Page 137 - Mechanical Engineer's Data Handbook

126 MECHANICAL ENGINEER'S DATA HANDBOOK

rotor rotates produces compression. Higher pressures (P, -Pi) V,
may be attained by using more than one stage. The Constant-volume work done W, = r1$
work is done partly isentropically and partly at
PZ
constant volume. Assuming ideal conditions: where r1 =-
Y Y-1 Pi
Isentropic work done Wi=-p, Vs(r(T)- 1)
(Y-1) Total work done per revolution W, = Wi + W,
Pi Pressure ratio: G8.5 normally
where: r = -
P1 20 in special cases.
Size: ,<150m3min-'
A two-stage vane compressor is shown in the figure.

I
Inlet

tlet

Two-stage vane compressor

3.12 Reciprocating air motor

Reciprocating air motors are used extensively for tools coal mines. The operating cycle is the reverse of that
such as breakers, picks, riveters, vibrators and drillers. for the reciprocating compressor.
They are useful where there is fire danger such as in

3.12.1 Power and flow rate p 6

Referring to the pV diagram:
[
Power P=N pI(Vl- V6)+ (P1 v1- Pz VZ)
n- 1

-
-p3(v3 v4)- (J'5v5--P4v'J]
n- 1
4
where n = index of expansion and compression.
V
Mass flow rate of air m = N Cut-off ratio =- '1 - '6
where: "=(?r and z=(?Y. '3 - '6

v5
P4 Clearance ratio = -
v3 - v5