I am looking for a formula to calculate specific temperature drop in duct based on airflow, knowing the specific power loss, airflow, thermal properties of the duct, and air temperatures. The goal is to be able to produce the graph attached.
I can digitize the curves and obtain a formula that will accurately reproduce them if that would help. It doesn't tell why or reveal the basis but it's something, perhaps a step toward achieving the ultimate goal.
The digitized curves are contained in the attached Excel spreadsheet. I'm not sure about the values of q in column A. Some are labeled (20, 10, 5, 2, 1, ...) but there are more curves than numbers and it isn't clear. If you change any of the q values in column A, the regression will automatically update. The regression is obtained using the Excel LINEST function in cells K2:P6 based on the ln(), which seemed most appropriate. The bottom figure is the regression. Clustering about the diagonal line (exact match) is better when plotting the ln(). The agreement may improve (larger R²) with the right values of q. You can experiment with the questionable values to get the best agreement.
Dudley J Benton Thank you so much! In the end, I managed to reach my goal based on the excel sheet you provided.
The maths of temperature drop are very simple.
The simple heat balance:
Overall Heat transfer coefficient x Surface area x mean temperature difference = Gas Flow x specific heat x temperature difference
is easy to calculate for any given duct : far more accurate than a simplified graphical method. This allows for indoor vs outdoor build, different insulation and face temperatures and varying ambient temperature.
- Badges
- Science topic
- Similar topics
- Geoscience
- Atmospheric Sciences
- Atmosphere
- Air