

Compressed air is free air that has been forced into a smaller volume and is at a pressure higherthan atmospheric. Some of the terms and definitions used when discussing air compressors are asfollows:

Absolute Pressure

The existing gage pressure plus the atmospheric pressure measured from absolute zero

Aftercooler

Device that dissipates heat caused by compression. This also effectively removes moisture downto the saturation temperature

Air Receiver

Tank into which compressed air is delivered and stored

Atmospheric pressure

Pressure at a specific altitude. At sea level this is 14.7 psia.


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Brake Horsepower

Total power input required to compress and deliver a given quantity of air, including losses due tofriction and other mechanical losses.

Capacity

SCFM - Standard cubic feet per minute. Delivered capacity in cubic feet of air measured at68 deg F and 14.7 psia. (per ASME Power Test Code, but this standard may vary).

ICFM - Inlet cubic feet per minute. The capacity entering the inlet filter in CFM at actualinlet conditions.

ACFM - Actual cubic feet per minute. Delivered CFM as measured at actual conditions atthe compressor suction downstream of the inlet filter. ACFM differs from ICFMprimary by seal losses and to a much lesser extent by the lower pressure condition

at the compressor suction due to pressure drop through the inlet filter. SinceACFM most realistically expresses the user's intent, it is recommended thatcompressors be specified in that unit.

Compression

The reduction of a specified volume, resulting in an increase in pressure

Compression Efficiency

Ratio of the theoretical to the actual required to compress air.

Compression Ratio

The ratio of the absolute discharge pressure to the absolute inlet pressure.

Compressor

A machine designed for compressing a gas or vapor from an initial pressure to ahigherdischarge pressure.

Design Pressure

Maximum continuous operating pressure. Also referred to as maximum workingpressure.

Design Speed

Maximum continuous operating speed of a compressor.


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Discharge Pressure

Total pressure at the discharge flange of the aftercooler.

Free Air

Air at atmospheric conditions. This may vary with altitude, barometric pressureand temperature.

Inlet Pressure

Total pressure at the inlet flange of the compressor or inlet filter

Inlet Temperature

Temperature at the inlet flange of the compressor or the inlet

filter.

Load Factor

The ratio of the average actual compressor output to the maximum rated outputfor a defined period of time.

IMoisture Separator

A devise designed to collect and remove moisture from the air during the coolingprocess

Pressure - Force per unit area

PSIG - Pressure above local atmospheric pressure

PSIA - Equal to gage pressure plus atmosphericpressure.

Pressure Drop - Loss of pressure commonly due to friction.

Rated Discharge Pressure - The highest continuous operating pressure tomeet thespecified conditions. It is lower thandesign pressure by 10% or 15 psig.

Slip

The internal leakage due to clearance.


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Speed

The number of revolutions per minute

Unloaded Horsepower

The power that is consumed to overcome frictional losses when operated in anunloaded condition.

Vacuum

Pressure below atmospheric

Volumetric Efficiency

The ratio of the actual quantity of air delivered to the displacement of the compressor.(For reciprocating compressors).

Types of Compression

A brief review of the thermodynamics of gas compression is in order at this point. Theperfect gas law expresses the equation of state for gases:

144pv = RT,

where p is the absolute pressure in psia,v is the specific volume in cubic feet per pound,R is a constant which depends on the nature of the gas, andT is the absolute temperature in deg F.

Specific heat is the amount of heat required to raise the temperature of one pound ofgas 1 deg F. The specific heat of a gas has two distinct values, depending on whetherthe volume or the pressure remain constant during the addition of heat:

Cp = specific heat at constant pressureCv = specific heat at constant volume

The factor or exponent k is the ratio between the specific heat at constant pressure tothe specific heat at constant volume.

k=Cp/cv

The value of k for air is commonly taken as 1.4.


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Adiabatic compression takes place when no heat is transferred into or out of the gasduring compression. For adiabatic compression,

pvk = constant

Adiabatic compression is further characterized by an increase in temperatureduring compression.

Isothermal compression occurs when the heat of compression is removed duringcompression, so that the temperature of the gas remains constant. The equationfor isothermal compression is:

pv = constant

Polytropic compression is characterized by the equation:

pv

n

= constant

When n=1, the polytropic compression is isothermal. When n=k, it is adiabatic.The slope of a pressure-volume curve is dependent on the value of n.

If a compressor is not cooled, and the compression takes place with 100%efficiency, it would beAdiabatic. However, the inefficiency of the compressor results inthe addition of heat during compression. As a result, the actual compression of an uncooledcompressor is polytropic, with a value of n greater than k.

Design Classifications

There are two broad classifications of compressors; positive displacement and dynamic.

Positive displacement compressors confine successive volumes of gas in an enclosed spacewhere pressure increases as the volume of the enclosed space decreases. They can be thought ofas constant volume-variable pressure machines; that is, they move a certain volume of gas witheach stroke, and the pressure is that of the system into which they discharge.

With dynamic compressors, the mechanical action of rotating impellers impart pressure andvelocity to the gas. They are constant pressure-variable volume machines.

Categories of Compressors

Compressors are generally categorized as: Reciprocating, Rotary, Centrifugal, and Axial. Theseare illustrated below in Figure 1. Figure 2 shows approximate ranges of capacity and pressure foreach compressor category.


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Figure 1 Illustration of various types of compressors.

Figure 2 Approximate ranges of application for reciprocating, centrifugal, and axial flowconditions.


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Reciprocating Compressors

Reciprocating compressors (abbreviated recips) may be considered for applicationsup to approximately 3000 ACFM. They are favored for low flow, high pressure

services. They can be arranged in configurations of two or more stages, withmaximum compression ratios per stage usually about 3:1 or 4:1. Intercoolers can beused between stages to remove moisture and to improve compressor efficiency. Moreon intercooling later. Single stage compressors may also be double acting. That isdischarging through both ends of the cylinder, thus doubling the capacity per stroke.

Discharge pressure is generally limited to 3000 psia for large machines, but can go upto 60,000 psia for small machines. Pressure rise per stage is usually limited by thevalve design, and is generally about 1000 psia per stage for large compressors.

Since reciprocating compressors are constant volume machines, the inlet ACFM

remains essentially unchanged if the compressor operates at a constant speed,regardless of the discharge pressure. A reciprocating compressor will operate at anydischarge pressure within the power limit of the driver. If there is insufficient aircapacity in the compressor forthe load to be handled, the air demand sets thepressure, which may be less than the rated pressure. If there is sufficient capacity,then a step control of the compressor can maintain the receiver pressure within a pre-set range. A step control unloads the compressor sequentially in several steps in 1 to2 psi increments as the load demand decreases. If two or more recips operate inparallel, the compressors should be controlled to unload sequentially, one after theother.

The cylinders of conventional reciprocating compressors require oil lubrication.Carry-overof lubricating oil can be a problem in some applications, such asinstrument air. There are "oil free" recips that can be used for these applications. Oilfree recips use teflon piston rings and valves, and so the air side is truly oil free exceptfor the small amount that carry over on the piston rods. The trouble is thatreciprocating compressors tend to be maintenance intensive in any case, and oil freerecips with teflon rings and valves are particularly demanding.

Rotary Compressors

Rotary compressors are positive displacement machines like recips, and although

they are very different configurations from recips, they do share the basic definingcharacteristics They compress gas by confining a specific volume of gas in a closedspace and increase the pressure by decreasing the volume of the space. They areconstant volume and variable pressure machines. They are configured as screw,sliding vane, lobe, and liquid piston compressors, as illustrated below.


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Figure 3 Illustration of a lobe compressor and a liquid piston compressor.

Sliding vane rotary compressors trap gas between vanes as the rotor passes an inletopening. As the rotor continues toward the discharge port, the volume of a cellbetween any two vanes decreases, causing the gas pressure to rise. The vanes slidein and out of slots as the rotor rotates, and are held against the casing by centrifugalforce. Capacities of sliding vane units range to 5000 CFM, and single stagecompression to about 50 psi. Multiple stages can by configured for higher pressures.

Lobe compressors come in two and three lobe design, with capacities from 5 to50,000 CFM. Pressures above 15 psi can be reached by connecting two or three lobe

compressors in series. Lobe compressors have identical impellers, or lobes, whichtrap and compress gas between the outer surfaces of the lobes and the outer casing.

Some screw and lobe compressors require large quantities of lubricant to be sprayedonto the rotating parts, and so are not suitable for applications requiring oil free air orgas. Other screw and lobe compressors have the rotating elements held in place withgears that prevent actual contact of the screws or lobes, so the air chamber canremain free of lubricating oil.Centrifugal Compressors

In centrifugal compressors, the gas travels essentially radially through one ormore stages of rotating impellers. The centrifugal action of the impeller producessome pressure rise and a large increase in air velocity. In the diffuser, velocityenergy is converted to static pressure. Velocity decreases, and pressure increases.Pressure / volume curves are represented in Figure 4. Each curve is for adifferent compressor speed.


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A compressor running at medium speed delivers a certain volume V1 at pressure P1.Increasing to high speed increases volume to V2 , or old volume V1 can be deliveredat a higher pressure P2.

Figure 4 Characteristic curves for a centrifugal pump.

The popularity of centrifugal compressors grew as the need for higher capacitiesreached and exceeded the practical limits of reciprocating compressors. Multistage


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centrifugal compressors can handle 150,000 CFM or more, and the lower capacity hasextended down to 500 CFM.

Figure 5 Sectional view of a centrifugal compressor.

A characteristic of centrifugal compressors is surge, or pumping. Surge occurs at reduced loads.At reduced capacities, impellers do not fully load, density is reduced, and full discharge pressure is

not developed. Since the pressure in the discharge line will then be momentarily higher,a reversesurge will occur. The impellers will then fill and again deliver full pressure. This cyclic effectwillcause pressure fluctuations within the compressor. These fluctuations can be destructive overa period of time.

Surge can occur at about 50% of rated capacity in single stage units designed for low compressionratios. Multistage units will surge at a higher capacity, perhaps 75 to 80%. A steeply risingpressure curve at reduced capacity will give an increased stable operating range at rated pressure.Such a curve can be obtained with a backward sloping impeller design.

Axial Flow Compressors

Axial flow compressors are dynamic compressors in which the air flow is parallel to theaxis of rotation. The most common usage is in combustion turbines and in ventilationsystems. They can handle very large volumes of air, and have a low pressureincrease per stage. Multistage axials can compress air to 150 psi. They deliver arelatively fixed amount of air over a range of pressures. To prevent surging at low


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loads, large axials can be fitted with a blowoff system to ensure that enough airpasses through the stages to remain stable.

Accessories

The compressor itself is the heart of the air or gas compressor system, but there are accessoriesthat are generally supplied as part of the system, either integral with the compressor, or as stand-alone devices.

Inlet Filters and Silencers

Filter-silencers can come as an integral package for each compressor. They are essential toprevent particulate matter from being ingested and damaging the compressor. Inlet filters usuallycome in variations of oil bath or dry cartridge types.

Intercoolers and Aftercoolers

Intercoolers are designed to remove the heat of compression between the stages of multi-stagecompressors. Aftercoolers serve the same function following the final stage of compression. Theycan be either water cooled or air cooled.

Atmospheric air contains moisture, and furthermore, the air may pick up oil vapor as it passesthrough some compressors. Cooling the air down to or below its initial temperature will removemoisture down to the dew point, improving the quality of the air.

Another purpose of intercooling is to improve the efficiency of compression. Refer to Figure 6

below, which is a pressure-volume diagram of compression. The work input to the compressor inthe case of isothermal compression (perfect cooling) is represented by the area ABCD, and is theleast possible work for a given compression. Perfect cooling means that all the heat ofcompression is removed, with no pressure drop in the intercoolers. The other extreme is adiabaticcompression (no cooling). All of the heat of compression is retained within the air. The work foradiabatic compression is represented by the area ABCE. The dotted curve is the approximateactual compression line, somewhere between isothermal and adiabatic, and represents a certaindegree of intercooling. The work input to the compressor with some intercooling is between thetwo extreme theoretical cases. The work saved by intercooling is represented by the area betweenthe actual curve and the adiabatic curve.


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Figure 6 Pressure/volume diagram of a compressor.

Separators

Intercoolers and aftercoolers can only remove water in proportion to their ability to lower thetemperature of the compressed air. Moisture separators are generally used between theaftercooler and the receiver, particularly in cases where moisture and oil cannot be tolerated in thecompressed air. Figure 7 below shows some of the separator designs in use. Some usecentrifugal force or rapid changes in direction of flow to throw out moisture particles. The airreceivers also contribute to the removal of moisture simply by providing a chamber of low airvelocity to allow the moisture to settle out and be removed by drain traps. In applications whereremoval of entrained oil droplets is essential, more sophisticated separators, such as coalescingfilters, may be installed after the receiver. Coalescing filters use a combination of baffles and acoalescing medium to remove the maximum amount of impurities. Some of these filter types areillustrated in Figure 7.


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Figure 7 Sketches of various moisture separator designs.


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Drives

Compressor drives are commonly electric motors, steam turbine or internal combustion engines.Recently, combustion turbines have also become popular. As previously mentioned, accessoriessuch as these often are purchased as part of the compressor package.

Control

Compressor controls are used to control output and load. Steam-driven compressors usually havecombination speed and pressure governors that vary air capacity by changing speed. There aretwo basic types of compressor controls: Throttling governors vary steam pressure, reducing it asair discharge pressure rises. Automatic cutoff governors change the cutoff point of valves withinthe steam cylinder.

Throttling governors are used with a manually adjustable cutoff in plants that have a steady supplyof exhaust steam, or where conditions are so nearly constant that automatic cutoff valves would

have little chance to function.

Motor-driven and other types of constant speed compressors usually have one of three types ofcontrols: (1) Constant speed control decreases compressor capacity in one or more steps bymeans of an unloader. (2) Automatic start and stop control uses a starter and pressure switch.This type of control works well where air demand is intermittent with long periods of no demand,and where precise pressure regulation is not necessary. (3) Dual control is a combination of (1)and (2) . It allows continuous operation when demand is nearly continuous, and automatic startand stop when demand is low.

Constant speed 5 step control unloads the compressor in five steps, reducing capacity from full

load to , , , and no load as demand decreases. There also are three step and one stepunloading controls for smaller compressors.

There are several other considerations in designing a compressed air or gas system, such ascompressor cooling, reliability requirements, and environmental concerns.

Conclusions

Compressors may be considered to be minor pieces of industrial equipment, but like electricmotors and pumps, they are essential to the reliable functioning of many complex functions and

mechanisms. As such, the design engineer and the operating engineer must be aware of theirapplications and limitations, and to be familiar with the definitions and terminology.

Air and Gas Compressors

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