The Frost number is a dimensionless parameter used in heat transfer analysis to determine whether or not frost will form on a surface. It is defined as the ratio of the rate of heat transfer through a solid surface to the heat of fusion of the condensing substance (usually water vapor). The Frost number is given by the formula:
Frost number = (h * L) / ΔH
Where:
- h is the convective heat transfer coefficient between the surface and the surrounding air
- L is the characteristic length of the surface
- ΔH is the heat of fusion of the condensing substance (e.g., water vapor)
The Frost number is a measure of the relative importance of heat transfer by conduction through the solid surface compared to the heat released during condensation. If the Frost number is below a certain threshold, frost is likely to form on the surface.
The Frost number can be lower than room temperature because it represents the balance between the heat transfer through the solid surface and the heat released during condensation. Even if the surface temperature is above room temperature, if the convective heat transfer coefficient (h) is high or the heat of fusion (ΔH) is low, the Frost number can still be low enough for frost formation.
Additionally, it's important to note that the Frost number is not an absolute temperature measurement but a dimensionless parameter that quantifies the likelihood of frost formation. It takes into account the heat transfer characteristics of the system rather than relying solely on the surface temperature.