VOLUMETRIC EFFICIENCY OF A COMPRESSOR

Instructed By :- Mr.S.Ganesh

Group Members :- R.S.V. Piyasena

P.P.G.C. Prasanna

R. Prasanthan

M.G.M.M. Premathilaka

A.B.D.S. Priyadarshana

G.P.N.S.G. Punchihewa

O.C. Ranawaka

U.I. Ranganath

Name :- T.G.P. PriyadarshanaIndex No :- 080382RCourse :- B.Sc. ENG. Mechanical Module :- ME 2030Practical No :-Date of conduct :- 12/07/2010Date of submission :- 19/07/2010

Discussion

A compressor is devise that take work input and store it as potential energy (pressure energy). Since it does not deliver any output power, efficiency cannot be defined using energy concept. As the air intake and delivering it at high pressure is the main job of the compressor. Because the air intake and exhaust is what mostly considered factors it is good approach to define a term as volumetric efficiency.

As noted above this concept does not consider the input energy, leakages, friction loss that plays a major role in overall performance of the machine. A term that considers some these factors will be a good term to describe the property of a good compressor.

Factors affecting Volumetric Efficiency

Delivery pressure. Clearance volume of the pistons. Number of stages in the compressor. Operating temperature. Valve operating speed. Cooling. How close to the isothermal process. Speed of rotation of the engine.

Temperature & Pressure variation of the delivery gas

The temperature variation of the delivery gas from the outlet from the compressor to the outlet of the delivery from the orifice under steady state condition can be given as following.

Temperature variation

Temperature drop during the flow of the outlet pipe (from outlet to external tank).

There is somewhat long pipeline from the compressor outlet to the tank so the outlet gas temperature drops due to heat transfer between the pipe and atmosphere. The pipe has colored while in order to reduce the heat transfer but there is always a drop in temperature.

Temperature drop at the pressure controlling gate.

The gate opening and closing create turbulence around the gate which in turn absorbs energy from the flow there is small temperature drop between the two ends of the gate.

Temperature drop at the storing tank.

Pressurized air is kept in the tank for some time since the air is at high temperature than atmosphere some amount of heat is rejected to atmosphere so there is a temperature drop.

If there are any valves, gates on the line then the temperature might get reduces due to throttling.

Pressure variation

Pressure changes at the outlet pipe due to the valve action.

Because the air is delivered at a high pressure than the tank pressure there is always a cyclic pressure variation.

Pressure drop at the controlling gates

Gates produce some irregularities in the cross section of the tube. That cause pressure drop between the two sides of the gate.

Pressure drop in the pipe line.

To get a flow in pipes there must be a pressure drop between the two ends of the pipe this drop is a gradual one that happen in the two pipes, in one from the compressor to tank and from tank to the orifice.

Measuring the pressure difference between the two ends of the orifice

Some methods can be adapted to measuring the pressure difference between two ends of the orifice with more accuracy using the following methods.

A liquid of low density (than water) can be used to get high sensitivity.

Colored alcohol can be used as the fluid for the manometer. Petroleum fuels can be used also, like kerosene.

Pressure sensors can be introduced to get a high accuracy.

Gauge pressure sensors can be used to get the gauge pressure sensors can be directly used to get the pressure. This is very high accurate than the normal measuring techniques.

Comparison of graphs

The given graph in the practical is much more like the expected graph there is some deviation in one value but overall they look same.

expected graphPressure

volumetric efficiency

0 10 20 30 40 50 60 70 800.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

pressure(psi)

Efficiency

practical graph