Reciprocating Air Compressor

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Reciprocating Air
Compressor

Prepared by

Prof.
Jagdish

S.
Talpada

151903
-

Fluid Power

Engineering

Lecture
-

1

prepared by Prof. Jagdish S. Talpada

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

Parts

Piston (Reciprocating)

The reci procati ng pi ston compressor i s the most wi del y used equi pment for gas

servi ce. The basi c desi gn consists of a pi ston i n a cyl i nder wi th pressure actuated

check val ves to control sucti on and di scharge fl ow through the cyl i nder. Standard

practi ce i s to have the pi ston dri ven by a rod passi ng through a packi ng case to

seal agai nst pressure l eaks. Wi th thi s doubl e acti ng desi gn, gas can be

compressed on both si des of the pi ston. The basi c desi gn i s more than a hundred

years ol d, and i s wel l devel oped. The throughput and l oadi ng can be adjusted by

speed vari ati on, addi tion of cl earance to the cyl i nders, deacti vati ng cyl inders to

reduce di spl acement or acti ve control of val ve cl osi ng, whi ch effecti vel y gi ves

vari able control of di spl acement. Effi ci enci es of thi s type of compressor can be

more than 85 percent for conversi on of horsepower i nput to pressure ri se.

Vane

A vane compressor consi sts of a cyl i ndrical chamber wi th a rotati ng paddl e wheel

type drum mounted off center i n the chamber. As the drum rotates, the sl i ding

paddl e wheel vanes secti on off vol umes, whi ch decrease i n vol ume as they move

toward di scharge. A sucti on port i s machi ned i nto the area where the chambers

have the hi ghest vol ume, and a di scharge port i s l ocated where the chambers have

the smal l est vol ume. Gas enters at the hi gher vol ume and i s compressed and

di scharged at the mi ni mum vol ume. Thi s type of compressor wi l l tol erate more

di rt than a reci procati ng uni t, and i s often used for natural gas producti on servi ces.

The maxi mum di fferenti al i s l imi ted by the strength of the paddl e wheel seal s, so

these uni ts are not appl i cable for hi gh pressures and di fferenti al s.



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.

Screw(Rotary)

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 cyl i nder i s a casti ng or forgi ng desi gned to safel y contai n some

maxi mum worki ng pressure. It i s machi ned to hol d compressor val ves and to

di rect gas fl ow to and from the cyl i nder cavi ty. In combi nati on wi th the cyl i nder

ends, i t must contai n the gas pressure, whi l e havi ng suffi cientl y l arge gas fl ow

passages so there are mi ni mal pressure drops due to gas fl ow. The cyl i nder and

ends may al so have water passages to stabi l ize temperature and di mensi onal

changes. Al l these requi rements i nvol ve compromi ses between si ze, strength, and

fl ow passage si ze (effi ci ency). Compressor cyl i nders are desi gned for some

operati ng range and servi ce. If condi ti ons change, they may not perform rel i abl y

or effi ci entl y. As an exampl e, a cyl i nder for gas transmi ssion has l arge fl ow

passages and val ve areas for effi ci ency at hi gh gas vol umes and l ow pressure

rati os, and wi l l not functi on at hi gh rati os. Si mi larly, a process cyl inder may be a

forgi ng wi th smal l passages, gi vi ng hi gher strength but l ow effi ci ency.

Piston/Rings

The compressor pi ston converts the energy/work suppl i ed by the engi ne, appl yi ng

i t to the gas to rai se i ts pressure. The pi ston must be strong enough to wi thstand

the pressures and forces appl i ed, but sti l l be as l i ght as possible, to mi ni mi ze

reci procati ng wei ghts and the resul ti ng shaki ng forces. The compressor ri ngs seal

gas pressure to avoi d l eaki ng from one si de of the pi ston to the other. The pi ston

may al so be fi tted wi th a ri der band, whi ch i s a l ow fri cti on materi al to keep the

metal pi ston from contacti ng the bore of the cyl i nder and causing scuffing and

wear. Materi al for the ri ngs and ri der bands i s sel ected to gi ve l ong l i fe and

mi ni mal wear wi th the typi cal pressures and gas compositi on of the compressor.

Whi l e thi s i s general l y a l ow fri cti on thermopl astic type materi al, ri ngs may be

made of bronze or other materi al s when temperatures are a probl em.

prepared by Prof. Jagdish S. Talpada

Valves

Compressor val ves are si mpl y fast acti ng check val ves wi th a l ow pressure drop.

They must be opti mi zed to bal ance the opposi ng demands for l ong operati ng l i fe

and mi ni mal pressure drop/fl ow l osses. They may al so have speci al features such

as center ports to al l ow cyl i nder unl oading.

The compressor val ve i s possibly the most cri ti cal component when determi ni ng

the requi rements for a compressor servi ce. The fl ow area i s sensi ti ve, as too smal l

an area wi l l gi ve l ow effi ci ency, but too l arge an area can resul t i n val ve fl utter

and earl y fai l ure. Si mi l arly, valve components must be desi gned for the expected

pressure and temperature condi ti ons.

Val ves have been desi gned wi th many confi gurati ons, parti cularly i n the seal i ng

el ements. These have progressed through steel, Bakel i te, gl ass fi lled Tefl on or

Nyl on, and hi gh strength pl asti cs. The most popul ar desi gns for seal ing el ements

are ri ng shaped stri ps, mushroom shaped
poppets
, and strai ght channel stri ps.

The desi gn of compressor val ves i ncl udes a number of vari ati ons to accommodate

cyl i nder fl ow and unl oadi ng requi rements. The si mpl est i s a singl e deck val ve,

shown on the l eft above, where gas fl ows i nto passages i n one face, across the

seal i ng el ements, and out through passages i n the back face of the val ve.

A modi fi cati on of thi s desi gn i s to have a l arge openi ng i n the center of the val ve.

Thi s al l ows adding a cyl inder deacti vator or cl earance vol ume to the cyl i nder.

Thi s added feature comes at the expense of reduced fl ow area and effi ci ency. To

compensate for thi s, two val ves may be assembl ed together wi th a fl ow passage

through the center. Thi s doubl e deck val ve desi gn has i mproved fl ow area and

effi ci ency. Thi s type of val ve can onl y be used i n a cyl i nder desi gned to accept

i ts i ncreased hei ght.


prepared by Prof. Jagdish S. Talpada

Packing

The compressor packi ng i s a seri es of pressure contai ning ri ngs l ocated i n the

crank end of a doubl e acti ng compressor cyl i nder. These seal agai nst the pi ston

rod and prevent l eakage, so that the cyl i nder can compress gas on both si des of

the pi ston. Agai n, as wi th compressor ri ngs, the packi ng materi al i s sel ected to

provi de best l i fe and seal i ng wi th expected condi ti ons. The packi ng i s general l y

pressure l ubri cated, and may have cool ant fl ow to remove fri cti on heat. There are

al so various specialty types to reduce gas l eakage around the rod. Thi s may be

i mportant when compressi ng hi ghl y fl ammable or toxi c gases. It i s al so

becomi ng more i mportant i n reduci ng gas l eakage and emi ssi on of “greenhouse

gases”.

Clearance
Unloaders

In many appl i cations, the vol ume of gas to be del i vered may change based on

ei ther gas suppl y or process demands. Al so, varyi ng pressure condi ti ons can

change the l oad on the dri ver, requi ri ng l oad control. Thi s may be accompl i shed

by speed vari ati on, deacti vati ng cyl inders or cyl inder ends, or by varyi ng cyl i nder

cl earance. Thi s l ast opti on i s hi ghl y preferred, as speed control may have a

l i mi ted range, and deacti vati ng cyl i nders or ends can cause mechani cal shaking or

acousti c pul sations. Cl earance
unl oaders

allow varyi ng throughput and l oad wi th

mi ni mal l oss of effi ci ency.
Unl oaders

are not actual l y a part of a compressor, but

are i ncl uded on many i nstal lati ons, to gi ve l oad and throughput control. Thi s may

be done by vol umes cast i nto the cyl i nder or heads, wi th a val ve to cl ose the

passageway. Other opti ons are val ve cap pockets and head end vari able pockets.

Added cl earance may have a si mpl e
handwheel

to control i ts operati on, or may

have pneumati c actuators, whi ch al l ow automati c operati on.

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

Lecture
-

-

2

prepared by Prof. Jagdish S. Talpada

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.

Crosshead

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

volume.

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

prepared by Prof. Jagdish S. Talpada

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

prepared by Prof. Jagdish S. Talpada

Volumetric Efficiency

As noted above, the cyl i nder does not bri ng gas i n through the enti re pi ston travel.

The percentage of stroke the sucti on val ve i s open, compared to the enti re stroke

i s cal led “vol umetri c effi ci ency”. If there were no cl earance (vol ume) l eft when

the pi ston compl eted i ts compressi on stroke, then cyl i nder pressure woul d

i mmedi atel y drop to sucti on pressure as the pi ston returned, gi vi ng 100 percent

vol umetri c effi ci ency.

Thus, the cyl i nder di splacement woul d be equal to the vol ume del i vered wi th each

stroke. However, due to gas re
-
expansi on, the sucti on val ve openi ng i s del ayed.

Thi s del ay becomes greater when the cyl i nder pressure rati o i ncreases or the

cl earance vol ume i ncreases. Thus, the cyl i nder del i vers a reduced vol ume to the

di scharge condi ti on.

The pi ctures bel ow i l l ustrate thi s effect, wi th the pi cture on l eft showi ng effect of

i ncreasing cl earance, and on ri ght the effect of i ncreasing pressure rati o. At hi gh

pressure rati os, or wi th l arge amounts of cl earance, the val ve openi ng may be

del ayed to the poi nt that the val ve does not open, and no gas fl ows through the

cyl i nder. Thi s condi ti on i s called zero vol umetri c effi ci ency, and can cause

seri ous cyl inder heati ng probl ems.

In normal operati on, fri cti on of ri ngs on the cyl i nder creates heat whi ch i s carried

away wi th the gas bei ng compressed. Si nce at zero vol umetri c effi ci ency, no gas

i s enteri ng or l eavi ng the cyl i nder, al l fricti on heati ng effects are contai ned wi thi n

the cyl i nder, causi ng an uncontrol led temperature ri se. As the hot gas i s

contai ned wi thi n the cyl i nder, normal temperature detecti on i n the di scharge l i ne

wi l l not be effecti ve.

prepared by Prof. Jagdish S. Talpada

prepared by Prof. Jagdish S. Talpada

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

clearance.

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

coatings.

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

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

compression.

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