1. Outline (1) Heat Exchanger Types (2) Heat Exchanger Analysis Methods
Overall Heat Transfer Coefficient
fouling, enhanced surfaces
LMTD Method
Effectiveness-NTU Method
(3) Homework #4
March 1, 2002

2. Introduction
Heat exchangers are devices that provide the flow of thermal energy between two or more fluids at different temperatures.
Used in wide range of applications
Classified according to:
Recuperators/Regenerators
Transfer processes: direct and indirect contact
Construction Geometry: tubes, plates, extended surfaces
Heat Transfer Mechanisms: single and two phase
Flow arrangements: parallel, counter, cross-flow
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3. HX Classifications
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4. HX Classifications
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5. Heat Exchanger Types
Concentric tube (double piped)
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6. Heat Exchanger Types Concentric tube (double piped)
One pipe is placed concentrically within the diameter of a larger pipe
Parallel flow versus counter flow
Fluid B
Fluid A
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7. Heat Exchanger Types Concentric tube (double piped)
used when small heat transfer areas are required (up to 50 m2)
used with high P fluids
easy to clean (fouling)
bulky and expensive per unit heat transfer area
Flexible, low installation cost, simple construction.
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8. Heat Exchanger Types
Shell and Tube
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9. Heat Exchanger Types Shell and Tube
most versatile (used in process industries, power stations, steam generators, AC and refrigeration systems)
large heat transfer area to volume/weight ratio
easily cleaned
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10. Heat Exchanger Types Compact Heat Exchangers
generally used in gas flow applications
surface density > 700 m2/m3
the surface area is increased by the use of fins
plate fin or tube fin geometry
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11. Heat Exchanger Types
Compact Heat
Exchangers
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12. Heat Exchanger Types Cross Flow finned versus unfinned
mixed versus unmixed
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13. Heat Exchanger Types
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14. Heat Exchanger Types
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15. Heat Exchanger Applications
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16. Heat Exchanger Analysis
Overall Heat Transfer Coefficient
LMTD
Effectiveness-NTU
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17. Overall Heat Transfer Coefficient
The overall coefficient is used to analyze heat ex-changers. It contains the effect of hot and cold side convection, conduction as well as fouling and fins.
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18. Fouling Factors
Heat exchanger surfaces are subject to fouling by fluid impurities, rust formation, or reactions between the fluid and the wall.
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19. Enhanced Surfaces
Fins are often added to the heat exchange surfaces to enhance the heat transfer by increasing the surface area. The overall surface efficiency is:
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20. Enhanced Surfaces
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21. Log-Mean Temperature Difference
Need to relate the total heat transfer rate to inlet and outlet fluid temperatures. Apply energy balance:
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22. Log-Mean Temperature Difference
We can also relate the total heat transfer rate to the temperature difference between the hot and cold fluids.
The log mean temperature difference depends on the heat exchanger configuration.
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23. LMTD Parallel-Flow HX Apply energy balance: Assume: insulated
no axial conduction
DPE, DKE negligible
Cp constant
U constant
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24. LMTD Parallel-Flow HX
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25. LMTD Parallel-Flow HX
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26. LMTD Counter-Flow HX DTlm,CF > DTlm,PF FOR SAME U: ACF < APF
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27. LMTD Counter-Flow HX DTlm,CF > DTlm,PF FOR SAME U: ACF < APF
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28. LMTD- Multi-Pass and Cross-Flow
Apply a correction factor to obtain LMTD
t: Tube Side
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29. LMTD Method SIZING PROBLEMS:
Calculate Q and the unknown outlet temperature.
Calculate DTlm and obtain the correction factor (F) if necessary
Calculate the overall heat transfer coefficient.
Determine A.
The LMTD method is not as easy to use for performance analysis….
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30. The Effectiveness-NTU Method
If the inlet temperatures are unknown then the LMTD method requires iteration.
Preferable to use the e-NTU method under these conditions.
Define Qmax
for Cc < Ch Qmax = Cc(Th,i - Tc,i)
for Ch < Cc Qmax = Ch(Th,i - Tc,i)
or Qmax = Cmin(Th,i - Tc,i)
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31. The Effectiveness-NTU Method
Now define the heat exchanger effectiveness, e.
The effectiveness is by definition between 0 & 1
The actual heat transfer rate is:
Q = eCmin(Th,i - Tc,i)
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32. The Effectiveness-NTU Method
For any heat exchanger:
e = f(NTU,Cmin/Cmax)
NTU (number of transfer units) designates the nondimensional heat transfer size of the heat exchanger:
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33. The Effectiveness-NTU Method
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34. The Effectiveness-NTU Method
PERFORMANCE ANALYSIS
Calculate the capacity ratio Cr = Cmin/Cmax and NTU = UA/Cmin from input data
Determine the effectiveness from the appropriate charts or e-NTU equations for the given heat exchanger and specified flow arrangement.
When e is known, calculate the total heat transfer rate
Calculate the outlet temperature.
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35. The Effectiveness-NTU Method
SIZING ANALYSIS
When the outlet and inlet temperatures are known, calculate e.
Calculate the capacity ratio Cr = Cmin/Cmax
Calculate the overall heat transfer coefficient, U
When e and C and the flow arrangement are known, determine NTU from the e-NTU equations.
When NTU is known, calculate the total heat transfer surface area.
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36. Homework Number 4
COMPLETE PARTs 1&2 BEFORE CLASS ON THURSDAY. WE WILL USE CLASS/LAB TIME ON Tuesday TO WORK ON PARTS 3 and 4.
Part 1:
Review Chapter 11 of Incropera and Dewitt. Work through Example Problems 11.3, 11.4, 11.5.
Part 2:
To ventilate a factory building, 5 kg/s of factory air at a temperature of 27 oC is exhausted and an identical flow rate of outdoor air at a temperature of -12 oC is introduced to take its place. To recover some of the heat of the exhaust air, heat exchangers are placed in the exhaust and ventilation air ducts and 2 kg/s of ethylene glycol is pumped between the two heat exchangers. The UA value of both of these crossflow heat exchangers is 6.33 kW/K. What is the temperature of the air entering the factory?
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37. Homework Number 4
Part 3:
Calculate the UA value for the cross flow heat exchanger specified above. The heat exchanger is 1m high by 1 m wide by 1 m long (in flow direction) and contains a tube bank heat transfer surface. The ethylene glycol flows through the tubes and the air flow around them (i.e. figure 11.2B in Incropera and Dewitt). The tubes are made of 1" diameter thin walled copper and are mounted in an aligned pattern (3 inches apart center to center – Review tube bank heat transfer data in Chapter 7 of Incropera and Dewitt). Use the data for internal tube heat transfer coefficient given in the Project 4 technical specifications.
Part 4:
Calculate the pressure drop for the ethylene glycol and the air. Use the data for ethylene glycol friction factor given in the Project 4 technical specifications.
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