Temperature drop in the duct

[ercegs]

Hi all,

when calculating temperature drop in the HVAC ducts, what type of flow do you consider that is outside duct?

1. Natural Convection

2. Forced Convection

Let's say it is normal building in question, and all ducts are inside the false ceiling, or in the room (if there is no false ceiling). I assume correct answer should be 1 (or at least this is what I've seen in most of the calcs)

In this case, when calculating Grashof number, what do you consider as charatheristic length for rectangular ducts in case

1. They are horizontal

2. They are vertical

How do you treat this? Do you consider that flow type is "horizontal plate" or "vertical plate"...or you use hydraulic diameter (in this case charatheristic length is actuall duct length if it is vertical or Dh if horizontal)? or how....



If there is similar topic before, please if someone can share the link.

Thanks

 

[EnergyProfessional]

I think if you ignore the "insulation value" of the airfilm on both sides and you take into account the insulation, you should be safe.
You could treat it as a heat exchanger and apply NTU (number of transfer units) method, or the Log-mean.... method

If you want to consider the airfilm, then inside the duct you have turbulent flow, so overall this is not much compared to the insulation. Outside treat it like what you would do for outside air film of a building in case of outdoor ducts.

 

[RandomUserName]

There is a formula for this calculation in the 2013 ASHRAE Fundamentals; Chapter 23 - Insulation for Mechanical Systems.