Page 99 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

96 Chapter 4 Shell and tube heat exchanger

4.5 Pressure drop estimation
Design problems specify the upper limits of pressure drop for the shell-side and the tube-side fluids.
As a general guide, the allowable pressure drop for liquids is 35 kPa for low viscosity (m < 1 mPa-s)
l
and 50e70 kPa for viscosity ranging from 1 to 10 mPa-s. For gases and vapours, the allowable
pressure drop is 0.4e0.8 kPa for high vacuum services, 0:1 absolute pressure for medium vacuum,
0:5 system gauge pressure for 1e2 bar pressure and 0:1 system gauge pressure above 10 bar
pressure. Several valid thermal design alternatives may meet the pressure drop limit(s). It is common to
consider 70 kPa as the pressure drop limit in general chemical processes.
Adopting a design with high pressure drop shall incur higher operating cost as fluid circulation
requires some form of pump or fan that shall consume additional power. Apart from the fluid prop-
erties, other factors, namely the nature of flow (laminar or turbulent) and the passage geometry,
strongly affect pressure drop. A fluid also experiences entrance loss as it enters the heat exchanger core
due to a sudden reduction in flow area and a loss due to a sudden expansion as the fluid exits the core.
In addition, if the density changes through the core as a result of heating or cooling, an acceleration or
deceleration in flow is experienced. This also contributes to the overall pressure drop (or gain).
The pressure loss components are therefore
Core loss: Pressure drop for flow with mass velocity G and mean density r is estimated based on
m
Fanning’s equation as
2
4fL G
(4.16)
Dp c ¼
2r
D h m
D h and L are the hydraulic diameter and length of the flow passage, f is the Fanning’s friction factor for
the flow condition.
If the core fluid density varies from r to r out , the pressure drop due to acceleration (or decel-
in
eration) incurred in the core is given by
G 2 1 1
(4.17)
Dp a ¼
2 r in r out
Entry and exit loss: Entry loss is estimated based on Bernoulli’s equation, coupled with a loss
coefficient K c associated with the flow area contraction ratio b as
in
G 2
2

in
Dp in ¼ 1 b þ K c (4.18)
2r in
b is defined as the ratio of the minimum area to the frontal area and r is density at inlet.
in
in
Exit loss is also estimated similarly using the following expression
G 2
2

e
2r e
Dp e ¼ 1 b þ K e (4.19)
Parameters with subscript ‘e’ refer to expansion in passage area at the exit.