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Introduction A heat exchanger is a device that is used to transfer thermal energy

Introduction
A heat exchanger is a device that is used to transfer thermal energy (enthalpy) between two or more fluids, between a solid surface and a fluid, or between solid particulates and a fluid, at different temperatures and in thermal contact. In heat exchangers, there are usually no external heat and work interactions. Typical applications involve heating or cooling of a fluid stream of concern and evaporation or condensation of single- or multicomponent fluid streams. In other applications, the objective may be to recover or reject heat, or sterilize, pasteurize, fractionate, distill, concentrate, crystallize, or control
a process fluid. In a few heat exchangers, the fluids exchanging heat are in direct contact. In most heat exchangers, heat transfer between fluids takes place through a separating wall or into and out of a wall in a transient manner. In many heat exchangers, the fluids are separated by a heat transfer surface, and ideally they do not mix or leak. Such exchangers are referred to as direct transfer type, or simply recuperators. In contrast, exchangers in which there is intermittent heat exchange between the hot and cold fluids—via thermal energy storage and release through the exchanger surface or matrix— are referred to as indirect transfer type, or simply regenerators. Such exchangers usually have fluid leakage from one fluid stream to the other, due to pressure differences and matrix rotation/valve switching. Common examples of heat exchangers are shell-and tube exchangers, automobile radiators, condensers, evaporators, air preheaters, and cooling towers. If no phase change occurs in any of the fluids in the exchanger, it is sometimes referred to as a sensible heat exchanger. There could be internal thermal energy sources in the exchangers, such as in electric heaters and nuclear fuel elements. Combustion and chemical reaction may take place within the exchanger, such as in boilers, fired heaters, and fluidized-bed exchangers. Mechanical devices may be used in some exchangers such as in scraped surface exchangers, agitated vessels, and stirred tank reactors. Heat transfer in the separating wall of a recuperator generally takes place byThe most common type of heat exchanger is a Shell and Tube in this design one fluid flows within small tubes that are then surrounded by an encasing called the shell where the other fluid flows within contacting the tubes and thereby initiating a heat transfer either for cooling or heating depending on the application. Due to their robustness and simplistic design, Shell and Tube heat exchangers continue to be used in industry for heat transfer purposes.
SWOT analysis is also considered as a best analysis for selection of the heat exchanger.

Fig. 1: SWOT analysis

Fig. 2, Heat Exchanger Classification (Shah, 1981)
According to the TEMA design and through evaluation it was found that the most appropriate design for the heat exchanger was a AFT heat exchanger with a pull-through floating head will be used as it was the best for cleaning and it is the most common configuration for chemical processes (Mukherjee, 1998).
The recommended design is to have two shell passes and four tube passes as outlined in graphs in (Sinnot, 2013). Whilst this report has been extensive in calculating the various design specifications found in the heat exchanger, it is advised that further study take place whether it be simulating or removing assumptions as these will lead to a more valid and reliable heat exchanger.
Classification based on Construction

Fig. 3 Construction based heat exchangers

Fig. 4 Shell and rear end based classification TEMA

Types of Baffles
Baffles may be classified as transverse and longitudinal types. The purpose of longitudinal baffles is to control the overall flow direction of the shell fluid such that a desired overall flow arrangement of the two fluid streams is achieved. For example, F, G, and H shells have longitudinal baffles. Transverse baffles may be classified as plate baffles and grid (rod, strip, and other axial-flow) baffles. Plate baffles are used to support the tubes during assembly and operation and to direct the fluid in the tube bundle approximately at right angles to the tubes to achieve higher heat transfer coefficients. Plate baffles increase the turbulence of the shell fluid and minimize tube-to-tube temperature differences and thermal stresses due to the crossflow. Single- and multi segmental baffles and disk and doughnut baffles. Single- and double-segmental baffles are used most frequently due to their ability to assist maximum heat transfer (due to a high-shell-side heat transfer coefficient) for a given pressure drop in a minimum amount of space. Triple and no-tubes-in-window
segmental baffles are used for low-pressure-drop applications. The choice of baffle type, spacing, and cut is determined largely by flow rate, desired heat transfer rate, allowable pressure drop, tube support, and flow-induced vibrations. Disk and doughnut baffles/ support plates are used primarily in nuclear heat exchangers. These baffles for nuclear exchangers have small perforations between tube holes to allow a combination of crossflow and longitudinal flow for lower shell-side pressure drop. The combined flow results in a slightly higher heat transfer coefficient than that for pure longitudinal flow and minimizes tube-to-tube temperature differences. The flow in a rod baffle heat exchanger is parallel to the tubes, and flow-induced vibrations are virtually eliminated by the baffle support of the tubes. One alternative to a rod baffle heat exchanger is the use of twisted tubes (after flattening the circular tubes, they are twisted).

Fig. 5 Baffles types
Methodology
Here are the steps for rating of Heat Exchanger
1. Compute Energy balance
2. Select heating medium (Given)
3. Compute utility flowrate
4. Compute physical properties
5. Allocate fluids side
6. Decide the exchanger type
7. Determine LMTD
8. Select value of overall coefficient, U
9. Estimate the tentative area requirement
10. Estimate the actual area (based on given geometry)
11. Calculate the shell and tube side heat transfer coefficient
12. Calculate the overall coefficient
13. Calculate heat transfer rate
Assumptions
1. Steam is at saturated condition
2. Steam outlet is only water
3. Temperature remain same in the shell side
4. Latent heat of steam remains same
5. Thermodynamics properties are calculated at each stream temperature conditions
6. Design calculations are performed based on the specifications given for the heat exchanger
7. Heat transfer coefficient for steam is assumed to be 8000 W/m2*C
8. Counter-current flow is assumed
9. Condensing steam is in the shell side due to changing in the volume requirement
10. No leakage is incorporated
Equations and Calculations:
Basis data is as follows:
Water mass flowrate = m1 = 154323 lb/hr
Inlet Temperature of water = 50 F
Outlet Temperature of water = 167 F
Total tubes = 124
Outer dia of tube = do = 1.9E-2 m
Length of tube = L = 4.05 m

1. Compute Energy balance
Using the below equation, we can calculate the heat transfer for the heat exchanger;

Q = 18084399 Btu/hr
2. Select heating medium (Given)
Saturated steam as a heating media at 4 bar
3. Compute utility flowrate
Steam flowrate = Q / (Lambda )
Steam flowrate = 19714.81 lb/hr
4. Compute physical properties
Stream # 1 2 3 4
T F 50 167 290 290
Cp Btu/lb-F 1.00158445 1.002 0.5252 1.02
Lambda Btu/lb 917.3 917.3 917.3 917.3
Density 62.3987293 60.83513
Viscosity cP 1.3
Thermal conductivity Btu/hr-ft-F 0.34 0.38 0.015 0.4
5. Allocate fluids side
Condensing steam is allocated in the shell side due to volume change and water is in the tube side.
6. Decide the exchanger type
The most used shell and tube type is chosen.
7. Determine LMTD

LMTD = 59.96 F

8. Select value of overall coefficient, U
Assumed value of overall heat transfer coefficient is 1200 Btu/ft2.F
9. Estimate the tentative area requirement

Area = 251.33 ft2
Area = 23.35 m2
10. Estimate the actual area (based on given geometry)

Actual Area = 29.97 ft2
11. Calculate the shell and tube side heat transfer coefficient

Re = 2428; Nu = 19.2

12. Calculate the overall coefficient

Uo = 8000 W/m2-C
13. Calculate heat transfer rate

14. Pressure drop for the tube side

15. Pressure drop for the shell side

Fig. 6: Counter-Current Heat Exchanger

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Result and Discussion
The result indicated that the heat exchanger is over designed as compared to the water heating requirement. The advantage of overdesigned heat exchanger is that it helps to maintain the heat exchanger operation longer regardless of the operational problem. If some tubes get blocked then can ultimately achieve that requirement due to extra tubes installed to process the same amount of material. Lastly, the rating of heat exchanger indicated the applicability of the procedure required to test whether heat exchanger can meet the requirements or not.

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Takeaways and Learning
This problem facilitated use to apply the heat transfer fundamentals as well as equations to some real-world problem. The concepts of the whole heat exchanger also become candid upon its practical application. Besides these, it helped us to find some new ways to figure out the problems of real world and how to apply engineering knowledge with particular focus on the chemical industries.
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Figures

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Nomenclature
Uo overall coefficient based on the outside area of the tube,
ho shell side heat transfer coefficient,
hi tube side heat transfer coefficient
hod shell side dirt coefficient (fouling factor),
hid tube side dirt coefficient (fouling factor),
kw thermal conductivity of the tube wall material,
di tube inside diameter
do tube outside diameter