A heat exchanger transfers heat from one medium to another without allowing the two media to mix. Heat naturally flows from higher to lower temperatures, so a heat exchanger brings two fluids of different temperatures into close contact while preventing mixing. Common applications include heating, cooling, and industrial processes. Key factors that impact heat transfer include material properties, surface area, flow rates, and temperature differences of the fluids. Heat exchangers are classified based on flow patterns, such as parallel, counter, and cross flow designs. The rate of heat transfer depends on the overall heat transfer coefficient, surface area, and log mean temperature difference between the fluids.

1. Heat Exchanger

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8. HEAT EXCHANGER
A heat exchanger is a piece of equipment built for
efficient from one heat transfer medium to another. The media may be
separated by a solid wall, so that they never mix, or they may be in direct
contact.
They are widely used in space heating, refrigeration, air conditioning,
power plants, chemical plants, petrochemical plants, petroleum refineries,
natural gas processing, and sewage treatment.
The classic example of a heat exchanger is found in an internal combustion
engine in which a circulating fluid known as engine coolant flows through
radiator coils and air flows past the coils, which cools the coolant and heats
the incoming air.

9. Principle Of Heat Exchanger
Heat exchangers work because heat
naturally flows from higher temperature to lower temperatures.
Therefore if a hot fluid and a cold fluid are separated by a heat
conducting surface heat can be transferred from the hot fluid to the
cold fluid.
Two fluids of different temperatures are brought into close contact but
are prevented from mixing by a physical barrier. The temperature of
the two fluids will tend to equalize. By arranging counter-current flow
it is possible for the temperature at the outlet of each fluid to approach
the temperature at the inlet of the other. The heat contents are simply
exchanged from one fluid to the other and vice versa. No energy is
added or removed

10. Heat transfer depends upon following factors:
•Type of the material between fluids
•Thickness of material
•Surface Area of material
•Type of fluid
•Gravity of fluid
•Flow rate of fluid

11. Classification of Heat Exchangers:
Types w.r.t Flow:
There are two primary classifications of heat exchangers according to their flow arrangement.
•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.

12. •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
from the heat (transfer) medium.
.Cross-flow heat exchangers:
In a cross-flow heat exchanger, the fluids travel
roughly perpendicular to one another through
the

13. Basic Equation defining the Heat Exchanger Principle:
Heat exchanger theory leads to the basic heat exchanger design equation:
Q = U AΔTlm,
where
•Q is the rate of heat transfer between the two fluids in the heat exchanger in
But/hr,
•U is the overall heat transfer coefficient in Btu/hr-ft2-oF,
•A is the heat transfer surface area in ft2,
ΔTlm is the log mean temperature difference in oF,
calculated from the inlet and outlet temperatures of
both fluids.

14. Log mean temperature difference (LMDT):
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. The larger the
LMTD, the more heat is transferred. The use of the LMTD arises straightforwardly from the analysis of
a heat exchanger with constant flow rate and fluid thermal properties.
We assume that a generic heat exchanger has two ends (which we call "A" and "B") at which the hot
and cold streams enter or exit on either side; then, the LMTD is defined by the logarithmic mean as
follows

15. where ΔTA is the temperature difference between the two streams at end A, and
ΔTB is the temperature difference beween the two streams at end B.
This equation is valid both for parallel flow, where the streams enter from the
same end, and for counter-current flow, where they enter from different ends.
Once calculated, the LMTD is usually applied to calculate the heat transfer in
an exchanger according to the simple equation:
Where Q = Heat transfer

16. Once calculated, the LMTD is usually applied to calculate the heat transfer in an
exchanger according to the simple equation
Where Q is the exchanged heat duty (in watts), U is the heat transfer coefficient (in
watts per kelvin per square meter) and A is the exchange area. Note that estimating
the heat transfer coefficient may be quite complicated.