Understanding HVAC Dampers and Flow Rates

[poont2]

Hello guys,

I recently got an internship at a engineering consultant firm doing HVAC work. I am kinda confuse on one thing. How does HVAC dampers work? I have always learned in school the continuity equation Q=AV=AV ,

Q = the volumetric flow rate
A = the cross sectional area of flow
V = the mean velocity

So how do you regulate flow in a duct system using damper when the flow is going to be the same even if you put a damper to block the air space, wouldn't it just make the flow kinda turbulent while the flow rate stay the same? wouldn't the velocity just increase at the damper and decrease after it passes the damper? I am kinda confuse, only way i can think of is it give more pressure making the air handling unit put out less air?

For example, if a pump is connected to A and B pipe. Where A pipe is 1" and B pipe is 5", of course the velocity is going to be larger on the smaller pipe, but the flow rate (Q) would be the same on both pipe rite?

Thanks for your help!

 

[SteamKing]

Which has the higher flow rate in your wash basin? When the valve is open or closed?

 

[russ_watters]

...only way i can think of is it give more pressure making the air handling unit put out less air?

That's the answer. The key is the pressure drop across the damper making it harder for the fan to move air.

 

[russlhenley]

A common misconception about HVAC air flow is that you need "back pressure" to make it work. Service techs will often close air dampers just to build pressure because they've been taught that pressure differential is the key to air flow. But efficiency demands mass flow which is reduced with back pressure. In the 70's a lot of homes were built with "high pressure" air distribution systems. This was accomplished by drastically reducing duct diameters. Unfortunately the resultant volume reduction resulted in poor heat distribution and efficiency. So, the less restriction the better and dampers should only be used to balance different areas always keeping in mind that the more open the damper the greater the efficiency.

 

[ameeno97]

I still don't get it. could you please illustrate more

 

[russ_watters]

What, exactly don't you get?

 

[ameeno97]

What, exactly don't you get?

I have put a post here about what confusing me :

https://www.physicsforums.com/showthread.php?t=508108


Thank you for you attention and I am waiting for satisfied answer

 

[ameeno97]

please put your replies here :

https://www.physicsforums.com/showthread.php?t=508108