# Heat Transfer

**Thermal Resistance**

blurb

**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 (m^{2}).

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

The following image graphically shows Heating and Cooling as Q (heat) being added or removed from an object.

The following formula is shown in two different ways. The first solves for time t and the second solves for 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**is the starting temperature in C.

_{s}**T**is the final temperature in C.

_{f}

**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 (m^{2}).**h** is the Heat Transfer Coefficient of the surface material in watts/meter^{2} C**T _{a}** is the Outside ambient air temperature in C

**T**is the Inside temperature in C

_{I}*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.

**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/meter^{2} C**A** is the exposed surface area in meters squared (m^{2}).**T _{s}** is the surface temperature in C

**T**is the ambient air temperature in C

_{a}

*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}