About Basics of heat exchangers by H. C. Sharma

1. HEAT EXCHANGERHEAT EXCHANGER
GUIDED BY:GUIDED BY: PREPARED BY:PREPARED BY:
Prof. S.C. SharmaProf. S.C. Sharma Aayush ToshniwalAayush Toshniwal
Harsh ChudgarHarsh Chudgar
Nikhil LeleNikhil Lele

2. FOULING FACTORFOULING FACTOR
The performance of heat exchanger usually deteriorates with
time as a result of accumulation of deposits on heat transfer
surfaces. The layer of deposits represents additional
resistance to heat transfer and causes the rate of heat
transfer in a heat transfer to decrease. The net effect of these
accumulation of heat transfer is represented by fouling factor
which is a measure of thermal resistance induced by fouling.

3. Fouling should be considered in the design and selection of
heat exchanger. In such application it may be necessary to
select a larger and expensive heat exchanger.
The periodic cleaning of heat exchanger and the resulting
down time are additional penalties associated with fouling.
The fouling factor depends on operating temperature and the
velocities of fluids and length of service. Fouling increases
with increasing temperature and decreasing velocities.

4. Fouling Factor
Precipitation of
solid deposits
Corrosion and
Chemicals fouling
Biological fouling

5. PRECIPITAION OF SOLIDPRECIPITAION OF SOLID
DEPOSITESDEPOSITES
These is specially found in the areas where the water is
hard. The scales of such deposits come off by scratching
and the surfaces can be cleaned of such deposits by
chemical treatment.
To avoid these problem, water in power and process plants
is extensively heated and its solid contents are removed
before it is allowed to circulates through the system.
The solid ash particles in the flue gases accumulating on
the surfaces of air pre-heaters create similar problems.

6. CHEMICAL FOULINGCHEMICAL FOULING
In this case surfaces are fouled by the accumulation of
the products of chemical reactions on the surfaces.
This form of fouling can be avoided by coating metal
pipes with glass or using plastic pipe instead of metal
ones.

7. BIOLOGICAL FOULINGBIOLOGICAL FOULING
Heat exchangers may also be fouled by the growth ofHeat exchangers may also be fouled by the growth of
algae in warn water. This type of fouling can bealgae in warn water. This type of fouling can be
prevented by chemical treatment.prevented by chemical treatment.

8. Analysis of heat exchangerAnalysis of heat exchanger
For analyzing of heat exchanger we will discuss the twoFor analyzing of heat exchanger we will discuss the two
methods.methods.
Log Mean Temperature Difference Method (LMTD)Log Mean Temperature Difference Method (LMTD)
effectiveness- NTU methodeffectiveness- NTU method
Heat exchangers usually operate for long periods of timeHeat exchangers usually operate for long periods of time
with no change in there operating condition. Therefore,with no change in there operating condition. Therefore,
they can be modeled asthey can be modeled as study flow device.study flow device.

9. AssumptionsAssumptions
The mass flow rate of each fluid remain constant.The mass flow rate of each fluid remain constant.
The fluid properties such as temp. and velocity at inlet and outletThe fluid properties such as temp. and velocity at inlet and outlet
remain same.remain same.
The fluid stream experienced little or no change in their velocitiesThe fluid stream experienced little or no change in their velocities
and elevations, and thus theand elevations, and thus the kinetic and potential energykinetic and potential energy changeschanges
are negligible.are negligible.
The specific heat of the fluid, in general, changes with temp. But, inThe specific heat of the fluid, in general, changes with temp. But, in
a specified temperature range it can be treated as a constant ata specified temperature range it can be treated as a constant at
some average value with little loss in accuracy.some average value with little loss in accuracy.
Axial heat conductionAxial heat conduction along the tube is usually insignificant and canalong the tube is usually insignificant and can
be considered negligible.be considered negligible.
Finally, the outer surface of the heat exchanger is assumed to beFinally, the outer surface of the heat exchanger is assumed to be
perfectly insulatedperfectly insulated..

10. Under these assumptions, the first law of
thermodynamucs requires that the rate of heat transfer
from the hot fluid be equal to the rate of heat transfer to
the cold one.That is,

11. Principle of heat exchangerPrinciple of heat exchanger

12. Counter flow Parallel flow
( ) ( )HC
CmCm  > ( ) ( )HC
CmCm  <

13. LMTD methods.LMTD methods.
The curse of the non-linear behaviorThe curse of the non-linear behavior
Due to the nonlinear behavior of the temperature difference cross the
heat exchanger. An appropriate average temperature difference has to
be adopted

14. The lmtd definitionThe lmtd definition

15. The correction factor F for multi-passThe correction factor F for multi-pass
andand
cross-flowcross-flow
The standard lmtd formulation is limited toThe standard lmtd formulation is limited to
the simple cases of parallel and counterthe simple cases of parallel and counter
flow configurations.flow configurations.
In more complex cases as cross flow andIn more complex cases as cross flow and
multi-pass the correction factor F has tomulti-pass the correction factor F has to
be considered.be considered.

16. F factors for various flow configurationsF factors for various flow configurations

17. F factors for various flowF factors for various flow
configurationsconfigurations

18. SELECTION OF HEAT EXCHANGERS
Heat exchangers are complicated devices and the result
obtained with the simplified approach presented should
be used with care.
It is natural to tend to overdesign the heat exchanger in
order to avoid unpleasant surprises.
Engineers in industry often find themselves in a position
to select heat exchangers to accompany certain heat
transfer tasks.

19. Heat transfer enhancement in heat exchanger is usually
accompanied by increased pressure drop and thus higher
pumping power. Therefore any gain from enhancement should
be weighed at the cost of the accompanying pressure drop.
Some thought should be given to which fluid should pass
through the tube side and which through the shell side. Usually
the more viscous fluid is suitable for shell side (larger passage
area and thus lower pressure drop) and fluid with higher
pressure for the tube side.

20. THE PROPER SELECTION DEPENDS ON FOLLOWING
FACTORS:
•HEAT TRANSFER RATE
•SIZE AND WEIGHT
•COST
•PUMPING POWER
•MATERIAL

21. HEAT TRANSFER
The heat exchanger should be capable of transferring heat at
the specified rate in order to achieve the desired temperature
change of the fluid at the specified mass flow rate
SIZE AND WEIGHT
Normally the smaller and lighter heat exchanger is the better
one. This is especially in the case of automotive and aerospace
industries.
Larger heat exchangers carry higher price tag. The space
available for the heat exchanger in some cases limits the length
of the tube that can be used.

22. COST
Budgetary limitations plays an important role in the selection
oh heat exchangers, except where money in not so important.
An Off-the-shelf heat exchanger has a definite cost
advantage over those made to order as in the cases where
heat exchangers are integral part of the overall device to be
manufactured.
The operating and maintenance costs of the heat exchanger
are also important considerations in assessing the overall
cost
OPERATING COST = (PUMPING COST, kW) X (HOURS OF
OPERATIONS, hrs) X (PRICE OF
ELECTRICITY, Rs/kWh)

23. PUMPING POWERPUMPING POWER
In heat exchangers, both fluids are usually forced to flow by
pumps or fans that consume electrical power.
Pumping power is the total electrical power consumed by the
motors of the pumps and fans.
MATERIALS
A temperature difference of 50
o
C or more between the tubes
and the shell will probably pose differential thermal
expansion problems and need to be considered. In case of
corrosive fluids, we may have to select expensive corrosion
resistance materials such as stainless steel.