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 performance of Heat Exchanger

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.

3

Effectiveness NTU method

• The trial and error can be avoided if we adopt

the alternative method called the Effectiveness

NTU method

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

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

6

Effectiveness of a heat exchanger

7

Effectiveness of a heat exchanger

• The two definitions are equivalent when,

• By definition, ε is a dimensionless quantity and

8

Effectiveness _Parallel Flow

If,

9

Effectiveness _Parallel Flow

But,

By substituting,

10

Effectiveness _Parallel Flow

Therefore the general equation becomes;

11

Effectiveness _Parallel Flow

Let’s define two dimensionless parameters…

0

𝑁 ≥ 0

12

Effectiveness _Parallel Flow

Substituting these definitions in to equations,

If we plot the effectiveness against the NTU it will give the following graph.

13

Effectiveness- NTU relationship for Parallel Flow

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Effectiveness- NTU relationship for Counter Flow

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Effectiveness- NTU relationship for Counter Flow

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Effectiveness- NTU relationship for Cross Flow

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Special Cases

• When the capacity rate is infinite,

C = 0

• Therefore,

21

Quiz

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.