This document discusses two types of heat exchanger problems - sizing problems where the area is to be determined given other parameters, and performance problems where the heat transfer rate is to be determined given other parameters including the area. It introduces the effectiveness-NTU method as an alternative to the trial and error approach for performance problems. It defines effectiveness as the ratio of actual to maximum possible heat transfer and derives equations for effectiveness as a function of NTU for different flow arrangements. It provides graphs of these relationships and discusses some special cases like when one capacity rate approaches infinity. The document aims to explain the effectiveness-NTU method for solving heat exchanger problems as an improvement over more complex dimensional approaches.

1. • Given: Flow rates and Inlet
outlet temperatures
• Find: The area
Sizing
the Heat
Exchanger
• Given: inlet temperature,
Area and flow rates.
• Find : The performance of
the heat exchanger
Finding the
performan
ce of Heat
Exchanger
1

2. • For the second type of problems,
q = ?
= ?
U - Known (Using the knowledge of convection or by experiment)
A - Size given
We will need a trial and error approach to solve this
type of problem.
2

3. Effectiveness NTU method
• The trial and error can be avoided if we adopt
the alternative method called the Effectiveness
NTU method
3

4. Effectiveness NTU method
• NTU method is not only a method which is
adopted to solve problems like case II
• It is a dimensionless approach which has many
more advantages over dimensional approach
4

5. Effectiveness of a heat exchanger
• Effectiveness of a heat exchanger is the ratio
of Rate of heat transfer in heat exchanger to
Maximum possible heat transfer rate
5

6. Effectiveness of a heat exchanger
6

7. Effectiveness of a heat exchanger
• The two definitions are equivalent when,
• By definition, ε is a dimensionless quantity
and
7

8. Effectiveness _Parallel Flow
If,
8

9. Effectiveness _Parallel Flow
But,
By substituting,
9

10. Effectiveness _Parallel Flow
Therefore the general equation becomes;
10

11. Effectiveness _Parallel Flow
11

12. Effectiveness _Parallel Flow
Substituting these definitions in to equations,
If we plot the effectiveness against the NTU it
will give the following graph.
12

13. Effectiveness- NTU relationship for Parallel Flow
13

14. Effectiveness- NTU relationship for
Counter Flow
14

15. Effectiveness- NTU relationship for Counter Flow
15

16. Effectiveness- NTU relationship for Cross Flow
16

17. 17

18. 18

19. 19

20. Special Cases
• When the capacity rate is infinite,
C = 0
• Therefore,
20

21. Quiz
21

22. Quiz
1. In a gas turbine power plant heat is transferred in a plate-
fin heat exchanger from the hot gases leaving the turbine
to the air leaving the compressor. Given the air flow rate
is 5000 kgh-1 and the flow rate of hot gases is 5075 kgh-1
. Given that Thi = 450 0C, Tci = 1700C, U = 52 Wm-2K-1 , A
= 50 m2, Cp = 1050 Jkg-1K-1 (for both sides). Flow
arrangement is single pass cross flow with both fluids
unmixed. Find The, Tce and q of the heat exchanger.
22