?p?0.5?9.81?104?200?1.013?105?75707Pa 760?p/?g=7.7m
The relation between the hole velocity and velocity of pipe
22 ?d0??1?V?V0???4.12????1m/s ?2??D?Friction loss
lu220012?0.025??5.1m Hf?4fd2g0.052?9.81 so
H=7.7+10+5.1=22.8m
2.3 . A centrifugal pump is to be used to extract water from a condenser in which the vacuum is 640 mm of mercury, as shown in figure. At the rated discharge, the net positive suction head must be at least 3m above the cavitation vapor pressure of 710mm mercury vacuum. If losses in the suction pipe accounted for a head of 1.5m. What must be the least height of the liquid Hg level in the condenser above the pump inlet? Solution:
From an energy balance,
p?pv Hg?o?Hf?NPSH?g
Where
Po=760-640=120mmHg Pv=760-710=50mmHg
Use of the equation will give the minimum height Hg as
po?pv?Hf?NPSH Hg??g
（0.12?0.05)?13600?9.81 ??1.5?3??3.55m1000?9.81
2.4 Sulphuric acid is pumped at 3 kg/s through a 60m length of smooth 25 mm pipe. Calculate the drop in pressure. If the pressure drop falls by one half, what will the new flowrate be ?
? Density of acid 1840kg/m3 ? Viscosity of acid 25×10-3 Pas
Solution:
Velocity of acid in the pipe:
mu?volumetricflowratem3?????3.32m/scross?sectionalareaofpipe?20.785?d20.785?1840?0.0252d4d?uReynolds number:
Re???0.025?1840?3.32?6109 ?325?10from Fig.1.22 for a smooth pipe when Re=6109, f=0.0085 pressure drop is calculated from equation 1.4-9
lu2603.322hf??4f?4?0.0085?450J/kg
?d20.0252?p?p?450?1840?827.5kPa
or friction factor is calculated from equation1.4-25
2lu2603.322?0.2lu?0.2hf??4f?4?0.046Re＝4?0.046?6109?426J/kg?d2d20.0252?p?p?426?1840?783.84kPa <