Lec 1 Compressors Overview

7/28/2019 Lec 1 Compressors Overview

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Watania Advanced Technology

Types of compressors

IntroductionWhat is a compressor?

A compressor is a device used to increase the pressure of acompressible fluid. The inlet pressure level can be any value from a deep vacuum toa high positive pressure. The discharge pressure can range fromsub-atmospheric levels to high values in the tens of thousands of pounds per square inch. The inlet and outlet pressure are related, corresponding with thetype of compressor and its configuration.

Compressed fluid The fluid can be any compressible fluid, either gas or vapor, andcan have a wide molecular weight range. Recorded molecularweights of compressed gases range from 2 for hydrogen to 352for uranium hexafluoride.

Applications

Applications of compressed gas vary from consumer products,such as the home refrigerator, to large complex petrochemicalplant installations.

Compression Modes

The different types of compressors can be subdivided into twobroad groups based on compression mode. There are two basicmodes:

Intermittent

The intermittent mode of compression is cyclic in nature, in thata specific quantity of gas is ingested by the compressor, actedupon, and discharged, before the cycle is repeated.

Continuous

The continuous compression mode is one in which the gas ismoved into the compressor, is acted upon, moved through thecompressor, and discharged without interruption of the flow atany point in the process.

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Compressors types

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Positive displacement compressors

Compressors using the intermittent compression mode are referred toas positive displacement compressors, of which there are two distinct

types: reciprocating and rotary.Dynamic

They are considered as Continuous Compression CompressorsEjectors

They are considered as Continuous Compression Compressors

Types of Positive displacement compressors

Reciprocating Compressors

It consists of a mechanical arrangement

in which a piston is free to move in acylinder. The displacing action of thepiston, together with the inlet valve orvalves, causes a quantity of gas to enterthe cylinder where it is in turncompressed and discharged. Action of the discharge valve or valves preventsthe backflow of gas into the compressor

from the discharge line during the next intake cycle.

Single-acting and double-acting

When the compression takes place on one side of the pistononly, the compressor is said to be single-acting.

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ReciprocatingCompressors

They are machines inwhich successive

volume of air or gasesare confined within a

closed space. Thepressure is increasedas the volume of the

closed space is

decreased.



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The compressor is double-acting when compression takes placeon each side of the piston.

Single-stage and multistage compressor ConfigurationsConfigurations consist of a single cylinder or multiple cylinderson a frame.When a single cylinder is used or when multiple cylinders on acommon frame are connected in parallel, the arrangement isreferred to as a single-stage compressor figure 1.1.When multiple cylinders on a common frame are connected inseries, usuallythrough a cooler,the arrangement isreferred to as amultistagecompressor.

Figure 1.1 Athree-stagesingle-actingreciprocatingcompressor

Rotary compressors

They add energy to the gas being compressed through a rotating shaftcarrying a single or multiple rotors. They perform the compression in anintermittent mode. They do not have inlet and discharge valves seefigure 1.2. Gas is trapped in the open area of the lobes as the lobe paircrosses the inlet port.

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Rotary Sliding VaneThey are machines in

which longitudinalvanes slide radially in a

rotor mountedeccentrically in a

cylinder.

Rotary LobeThey are machines in

which two mating lobeimpeller revolve within

a cylinder andprevented from making

contact to each otherby timing gears

Rotary Liquid PistonThey are machines inwhich water or otherliquid are used to in a

single rotating elementto displace the air or

the gas.



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Figure 1.2 rotary elements

Types of dynamic compressors

Ejectors

They are considered as Continuous Compression Compressors. The ejector

can first be identified as having no moving parts figure3.

Figure 1-3

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Cross section of an ejector

The ejector is operated directly by a motive gas or vapor source. Air and

steam are probably the two most common of the motive gases. The ejector uses a nozzle to accelerate the motive gas into the suctionchamber where the gas to be compressed is admitted at right angles tothe motive gas direction. In the suction chamber, also referred to as themixing chamber, the suction gas is entrained by the motive fluid. Themixture moves into a diffuser where the high velocity gas is graduallydecelerated and increased in pressure.

Dynamic Compressors

In which air or gases are compressed bydynamic action of rotating vans orimpellers imparting velocity andpressure to the flowing gas.In dynamic compressors, energy istransferred from a moving set of bladesto the gas. The energy takes the form of velocity and pressure in the rotatingelement, with further pressureconversion taking place in the stationaryelements. Because of the dynamic nature of these compressors, thedensity and molecular weight have an influence on the amount of pressure the compressor can generate. The dynamic compressors are further subdivided into three categories,based primarily on the direction of flow through the machine. These areradial, axial, and mixed flow.

The radial-flow or centrifugal compressor

A typical multistage centrifugal compressor can be seen in Figure1.4. The compressor uses an impeller consisting of radial or

backward-leaning blades and a front and rear shroud. The frontshroud is optionally rotating or stationary depending on thespecific design.

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Centrifugalcompressor

They are machines inwhich velocity and

pressure are given tothe air or the gas in

radial direction by oneor more impeller-

diffuser combination



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Figure 1.4Radial-flow horizontally split multistage centrifugal

compressor

As the impeller rotates, gas is moved between the rotatingblades from the area near the shaft and radially outward todischarge into a stationary section, called a diffuser.Energy is transferred to the gas while it is traveling through theimpeller. Part of the energy converts to pressure along the bladepath while the balance remains as velocity at the impeller tip

where it is slowed in the diffuser and converted to pressure. Thefraction of the pressure conversion taking place in the impeller isa function of the backward leaning of the blades. The more radialthe blade, the less pressure conversion in the impeller and themore conversion taking place in the diffuser. Centrifugalcompressors are quite often built in a multistage configuration,where multiple impellers are installed in one frame and operatein series.

Axial compressors

Axial compressors are large-volume compressors that arecharacterized by the axial direction of the flow passing through themachine. The energy from the rotor is transferred to the gas byblading (see Figure 1.5). The rotor consists of multiple rowsof un-shrouded blades. Before andafter each rotor row is a stationary(stator) row. For example, a gas

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Axial compressorThey are machines in

which velocity andpressure are given tothe air or the gas in

axial direction by setsof moving and

stationary rows of blades



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particle passing through the machine alternately moves through astationary row, then a rotor row, then another stationary row, untilit completes the total gas path. A pair of rotating and stationary

blade rows defines a stage. One common arrangement has theenergy transfer arranged to provide 50% of the pressure rise in therotating row and the other 50% in the stationary row. This design isreferred to as 50% reaction.Axial compressors are smaller and are significantly more efficientthan centrifugal compressors when a comparison is made at anequivalent flow rating. The exacting blade design, whilemaintaining structural integrity, renders this an expensive pieceof equipment when compared to centrifugals. But it is generally justified with an overall evaluation that includes the energy cost.

Figure 1.5 Axial-flow compressorsCompressor Performance

Compressor capacity is the volume of gas moving through thecompressor is a given period of time. Speed control is the easiest andmost economical way to control capacity. Recycling gas also affectscompressor performance and enables the operator to control theperformance of the compressor. The performance curves of compressors are presented in terms of head

(percent pressure ratio versus percent flow). Fig. 1.6 shows thecharacteristics of positive displacement, axial flow and centrifugalcompressors.From figure 1.6 the basic pressure-capacity constant speedcharacteristic shows:

Centrifugal compressor is a variable capacity, constantpressure machine.

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Axial and positive displacement compressors are constantcapacity, variable pressure machine.

Positive displacement machines are for small capacities

Centrifugal compressor machines are for medium capacities

Axial machines are for large capacities

There is considerable overlap of capacity range betweenthese different types.

Figure 1.6 General performance curves.

Compressors capacities

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Standard compressors symbols

The table shows the typical areas of work for various standardtypes of compressors.

Type Symbol Op. diagramPressure

range[bar]

Volumeflow

[m³/h]

Plungercompress

or

10 (1-stage)35 (2-stage)

120600

Crosshead

compressor

10 (1-stage)35 (2-stage)

120600

Diaphragm

compress

or <

low low

Freepiston

compressor

limiteduse as

gasgenerator

Rotaryvane

compressor

16 4 500

Liquidring

compressor

10 <BR>

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Screw

compressor

22 750

Rootcompress

or1,6 1 200

Axialcompress

or10 200 000

Radialcompress

or10 200 000

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Lec 1 Compressors Overview

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