Fundamentals of heat exchangers, types of heat exchangers and certain formulas

1. By

2. WHAT IS HX……???
 Heat exchangers are equipment that transfer
heat from one medium to another.
E.g.
 From hot water to cold water,
 From hot steam to cold water,
 From hot gas to cold water,
 From hot water or unsaturated steam to cool air.

3. WHAT IS HX…..???
 A heat exchanger is a component that allows the transfer of
heat from one fluid (liquid or gas) to another fluid.
Reasons for heat transfer include the following:
1. To heat a cooler fluid by means of a hotter fluid
2. To reduce the temperature of a hot fluid by means of a cooler
fluid
3. To boil a liquid by means of a hotter fluid

4. WHAT IS HX….???
4. To condense a gaseous fluid by means of a cooler fluid
5. To boil a liquid while condensing a hotter gaseous fluid
Regardless of the function the heat exchanger fulfils, in order
to transfer heat the fluids involved must be at different
temperatures and they must come into thermal contact.
 Heat can flow only from the hotter to the cooler fluid.

5. TYPES
 Although heat exchangers come in every shape
and size imaginable, the construction of most
heat exchangers fall into one of two categories:
 tube and shell,
 or plate.

6. TUBE AND SHELL
 The most basic and the most common type of heat
exchanger construction is the tube and shell, as shown in
Figure.
 This type of heat exchanger consists of a set of tubes in a
container called a shell. The fluid flowing inside the
tubes is called the tube side fluid and the fluid flowing on
the outside of the tubes is the shell side fluid.
 At the ends of the tubes, the tube side fluid is separated
from the shell side fluid by the tube sheet(s). The tubes
are rolled and press-fitted or welded into the tube sheet
to provide a leak tight seal.

7. Shell & Tube HX

8. TUBE AND SHELL
 In systems where the two fluids are at vastly different
pressures, the higher pressure fluid is typically directed through
the tubes and the lower pressure fluid is circulated on the shell
side.
 This is due to economy, because the heat exchanger tubes can
be made to withstand higher pressures than the shell of the heat
exchanger for a much lower cost.
 The support plates shown on Figure also act as baffles to direct
the flow of fluid within the shell back and forth across the
tubes.

9. PLATE HX….?
 A plate type heat exchanger, as illustrated in Figure 2, consists of
plates instead of tubes to separate the hot and cold fluids.
 The hot and cold fluids alternate between each of the plates. Baffles
direct the flow of fluid between plates.
 Because each of the plates has a very large surface area, the
plates provide each of the fluids with an extremely large
heat transfer area.
 Therefore a plate type heat exchanger, as compared to a similar
ly sized tube and shell heat exchanger, is capable of transferring
much more heat.
 This is due to the larger area the plates provide over tubes.

10. Plate Type Heat Exchanger

11. FLOW OF ARRANGEMENT
1. Parallel Flow
2. Counter Flow
3. Cross Flow
 In parallel-flow heat exchangers, the two fluids enter the
exchanger at the same end, and travel in parallel to one
another to the other side.
 In counter-flow heat exchangers the fluids enter the
exchanger from opposite ends. The counter current design
is most efficient, in that it can transfer the most heat.

12. Parallel Flow, Counter
Flow, Cross Flow Types
of HX

13. FLOW OF ARRANGEMENT
 Ina cross-flow heat exchanger, the fluids travel
roughly perpendicular to one another through the
exchanger.
 The exchanger's performance can also be
affected by the addition of fins or corrugations in
one or both directions, which increase surface
area and may channel fluid flow or induce
turbulence.

14. REGENERATIVE HEAT EXCHANGER
 A third type of heat exchanger is the regenerative heat
exchanger.
 In this, the heat from a process is used to warm the fluids to be
used in the process, and the same type of fluid is used either
side of the heat exchanger (these heat exchangers can be either
plate-and-frame or shell-and-tube construction).
 These exchangers are used only for gases and not for liquids.
The major factor for this is the heat capacity of the heat transfer
matrix.

15. Regenerative Heat Exchanger
Recuperator type Heat
Exchanger

16. RECUPERATORS
 A fourth type of heat exchanger uses an intermediate fluid or
solid store to hold heat, which is then moved to the other side
of the heat exchanger to be released.
 Two examples of this are adiabatic wheels, which consist of a
large wheel with fine threads rotating through the hot and cold
fluids, and fluid heat exchangers.
 This type is used when it is acceptable for a small amount of
mixing to occur between the two streams.

17. LMTD
 The log mean temperature difference (LMTD)
is used to determine the temperature driving
force for heat transfer in flow systems (most
notably in heat exchangers).
 The LMTD is a logarithmic average of the
temperature difference between the hot and cold
streams at each end of the exchanger.

18.  For Counter current flow (i.e. where the hot stream, liquid or gas,
goes from say left to right, and the cold stream, again liquid or gas
goes from right to left), is given by the following equation:
 And for Parallel flow (i.e. where the hot stream, liquid or gas, goes
from say left to right, and so does the cold stream), is given by the
following equation:
 T1 = Hot Stream Inlet Temp.
 T2 = Hot Stream Outlet Temp.
 t1 = Cold Stream Inlet Temp.
 t2 = Cold Stream Outlet Temp.

19. NTU METHOD
 The Number of Transfer Units (NTU) Method is used to calculate
the rate of heat transfer in heat exchangers (especially counter
current exchangers) when there is insufficient information to
calculate the Log-Mean Temperature Difference (LMTD).
 The method proceeds by calculating the heat capacity rates (i.e. flow
rate multiplied by specific heat) Ch and Cc for the hot and cold fluids
respectively, and denoting the smaller one as Cmin.
 value of qmax is the maximum heat which could be transferred
between the fluids.

20. EFFECTIVENESS OF HX
E is then defined in terms of that maximum:
E can be calculated using correlations in terms of the
'heat capacity ratio‘
 and the number of transfer units, NTU

21. PURPOSE OF THE PERFORMANCE TEST
 Todetermine the overall heat transfer coefficient
for assessing the performance of the heat
exchanger.
 Any deviation from the design heat transfer
coefficient will indicate occurrence of fouling.

22. CALCULATIONS
 Step A - monitoring and reading the steady state
parameters like temperature and pressure (inlet , outlet,
hot and cold)
 Step B – with monitored data the physical properties of
stream is determined like
1. Density
2. Viscosity
3. Specific heat etc.

23. CALCULATIONS
 Step C - The thermal parameters are calculated
and tabulated like the temperature and pressure
range.

24. CALCULATIONS
 Step D – finally all the thermal parametrs are
evaluated like,
1. Capacity
2. Effectiveness
3. NTU
4. LMTD
5. U = overall heat transfer by the use of various
heat transfer formulaes.