Week-1, Lecture-5 Basic Design Parameters

Shabina Khanam
Associate Professor
Department of Chemical Engineering

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 Basic Design Parameters
FT correction factor
FT correction factor is usually correlated in terms of dimensionless
ratios, the thermal effectiveness of the exchanger (P) and the ratio of
two heat-capacity flowrate (R) as

P = (THi - THO) / (THi – Tci)

R = CPH / CPC = (TCo – Tci) / (THi – THo)

Where, THi = Hot stream inlet temperature (oC)

THo = Hot stream outlet temperature (oC)
TCi = cold stream inlet temperature (oC)
TCo = cold stream outlet temperature (oC)
 Basic Design Parameters
Three basic situations encountered
Temperature Approach - Final temperature of hot stream is
higher than final temperature of the cold, shown in figure. This
is called temperature approach as outlet temperature of hot
stream approaches to the outlet temperature of cold stream.
This situation is straightforward to design, since it can always
be accommodated in a single 1 – 2 shell.
 Basic Design Parameters
Three basic situations encountered
Temperature Cross - The outlet temperature of the hot
stream is slightly lower than the outlet temperature of the
cold stream, as illustrated in figure. This is called
temperature cross. This situation is usually
straightforward to design, provided the temperature cross
is small, as it can be accommodated in a single shell.
 Basic Design Parameters
Three basic situations encountered
Large Temperature cross - As the amount of temperature cross increases,
problems are encountered as illustrated in figure below:

In this case, the FT decreases significantly, causing a dramatic increase in the heat transfer area
requirement leading to an infeasible design. Thus, for a given R, the design of the heat exchanger
becomes less and less efficient as the asymptotic region of the FT curve is reached.
 Basic Design Parameters
FT correction factor
Infeasible exchanger designs return FT≤0. Having FT>0, however, is not
enough to make a design practical. A commonly used rule of thumb
requires FT≥0.75 for the design to be considered practical.

It is well known fact that for multipass exchangers the heat recovery is
limited by LMTD correction factor, FT. As temperature approach
decreases, FT decreases rapidly. If FT<0.75 one should increase the
number of shells till FT becomes greater that 0.75.
 Basic Design Parameters
Shells in series reduces the temperature cross in exchangers
 Basic Design Parameters
Fluid allocation: shell or tubes
Where no phase change occurs, the following factors will determine the allocation of
the fluid streams to the shell or tubes.
• Corrosion: The more corrosive fluid should be allocated to the tube-side. This will
reduce the cost of expensive alloy.
• Fouling: The fluid that has the greatest tendency to foul the heat-transfer surfaces
should be placed in the tubes. This will give better control over the design fluid
velocity, and the higher allowable velocity in the tubes will reduce fouling. Also, the
tubes will be easier to clean.
• Fluid temperatures: If the temperatures are high enough to require the use of
special alloys placing the higher temperature fluid in the tubes will reduce the
overall cost.

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 Basic Design Parameters
Fluid allocation: shell or tubes
Operating pressures: The higher pressure stream should be allocated to the
tube-side. High-pressure tubes will be cheaper than a high-pressure shell.
Viscosity: Generally, a higher heat-transfer coefficient will be obtained by
allocating the more viscous material to the shell-side, providing the flow is
turbulent. The critical Reynolds number for turbulent flow in the shell is in the
region of 200. If turbulent flow cannot be achieved in the shell it is better to place
the fluid in the tubes, as the tube-side heat-transfer coefficient can be predicted
with more certainty.

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 Basic Design Parameters
Fluid allocation: shell or tubes
Stream flow-rates: Allocating the fluids with the lowest flow-rate to the shell-
side will normally give the most economical design.
Criteria for fluid placement, in order of priority
Tube-side fluid Shell-side fluid
Corrosive fluid Condensing vapor (unless corrosive)
Hotter fluid High viscous fluid
Fouling fluid Low flow rate stream
Less viscous fluid
Higher-pressure stream

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 Basic Design Parameters
Shell and tube fluid velocities

High velocities will give high heat-transfer coefficients but also a

high-pressure drop. The velocity must be high enough to prevent
any suspended solids settling, but not so high as to cause
erosion. High velocities will reduce fouling.

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 Basic Design Parameters
Shell and tube fluid velocities Typical design velocities are:
Liquids
• Tube-side, process fluids: 1 to 2 m/s, maximum 4 m/s if required to reduce
fouling; water: 1.5 to 2.5 m/s.
• Shell-side: 0.3 to 1 m/s.
Vapours
For vapours, the velocity used will depend on the operating pressure and fluid
density:
Vacuum 50 to 70 m/s
Atmospheric pressure 10 to 30 m/s
High pressure 5 to 10 m/s

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 Basic Design Parameters
Stream temperatures
The closer the temperature approach used (the difference between the outlet
temperature of one stream and the inlet temperature of the other stream) the larger
will be the heat-transfer area required for a given duty. The optimum value will
depend on the application, and can only be determined by making an economic
analysis of alternative designs. As a general guide the greater temperature
difference should be at least 20C, and the least temperature difference 5 to 7C for
coolers using cooling water, and 3 to 5C using refrigerated brines. The maximum
temperature rise in recirculated cooling water is limited to around 40C.

Care should be taken to ensure that cooling media temperatures are kept well
above the freezing point of the process materials. When the heat exchange is
between process fluids for heat recovery the optimum approach temperatures will
normally not be lower than 20C.
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 Basic Design Parameters
Pressure drop
In many applications the pressure drop available to drive the fluids through
the exchanger will be set by the process conditions, and the available
pressure drop will vary from a few millibars in vacuum service to several
bars in pressure systems.

When the designer is free to select the pressure drop an economic

analysis can be made to determine the exchanger design which gives the
lowest operating costs, taking into consideration both capital and pumping
costs. However, a full economic analysis will only be justified for very large,
expensive, exchangers. The values suggested below can be used as a
general guide, and will normally give designs that are near the optimum.

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 Basic Design Parameters
Pressure drop
The values suggested below can be used as a general guide, and will normally
give designs that are near the optimum.
Liquids
• Viscosity <1 mN s/m2 35 kN/m2
• 1 to 10 mN s/m2 50-70 kN/m2
Gas and vapours
• Vacuum 0.1 - 0.8 kN/m2
• 1 to 2 bar 0.5  system gauge pressure
• Above 10 bar 0.1  system gauge pressure

When a high-pressure drop is utilised, care must be taken to ensure that the
resulting high fluid velocity does not cause erosion or flow-induced tube vibration.
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 Summary of the video
 Basic design equation is discussed in details.
 Overall heat transfer coefficient, dirt factor and mean
temperature difference is discussed.
 Ft correction factor is detailed.
 Guidelines to allocate fluid in shell and tube sides are shown.
 Permissible limits of fluid velocity and pressure drop in shell and
tube sides are mentioned.

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 References
1 Backhurst, J.R. and Harker J.H., “Coulson and Richardson Chemical Engineering”,
Vol. II, 5th Ed., 2002, Butterworth-Heinemann.
2 Sinnott, R.K., “Coulson and Richardson’s Chemical Engineering Series: Chemical
Engineering Design”, Vol. VI, 4th Ed., 2005, Elsevier Butterworth-Heinemann.
3 Serth, R.W., “Process Heat Transfer: Principles and Applications” 2007, Elsevier Ltd.
4 Shah, R.K. and Sekulic, D.P., “Fundamentals of heat Exchanger Design”, 2003, John
Wiley & Sons.
Thank You!

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