Introduction to Heat Exchanger

Heat exchanger
What is the heat exchanger?
Heat exchangers are devices that facilitate the exchange of heat between two fluids that are at
different temperatures
Prevent the two fluids from mixing with each other
Heat transfer in a heat exchanger involves:
Convection in each fluid
Conduction through the wall separating the fluids

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The Overall Heat – Transfer Coefficient

The overall heat-transfer coefficient is discussed previously , the heat transfer through the plane wall .
Where:
TA and TB are the fluid temperatures on each side of the wall. The overall heat-transfer coefficient U is defined by the relation


The overall heat transfer is obtained from the thermal network


The overall heat-transfer coefficient may be based on either the inside or outside area of the tub.




Where:
i and o inside and outside of the smaller inner tube.

Example 1:

Hot water at 98 ? flows through a 2-in schedule 40 horizontal steel pipe [k =54 W/m• ?C] and is exposed to atmospheric air at 20?. The water velocity is 25 cm/s. Calculate the overall heat transfer coefficient for this situation, based on the outer area of pipe.

Solution:
From Appendix A the dimensions of 2-in schedule 40 pipe are

ID = 2.067 in=0.0525 m
OD = 2.375 in=0.06033 m

The properties of water at 98 ? are ?=960 kg/m3 ?=2.82×10?4 kg/m• s
k =0.68 W/m•? Pr=1.76


Where:
To is the unknown outside pipe surface temperature. The inner pipe surface as Ti and the water temperature as Tw; then the energy balance requires

Combining Equations


As a result, the outside heat-transfer coefficient and thermal resistance are

The overall heat-transfer coefficient based on the outer area is written in terms of these resistances as

With numerical values inserted,

In this calculation we used the outside area for a 1.0-m length as

FOULING FACTORS

This factor is take place after a period of operation the heat-transfer surfaces for a heat exchanger may become coated with various deposits present in the flow systems .Therefore, the interaction between the fluids and the material used for construction of the heat exchanger.
Fouling factors must be obtained experimentally by determining the values of U for both clean and dirty conditions in the heat exchanger. The fouling factor is thus defined as