Summer 2018 EC-II Complete Solutions 9
Winter 2017 EC-II Complete Solutions 11
Summer 2017 EC-II Complete Solutions 12
Winter 2016 EC-II Complete Solutions 12
Winter 2017 – Q1 [Preview]
Q. 1 a) Classify the compressor in detail.[3M]
Answer:- The air compressors can broadly be classified as
Reciprocating compressor: A reciprocating compressor is used to produce high-pressure air. It uses the displacement of the piston in the cylinder for compression. It handles a low mass of air and a high-pressure ratio.
Rotary compressor: Rotary compressors are used for low and medium pressures. they usually consist of the bladed wheel that spins inside a circular housing. The rotary compressor handles a large mass of air.
Centrifugal compressor: Centrifugal compressors are dynamic action compressors. Centrifugal compressors are better suited for applications requiring very high capacities, typically above 3000 m3/min and a moderate pressure ratio of 4 to 6.
Axial compressor: Axial compressors are dynamic action compressors and suitable for a higher pressure ratio of 8 to 10. The axial compressors are more efficient than centrifugal compressors.
Q. 1 b) Derive the condition for minimum work in a multistage compressor with perfect intercooling.[7M]
Multistage compressor with intercooling
The intermediate pressure P2 influences the work to be done on the air and its distribution between stages. The intermediate pressure, which makes work input minimum is always important.
The total power input to a two stage reciprocating air compressor with perfect intercooling is given by equation
If P1, T1 and P3 are fixed then the optimum value of intermediate pressure P2 for minimum work input can be obtained by applying condition of minima, that is
This is the condition for minimum work in multistage compressor with perfect intercooling.
Q. 1 c) Define volumetric efficiency of a reciprocating compressor. Derive the equation for the same. [4M]
Answer:- The volumetric efficiency of the reciprocating compressor is defined as the ratio of actual volume of air sucked into the compressor, measured at atmospheric pressure and temperature to the piston displacement volume.
The volumetric efficiency can be expressed in terms of effective volume and piston displacement volume as;
since, V1 = Total volume = Vs + Vc
Introducing as clearance ratio and using Vc = V3, then
for expansion of gas in clearance volume
The volumetric efficiency decreases with pressure ratio in compressor.