Ministry of Higher Education
King Soud University
College of Engineering
Chemical Engineering Department
Research Title :-
Shell and Tube
Heat Exchanger
Name:- Moh'd Saad Jalmood
No.:- 425104886
Dr.:- Malek Al Ahmed
Subject :- ChE 313 (( Heat Transfer ))
Introduction:A shell and tube heat exchanger is a class of heat exchanger designs. It is the most
common type of heat exchanger in oil refineries and other large chemical processes,
and is suited for higher-pressure and higher-temperature applications. As its name
implies, this type of heat exchanger consists of a shell (a large pressure vessel) with a
bundle of tubes inside it. One fluid runs through the tubes, and another fluid flows
over the tubes (through the shell) to transfer heat between the two fluids. The set of
tubes is called a tube bundle, and may be composed by several types of tubes: plain,
longitudinally finned, etc.
A heat exchanger is a device for transferring heat from one fluid to another, where a
solid wall separates the fluids so that they never mix. They are widely used in
refrigeration, air conditioning, space heating, power production, and chemical
processing. One common example of a heat exchanger is the radiator in a car, in
which the hot radiator fluid is cooled by the flow of air over the radiator surface. (2)
Theory and Application:Two fluids, of different starting temperatures, flow through the heat exchanger. One
flows through the tubes (the tube side) and the other 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. In order to transfer heat efficiently,
a large heat transfer area should be used, leading to the use of many tubes. In this
way, waste heat can be put to use. This is an efficient way to conserve energy.
Heat exchangers with only one phase (liquid or gas) on each side can be called onephase or single-phase heat exchangers. Two-phase heat exchangers can be used to
heat a liquid to boil it into a gas (vapor), sometimes called boilers, or cool a vapor to
condense it into a liquid (called condensers), with the phase change usually occurring
on the shell side. Boilers in steam engine locomotives are typically large, usually
cylindrically-shaped shell-and-tube heat exchangers. In large power plants with
steam-driven turbines, shell-and-tube surface condensers are used to condense the
exhaust steam exiting the turbine into condensate water which is recycled back to be
turned into steam in the steam generator.
Shell and tube heat exchanger Types:There can be many variations on the shell and tube design. Typically, the ends of each
tube are connected to plenums (sometimes called water boxes) through holes in
tubesheets. The tubes may be straight or bent in the shape of a U, called U-tubes.
U-Tubes
The Shell and Tube (u-tube) is the most common type of heat exchanger used in the
process, petroleum, chemical and HVAC industries, it contains a number of parallel
u-tubes inside a shell. Shell Tube heat exchangers are used when a process requires
large amounts of fluid to be heated or cooled. Due to their design, shell tube heat
exchangers offer a large heat transfer area and provide high heat transfer efficiency.
In nuclear power plants called pressurized water reactors, large heat exchangers called
steam generators are two-phase, shell-and-tube heat exchangers which typically have
U-tubes. They are used to boil water recycled from a surface condenser into steam to
drive the turbine to produce power. Most shell-and-tube heat exchangers are either 1,
2, or 4 pass designs on the tube side. This refers to the number of times the fluid in the
tubes passes through the fluid in the shell. In a single pass heat exchanger, the fluid
goes in one end of each tube and out the other.
Surface condensers in power plants are often 1-pass straight-tube heat exchangers.
Two and four pass designs are common because the fluid can enter and exit on the
same side. This makes construction much simpler.
Straight-Tube 1-pass
One pass means that the fluid enter on one side and exit on the other side of the heat
exchanger
There are often baffles directing flow through the shell side so the fluid does not take
a short cut through the shell side leaving ineffective low flow volumes.
Counter current heat exchangers are most efficient because they allow the highest log
mean temperature difference between the hot and cold streams. Many companies
however do not use single pass heat exchangers because they can break easily in
addition to being more expensive to build. Often multiple heat exchangers can be used
to simulate the counter current flow of a single large exchanger.
Straight-Tube 2-pass
Two pass heat exchanger means that the fluid enters and exit on the same side of the
heat exchanger.
There are many different types or designs of shell and tube heat exchangers to meet
various process requirements. Shell and Tube heat exchangers can provide steady heat
transfer by utilizing multiple passes of one or both fluids. SEC shell and tube heat
exchangers come in two (2) and four (4) pass models standard, and multi-pass custom
models.
Shell and Tube heat exchangers use baffles on the shell-side fluid to accomplished
mixing or turbulence. Without the use of baffles, the fluid can become stagnant in
certain parts of the shell.
Selection of tube material:To be able to transfer heat well, the tube material should have good thermal
conductivity. Because heat is transferred from a hot to a cold side through the tubes,
there is a temperature difference through the width of the tubes. Because of the
tendency of the tube material to thermally expand differently at various temperatures,
thermal stresses occur during operation. This is in addition to any stress from high
pressures from the fluids themselves. The tube material also should be compatible
with both the shell and tube side fluids for long periods under the operating conditions
(temperatures, pressures, pH, etc.) to minimize deterioration such as corrosion. All of
these requirements call for careful selection of strong, thermally-conductive,
corrosion-resistant, high quality tube materials, typically metals. Poor choice of tube
material could result in a leak through a tube between the shell and tube sides causing
fluid cross-contamination and possibly loss of pressure. (1)
Standard heat exchangers particularly made for heavy duty ever with medium
pressure ranges. The composition can be made with different choice of materials, all
sort of combinations make this type of exchangers versatile enough to solve any
problem of fluids, flows, even with high duty rating.
(2)
Baffles:
Punched from steel plates, with supporting lips for higher thermal efficiency and more
safety in case of vibration.
Shell & Tubesheets:
Welded carbon steel construction, to give most rugged exchanger, adequate thickness
for t trouble free long life. Shell side connections made also for customers
requirement, over the standard threaded connections we propose flanges
How To Make a Shell and Tube Heat Exchanger ?
(3)
When preparing to design a heat exchanger, do
you ever wonder where to start?
1. Is there a phase change involved in my
system?
A quick look at the boiling points compared
with the entrance and exit temperatures will help
you answer this question.
2. How many "zones" are involved in my
system?
"Zones" can best be defined as regimes of
phase changes where the overall heat transfer
coefficient (Uo) will vary. Using T-Q
(Temperature-Heat) diagrams are the best way to
pinpoint zones. The system is defined as cocurrent or countercurrent and the diagram is
constructed. The diagram on the left illustrates
the use of T-Q diagrams. These diagrams should
accompany your basic (input-output) diagram of
the heat exchanger. Chemical #1 enters the shell
at 200 0C as a superheated vapor. In Zone 1, it
releases heat to the tubeside chemical (Chemical
#2). Zone 1 ends just a Chemical #1 begins to
condense. The tubeside (Chemical #2) enters as a
liquid or gas and does not change phase
throughout the exchanger. Chemical #1 leaves
Zone 1 and enters Zone 2 at its boiling
temperature, Tb1. T* marks the temperature of
Chemical #2 when Chemical #1 begins to
condense. In Zone 2, Chemical #1 condenses to
completion while Chemical #2 continues to
increase in temperature. The temperature of
Chemical #2 when Chemical #1 is fully
condensed is denoted at T**. Finally, in Zone 3,
both chemicals are liquids. Chemical #1 is
simply liberating heat to Chemical #2 as it becomes a subcooled liquid and exits the
shell at 100 0C.
Defining zones is one of the most important aspects of heat exchanger design. It is
also important to remember that if your process simulator does not support zoned
analysis (such as Chemcad III), you should model each zone with a separate heat
exchanger. Thus, the previous illustration would require 3 heat exchangers in the
simulation. BUT, do not draw 3 exchangers on your PFD (Process Flow Diagram).
This is all happening in one exchanger.
3. What are the flowrates and operating pressures involved in my system?
This information is critical in establishing the mass and energy balance around the
exchanger. Operating pressures are particularly important for gases as their physical
properties vary greatly with pressure.
4. What are the physical properties of the streams involved?
If you're using a process simulator, obtaining the physical properties of your
streams should be just a click of the mouse away. However, if performing the
calculation by hand, you may have to do some estimating as the streams may not be
of pure substances. Also, you should get the physical properties for each zone
separately to ensure accuracy, but in some cases it is acceptable to use an average
value. This would be true of Chemical #2 in the tubes since it is not changing phase
or undergoing a truly significant temperature change (over 1000C). Physical
properties that you will want to collect for each phase of each stream will include:
heat capacity, viscosity, thermal conductivity, density, and latent heat (for phase
changes). These are in addition to the boiling points of the streams at their respective
pressures.
5. What are the allowable pressure drops and velocities in the exchanger?
Pressure drops are very important in exchanger design (especially for gases). As
the pressure drops, so does viscosity and the fluids ability to transfer heat. Therefore,
the pressure drop and velocities must be limited. The velocity is directly proportional
to the heat transfer coefficient which is motivation to keep it high, while erosion and
material limits are motivation to keep the velocity low. Typical liquid velocities are
1-3 m/s (3-10 ft/s). Typical gas velocities are 15-30 m/s (50-100 ft/s). Typical
pressure drops are 30-60 kPa (5-8 psi) on the tubeside and 20-30 kPa (3-5 psi) on the
shellside.
6. What is the heat duty of the system?
This can be answered by a simple energy balance from one of the streams.
7. What is the estimated area of the exchanger?
Unfortunately, this is where the real fun begins in heat exchanger design! You'll
need to find estimates for the heat transfer coefficients that you'll be dealing with.
These can be found in most textbooks dedicated to the subject or in Perry's Chemical
Engineers' Handbook. Once you've estimated the overall heat transfer coefficient, use
the equation Q=UoADTlm to get your preliminary area estimate. Remember to use
the above equation to get an area for each zone, then add them together.
8. What geometric configuration is right for my exchanger?
Now that you have an area estimate, it's time to find a geometry that meets your
needs. Once you've selected a shell diameter, tubesheet layout, baffle and tube
spacing, etc., it's time to check your velocity and pressure drop requirements to see if
they're being met. Experienced designers will usually combine these steps and
actually obtain a tube size that meets the velocity and pressure drop requirements and
then proceed. Some guidelines may be as follows: 3/4 in. and 1.0 in. diameter tubes
are the most popular and smaller sizes should only be used for exchangers needing
less than 30 m2 of area. If your pressure drop requirements are low, avoid using four
or more tube passes as this will drastically increase your pressure drop. Once you
have a geometry selected that meets all of your needs, it's on to step #9.
9. Now that I have a geometry in mind, what is the actual overall heat transfer
coefficient?
This is where you'll spend much of your time in designing a heat exchanger.
Although many textbooks show Nu=0.027(NRE)0.8(NPR)0.33 as the "fundamental
equation for turbulent flow heat transfer", what they sometimes fail to tell you is that
the exponents can vary widely for different situations. For example, condensation in
the shell has different exponents than condensation in the tubes. Use this fundamental
equation if you must, but you should consult a good resource for accurate equations. I
highly recommend the following: Handbook of Chemical Engineering Calculations,
2nd Ed., by Nicholas P. Chopey from McGraw-Hill publishers (ISBN 0070110212).
Also, don't forget to include the transfer coefficient across the tube wall and the
fouling coefficient. These can be very significant!
10. What is the actual area of the exchanger using the 'actual' heat transfer
coefficient?
If you recall, you used estimated heat transfer coefficients to get an initial area.
Now it's time to recalculate the area.
REFERENCES :1- From Wikipedia Shell and tube heat exchanger, Perry, Robert H. and Green,
Don W. (1984). Perry's Chemical Engineers' Handbook, 6th Edition,
McGraw-Hill.
2- www.indiamart.com/multitechsales/our-collection.html
3- www.secshellandtube.com