In a compressor actual volume of air inside the cylinder for compression is less than the theoretical calculated volume because of the clearance volume. When the air is compressed, and is delivered by the discharge valve some volume of the air remains inside the clearance volume at high discharge pressure. When the piston starts moving downwards, the suction valve does not opens immediately because of this high pressure gas. When the piston moves downwards the pressure of the gas in the clearance volume starts reducing due to expansion. Only when the pressure of this air reaches to certain minimum level, the suction valve opens. Thus the portion of the suction stroke of the piston goes unused and during this certain volume of the air is not sucked. This reduces the volume of the air that can be sucked by the cylinder or the compressor. Factors that affect volumetric efficiency
Delivery pressure.
How close to the isothermal process.
Compression ratio
Number of stages in the compressor.
Operating temperature.
Clearance volume of the pistons.
Valve operating speed.
Cooling.
How close to the isothermal process. Speed of rotation of the engine.
Temperature and pressure variation along the line from the compressor outlet to orifice meter .
Compressor
Orifice meter
We can assume the tube is uniform and the frictional losses are negligible . By using steady flow energy equation q – w = ∆h + ∆ K.E +∆ P.E P.E We can neglect the change of kinetic energy and potential energy .
W=0 ;
∆ K.E=0 ;
∆ P.E=0
q = ∆h( enthalpy change) but heat loses along the pipe. So the enthalpy decreases along the pipe . Temperature decreases . Assuming ideal gas behavior
v= A.u
A- cross section ; u – velocity
PαT
Variation of pressure and temperature Pressure Temperature
(length )
Note There are some other ways that can affect the pressure and temperature
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.
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 always a cyclic pressure variation.
is
Methods to measuring the pressure difference between the two ends of the orifice
Pressure sensors can be used
Gauge pressure sensors can be used to get the gauge pressure .Sensors can be directly used to get the pressure more accurately .
