Fan Re-speed, Fan Laws not enough

[fiberstress]

Hi,

Been reading a few threads about using fan laws, but I think they're incorrect and I need some reassurance.

If you have a fan and change its speed while keeping the same pressure change over the fan, then you can use the 1st fan law. But if you have it connected to a ducting system, you have to follow the system curve with the new fan curve from the different speed. For example, if I cut the RPM in half, my new CFM will be more than half the first CFM because the back pressure is also reduced (lower air flow=> lower friction loss).

Sound right? Do I incorporate the new expected system pressure (second fan law) as well?

 

[willard3]

There are more than one fan law and you can solve for any variable you want.

You may be getting incorrect results which makes me question your inputs........

 

[MintJulep]

Yes, you are right.

Fan laws apply only "when all other things remain the same".

 

[fiberstress]

I answered my own question. I had the wrong premise. If you change the speed of a pump/fan that operates at one flow and head, both the new flow and new head at the new speed will be known using the fan laws. You don't need to keep the other parameters equal.

Since my book says (and books are never wrong...) turbulent air ducting systems follow the equation below closely with minimal error:

dp1/dp2 = (Q1/Q2)^2

if you plug in variables from the fan affinity laws for pressure and flow you get the same result:

Affinity law 1: Q1/Q2 = (D1/D2)^3(N1/N2)
Affinity law 2: p1/p2 = (D1/D2)^2(N1/N2)^2

dp1/dp2 = (Q1/Q2)^2 = (N1/N2)^2 (for same fan, same D)

So you get your system curve by plotting my first equation and that happens to be the same parabola you would get from plotting the fan laws.

In other words, the fan laws apply (as long as you keep your system the same as before).