As a chemical engineer working in industries, I often come across equipment where heat transfer is happening continuously, whether it’s in heat exchangers, reactors, furnaces, or cooling systems.
There are three modes of heat transfer: conduction, convection, and radiation. In this article, we will learn the types of heat transfer with examples and laws.
Before we get into our topic about types of heat transfer, let’s have a brief introduction about heat transfer.
Table of Contents
What is heat transfer?
Heat transfer is a form of energy transfer in which heat is transferred due to the temperature difference of two substances or objects. It occurs in the direction of decreasing temperature, i.e. from high to low. Heat is being transferred by three methods: conduction, convection, and radiation.
Now that we have learned the definition in brief, let’s examine the types of heat transfer in detail.
What are the different Modes of heat transfer
Modes of heat transfer, or in other words, types of heat transfer, are listed below
- Conduction
- Convection
- Radiation
Now, let us understand all the modes of heat transfer in detail with examples
1. Conduction
When two bodies are in physical contact with each other, heat conduction occurs from the hotter body to the colder body. This transfer of heat is known as conduction heat transfer. It occurs due to the collision and vibration of molecules within the solid.
Example
When a metal spoon is placed in hot soup, the part that is immersed gains heat, and its temperature rises. Due to the temperature difference between the hot end (in the soup) and the cooler handle (open to air), heat flows along the length of the spoon.
This transfer occurs through conduction, where thermal energy moves from high to low temperature via molecular interactions and free electron movement in the metal.
Fourier laws of heat transfer
Where:
- Where:
= Total heat transferred (Joules)
= Rate of heat transfer (Watts or J/s)
= Thermal conductivity of the material (W/m·K)
= Area of heat transfer surface (m2)
= Temperature on the hot side (°C or K)
= Temperature on the cold side (°C or K)
= Time duration for heat transfer (seconds)
= Thickness or distance between hot and cold surfaces (m)
= Rate of heat transfer (Watts or J/s)
= Thermal conductivity of the material (W/m·K)
= Time duration for heat transfer (seconds)
= Thickness or distance between hot and cold surfaces (m)2. Convection
In convection, heat is transferred through the movement of a fluid in liquids and gases. It happens when faster-moving, hotter particles collide with slower-moving, colder particles and pass on their energy.
Example
When water is heated in a pot. As the base of the pot is heated by a flame, the water molecules in contact with the hot surface gain energy and become less dense. This warmer, lighter water rises upward, while the cooler, denser water at the top sinks to the bottom to replace it. This continuous cycle creates what’s known as a convection current.
This process is efficient because it combines both heat conduction (from the pot to the water molecules in contact with it) and fluid motion. The overall heat transfer depends on factors like the surface area of contact, the temperature difference between the surface and fluid, and the nature of the fluid itself. This relationship is described by Newton’s Law of Cooling,
Newton’s Law of Cooling
Q = 
Where,
= Rate of heat transfer (Watts, W)
= Convective heat transfer coefficient (W/m2 \cdot K)
= Surface area through which heat is transferred (m2)
= Temperature of the hotter surface (°C or K)
= Temperature of the cooler fluid or surface (°C or K)
3. Radiation
In this mode, heat is transferred from a hot body to a cold body by using electromagnetic waves without relying on a medium. It doesn’t need air or any other medium.
Example
The heat you feel from the Sun. Even though space is a vacuum (with no air or particles), the Sun’s heat travels millions of kilometres and reaches Earth. This is possible because the Sun emits energy in the form of infrared radiation, a type of electromagnetic wave.
When these waves hit our skin or any object on Earth, they are absorbed and converted into heat. That’s why you feel warm standing in sunlight, even if the air around you is cold. Radiation doesn’t need solids, liquids, or gases to transfer heat it happens purely through wave propagation.
Stefan–Boltzmann law
The Stefan–Boltzmann law states that the total thermal radiation emitted per unit surface area of an ideal blackbody is directly proportional to the fourth power of its absolute temperature. In equation form:
Where:
= Radiative heat transfer (W)
= Stefan–Boltzmann constant (
)- = Surface area (m^2)
, 
, 
= Absolute temperatures (K)
= Radiative heat transfer (W)
= Stefan–Boltzmann constant (
)
, 
, 
= Absolute temperatures (K)According to the Stefan–Boltzmann Law, the emission from a single surface is given by
E = 
Real surfaces don’t radiate as much heat as a perfect blackbody.
- That’s because they have an emissivity value ε between 0 and 1.
Emissivity shows how efficiently a surface gives off heat as radiation.
ε < 1: Real surface (e.g., metal, glass, painted surfaces)
ε = 1: Ideal blackbody
FAQ (Modes of heat transfer)
1. What are conduction, convection, and radiation?
These are the types of heat transfer, in other words, modes of heat transfer.
2. What is Fourier’s law of conduction?
Fourier’s Law states that the rate of heat transfer through a solid is proportional to the negative temperature gradient and the area perpendicular to the flow
3. What is the unit of the Fourier’s law?
For heat transfer rate ( q ): Watt (W)
For heat flux ( qn ): Watt per square meter (W/m2)
4. What is the law of convection?
Newton’s Law of Cooling states that the rate of heat transfer between a surface and a fluid is proportional to the temperature difference between them.
5. What is the radiation heat transfer?
Radiation heat transfer is the process of heat transfer through electromagnetic waves. It doesn’t need a medium
6. What is the first law of radiation?
Stefan–Boltzmann law is the first law of radiation.
7. What are the Laws of heat transfer
Fourier’s law: Governs conduction
Newton’s law of cooling: Governs convection
Stefan Boltzmann’s law: Governs radiation
additional laws are
Plank’s law: The law of heat transfer for radiation
Wien’s displacement law: The law of heat transfer for radiation
8. What is k in Fourier’s law?
k is thermal conductivity (W/m·K), which describes how readily heat moves through a material: it’s the amount of heat transferred across a given thickness and area for each degree of temperature difference.
Wrapping Up
These are the three modes of heat transfer. If you have any doubts regarding this topic, feel free to use the comment section or in case you want us to cover any specific topic, reach out to us. Till then, check out our other articles regarding industrial safety and chemical engineering.
