exercise comp

Thermodynamics II Laboratory Instructions

2012 Division of Thermodynamics, Institute of Heat Engineering

Compressor

Exercise No 11

Introduction Objective of the exercise is to investigate thermodynamic processes occurring in a reciprocating

compressor, determine key components of its energy balance and compare obtained results with

theoretical description.

In an ideal reciprocating compressor the compression process is isothermal. In such a process the

work needed to compress gas would be minimal. Real compressors however do not work that way;

compression process is polytropic, almost adiabatic. This results with increased energy consumption

and increased outlet gas temperature, which may cause some problems. This is one of the reasons of

utilising multi-stage compressors with intercooling. Intercooling allows to decrease final gas

temperature (and also its final volume), decrease dimensions of further stages and lower work

needed for entire compression process. Figure below shows theoretical single-stage compression in

a reciprocating compressor.

1-2 is the actual compression process, 3-2 is gas removal from the cylinder, 3-4 is decompression of

remaining gas (there is always some gas remaining after the outlet valve is closed) and 4-1 is supply

of another batch of low pressure gas.

Area inside the curve reflects the work theoretically needed to complete entire compression cycle.



The figure above shows real-life chart. The differences include:

Processes 2-3 and 4-1 are not isobaric, there are visible pressure variations near points 2 and

4 related to valve opening

Compression/expansion processes are not adiabatic.

Due to those facts the actual work (indicated work) is higher than theoretical value for the same

pressure difference and volume.

Investigated machinery The investigated piece of equipment is a low-speed two-stage reciprocating compressor with both

pistons installed on common connecting rod, driven by a three-phase AC electric motor via belt

transmission system. The ambient air is sucked into the 1st stage, compressed there, then cooled in

an intercooler and further compressed with the 2nd stage, which has additional jacket cooling. From

the 2nd stage the air is directed to an equalising tank and then to a system of pipelines with flow

measurement devices installed.

The main dimensions of the compressor are:

Stroke s = 0.15 m

1st stage bore D1 = 0.30 m

2nd stage bore D2 = 0.24 m.

Both intercooler and 2nd stage jacket are cooled with water (parallel connection on the water side).

There are following measurements:

Motor voltage

Motor current

Motor power (currently unserviceable, use power factor value from the motors plate)



Speed

Air pressure after 1st stage

Air pressure after 2nd stage

Inlet water temperature

Intercooler outlet water temperature

Jacket outlet water temperature

Intercooler water flow

Jacket water flow.

The compressor is also equipped with an indicator a mechanical device which draws p-V diagrams

for both stages.

Measurements and calculations For all formulas use values in main SI units, unless otherwise indicated.

Flow measurement Flow measurement may be calculated according to the displacement volume and number of cycles in

a unit of time (speed):

sn4

DV

2

1s

where : D1 1st stage bore

s Stroke

n Speed

0s Vm

The air density shall be calculated for measured ambient conditions, using ideal gas law and

gas constant of R = 287 J/(kg K).

Flow in the pipeline downstream from the compressed air tank may be measured with measurement

nozzle:

v

pdKm 2

where: d nozzle diameter ( d = 0.031 m)

p nozzle pressure drop [Pa]



v air specific volume for conditions inside pipeline

K empirical coefficient.

K = 1.15292967

While both values should be identical (in a steady state and with valves at the pipeline open, i.e.

when the flows into the tank and from the tank are equal), but there will be a noticeable difference.

This is primarily caused by:

Fact that the piston never actually reaches cylinder head, so some of the compressed air is

not pushed out of the cylinder, but gets expanded instead (see Vs on charts above)

Possible leaks in the system.

Indicated power measurement The indicated power shall be determined from indicator diagrams crated during operation. Using

these you may obtain average indicated pressure pi. This is the highest pressure which would occur in

a rectangular cycle with an area equal to that of the real one, and the same base length. As the

length of the diagram l is equal to the piston stroke, the average indicated pressure may be

calculated as:

al

Fpi

where: F measured area of the indicator diagram

[ mm2 ] ;

l diagram length along horizontal axis [ mm ] ;

a indicators spring coefficient [mm/Pa], different for each stage:

a1 = 12 10-5 mm/Pa,

a2 = 3 10-5 mm/Pa.

After average indicated pressure is determined for each stage, indicated power Ni may be calculated

as:



i222i2i121I pDppD4

snN

where: D1 1st stage bore

D2 2nd stage bore

s stroke

n speed

pi1 1st stage average indicated pressure

pi2 2nd stage average indicated pressure.

Other components of the energy balance Electric power may be calculated from measured voltage U and current I for the three-phase motor:

3 coselN U I

where cos = 0.86.

Compressor shaft power is therefore:

s e m tN N

where: m electric motor efficiency m = 0. 86

t transmission efficiency t = 0. 97

Thus mechanical efficiency of the system is:

im

s

N

N

Thermal power of the water cooling WQ is calculated as:

)tt(cm)tt(cmQ 1w3ww2w1w2ww1ww

where: 1wm intercooler water flow

2wm jacket water flow



1wt water inlet temperature

2wt intercooler water outlet temperature

3wt jacket outlet water temperature

wc water specific heat capacity wc = 4.19 kJ/(kg K)

Report The report should contain measurement results, calculation of individual energy balance components

and conclusions. Detailed information will be provided by the instructor.

exercise comp

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