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151903 - Fluid Power Engineering. Reciprocating Air Compressor. Prepared by Prof. Jagdish S. Talpada. Lecture - 1. Reciprocating/Positive Displacement Compressors. Gas compression has been one of the anchor points of the industrial revolution ,

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lecture 1
Lecture - 1

prepared by Prof. Jagdish S. Talpada

reciprocating positive displacement compressors
Reciprocating/Positive Displacement Compressors

Gas compression has been one of the anchor points of the industrial revolution,

beginning with low pressure air supply for iron and steel refining, through higher

pressure air supply for drilling and plant operating equipment, to high pressures as

required for chemical synthesis, storage and pipeline deliveries of fuel gases. The

positive displacement compressors in use today can trace their ancestry back to

the original pumping machines invented by James Watts, or the bellows and

blowers of blacksmiths.

Piston type compressors have a solid position in this field: the technology is

mature (more than a century of development), the fabrication process is straight

forward, and the equipment is extremely scalable, ranging from miniature

emergency tire inflation pumps to compressors of 10,000 horsepower or more.

These latter are particularly used in the chemical process and gas transmission

industries. There the requirements for high reliability, extreme range in

throughput volume, and flexibility in operating pressures make an excellent fit for

reciprocating piston compressors. This module describes the operating

characteristics of various positive displacement compressors and develops the

theory, basic calculations and rudiments of control for the piston type

reciprocating compression process. While some references are to the gas

compression and transmission industry, the same equipment construction and

control methods are used in process compressors for industries such as

petrochemicals and chemical synthesis.

prepared by Prof. Jagdish S. Talpada

p arts

Piston (Reciprocating)

The reciprocating piston compressor is the most widely used equipment for gas

service. The basic design consists of a piston in a cylinder with pressure actuated

check valves to control suction and discharge flow through the cylinder. Standard

practice is to have the piston driven by a rod passing through a packing case to

seal against pressure leaks. With this double acting design, gas can be

compressed on both sides of the piston. The basic design is more than a hundred

years old, and is well developed. The throughput and loading can be adjusted by

speed variation, addition of clearance to the cylinders, deactivating cylinders to

reduce displacement or active control of valve closing, which effectively gives

variable control of displacement. Efficiencies of this type of compressor can be

more than 85 percent for conversion of horsepower input to pressure rise.


A vane compressor consists of a cylindrical chamber with a rotating paddle wheel

type drum mounted off center in the chamber. As the drum rotates, the sliding

paddle wheel vanes section off volumes, which decrease in volume as they move

toward discharge. A suction port is machined into the area where the chambers

have the highest volume, and a discharge port is located where the chambers have

the smallest volume. Gas enters at the higher volume and is compressed and

discharged at the minimum volume. This type of compressor will tolerate more

dirt than a reciprocating unit, and is often used for natural gas production services.

The maximum differential is limited by the strength of the paddle wheel seals, so

these units are not applicable for high pressures and differentials.

prepared by Prof. Jagdish S. Talpada


Blower (Rotary)

In this compressor, two intermeshing elements rotate in an ellipsoidal chamber

with intake and exhaust ports on opposite sides. As they rotate, gas is trapped in

spaces formed between the chamber and moved to the opposite side of the

chamber, where it is delivered to the discharge. This action is similar to the vane

compressor, but is even more tolerant of liquids and dirt. For high pressure ratios,

oil may be injected into the suction to improve the seal of the rotors and remove

some of the heat of compression.


The operation of a screw compressor is similar to the blower, except that the

compression chambers are formed between two intermeshed elements similar to

worm gears or screw threads. This compressor also requires oil injection for

sealing and cooling. It is designed for high pressure ratios but is usually limited

to discharge pressures below 250 Psig.

prepared by Prof. Jagdish S. Talpada


Cylinder and Ends

The compressor cylinder is a casting or forging designed to safely contain some

maximum working pressure. It is machined to hold compressor valves and to

direct gas flow to and from the cylinder cavity. In combination with the cylinder

ends, it must contain the gas pressure, while having sufficiently large gas flow

passages so there are minimal pressure drops due to gas flow. The cylinder and

ends may also have water passages to stabilize temperature and dimensional

changes. All these requirements involve compromises between size, strength, and

flow passage size (efficiency). Compressor cylinders are designed for some

operating range and service. If conditions change, they may not perform reliably

or efficiently. As an example, a cylinder for gas transmission has large flow

passages and valve areas for efficiency at high gas volumes and low pressure

ratios, and will not function at high ratios. Similarly, a process cylinder may be a

forging with small passages, giving higher strength but low efficiency.


The compressor piston converts the energy/work supplied by the engine, applying

it to the gas to raise its pressure. The piston must be strong enough to withstand

the pressures and forces applied, but still be as light as possible, to minimize

reciprocating weights and the resulting shaking forces. The compressor rings seal

gas pressure to avoid leaking from one side of the piston to the other. The piston

may also be fitted with a rider band, which is a low friction material to keep the

metal piston from contacting the bore of the cylinder and causing scuffing and

wear. Material for the rings and rider bands is selected to give long life and

minimal wear with the typical pressures and gas composition of the compressor.

While this is generally a low friction thermoplastic type material, rings may be

made of bronze or other materials when temperatures are a problem.

prepared by Prof. Jagdish S. Talpada



Compressor valves are simply fast acting check valves with a low pressure drop.

They must be optimized to balance the opposing demands for long operating life

and minimal pressure drop/flow losses. They may also have special features such

as center ports to allow cylinder unloading.

The compressor valve is possibly the most critical component when determining

the requirements for a compressor service. The flow area is sensitive, as too small

an area will give low efficiency, but too large an area can result in valve flutter

and early failure. Similarly, valve components must be designed for the expected

pressure and temperature conditions.

Valves have been designed with many configurations, particularly in the sealing

elements. These have progressed through steel, Bakelite, glass filled Teflon or

Nylon, and high strength plastics. The most popular designs for sealing elements

are ring shaped strips, mushroom shaped poppets, and straight channel strips.

The design of compressor valves includes a number of variations to accommodate

cylinder flow and unloading requirements. The simplest is a single deck valve,

shown on the left above, where gas flows into passages in one face, across the

sealing elements, and out through passages in the back face of the valve.

A modification of this design is to have a large opening in the center of the valve.

This allows adding a cylinder deactivator or clearance volume to the cylinder.

This added feature comes at the expense of reduced flow area and efficiency. To

compensate for this, two valves may be assembled together with a flow passage

through the center. This double deck valve design has improved flow area and

efficiency. This type of valve can only be used in a cylinder designed to accept

its increased height.

prepared by Prof. Jagdish S. Talpada



The compressor packing is a series of pressure containing rings located in the

crank end of a double acting compressor cylinder. These seal against the piston

rod and prevent leakage, so that the cylinder can compress gas on both sides of

the piston. Again, as with compressor rings, the packing material is selected to

provide best life and sealing with expected conditions. The packing is generally

pressure lubricated, and may have coolant flow to remove friction heat. There are

also various specialty types to reduce gas leakage around the rod. This may be

important when compressing highly flammable or toxic gases. It is also

becoming more important in reducing gas leakage and emission of “greenhouse


Clearance Unloaders

In many applications, the volume of gas to be delivered may change based on

either gas supply or process demands. Also, varying pressure conditions can

change the load on the driver, requiring load control. This may be accomplished

by speed variation, deactivating cylinders or cylinder ends, or by varying cylinder

clearance. This last option is highly preferred, as speed control may have a

limited range, and deactivating cylinders or ends can cause mechanical shaking or

acoustic pulsations. Clearance unloaders allow varying throughput and load with

minimal loss of efficiency. Unloaders are not actually a part of a compressor, but

are included on many installations, to give load and throughput control. This may

be done by volumes cast into the cylinder or heads, with a valve to close the

passageway. Other options are valve cap pockets and head end variable pockets.

Added clearance may have a simple handwheel to control its operation, or may

have pneumatic actuators, which allow automatic operation.

prepared by Prof. Jagdish S. Talpada


Distance Piece Compartment(s)

A distance piece section may be placed between the crosshead and cylinder to

prevent leakage of gas from the compressor packing entering the compressor

crankcase. At the crosshead end, an oil seal around the compressor rod prevents

oil from migrating to the cylinder, and gas from entering the crankcase. This

distance piece is normally vented to remove any gas which leaks from the

packing. In cases of explosive or toxic gases there may be two distance pieces in

series, to assure containment of the gases.

prepared by Prof. Jagdish S. Talpada

l ecture 2
Lecture- - 2

prepared by Prof. Jagdish S. Talpada

definition of terms
Definition of Terms

Single and Double Acting Compressor

A Single Acting piston compresses gas on only one face, either by design

or by deactivating valves on one side of a double acting cylinder

Double Acting – Piston compresses gas alternately on both faces.

Connecting Rod

A compressor element connecting the crankshaft to the compressor piston

or crosshead. The connecting rod converts the rotation of the crankshaft

into linear motion to drive the compressor piston.


A crosshead is a sliding component at the outer end of the connecting rod,

which converts the eccentric motion of the connecting rod to pure linear,

eliminating side forces on the compressor piston.

prepared by Prof. Jagdish S. Talpada


Wrist Pin/Crosshead Pin

The wrist or crosshead pin connects the outer end of a connecting rod to

either a single acting, trunk type piston (wrist pin) or to a crosshead

(crosshead pin)

Compressor Rod/Piston Rod

A cylindrical rod which connects the compressor piston to a crosshead,

normally passing through a packing case to seal compression pressure into

the cylinder

Compressor Piston

A reciprocating component, normally fitted with piston rings which

changes the volume of a cylinder, providing compression. It may be a

simple trunk type piston directly connected to the connecting rod, or

double acting, driven by a compressor rod.

Compressor Rings

Compressor rings provide a seal between the compressor piston and

cylinder wall, preventing gas leakage either into or out of the cylinder


Rider Rings and Rider Bands

Rider rings or bands are normally provided on a double acting piston to

prevent contact of the piston with the cylinder wall. Rider rings/bands are

normally made of carbon filled Teflon or other low friction material.

prepared by Prof. Jagdish S. Talpada


Compressor Packing

Compressor packing is used in a double acting cylinder to seal around the

compressor rod, preventing gas leakage from the cylinder. Packing is

normally a series of segmented metallic rings, assembled and held in the

end of the cylinder by the packing case.

Compressor Valves

Compressor valves are high speed check valves, controlling flow of gas

into the cylinder (suction valve) or out of the cylinder (discharge valve).

They are designed for minimal pressure loss and maximum reliability

Cylinder Clearance (Mechanical)

Clearance must be provided at the end of the piston stroke to avoid contact

between the piston face and the compressor cylinder head. This clearance

is expressed in linear measurement (inches or mm.).

Cylinder Clearance (Volume)

Volumetric clearance is space left at the end of a piston stroke, both due to

mechanical clearance and volumes above suction and discharge valves to

allow for good gas flow. Clearance may also be added for control of

throughput volume and/or load control (unloaders or clearance pockets).

prepared by Prof. Jagdish S. Talpada


Compression Ratio

Compression ratio is the measure of increase in pressure across a

compressor cylinder. It is determined by dividing the discharge pressure

by suction pressure (both pressures must be absolute rather than gauge)

Pressure – Absolute and Gauge

Gauge pressure is the value which would be measured by a gauge

calibrated to indicate zero pressure when exposed to atmosphere.

Absolute pressure is pressure which would be read from a gauge

calibrated to read zero when exposed to complete vacuum. Normally

absolute pressure is gauge pressure + 14.73 PSI.

prepared by Prof. Jagdish S. Talpada

lecture 3
Lecture - 3

prepared by Prof. Jagdish S. Talpada

cycle events
Cycle Events

In a reciprocating compressor, the process follows four main events –

compression, discharge, re-expansion and intake. The first two are accomplished

as the piston moves forward, reducing cylinder volume, while the second takes

place as the piston moves back down the cylinder.

For a more complete picture, assume starting the cycle with the compressor at the

bottom of its stroke, with maximum cylinder volume. The cylinder is full of gas

at suction pressure, and both suction and discharge valves are closed by gas

pressure. As the piston moves forward, the cylinder volume decreases and

pressure rises. When the cylinder pressure rises slightly above discharge

pressure, the discharge valve opens and gas is pushed into the discharge piping for

the rest of the stroke. At top center, the discharge valve closes. As there must be

clearance between the piston face and cylinder head to prevent parts hitting each

other, some volume of gas is trapped in the cylinder at discharge pressure. As the

piston moves back down the cylinder, this gas re-expands until it reaches suction

pressure. At this point, the suction valve opens and a fresh charge of gas flows

prepared by Prof. Jagdish S. Talpada

volumetric efficiency
Volumetric Efficiency

As noted above, the cylinder does not bring gas in through the entire piston travel.

The percentage of stroke the suction valve is open, compared to the entire stroke

is called “volumetric efficiency”. If there were no clearance (volume) left when

the piston completed its compression stroke, then cylinder pressure would

immediately drop to suction pressure as the piston returned, giving 100 percent

volumetric efficiency.

Thus, the cylinder displacement would be equal to the volume delivered with each

stroke. However, due to gas re-expansion, the suction valve opening is delayed.

This delay becomes greater when the cylinder pressure ratio increases or the

clearance volume increases. Thus, the cylinder delivers a reduced volume to the

discharge condition.

The pictures below illustrate this effect, with the picture on left showing effect of

increasing clearance, and on right the effect of increasing pressure ratio. At high

pressure ratios, or with large amounts of clearance, the valve opening may be

delayed to the point that the valve does not open, and no gas flows through the

cylinder. This condition is called zero volumetric efficiency, and can cause

serious cylinder heating problems.

In normal operation, friction of rings on the cylinder creates heat which is carried

away with the gas being compressed. Since at zero volumetric efficiency, no gas

is entering or leaving the cylinder, all friction heating effects are contained within

the cylinder, causing an uncontrolled temperature rise. As the hot gas is

contained within the cylinder, normal temperature detection in the discharge line

will not be effective.

prepared by Prof. Jagdish S. Talpada


Clearance Control

As noted above, cylinder clearance will significantly affect throughput and

horsepower of a compressor. Some amount of volumetric clearance is built into

the cylinder to prevent the compressor cylinder from contacting the heads at the

extremes of piston travel, and to provide a smooth gas flow path into and out of

the cylinder.

Beyond this, additional clearance can be introduced by providing clearance

pockets or passages which open into the cylinder cavity. These have valves

which can be opened or closed to add or remove the clearance from the

compression process. Also, some cylinders may be equipped with a variable

clearance pocket on the outboard cylinder head. These have a piston positioned

by a screw and hand wheel, which will add a variable amount of clearance.

Work of Cycle

The familiar definition of work is force times distance. In the pressure-volume

cards shown above, piston movement or change in volume defines a distance. As

the force against the piston changes as pressure increases and decreases, the area

of the card defines the work involved in the cycle.

A key point to note is that for a given pressure differential, changing the

volumetric efficiency changes both the volume delivered and the work of the

cycle. This is the basis for load control of compressors by changing the cylinder


prepared by Prof. Jagdish S. Talpada


Pressure Ratio

Pressure ratio is the discharge pressure of the compressor divided by the suction

pressure. These pressures must be in absolute (Psia) rather than gauge (Psig)

pressure. As most operating gauges read in Psig, atmospheric pressure must be

added. This is normally about 14 Psi.

A reciprocating compressor may be able to operate at high pressure ratios, but is

usually limited by other conditions, particularly temperature. A compressor’s

discharge temperature increases with pressure ratio. For example, at a pressure

ratio of four and a suction temperature of 60 degrees, discharge temperature

would be about 310 degrees. This is a safe practical limit for most compressor

components. Consequently, pressure ratios across any single compressor

cylinder rarely are allowed to exceed four to one.

Temperature Rise – Ratio Effect

When a gas is compressed, its temperature rises in proportion to the pressure

ratio. For low pressure ratios, the discharge temperature may be only twenty to

prepared by Prof. Jagdish S. Talpada


fifty degrees higher than suction temperature. When the pressure ratio is high,

such as on storage or production service, the discharge temperature may be more

than a hundred degrees higher than the suction.

This is true for all types of compressors. This temperature rise may limit the

amount of pressure rise allowable across a compressor, or require special

components to withstand the temperature. This temperature must be reduced

before gas is put into underground pipelines, to prevent melting their protective


In most cases, the discharge temperature from a compressor station must be kept

below 1250F, requiring gas coolers at higher pressure ratios. This is particularly

the case at storage and production stations, where high pressure ratios give

extreme discharge temperatures.

prepared by Prof. Jagdish S. Talpada

lecture 4
Lecture - 4

prepared by Prof. Jagdish S. Talpada

multi staging

Sharing Differential

The limits of operation listed above show that a reciprocating compressor has a

number of mechanical limits, most of which are related to pressure differential.

Often differentials are required greater than can be accomplished with a single

stage of compression. In this case, it is necessary to have multiple stages of


This is accomplished by having a cylinder or cylinders which take gas in at a low

pressure, compress and discharge to an intermediate pressure, then repeat with

additional cylinders to take the gas to the discharge pressure. In this process,

pressure differential and temperature rise across each cylinder can be controlled to

a reasonable level. The gas may be cooled between stages to minimize discharge

temperatures. Normally this is done with two or more cylinders on the same

compressor unit, with gas cooling between stages.

prepared by Prof. Jagdish S. Talpada


Efficiency Increase

When gas is compressed, the temperature rise effectively creates higher volume at

the discharge conditions. This requires more energy (work) for compression. In

multiple stage compression with cooling, the temperature rise is minimized,

which reduces the total work required to compress to the final discharge.

Operating Difficulties

Multiple stage compression presents challenges for both design and operation. At

the design stage, cylinders must be sized so that all stages are operating within

their limits. In operation, the pressure balance between stages must be maintained

by following a specified unloading sequence when pressures change, or when

controlling engine load.

Mechanical failures such as leaking compressor valves or rings can cause pressure

unbalance, which may put excessive differentials or temperatures on other stages.

The compressor piping and pulsation bottles will also be more complex, which

will probably require an electric analog or digital evaluation to avoid pulsation or

vibration problems.

prepared by Prof. Jagdish S. Talpada