Heat transfer in humid climates 2

[Tunalover]

Folks-
Does humid air at temperature T carry away more heat by convection than dry air at temperature T? Why?


Tunalover

 

[25362]

I'd say that when considering:

1. The small mass % of moisture in air,
2. The "cancelling" effect of the differing thermophysical properties of dry air and water vapor at "moderate" (300K-600K) temperatures and atmospheric pressure,
3. And assuming no mass transfers with their attached latent heat of vaporization or condensation as by humidification and dehumidification,

In ceteris paribus, dry and humid air should behave equally on heat transfer by convection.

Besides, in the case of simultaneous heat and mass transfer, the overall HT coefficient increases when both of these transfers move in the same direction as in air dehumidification with cooling, or when heating air with humidification. It decreases when these tranfers are in opposite directions as with air humidification with cooling.

 

[tkordyban]

25362 is right. Even at 100% relative humidity, the amount of water vapor in air is very small, and has only a tiny effect on the material properties of air.

But assuming you still care about that small effect, water vapor is LESS DENSE than dry air, so humid air is less dense than dry air. Convective heat transfer coefficients are proportional to the density, so heat transfer actually goes down a little bit in humid air

This seems the opposite of our intuition. In high humidity, the air feels "heavy" to us, because we can't lose heat by sweating. But to an object that doesn't sweat, the humidity doesn't matter very much.

 

[25362]

In natural (free) convection it appears the HTC for dry air is a bit better than for moist air. In forced convection the opposite appears to be true. Anyway, the margin of error is large enough to neglect these small "theoretical" differences.