Design Considerations for Shell and Tube Heat Exchanger

Introduction:
Transfer of heat from one fluid to another is an important process in chemical
industries and refrigeration industries. Shell and tube heat ex changer are one most
efficient equipment for this application. For chemical industries chemical and water
are two fluid where as for Refrigeration industries refrigerant and water can be the
both fluids.
The shell and tube type heat ex changer are divided into two category.
 Fixed Tube Heat Changer: This is the cheapest type heat ex changer, here the
tube sheet is welded to the shell and cannot be removed for maintenance. Fpr
cleaning purpose special type tools are required. This type of heat ex changer is
used where the fluids are non corrosive in nature.
 Removable type Heat ex changer: This is bit costly. The tube sheet can be
removed from the shell also there will be difficulty in leakage arrest of the fluid
through different cross-section. This type heat ex changer can be floating head
type and u tube type.
The design of the heat changer is based on TEMA (US) specification.
Fig 1: Heat Ex Changer Classification

Heat Changer Nomenclature:
Fig 2: Heat ex changer Nomenclature
Design Consideration:
Thermal Design Consideration:
For thermal design we need to study the heat transfer area, no of tubes, tube length
and diameter, tube layout, tube pitch, number of baffles, shell and tube pass and
pressure drop across the shell and tube.
Shell Design Consideration:
The tube bundle is placed in the shell. This is fabricated from the steel pipe. The
major consideration is the clearance between the tube bundle and shell inner wall.
Tube Design Consideration:
The OD of the tube is fabricated from 3/4” and 1” pipe. This is designed to maximize
the number of tube to increase the turbulence. Also the thickness the tubes is
designed to withstand the internal pressure.
Tube Pitch and Tube layout:
Tube pitch is the shortest distance between two tubes and Tube layout is the
arrangement of the tubes in the shell.
Fig 3: Tube Layout types

Tube Sheet Design:
It is a fixed sheet that behaves as a barrier between the tubes and the shell fluid. It is
designed to perform as a tight seal. It is designed as per TEMA to have minimum
thickness.
Baffle Design:
Baffles are used to increase the fluid velocity by diverting the flow across the tube
bundle to obtain higher transfer co-efficient. The distance between adjacent baffles
is called baffle-spacing. These baffles are also used to limit the vibration of the tube
while fluid in motion inside the tubes.
Design Calculation:
Heat transfer Co-efficient
Another important thing is rate of heat transfer between the hot fluid and the cold
fluid.
The total heat transfer q
ΔT s=T i−To
ΔT t =ti −to .
Rate of heat transfer related to heat transfer co-efficient
In a shell-and-tube heat exchanger, the area for heat transfer is
A=nt π do L .
The correction factor, F, is needed due to the fact the theory was originally
developed for the case of pure counterflow. In a shell-and-tube heat exchanger,
there is usually one shell pass and some multiple of two tube passes. The shell-and-
tube heat exchanger used in experimentation has one shell pass and two tube
passes. For this case, the correction factor, F, becomes

where the equations for each of the variables in the equation are
inally, the log mean temperature difference is defined as the mean between the inlet
and outlet temperatures for both the shell-and-tube. The equation for the log mean
temperature difference is
Another important consideration in the development of the theory behind a
heat exchanger is the Reynolds number, a ratio between the inertial and viscous
forces in flow. The value of this dimensionless group denotes whether the flow in
the heat exchanger is laminar or turbulent. Naturally, turbulent flow is preferred, as
heat transfer in turbulent flow is much greater. The Reynolds number for flow
through tubes is given by
Retube= di ̇mt
μt at (13)
with at, the tube flow area, given by
at=
nt ( π
4 di
2
)
N p
.
(14)
The Reynolds number for flow through the shell is quite a complicated expression