Heat exchanger and its type,ntu method

heat exchanger

1. 1.Introduction of heat exchanger
2.Type of heat exchanger
3.NTU (Number of Transfer Unit)
Method
4.Heat exchanger design
Content
Prepared By GROUP 3 SECTION D1

2. 1.IntroduCtIon of heat exChanger
 are device built for efficient heat transfer from one fluid
to another.
may be define as an equipment which transfer the energy from a hot
fluid to a cold fluid with maximum rate and minimum investment
and running cost.
Example
q intercoolers and preheaters
q condenser and boiler in steam plant
q evaporator and condenser refrigeration unit

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3. 2.tYPe of heat exChanger
Ø Classification of heat exchanger according nature of heat exchanger, relative
direction of fluid motion, design and construction features, physical state of fluid.
i. Direct contact heat exchanger
ii. Indirect contact heat exchanger
(A) Regenerator (B) Recuperator

(i) Direct contact heat exchanger
In a direct contact or open or open heat exchanger
the exchanger of heat takes place by direct mixing
of hot and cold fluids and transfer of heat and
mass takes place simultaneously.
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4. (ii) Indirect contact heat exchanger
In this type of heat exchanger the heat transfer between two fluids could be carried
out by transmission through wall which separates the two fluids. This type include
(a) Regenerators
(b) Recuperators or surface exchanger
(a) Regenerator: In a regenerator type of heat exchanger the hot and cold fluid
pass alternately through a space containing solid particle (matrix) these particle
providing alternately a sink and a source for heat flow.
Example:- I.C. engine and gas turbines, Open hearth and glass melting furnaces
(b) Recuperator: is the most important type of heat exchanger in which the
flowing fluid exchanging heat are on either side of dividing wall (in the form of
pipes or tube generally). These heat exchangers are used when two fluids cannot be
allowed to mix.
Example:- Automobile radiators, Oil coolers, intercoolers, air preheaters
i. Parallel flow or unidirectional flow
ii. Counter flow
iii. Cross flow 4

5. (i) Parallel flow heat exchanger
In parallel-flow heat exchanger the two fluids enter the exchanger at the same
end and travel in parallel to one another to the other side.
Example:- Oil coolers, oil heaters, water heaters etc.
(ii) Counter -flow heat exchanger
In counter-flow heat exchangers the fluids enter the exchanger from opposite end.
The counter current design is the most efficient.



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6. (iii) Cross-flow heat exchanger
In a cross-flow heat exchanger the fluids travel roughly perpendicular to one
Another through the exchanger.

Example:- the cooling unit of refrigeration system, automobile radiator etc.
i. Concentric tube
Shell and tube
Multiple shell and tube passes
Compact heat exchangers
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7. (i) Concentric tube
Two concentric tube are used each carrying one of the fluid. The direction of
flow May be parallel.


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8. (ii) Shell and tube
In this type of heat exchanger one of the fluids flows through a bundle of tubes
enclosed by a shell. The other fluid is forced through the shell and it flows over
the outside surface of the tube.


(iii) Multiple shell and tube passes
Multiple shell and tube passes are used for enhancing the overall heat transfer.
(iv) Compact heat exchanger
There are special purpose heat exchanger and have a very large transfer surface
area per unit volume of the exchanger.
i. Condensers
ii. Evaporators8

9. (i) Condenser
In a condenser the condensing fluid remain at constant temperature throughout
the exchanger while the temperature of the colder fluid gradually increase from inlet
to outlet.
(ii) Evaporators
In this case the boiling fluid (cold fluid) remain at constant temperature while the
temperature of hot fluid gradually decrease from inlet to outlet.


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10. 3.ntu (number of transfer unIt) method
The heat exchanger effectiveness (ℇ) is defined as the ratio of actual heat transfer to
the maximum possible heat transfer. Thus



Ø By compiling non-dimensional grouping, ℇ can be expressed as function of three
Ø non-dimensional parameter, this is known as NTU method.
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The heat exchanger dQ through an area dA of the heat exchanger is given by
From expression we have

12. 12
We have expression for effectiveness

13. 13
vWe find the effectiveness of parallel flow and counter flow heat exchanger is
Given by the following expression

14. 4.heat exChanger desIgn
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Apart from predicting heat transfer a host of considering arise in the design of heat
exchanger two primary ones are minimizing pumping power and fixed cost.
oThe pumping power mathematically expressed as

where:- is the mass flow rate of stream
the pressure drop of the stream as it passes through the exchanger
the fluid density
Determining the pressure drop can be relatively straight forward in a single-pass
pipe in tube heat exchanger. The pressure drop in a long straight run of pipe is
given by
where:- is the length of
is the hydraulic diameter
is the mean velocity of the flow in the pipe

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f is the Darcy-weisbach friction factor
oOptimizing the design of an exchanger it can reduce the fixed cost of an
exchanger by increasing U and reducing the required area.
oEarly in the process the designer should assess the cost of the calculation in
comparison with
(a) The converging accuracy of computation
(b) The investment in the exchanger
(c) The cost of miscalculation
oSpecification of inlet and outlet temperature