The basic purpose of developing an excel model is defeated & it can not give the flexibility & options you need for identifying all possible scenarios. Speed of work is also slow....What to do?
You need to develop some correlation for that relevant property so that a formula can be used which can give you all results at just a click of mouse.
Here is one such property i.e. thermal conductivity of air vs its correlation with temperature. The equation is...
k = 1.5207 x 10^(-11) x T^3 - 4.8574 x 10^(-8) x T^2 + 1.0184 x 10^(-4) x T - 0.00039333
Where k is in W/m/K
and T is in K
The range of use is T = 100 to 1600 K
Now you can use it in any calculation for example if you designing any air heater, you can use it to find out Nusselt no of air side & accordingly your U will vary in the Excel sheet automatically. Thus you will get better results.