Solids, liquids and gases, represent three different states (a.k.a. phases). Solids are almost totally incompressible and have a definite shape, whilst liquids are nearly incompressible, and gases are compressible. For practical purposes, solids and liquids compress by so little that they are often assumed to be incompressible. Water can exist as a solid (ice), liquid (water), or gas (steam).

Heat is defined as ‘thermal energy transferred between two systems that are in direct contact with each other, but at different temperatures.’ If heat is transferred to a substance, the substance will change temperature, or change state.

Change of State Example

A large amount of heat is transferred to a block of ice, which causes it to melt and become water.

Change of Temperature Example

A small amount of heat is transferred to some water, its temperature increases, but its state does not change.

The important difference in both examples is that energy can be transferred to a substance to change the temperature of the substance, or change the state of the substance. The differences between the two energy forms is clarified in the next section.


Sensible and Latent Heat

The term heat (a.k.a. heat energy) describes two types of heat, sensible heat and latent heat.


Sensible Heat can be measured using a thermometer and sensed by a human (‘sense-ible heat’). A change in sensible heat is not accompanied by a change in state.

Latent Heat can be identified by a substance’s change in state/phase, but not by a change in temperature. Evaporation and condensation are forms of latent heat.


The below graph shows that heat energy added does not always cause a temperature change. Sensible heat is seen whenever heat is added and the temperature changes proportionately (sloped lines on the graph). Latent heat is seen whenever heat is added and no change of temperature occurs (horizontal lines on the graph).

Sensible and Latent Heat Diagram

Sensible and Latent Heat Diagram

Adding sensible heat to water will gradually increase its temperature, although it will only boil once it reaches its saturation temperature (boiling point). At saturation temperature, no more sensible heat can be added, but more heat can be added in the form of latent heat.

The additional latent heat causes the water to evaporate and change state to a gas. The resultant steam contains both the sensible and latent heat energy that was transferred into it, however, changing water into gas requires far more energy than simply heating the water, thus the steam contains much more latent heat than sensible heat. The total heat contained by the steam is very important, as this represents the amount of energy that can later be used by the end consumer.


A liquid at saturation temperature (temperature at which it boils), or saturation pressure (pressure at which it boils), is literally saturated with sensible heat i.e. it cannot hold any more heat without beginning to boil.


Transferring Heat

As stated in the second law of thermodynamics, heat energy (a.k.a. heat) is transferred from hot to cold. The temperature difference between two substances dictates the heat transfer rate.


Substances with a high temperature difference will have a higher heat transfer rate than substances with a low temperature difference.


Heat is transferred via conduction, convection or radiation. In most industrial settings, heat is transferred via a mixture of one or more of these heat transfer means, rarely by a single mean.

  • Conduction – heat is transferred directly from one molecule to another. Conduction occurs in solids, liquids and gases.
  • Convection – heat is transferred by molecules in a fluid state. Convection may be forced (using a pump or fan), or, natural, due to temperature and density differences in the fluid. The two types of convection are known as forced convection and natural convection.
  • Radiation – heat is transferred via radiant energy (electromagnetic waves). Radiant energy is only transferred to opaque objects i.e. objects that do not allow light to pass through.

Conduction, Convection and Radiation

Conduction, Convection and Radiation



Conduction occurs in solids, liquids and gases. A substance’s ability to absorb heat via conduction is referred to as its thermal conductivity. Generally, solids have higher thermal conductivity than liquids because the molecules are closer together. Likewise, liquids generally have higher thermal conductivity than gases. Air has low thermal conductivity, which is why insulating materials often have large air spaces/pockets.

Note that thermal conductivity is based upon the material, not its state/phase. For example, many types of wood have a lower thermal conductivity rating than water, but wood is a solid and water is a liquid. 

Conduction Example

If one end of a metal rod is heated in a fire, whilst the other end is not, the heated metal rod molecules will pass their heat to their neighbouring -cooler- molecules. The process continues allowing heat to be conducted from the hot end of the rod to the cold end.

In the case of boilers, conduction occurs when heat is transferred from the internal heating surfaces of the boiler to the external heating surfaces.



Convection may be forced or natural. Forced convection requires a pump or fan etc. Natural convection occurs due to temperature differences in the fluid (hot molecules are less dense than cold molecules, which is why they have a natural tendency to rise above them). 

Convection Example

Heating water in a pot causes the heated molecules to become less dense and thus rise to the top of the pot due to natural convection; cooler more dense molecules then occupy the space where the heated molecules were.

The stack effect (a.k.a. chimney effect) is an example of natural convection. The below image shows a natural draft cooling tower. The cooling tower allows cooler air to enter through the base, where it is then heated by hot water. The hot -less dense- air then rises to the top of the tower, which causes cooler air to be drawn in through the base of the tower. In this manner, its possible to use very large volumes of air to cool a process without using pumps or fans.

Hot (less dense) Air Rises and is Replaced by Cooler (more dense) Air

Hot (less dense) Air Rises and is Replaced by Cooler (more dense) Air

In the case of boilers, convection occurs when the water closest to the internal heating surfaces is heated and becomes less dense. The less dense water flows upwards and is replaced by cooler, more dense water; the process is continuous.



Radiant energy is only transferred to heat energy when electromagnetic waves impact upon a substance that does not allow light to pass through it i.e. the substance is opaque. For example, the sun transmits radiant energy, but this energy only becomes heat energy once it encounters an opaque object i.e. the earth.

The amount of energy a substance can transfer via radiation depends largely upon its temperature and emissivity. A surface’s ability to emit thermal radiation is measured by its emissivity. Generally, rough and dark surfaces have a higher emissivity coefficient than smooth and shiny surfaces.

In the case of boilers, radiant energy is released during combustion and is transmitted onto the opaque internal heating surfaces of the boiler, where it changes to thermal energy. Note that radiant energy is only transferred by line of sight.

Radiation Example

If a boiler operator opens the boiler furnace door, he/she will feel the heat because the electromagnetic waves travel outwards from the furnace to the operator. If the door is then closed, the electromagnetic waves can no longer travel outwards and the heat is no longer felt. Air between the furnace and operator is not heated by the electromagnetic waves because air allows light to travel through it (its not opaque). Only opaque substances with a direct line of sight to the place of combustion will be impacted by the emitted electromagnetic waves.

Heat Felt Due to Radiation

Heat Felt Due to Radiation


Heat Units

Heat is a form of energy and is often stated in British thermal units (Btus) or Calories (Cal).

  • Btu – the amount of heat required to increase the temperature of one pound of water by one degree Fahrenheit.
  • Calorie – the amount of heat required to increase the temperature of one gram of water by one degree Celsius.

In addition to the units of Btu and Calorie, the unit of Joule is often used.


Specific Heat

Different substances require different amounts of heat to change temperature. In order to make a comparison between substances possible, substances are given a specific heat (S.H.) value, these values are then compiled into tables. The values in the compiled tables are approximations only, the true specific heat value of a fuel is determined by sampling and analysing the fuel in a laboratory.

Specific heat is the amount of heat required to change one unit of a substance’s mass by one unit in temperature. The units used depend upon if imperial or metric units are favoured. The energy units of British thermal units (Btu) and Joules (J) are often used in specific heat calculations.


The term specific heat capacity has the exact same meaning as specific heat, the terms are used interchangeably.


Example 1 (Imperial)

Specific heat may be calculated as the amount of heat required to change one pound of a material by one degree Fahrenheit. Imperial specific heat capacity units are Btu/lb°F.

Example 2 (Metric)

Specific heat may be calculated as the amount of heat energy required to change one kilogram of a material by one degree Kelvin. Metric specific heat capacity units are J/kgK. Note that one Celsius is equivalent to one Kelvin in magnitude, so the unit J/kg°C yields the same specific heat capacity values as when quoted in J/kgK.

The specific heat values of various substances are given in the below table.



Imperial Specific Heat (S.H.) Value


Metric Specific Heat (S.H.) Value


Fresh Water









Fire Brick



Boiler Scale









Specific Heat Values of Various Substances


Additional Resources