Modes of Heat Transfer Study Notes for Chemical Engineering

By BYJU'S Exam Prep

Updated on: September 25th, 2023

In the field of chemical engineering, the study of heat transfer is of utmost importance. Understanding the different modes of heat transfer is essential for designing efficient processes, optimizing energy usage, and ensuring the safety and reliability of various industrial systems. Heat transfer refers to the movement of thermal energy from one object or substance to another, resulting in a change in temperature.

The following study notes aim to provide chemical engineering students with a comprehensive understanding of the different modes of heat transfer which will be helpful for various competitive exams like GATE and ESE. The three primary modes of heat transfer are conduction, convection, and radiation. Each mode operates through distinct mechanisms and plays a vital role in different engineering applications.

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Modes of Heat Transfer

The heat transfer processes have been categorized into three basic modes: Conduction, Convection, and Radiation.


  • Conduction is the flow of heat in a substance due to the exchange of energy between molecules having more energy and molecules having less energy.
  • Molecules having more energy have a higher temperature
  • Molecules having less energy have lower temperature
  • In a solid structure, molecules undergo lattice vibration molecules with high temperatures will have high lattice vibration about their equilibrium position and vice-versa. However, the movement of free electrons in the metal is also responsible for conductive heat transfer.


  • It is the energy transfer due to random molecular motion along with the macroscopic motion of the fluid particles.
  • The macroscopic movement of the particles can be created in two ways.
  • When the motion is due to the temperature difference which creates the density difference, the heat transfer is by natural convection.
  • Whereas, if the macroscopic motion is forced by some mechanical device (agitator, blower, compressor, pump), the heat transfer takes place by forced convection. Convection


  • It is the energy emitted by matter which is at a finite temperature. All forms of matter at certain temperatures emit electromagnetic radiation in all directions.
  • The transfer of energy by conduction and convection requires the presence of a material medium as well as the temperature difference, whereas radiation does not require any material/medium or temperature difference.
  • In fact, radiation transfer is most efficient in a vacuum.
  • Characteristics of thermal radiation
    • Radiation increases with temperature.
    • No medium or material is required for energy transfer by radiation

Difference Between Thermodynamics and Heat Transfer

  • Thermodynamics is mainly concerned with the conversion of heat energy into other useful forms of energy and is based on
    • The concept of thermal equilibrium (Zeroth Law)
    • The First Law (the principle of conservation of energy)
    • The Second Law (the direction in which a particular process can take place)
  • Thermodynamics is silent about the heat energy exchange mechanism.
  • The transfer of heat energy between systems can only take place whenever there is a temperature gradient.
  • Heat transfer is basically a non-equilibrium phenomenon.
  • The Science of heat transfer tells us the rate at which the heat energy can be transferred when there is a thermal non-equilibrium i.e. the science of heat transfer seeks to do what thermodynamics is inherently unable to do.

Basic Laws of Heat Transfer

Conduction (Fourier’s Law)

  • The transfer of heat energy by conduction takes place within the boundaries of a system, or across the boundary of the system into another system placed in direct physical contact with the first, without any appreciable displacement of matter comprising the system, or by the exchange of kinetic energy of motion of the molecules by direct communication, or by drift of electrons in the case of heat conduction in metals.
  • The rate equation which describes this mechanism is given by Fourier’s Law



  • Q = rate of heat flow in X-direction by conduction in J/s or W,
  • k = thermal conductivity of the material. It quantitatively measures the heat conducting ability and is a physical property of the material that depends upon the composition of the material, W/m-K,
  • A = cross-sectional area normal to the direction of heat flow, m2,
  • dT/dx = temperature gradient at the section.

The negative sign is included to make the heat transfer rate Q positive in the direction of heat flow (heat flows in the direction of decreasing temperature gradient (dT/dx = – ve)Conduction


  • The basic law of heat convection is Newton’s law of cooling.
  • This law states that the heat flux by convection is directly proportional to the temperature difference between the solid surface and the fluid

(Q/A) α ΔT


  • Let us consider a solid surface of uniform temperature Tw. A fluid over the solid surface is having a bulk temperature of Tf

(Q/A) = q = hA(Tw-Tf)

  • If Tw > Tf, heat flows from the wall to the fluid Tf> Tw, heat flows from the fluid to the wall

Where, h = heat transfer coefficient for film coefficient ( W/m2K)

  • The heat transfer coefficient depends on the shape of the surface, the roughness of the solid surface, the velocity of the fluid flowing over the surface, the state of the fluid, surface purity, etc.

Radiation(Stefan – Boltzmann law)

  • This law states that the thermal radiation emitted from a body is directly proportional to the fourth power of absolute temperature i.e

(Q/A) α T4

(Q/A) = q σT4

where, σ = Stefan Boltzmann constant = 5.67 × 10-3 W/m2K4.

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