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Handbook of Turbomachinery Second Edition Revised - Ventech!

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exact agreement may not occur in all instances. Since we expect a higher<br />

efficiency with the radial-inflow turbine, we will assume an overall efficiency,<br />

including tip clearance effects, <strong>of</strong> 0.85. As with the axial-flow turbine, the<br />

vector diagram needs to be calculated using the zero-clearance work. We<br />

assume that the tip clearance loss is 5%. The actual enthalpy drop is<br />

DhOA ¼ð0:85Þ 49:14 Btu<br />

lbm<br />

and the zero-clearance work is<br />

DhOAZC ¼ 0:85<br />

0:95<br />

49:14 Btu<br />

lbm<br />

¼ 41:77 Btu<br />

lbm<br />

¼ 43:97 Btu<br />

lbm<br />

The required turbine flow is found using Eq. (7):<br />

_m ¼ P ð100 hpÞð0:7069 Btu=sec=hpÞ<br />

¼ ¼ 1:693 lbm= sec<br />

DhOA 41:77 Btu=lbm<br />

The mass flow rate is needed to calculate turbine flow area and is also a<br />

system requirement.<br />

We calculate the tip speed <strong>of</strong> the turbine based on the optimum value<br />

(0.7) <strong>of</strong> blade-jet speed ratio U1=C0. From Eq. (42) we have<br />

C0 ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />

p<br />

ft lbm<br />

2Dhisentropic ¼ ð2Þ32:174 lbf sec2 ft lbf<br />

778:16<br />

Btu<br />

¼ 1569 ft= sec<br />

For U1=C0 ¼ 0:7, the wheel speed is calculated to be<br />

U1 ¼ð0:7Þð1569 ft= secÞ ¼1098 ft= sec<br />

49:14 Btu<br />

lbm<br />

Assuming zero exit swirl, we calculate the rotor inlet absolute<br />

tangential velocity component using Eq. (9):<br />

Vy1 ¼ DhOA ZC<br />

U1<br />

¼<br />

¼ 1002 ft= sec<br />

32:174<br />

ft lbm<br />

lbf sec 2<br />

ft lbf 778:16 Btu<br />

1098 ft= sec<br />

43:97 Btu<br />

lbm<br />

The required rotor blade count is obtained from Eq. (61):<br />

Zr ¼<br />

1<br />

2<br />

ðVy1=U1Þ ¼<br />

1<br />

2<br />

¼ 23<br />

ð1002=1098Þ<br />

This blade count is much higher than the 10–14 guideline given by Rodgers<br />

Copyright © 2003 Marcel Dekker, Inc.<br />

1=2

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