What Is Heat Transfer

[caption id="attachment_198" align="alignright" width="300" caption="what is heat transfer"][/caption]

Heat transfer is considered as one of the most basic discipline of chemical engineering & technology. It generally concerns with generation, consumption and conversion of heat energy in the system. Heat transfer; it self plays a very important role in process industries, and it is always better for the process industries, to optimize their heat transfer processes [may include furnaces, evaporators, distillation units, dryers, reaction vessels etc] by the selection of proper heat exchangers, preventing heat losses and controlling the heat flow rate.

Heat transfer under normal conditions always flows from a hotter region to a cooler region, i.e. heat transfer follows the temperature gradient between the two systems [or system and surrounding], until a thermal equilibrium is maintained between the two systems.  However, according to the “Clausius statement of second law of thermodynamics”, if the work is done on the system, the heat can flow from a colder region to a hotter region. This Clausius statement is the basic principle behind the working of refrigerators.

Now how to calculate heat transfer :

\[Q=UAΔT\]

Where,

Q = transfer of heat per unit time
A = heat transfer area
ΔT = temperature difference between two systems

This is the most basic expression to define heat transfer, there are several other heat transfer formulas derived from this expression. Picture at the right side is taken from images by dan.

There are three forms of heat transfer:

1) Conduction

Conduction is generally considered as the heat transfer phenomena for the solids, however conduction can also occur in fluids too [in microscopic level, like diffusion phenomena]. In solids, Conduction is the result of transfer of vibration energy from one molecule to other, while in fluids, it occurs in addition as a result of transfer of kinetic energy. Conduction follows the Fourier Law of Heat Conduction:

\[Q=-kAΔT\]

Where,

Q = transfer of heat per unit time
k = conductive heat transfer coefficient
A  = heat transfer area
ΔT = temperature difference between two systems

  The thermal conductivity units in SI system is W/mK.

2) Convection

Convective heat transfer occurs when the heat is transferred from a solid surface to a moving fluid owing to the temperature difference between the solid and the fluid. Convection follows the Newton’s Cooling Law of Heat Convection:

\[Q=hA(T-T_{'})\]

Where,

Q = transfer of heat per unit time
h = convection heat transfer coefficient
A  = heat transfer area
T = temperature of fluid
T'= temperature of solid

3) Radiation

All materials radiate thermal energy in the form of electromagnetic waves. So radiation is the transfer of heat by the emission of electromagnetic waves. When they fall on the body, they may partially be reflected, transmitted or absorbed. Radiation is that fraction which falls and absorbed by the body.

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