Heat Transfer Equations

Thermal Resistance

Where;

R is thermal resistance in C/watt
L is the length in meters (m)
K is the Thermal Conductivity of the material in watts/meter C
A is the cross sectional area in meters squared (m2).

Example

Time Required to Cool (or heat) an Object [or, Q required to cool or heat an object within a specific time]

Where;

Q is the heat added (or being removed) from the object in watts
m is the mass (weight) of the object in Kg
Cp is the Specific heat of the object material in J/Kg C
t is the time required to cool down (or heat up) the object in seconds
Ts is the starting temperature in C
Tf is the final temperature in C

Example

Heat Gain (or loss) - through the walls of an insulated container

Where;

Q is the heat lost or gained in watts
L is the thickness of insulation in meters (m)
K is the Thermal Conductivity of the insulation material in watts/meter K (or C)
A is the outside surface area of the container in meters squared (m2).
h is the Heat Transfer Coefficient of the surface material in watts/meter2 C
Ta is the Outside ambient air temperature in C
TI is the Inside temperature in C

Natural or Free Convection is essentially still to slightly stirred air with h values ranging from 1 to 25.
Forced Convection is air moved by a fan or other active method. h values range from 25 to 100.

Excel Calculator Worksheet

Heat Loss (or gain) - by Convection

Where;

Q is the heat lost (or gained) in watts
h is the Heat Transfer Coefficient of the surface material in watts/meter2 C
A is the exposed surface area in meters squared (m2).
Ts is the surface temperature in C
Ta is the ambient air temperature in C

Natural or Free Convection is essentially still to slightly stirred air with h values ranging from 1 to 25.
Forced Convection is air moved by a fan or other active method. h values range from 10 to 100.

Heat is "gained" or added to a surface when the ambient air (Ta) is higher than the surface temperature (Ts).

Example