Transcript UNIT-6


A 'heat exchanger' may be defined as an equipment
which transfers the energy from a hot fluid to a cold
fluid. Here, the process of heating or cooling occurs.
In heat exchangers the temperature of each fluid
changes as it passes through the exchangers.
General design of heat exchange equipment :
 The design of heat exchange equipment is based on
general principles.
 From mass and energy balance HT area is calculated.
 Quantities to be evaluated are U,LMTD.
 In simple devices these quantities can be calculated
accurately but in complex processing units the
evaluation may be difficult and the final design is
always a compromise
based on engineering
judgment to give best overall performance.
TYPES OF HEAT EXCHANGERS:
1.Doble pipe heat exchangers:

It consists of concentric pipes with standard return
bends.
 One fluid flows through inside metal pipe and the
second fluid flows through the annulus between the
outside pipe and inside pipe.
 The flow directions may be either parallel or counter
fashions.
 These exchangers are used when heat transfer area
required is not more than 150 sq.ft
Advantages;
• 1. Simple in construction
• 2. Cheap
• 3. Very easy to clean
• 4. Very attractive when required Heat transfer areas are small.
•
Disadvantages:
• 1. The simple double pipe heat exchanger is inadequate for
large flow rates
• 2. If several double pipes are used in parallel, the weight of
metal required for the outer tubes becomes so large.
• 3. Smaller heat transfer area in large floor space as compared
to other types
• 4. Leakage are more.
•
Shell and tube heat exchanger :
 The simple double pipe heat exchanger is inadequate
for large flow rates. If several double pipes are used
in parallel, the weight of metal required for the outer
tubes becomes so large.
 When large areas are required we go for shell and
tube heat exchangers.
 It is the most common type of heat exchanger in oil
refineries and other large chemical processes.

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Shell and tube heat exchanger consists of a shell
with a bundle of tubes inside it.
One fluid flows through the tubes (the tube side)
and the other fluid flows outside the tubes but
inside the shell (the shell side).
Heat is transferred from one fluid to the other
through the tube walls, either from tube side to
shell side or vice versa.
The fluids can be either liquids or gases on either
the shell or the tube side.
Construction and parts:
Shell:
It is usually a cylindrical casing through which one of
the fluid flows. Shell is commonly made of carbon
steel. The minimum thickness of shell made of
carbon steel varies from 5 mm to 11 mm depending
upon the diameter.
Tubes:
 Standard heat exchanger tubes which are used in
many industrial processes may be of various sizes
and lengths.
 The wall thickness of tubes is usually expressed in
terms of Birmingham Wire Gauge (BWG).
 The thickness depends upon material of construction
and diameter.
 Standard lengths of tubes for heat exchanger
construction are 8, 12, 16 and 20 ft.
Tube pitch:
 The shortest centre-to-centre distance between the
adjacent tubes is called as tube pitch.
 Tubes arranged in a triangular or square layout,
known as triangular or square pitch.
 Square pitch gives lower shell side pressure drop
than triangular pitch.
 Square pitch is good for easy cleaning whereas
triangular pitch gives more number tubes for same
space available
 Unless shell side fluid fouls badly, triangular pitch is
used.
TEMA standards specify a minimum center to
center distance 1.25 times outside diameter of the
tubes for triangular pitch and a minimum cleaning
lane of ¼ inch for square pitch.
Tube sheet:
 It is essentially a flat circular plate. A large number
of holes are drilled in the tube sheet according to the
pitch requirements.
Baffles:
The baffles are installed in the shell
1) To increase the rate of heat transfer by increasing the
velocity and turbulence of the shell side fluid
2) It helps as structural supports for tubes and dampers
against vibration.
3) The baffles cause the fluid to flow through shell at
right angles to the axes of the tubes (Cross flow).
OR
They promote cross flow

To avoid the bypassing of the shell side fluid the
clearance between the baffles and shell, and baffles
and tubes must be minimum.
 The centre-to-centre distance between adjacent
baffles is known as baffle spacing or baffle pitch.
 The baffle space should not be greater than the inside
diameter of the shell and should not less than the
one-fifth if the inside diameter of the shell.
 The optimum baffle spacing is 0.3 to 0.50 times the
shell diameter
25% cutoff baffles

Single pass 1-1 exchanger:
Limitations:
 It occupy more space
 Cannot obtain high velocities hence low heat transfer
coefficients.
 No solution for expansion problems.
MULTI PASS HEAT EXCHANGERS:

Multi pass construction decreases the cross section
of the fluid path and increases the fluid velocity and
corresponding HT Coefficient
Advantages:
1) High velocities
2) Short tubes
3) Solution to expansion problems
Disadvantages:
1) Exchanger is more complicated
2) Friction loss are increased because of high
velocities, longer path ,multiplication of entrance
and exit losses
1-2 heat exchangers:
2-4 Heat exchangers:
 1-2 heat exchanger has an important limitation.
Because of parallel flow pass ,the exchanger is
unable to bring one of the fluid very near to the
entrance temperature of the fluid.
OR
The heat recovery is poor.
So we go for 2-4 heat exchanger
It gives high velocity and large HTC than 1-2
Exchanger with same flow rates.
Heat transfer coefficients in shell and tube heat exchangers:
In a shell-and-tube exchanger, the shell-side and tube
side heat transfer coefficients are of comparable
importance and both must be large if a satisfactory
overall coefficient is to be attained.
Tube-side coefficient:
The heat transfer coefficient for inside the tubes (hi) can be calculated using
the Sieder-Tate equation for turbulent flow in a constant diameter pipe:
0.8
0.14
Cp  0.333   
DG 
hD
  
 0.023  
 k 
    k  w 
Shell-side coefficient:
The heat transfer coefficient for the shell side cannot be calculated
using the correlations discussed so far since the direction of flow is
partly perpendicular to the tubes and partly parallel.
An approximate equation for predicting shell-side coefficients is the
Donohue equation:
The Donohue equation is based on the weighted average of the mass velocity of
the shell-side fluid flowing parallel to the tubes (Gb) and the mass velocity of the
shell-side fluid flowing across the tubes (Gc):
0.6
D G 
h D

 o o 
  0.2 o e 
 k 
  
0.14
0.33
C p     
where

  
 k   w 
Ge = (GbGc)1/2
 / Sb ,
Gb  m
Ds2
D 2o
Sb  fb
 Nb
4
4
 / Sc
Gc  m
 D 
Sc  PDs 1  o 
p 

fb = fraction of the shell cross-section
occupied by the baffle window.
Nb = number of tubes in baffle window
 is the mass flow rate of the shell-side fluid
m
Do = outside diameter of tubes
Ds = inside diameter of the shell
P = baffle spacing
p = tube pitch
Gb
Gb
Gc
Exchanger Fouling
Electron microscope image showing fibers, dust, and other deposited material on a
residential air conditioner coil and a fouled water line in a water heater.
Exchanger Fouling