Page 246 - Chemical engineering design

222
CHEMICAL ENGINEERING
The approximate equations should not be used for steam, as the quality of steam depends
on its pressure, and hence the pressure drop.
Nolte (1978) gives detailed methods for the selection of economic pipe diameters,
taking into account all the factors involved. He gives equations for liquids, gases, steam
and two-phase systems. He includes in his method an allowance for the pressure drop
due to fittings and valves, which was neglected in the development of equation 5.12, and
by most other authors.
The use of equations 5.14 and 5.15 are illustrated in Examples 5.6 and 5.7, and the
results compared with those obtained by other authors. Peters and Timmerhaus’s formulae
give larger values for the economic pipe diameters, which is probably due to their low
value for the installation cost factor, F.

Example 5.6
Ž
Estimate the optimum pipe diameter for a water flow rate of 10 kg/s, at 20 C. Carbon
3
steel pipe will be used. Density of water 1000 kg/m .

Solution
d, optimum D 293 ð 10 0.53 1000 0.37 5.14
D 77.1mm
use 80-mm pipe.

Ž
2
Viscosity of water at 20 C D 1.1 ð 10 3 Ns/m ,
4G 4 ð 10 5
Re D D D 1.45 ð 10
d ð 1.1 ð 10 3 ð 80 ð 10 3
>4000, so flow is turbulent.
Comparison of methods:

Economic diameter
Equation 5.14 180 mm
Peters and Timmerhaus (1991) 4 in. (100 mm)
Nolte (1978) 80 mm

Example 5.7
Ž
Estimate the optimum pipe diameter for a flow of HCl of 7000 kg/h at 5 bar, 15 C,
Ž
3
stainless steel pipe. Molar volume 22.4 m /kmol, at 1 bar, 0 C.
Solution

Molecular weight HCl D 36.5.
36.5 5 273 3
Density at operating conditions D ð ð D 7.72 kg/m
22.4 1 288