CHEMICAL ENGINEERING

116 CHEMICAL ENGINEERING VOLUME 1 SOLUTIONS

In this problem,ND10 Hz,QD 0 .4m^3 /s, andhD20 m.

Thus: NsD 10 ð 0. 4 ^0.^5 / 9. 81 ð 20 ^0.^75 D 0. 121

Reference should be made to specialist texts on pumps where classifications of pump
types as a function of specific speed are presented. A centrifugal pump is suggested here.

Q/NandQ 1 /Q 2 DN 1 /N 2 (equation 8.15)

and: h/N^2 andh 1 /h 2 DN 1 /N 2 ^2 (equation 8.16)

Thus:  20 / 30 D 10 /N 2 ^2 from whichN 2 D 12 .24 Hz

and: 0. 4 /Q 2 D 10 / 12. 24 from whichQ 2 D 0 .49 m^3 /s

Power requiredD 1 /nmass flowðheadðg

D 1 / 0. 7  0. 49 ð 1000 ð 30 ð 9. 81 D206 W

PROBLEM 8.12

A centrifugal pump is to be used to extract water from a condenser in which the vacuum
is 640 mm of mercury. At the rated discharge, the net positive suction head must be at
least 3 m above the cavitation vapour pressure of 710 mm mercury vacuum. If losses in
the suction pipe account for a head of 1.5 m, what must be the least height of the liquid
level in the condenser above the pump inlet?

Solution

The system is illustrated in Fig. 8c. From an energy balance, the head at the suction point
of the pump is:
hiDP 0 /hCxu^2 i/ 2 ghf

Liquid level

Condenser pressure

= P 0 kN/m^2

x m

Pump

Velocity

= ui

Figure 8c.