1. MER 439 - Design of Thermal Fluid Systems
Heat Exchanger Analysis
Professor Anderson
Spring Term 2012

2. Outline (1) Heat Exchanger Types (2) Heat Exchanger Analysis Methods
Overall Heat Transfer Coefficient
fouling, enhanced surfaces
LMTD Method
Effectiveness-NTU Method

3. HX Classifications Recuperators/Regenerators
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

4. HX Classifications

5. Concentric tube (double piped)
Heat Exchanger Types
Concentric tube (double piped)

6. Concentric tube (double piped)
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
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

7. Shell and Tube 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

8. Compact Heat Exchangers Heat Exchanger Types
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

9. Cross Flow Heat Exchanger Types finned versus unfinned
mixed versus unmixed

10. Heat Exchanger Types

11. Heat Exchanger Types

12. Heat Exchanger Analysis
Overall Heat Transfer Coefficient
LMTD
Effectiveness-NTU

13. 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.
Heat exchanger surfaces are subject to fouling by fluid impurities, rust formation, or reactions between the fluid and the wall.

14. 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:

15. Log-Mean Temperature Difference
To relate the total heat transfer rate to inlet and outlet fluid temperatures. Apply energy balance:

16. Log-Mean Temperature Difference
We can also relate the total heat transfer rate to the temperature difference between the hot and cold fluids. .

17. The log mean temperature difference depends on the heat exchanger configuration
Th,in
Th,in
Th,out
Th,out
Tc,out
Tc,out
Tc,in
Tc,in

18. LMTD Parallel-Flow HX

19. LMTD Counter-Flow HX
DTlm,CF > DTlm,PF FOR SAME U: ACF < APF

20. LMTD- Multi-Pass and Cross-Flow
Apply a correction factor to obtain LMTD
t: Tube Side

21. LMTD Method Sizing a Heat Exchanger:
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….

22. The Effectiveness-NTU Method
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)
Q = eCmin(Th,i - Tc,i)

23. 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:

24. The Effectiveness-NTU Method

25. 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.

26. 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.

27. The Homework