Technical Info - Heat Transfer Equations

Heat Transfer Equations

Thermal Resistance

R=L/KA

Where;

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 [Also known as Newton's Law of Cooling]

t=mC_p(T_s-T_f)/Q

Where;

Q is the heat added (or being removed) from the object in watts
m is the mass (weight) of the object in Kg
C_p 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
T_s is the starting temperature in C
T_f is the final temperature in C

Example

Heat Loss (or gain) - through the walls of an insulated container

Q=A((T_O-T_I)/(L/K + 1/h))

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 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
T_O is the Outside temperature in C
T_I 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 10 to 100.

Example

Heat Loss (or gain) - by Convection

Q=hA(T_s-T_a)

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).
T_s is the surface temperature in C
T_a 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 (T_a) is higher than the surface temperature (T_s).

Example