PNEUMATIC AND HYDRAULIC EQUIPMENT

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MODULE TITLE : APPLICATIONS OF PNEUMATICS AND

HYDRAULICS

TOPIC TITLE : PNEUMATIC AND HYDRAULIC EQUIPMENT

LESSON 2 : AIR COMPRESSOR TYPES AND CONTROL

APH - 2 - 2

Published by Teesside University Open Learning (Engineering)

School of Science & Engineering

Teesside University

Tees Valley, UK

TS1 3BA

+44 (0)1642 342740

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________________________________________________________________________________________

INTRODUCTION ________________________________________________________________________________________

At the heart of any production plant using compressed air you will find the

compressor and its associated control equipment.

This lesson sets out to deal with the operation of the more commonly found

types of air compressor and the methods by which their output capacity is

controlled.

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YOUR AIMS ________________________________________________________________________________________

On completion of this lesson you should be able to:

• describe the difference in operational principle between positive

displacement and roto-dynamic compressors

• describe and illustrate the construction of;

– a reciprocating-piston compressor

– a diaphragm compressor

– a rotary-vane compressor

– a rotary-screw compressor

• state and describe four methods by which output capacity can be

controlled.

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TYPES OF MACHINE ________________________________________________________________________________________

Compressors can be broadly divided into two categories:

Roto-dynamic types

These are rotary continuous-flow machines in which a high-speed rotating

element accelerates the air, increasing its kinetic energy. This increase in

kinetic energy is then converted into increased pressure energy prior to being

discharged from the machine. Dynamic machines are not economical at

capacities lower than 600 m3 min–1 and cannot attain pressures necessary for

pneumatic control applications unless built in multistage designs. For these

reasons they are seldom encountered in pneumatic service.

Positive-displacement types

These are compressors in which successive volumes of air are enclosed and

then elevated to a higher pressure by reducing their volumes. They appear in

reciprocating and rotary types, are supplied in a number of designs, and are the

most frequently employed in compressed-air plant, supplying air for pneumatic

service applications.

Most types of compressor can be multi-staged and can benefit from this effect,

as was shown in Lesson 1.

The two broad categories may be further sub-divided as indicated by the chart

in FIGURE 1, which illustrates their relationship.

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FIG. 1 Types of Compressor

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RECIPROCATING PISTON COMPRESSORS ________________________________________________________________________________________

The reciprocating piston compressor is still the most commonly used and most

versatile compressor available, although there is an increasing trend towards

the screw compressor due to its quieter operation and low pulsation

characteristics. Reciprocating piston compressors may be either single or

double-acting.

Single-acting machines have only one compression stroke per revolution of the

crankshaft and use only the top of the piston to compress the air. FIGURE 2

shows this design.

COMPRESSORS

Roto-dynamic Positive-displacement

Radial Axial

Rotary Reciprocating

Vane Screw

Single acting piston

Double acting piston

Diaphragm

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FIG. 2 Single-acting Compressor

This machine has limited pressure capability as was shown in Lesson 1. It is

suitable for small workshops, garages, etc. It is simple in design and therefore

it is easy to maintain and to control its output.

Double-acting compressors use both sides of the piston to compress the gas.

This gives two working strokes for every revolution of the crankshaft which

increases the delivery rate of the machine to almost double that of the single-

acting type of the same size.

Delivery valve Inlet valve

Air inlet Delivery of

compressed air

Coolant outlet

Cooling jacket

Coolant inlet

Piston

Crankshaft

Air being compressed

and delivered

Connecting rod

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The design of this type of machine differs from the single-acting compressor in

several ways.

• Two extra valves are required to control the intake and delivery of the

underside of the piston.

• The bottom of the cylinder requires sealing to allow compression to be

undertaken on the downstroke. This requires the use of a linear piston-

rod and a cross-head guide, as shown in FIGURE 3, which is attached to

the connecting rod.

FIG. 3 Single-stage, Double-acting Compressor

Piston

Piston Rod

Delivery (outlet)

Delivery valve 2

Gland

Cross head guide

Inlet valve 2

Suction (inlet)

Entry chamber

Inlet valve 1

Delivery valve 1

Delivery chamber

Cooling jacket

Pivot

Connecting rodCross head

Crankshaft

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The double-acting compressor cycle consists of the following.

• The 'up' stroke: initially valves (1) remain closed to allow compression to

take place. When the delivery pressure is reached, the delivery valve (1)

opens. During the up-stroke the inlet valve (2) is open, allowing a fresh

charge into the bottom of the cylinder ready for compression on the

down-stroke.

• The 'down' stroke: initially the delivery valve (1) closes and the inlet

valve (1) opens allowing a fresh charge into the top of the cylinder. Both

inlet and delivery valves (2) are closed until compression is complete,

when the delivery valve (2) opens, allowing the second discharge into the

system.

The double-acting compressor is used when higher flowrates are required: it

succeeds in achieving this at the expense of increased complexity of design.

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RECIPROCATING DIAPHRAGM COMPRESSORS ________________________________________________________________________________________

These machines use the principle of the single-acting piston compressor but,

instead of using a piston to provide the compressing action, a diaphragm is

used. The advantage of this arrangement is that no lubrication of the piston is

required and 100% seal is obtained with the diaphragm. This gives the

advantage of producing oil-free air. This type of machine is normally used to

supply instrument air at lower pressures up to a maximum of approximately

7 bar.

FIGURE 4 illustrates the construction of a diaphragm compressor.

FIG. 4 Diaphragm Compressor

Air inlet Air outlet at

pressure

Flexible diaphragm

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________________________________________________________________________________________

ROTARY COMPRESSORS ________________________________________________________________________________________

Rotary-type displacement compressors deliver air which is less pulsating than

the piston type and they are also quieter in operation. The air supplied from

these machines is often 'oil-loaded' and requires oil removal before use.

Because of the continuous rotary action this kind of machine is smaller for a

given flow than a reciprocating type.

ROTARY VANE COMPRESSORS

The vane-type compressor shown in FIGURE 5 consists of a rotor containing

sliding vanes mounted eccentrically to the main housing. The vanes are driven

out to the walls of the circular housing by centrifugal force during rotation,

sometimes assisted by springs. The vanes together with the circular housing

form the compression spaces. Inlet and delivery ports are included in the

circular housing. Air is trapped in pockets formed between each adjacent pair

of vanes and the air volume within each pocket is continually being expanded

and then compressed on each revolution (as with the piston type). Air enters a

pocket through the inlet port during expansion, is then compressed as the

pocket gets smaller, and is finally delivered into the system via the delivery

port.

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FIG. 5 Rotary Vane Compressor

FIGURE 6 illustrates the compression process for the rotary-vane machine. As

the air in a pocket between the vanes expands from point A, its pressure is

reduced. Atmospheric pressure causes an inflow of air through the inlet port

which continues to point B. From point B to point C the pockets are reduced

in volume and hence compression occurs. As the vanes pass point C the

delivery port is uncovered and the compressed gas is delivered into the system.

Sliding vanes

Inlet port

Air inlet

Rotor

Oil injection

Delivery to oil filter

Discharge port

Cooling jacket

Air pocket

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FIG. 6 Compression Process for Rotary Vane Compressor

The vanes of most compressors are made of non-metallic substances, often

fibre or carbon.

To aid sealing between the vanes and the casing, oil is injected into the

compressor space. This also aids cooling by carrying away generated heat and

acts as a lubricant. Although the oil is essential for the efficiency of the

compressor, it results in oil-loaded air being produced which must be removed

before delivery into the system. This is done by passing the air through an air-

oil separator. The oil which is removed is then cooled and re-circulated into

the oil injection system.

The major advantages of rotary-vane compressors are:

• little vibration

• the air produced is relatively pulse-free

• low noise level.

Discharge

Intake

m pr

es si

Inlet

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The above advantages make the rotary-vane machine suitable for mobile

compressor applications.

SCREW COMPRESSORS

Rotary-screw compressors have replaced reciprocating compressors for many

applications. They offer several advantages over reciprocating machines due

to the following:

• pulsation-free air is produced

• they can be used without receivers

• noise level is generally low

• vibration is minimal.

This type of machine consists of two intermeshing helical screw rotors which

are arranged to rotate in opposite directions. The rotors are of different

designs, one male and the other female. FIGURE 7 shows the plan and end

views of these screw rotors.

The male screw usually has one or two less lobes than the female has flutes.

This allows for an increased root diameter on the female screw which increases

its strength.

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FIG. 7 Rotary Screw Compressor

Bearings

Delivery casing

'Male' splined rotor

'Step-up' helical gears

'Female' rotor

Inlet casing

Plan view

Female rotor

Male rotor

Casing

End view

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There are two distinct designs of screw compressor:

• Oil free: this design uses a set of timing gears to drive both rotors in

synchronisation and at the same time stop metal-to-metal contact. This

allows oil-free air to be produced. As there is no metal-to-metal contact

between the rotors or the casing, friction is very low, but at the expense of

compression efficiency due to leakage between the screws and the casing.

• Oil-loaded: this design does not generally employ timing gears and one

rotor is used to drive the other. This is possible due to the oil flooding

giving good lubrication. Good sealing is also obtained between the rotors

and the casing. The oil which is carried through the machine with the air

also picks up some of the heat of compression. When this oil is removed

from the air prior to entry into the system this heat is removed and the oil

is re-circulated.

As rotation occurs, air is drawn into the inlet and into the spaces between the

screws. This forms a helical column of air which is compressed up to the

delivery port and discharged into the system.

A recent development of the oil-flooded design has been to coat the rotors in

poly-tetra-fluoro-ethylene (PTFE). This has a very low coefficient of friction

and allows the simple design of one rotor driving the other without the need

for timing gears and lubrication.

These machines are very popular for mobile compressor applications where

large capacities are required. They are, however, gaining popularity in many

traditional compressed-air plant situations due to the previously-mentioned

advantages.

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________________________________________________________________________________________

ROTO-DYNAMIC COMPRESSORS ________________________________________________________________________________________

Roto-dynamic compressors fall into two main categories:

• centrifugal flow

• axial flow.

They are essentially high flow, low pressure machines, although high pressures

can be obtained by multi-staging.

Advantages of dynamic compressors are:

• oil-free air

• pulsation-free air

• high air delivery capacity.

CENTRIFUGAL FLOW COMPRESSOR

This machine consists of an impeller mounted on a shaft between bearings and

housed in a volute casing. Air is taken in at the centre or 'eye' of the impeller

and 'flung' outward by the centrifugal force set up by the high rotational speed.

This action speeds up the air increasing its kinetic energy. When the air is

slowed down by the action of the diffuser ring and the volute casing, the

kinetic energy is converted into increased pressure energy.

FIGURE 8 shows a simple single-stage centrifugal machine. The pressure

increase in this system would be very low.

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FIG. 8 Single-stage Centrifugal Compressor

To achieve higher pressure ratios the machine is multi-staged. This is

normally done by increasing the number of impellers on the shaft and

incorporating them within a special casing. FIGURE 9 shows a multi-stage

centrifugal compressor. Each stage of the machine accelerates the air and

imparts kinetic energy. Between stages the air is slowed down and its pressure

is hence increased. The air then enters the eye of the next impeller at a higher

pressure and the process is repeated.

These compressors are rarely found in pneumatic power situations but are

more suited to high flow, low pressure applications.

Gas exit

Gas in at impeller 'eye'

Diffuser vanes

Impeller blades

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FIG. 9 Multi-stage Centrifugal Compression

AXIAL FLOW

The most basic type of axial flow compressor is the fan. It takes in air from

one side, increases its velocity and passes it to the other in an axial direction,

coupled with some rotation.

To obtain usable pressure ratios, this design requires multi-staging as with the

centrifugal type. It differs from the centrifugal type by having alternate rows

of fixed and moving blades.

Shaft driven by electric motor

1st Stage 2nd Stage 3rd Stage

Impeller

Suction ( gas in )

Discharge ( gas out )

3rd Compression

1st Compression

2nd Compression

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The purpose of the fixed blades between moving blades is to:

• convert the velocity energy into pressure energy

• change the direction of the air so that it approaches the next set of rotating

blades at the optimum angle.

FIG. 10 Multi-stage Axial Flow Compressor

Again, axial flow compressors are not normally found in air power situations

but are used more often in high flowrate and low pressure applications.

Air delivery

Drive shaft

Last row of fixed blades

Casing, or stator

Air inlet

Inlet fairing

Row 1 of moving blades

Row 1 of fixed blades

Rotor

1 stage of compression = 1 row of moving blades + 1 row of fixed blades

Row 1

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COMPRESSOR CONTROL ________________________________________________________________________________________

If a compressor is allowed to run continuously and user demand for air falls,

the pressure in the system will rise until such time as a system relief valve

opens or the compressor driving source stalls. To prevent this happening,

some form of control over the compressor output flow is required to aid the

matching of air supply to user demand.

There are four common methods employed to achieve this:

• control of compressor speed

• stop/start control

• intake throttling

• inlet valve lifting.

The above systems of control are almost always activated by sensing the

delivery pressure in the system. When this pressure rises, the compressor

output is reduced; when it falls, the output is increased.

CONTROL OF COMPRESSOR SPEED

A pressure-sensing device is used to control the speed of the prime mover

(motor, engine or turbine). This type of control is most suitable for control of

compressors driven by internal combustion engines. It can be adapted to

control the output of all types of compressor and is often found on dynamic

machines.

FIGURE 11 shows a circuit using this method of control.

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FIG. 11 Compressor Speed Control Circuit

STOP/START CONTROL

This is a convenient method of control when using small electrically-driven

machines. A rise in the delivery pressure above a pre-determined level

operates a pressure switch which in turn switches off the compressor drive

motor. When this pressure falls, the motor restarts. This method is most

suitable for small piston compressors.

FIGURE 12 shows a circuit employing stop/start control.

FIG. 12 Compressor Stop-Start Control Circuit

Compressor

Pressure sensing line

To system

Receiver

Air intake

Motor

Variable pressure on-off switch

Speed governor

Compressor

Pressure feedback control line

To system

Receiver Air intakePrime mover

Delivery

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INTAKE THROTTLING

With this method a valve in the compressor's air intake is used to restrict the

flow of air to the compressor. As the delivery pressure rises the valve moves

towards the closed position; as it falls the valve is opened. When the valve is

fully closed the compressor still runs at normal speed, but because no air is

being introduced less work is done. Because compression of air trapped within

the machine still takes place, energy is still being consumed.

This type of control can be used for vane, screw and some dynamic machines.

Its main advantage is that it responds very quickly. A disadvantage is that if

the throttle is closed for long periods, overheating may occur.

FIGURE 13 illustrates a circuit using intake throttling.

FIG. 13 Compressor Intake Throttling Control Circuit

Compressor

Pressure sensing line

To system

Receiver

Air intake

Motor

Variable throttle valve

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INLET VALVE LIFTING

This method only applies to piston compressors. Control is achieved by

holding open the inlet valve so that compression cannot take place. Because

compression does not take place and pressure in the cylinder is very low, little

power is used, making it an efficient means of control.

When the pressure in the system rises above a pre-determined level, a valve

opens allowing system pressure to activate an inlet valve lifting device, hence

unloading the compressor. When system pressure falls, the inlet valve is

allowed to close and the compressor operates normally.

FIGURE 14 shows a system employing inlet valve lifting.

FIG. 14 Compressor Control using Inlet Valve Lifting

Valve seat

Unloading arm

Valve plate

Valve guard

Diaphragm

Pilot signal

Air intake

Spring

Piston

Inlet valve control

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________________________________________________________________________________________

SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. Describe briefly the difference between the operating principles of roto-

dynamic and positive-displacement types of compressors.

2. Why are roto-dynamic machines not normally used for pneumatic plant

services?

3. What is the function of the crosshead and crosshead guide in a double

acting compressor?

4. Why is it necessary to inject oil into the internals of a rotary-vane type

compressor? What feature of this design provides automatic

compensation for vane wear?

5. Describe the basic difference in design between an oil-free and an oil-

loaded rotary-screw compressor.

6. List the four commonly used methods of pressure control for air

compressors and, with the aid of a simple sketch, describe the method

most suitable for small electric motor-driven machines.

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________________________________________________________________________________________

NOTES ________________________________________________________________________________________

...................................................................................................................................................

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________________________________________________________________________________________

ANSWERS TO SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. In a roto-dynamic compressor, the pressure increase is mainly due to the

velocity, and therefore the kinetic energy, of the fluid being increased by a

high-speed rotating element. This increased speed is then converted to

increased pressure by another mechanical device – either a diffuser ring,

volute casing or stator, dependent upon the type of dynamic machine.

In a positive-displacement compressor, a fixed volume of air is enclosed

in a compression chamber and a force applied via a piston or rotating

component to reduce the volume of the gas, this is responsible for

increasing the pressure energy of the air.

2. Roto-dynamic compressors are high mass-flowrate machines which are

not economical for use at capacities less than about 600 m3 min–1. They

also cannot attain the pressures required for pneumatic control

applications unless built in multi-stage designs. This also adds to the

expense of the installation. As a consequence, most pneumatic plant

servicing compressors will be of the positive-displacement type.

3. The function of the crosshead and crosshead guide in a double-acting

reciprocating compressor is to convert the rocking action of the crank

connecting rod into true linear motion of the piston rod. The piston rod

can then be sealed and the piston can be designed to compress on both its

upward and downward strokes.

4. It is necessary to inject oil into the internal parts of a rotary-vane

compressor to reduce the sliding friction generated by the vanes of the

rotor passing over the inside surface of the cam ring at high speed. It is

also used to improve the sealing between the tips of the vanes and the

cam ring, so increasing the volumetric efficiency of the compressor.

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The high centrifugal force, generated by turning the rotor at a high speed,

will tend to hold the vanes in contact with the inner surface of the cam

ring. If the vane tips do wear, then centrifugal force acting on the vanes

will immediately throw them out further to compensate. In some designs,

springs are used to hold the vanes in contact with the cam ring at lower

speeds.

5. In an oil-free design of screw compressor the two rotating screws are

driven externally by gears in such a manner that they do not come into

contact with each other in the compression chamber. This will have the

effect of reducing the volumetric efficiency of the machine due to

increased internal leakage but will remove the need for the air to be oil-

loaded.

In the oil-loaded design only one rotating screw is externally driven and

this then imparts the drive to the other screw. This results in a degree of

sliding friction being generated between the two. For this reason, it is

necessary to inject oil to reduce the friction.

6. The four most commonly used methods of compressor capacity control

are:

• control of compressor speed

• stop/start control

• intake volume throttling

• inlet valve lifting.

The method most suitable for the control of a small electric motor-driven

installation would be stop/start control, as illustrated in the sketch

overleaf.

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FIG. 12 (Reproduced)

Compressor

Pressure sensing line

To system

Receiver

Air intake

Motor

Variable pressure on-off switch

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________________________________________________________________________________________

SUMMARY ________________________________________________________________________________________

Air compressors fall into two main types, positive-displacement machines and

roto-dynamic machines. However, it is normally only the displacement types

that are used to supply air for the operation of pneumatic control systems

because dynamic machines have in the main very large outputs and low

compression ratios which make them unsuitable for such duties.

Of the displacement types, the reciprocating-piston compressor is still

probably the most widely-used either in its single-stage or multi-stage form.

However, it is gradually being replaced by the rotary-displacement designs,

notably the rotary-vane type and the rotary-screw type.

Both of these designs have certain advantages over reciprocating-piston

machines, in that they are quieter and transmit less vibration in operation, and

the air that they produce is virtually pulsation free. The air that is produced by

this type of machine, however, can be quite highly oil-loaded and some

provision must be made for the removal of this unwanted side effect.

At the conclusion of this lesson you should also be able to appreciate the need

for some form of control of the output capacity of the machine, so that supply

can be more closely matched to demand.

The most common methods of controlling compressor output are by:

• motor speed control

• stop/start control

• intake throttling

• inlet valve lifting.

Each method is particularly suited to a different application.

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