To compute the incident heat flux on a surface it is necessary to sum the kinetic energy of the incident molecules. On average each molecule carries kinetic energy, qm, given by:

Equation 3-15 can be used to shown that:

The total kinetic energy arriving at the surface is calculated by multiplying the total flux arriving from a given direction, multiplying this quantity by Equation 3-22, and then integrating over all directions. From Equation 3-17 the appropriate integral is given by:

This is the heat flux that flows into the solid walls of the vacuum chamber.