Condensation formation on Underside of roof

[AlexLudgate]

Hello,

I am attempting to calculate the amount of condensation formation on the underside of a roof (steel sheet). moist air is on the underside of the roof, ambient air on the upper side

I know the following properties;

RH of moist air
Temperature of moist air, T1
Temperature of outside air, T2

I work out the heat transfer through unit area of the roof, Q=U(T1-T2)

I assume that all heat lost though the roof causes firstly enthalpy reduction of moist air to a point where it reaches 100% RH and past this point condensation of the moisture.

The amount of condensation formed is heat loss (past reaching 100% RH) (kW/m^2) divided by latent heat of vaporisation (kJ/kg), giving me moisture formed per unit area per second (kg/m^2.s)

also, condensate will only form if inside roof temperature is below that of moist air temp T1.

This assumes a lot of things - am i going wrong already?

My problem is in obtaining a good approximation of the U value for heat transfer through the roof.

I know that the basic formula for calculating the U value, which includes convection heat transfer through the film of air on the underside of the roof, conduction through the roof, and convection through the air film on the outside of the roof.

Should I also account for radiation? (average temps are T1 = 70C, T2 = 10C)

In working out the U value, I need heat transfer coefficents for the air films inside and outside.
I want to include forced convection due to air flow.

If there is a significant airflow of moist air under the roof (exhausting through windows in walls) then will this effect condensation formation?
Is there a relationship bewteen airflow and condensation? I know that With evaporation, an airflow will increase the rate of evaporation.

There is a lot of information here, some vague, and a lot of questions - feel free to pick up any of the points I have raised, I will be checking the forum regularly to add more details as you request it!
Thankyou for reading

 

[Navin A S]

.The first step to solving your problem is to calculate the U-value for the roof. To do this, you will need to consider the heat transfer through convection, conduction, and radiation. The convection and conduction will be from both the air films on the underside and outside of the roof. The radiation will depend on the temperatures of the moist air and outside air, which you have already noted. You will also need to consider the forced convection due to the airflow of the moist air under the roof. The relationship between the airflow and condensation formation is that the airflow will increase the rate of condensation formation, but only up to a certain point. Beyond that point, the rate of condensation formation will decrease.