RECIPROCATING_AIR_COMPRESSOR.ppt

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THERMODYNAMICS - II
RECIPROCATING AIR COMPRESSOR

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Reciprocating compressor
 The function of a compressor is to take a definite quantity of fluid and
deliver it at a required pressure.
 The most efficient machine is one which will accomplish this with a
minimum input of mechanical work.
 All compressing devices are power absorbing devices, which require an
external power source.
 The external power source may be electrical motor or IC engines
connected directly to the compressor using coupling.
 Reciprocating compressors are similar to IC engines construction but
without any fuel delivery system.

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USES OF COMPRESSED AIR:
1) In mines due to danger of fire no electrical or fuel motors are used. The
compressed air after cooling passed down to mines for operating air
motors, pneumatic drills, pumps, drill sharpeners, coal cutters, air
motors, pneumatic drills, pumps, drill sharpeners etc.,
2) It is used for inflating tubes in automobiles, spray painting, and air
washing of vehicles in service stations.
3) It is used in operating the lifts and hoists.
4) It is used in starting and supercharging of the internal combustion
engines.

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5. It is used in pneumatic brakes in automobiles, locomotives, railway
carriages etc
6. It is used in sand blasting in industries, chemical processing, blast
furnaces, chemical processing.
7. It is used in combustion of fuels in gas turbines

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THERMODYNAMICS - II
CLASSIFICATION:
Depending upon energy transfer, compressors are classified as
(A) According to type of compression
i) Positive displacement machines
ii) Dynamic machines
(B) Depending upon the pressure ratio
i) Fans (pressure ratio <1.1)
ii) Blowers (pressure ratio > 1.1 <2.3)
iii) Compressors (pressure ratio > 2.3)

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(C) According to use of piston sides
i) Single acting
ii) Double acting
(D) According to number of stages
i) Single stage
ii) Multi stage

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THERMODYNAMICS - II
SINGLE STAGE RECIPROCATING COMPRESSOR WITHOUT CLEARANCE
• Consider a single stage reciprocating air
compressor without flow resistance at inlet and
delivery and without clearance.
• Assuming working fluid is air which is a
perfect gas
• Let P1=initial pressure before compression.
• V1= initial volume
• V2=volume after compression
• P2=Final pressure

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• There are 3 possible processes with n=1, n = γ = and 1<n< γ
 When n=1, PV=C the compression is called isothermal
 When n= γ, the compression is called rev. adiabatic
 When 1<n< γ, the compression is called polytropic
• The work done on the air is given by the area of the indicator
diagram and work done will be minimum when the area of indicator
diagram is minimum.
• The height of the indicator diagram is fixed by the required
pressure ratio and the length 4-1 by the cylinder volume.
• The variable which can influence the area of the diagram is 1-2
which is decided by the index ‘n’ in the equation PVn=C.

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(i) Work done in a polytropic process

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THERMODYNAMICS - II
EXPRESSION FOR WORK OF COMPRESSION – WITH CLEARANCE

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VOLUMETRIC EFFICIENCY:
• Due to clearance volume intake capacity of air compressor reduces.
• The ratio of actual volume sucked to the theoretical volume swept by the piston is known
as volumetric efficiency.
• It is the tool to gauge the performance of a reciprocating compressor.
Free Air Delivered: Free air delivered is the
actual volume of air delivered at stated
pressure reduced to standard temperature and
pressure i.e., 1 bar and 15oC

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Volumetric efficiency in terms of %age of clearance and pressure ratio:
Consider a single stage reciprocating compressor
with clearance as shown in the figure

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THERMODYNAMICS - II
MULTISTAGING OF RECIPROCATING AIR COMPRESSORS
 In a single stage air compressor as the pressure ratio
increases leakage past the piston increases and
compressor requires robust construction. A high
compression process, high temperature affects the
operation of delivery valve and lubrication problem arises.
 As the pressure ratio increases work of compression
increases.
 It is for these reasons that if a higher compression is needed
the overall pressure ratio can be divided into two or more
stages.

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ADVANTAGES OF MULTISTAGE COMPRESSION:
 Multistage compressor requires less power as compared to single stage compressor
for the same delivery pressure and same quantity free air due to intercooling.
 Volumetric efficiency will be higher due to lower temperature and pressure ranges
in each stage.
 Due to phasing of operation into stages, a better mechanical balance is possible
with multistage compression.
 Due to lower temperature range in each stage lubrication will be better in multistage
compression.
 Because of lower pressure range lower pressure cylinder is made thin and high pressure
cylinder is made thicker, so that overall saving in material is possible compared t single
stage.

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Heat Rejected per Kg. & per stage in multi stage compression:
1-2-polytropic compression in the first stage
2-2’-constant pressure cooling process in the intercooler
2’-3- Polytropic compression in the second stage
1-3”-isentropic compression in one step (stage)
1-3’- Polytropic compression in one stage
The given figure shows graphical
representation of heat rejected during
compression and in the intercooler.

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If the air is cooled to its initial temperature the whole of the work done in
the compressor must be rejected to the cooling medium

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