Easy Fan Question
Easy Fan Question
(OP)
I feel silly asking, but I am having a brain freeze. For a centrifugal blower on an exhaust system, what happens to the suction-side pressure as I close down a damper on the discharge side?
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RE: Easy Fan Question
RE: Easy Fan Question
RE: Easy Fan Question
RE: Easy Fan Question
So what happens to the discharge pressure then (between the fan and the damper that I just started closing)? Does it increase? If so, is it by the same amount that the suction side decreased?
RE: Easy Fan Question
It will be highly dependent on the shape of the fan curve (which depends on the type of blower (forward curved, backward inclined, radial...), and where you were originally operating with respect to the hump. Get too close to the hump, and the entire system may become unstable, and start surging.
RE: Easy Fan Question
RE: Easy Fan Question
As the flow decreases due to increased damping, the pressure across the fan will increase and both suction and discharge sides will increase algebraically, but the discharge side will increase more.
RE: Easy Fan Question
If you are saying the pressure at the fan inlet will increase, then I agree. I was saying that the pressure drop across the entire inlet line will be less and so the pressure at the fan inlet will increase.
RE: Easy Fan Question
The pressure in the suction duct is (atmospheric pressure -the resistance offered by the suction duct and accessories in the suction duct at a specific flowrate-velocity head) and the pressure in the discharge duct is the resistance offered by the system at corresponding flowrate. As the resistance is proportional to the square of velocity (and thus square of flowrate), when you control the discharge damper, the suction pressure moves towards atmosphere. It is simply atmospheric pressure at shut off condition. This is similar to what wfn217 said in their 3Jan post.
When you control the discharge damper, you are creating more resistance and thus the static pressure increases but total pressure remains same.
In your example, suppose -2" is gauge pressure and if new gauge pressure is -1.5" then the increase in pressure is 0.5"(25%) and this approximately corresponds to 5% decrease in volume flowrate. Now, check what discharge pressure creates 5% drop in volume flowrate from the fan curve. This is your new discharge pressure.
RE: Easy Fan Question
The reason I ask this question is that my company has inherited an exhaust system that the plagued with problems - mostly due to fans being too small. The fans serve multiple lab hoods and are on VFDs. The system has Iris dampers on the fan discharge ducts which throttle down in order to reduce duct area and maintain required discharge velocity as the VFDs slow down.
As it turns out, the Iris dampers are not motorized and will be set in fixed positions to maintain min velocity at the min VFD setting. We are still trying to find that minumum balance point. Long story short, I just want to make sure that my suction duct pressure will increase (i.e. become less negative) when I throttle down the Iris damper. That way, my VFD will speed up the fan in order to maintain my suction static setting. At the same time, I will get back the flow lost by throttling the damper and still maintain my discharge velocity.
RE: Easy Fan Question
As velocity and pressure are a function of flow volume, when you close the outlet damper, inlet pressure becomes less negative.
RE: Easy Fan Question
You may need to provide a nozzle at the tip of the stack so you can have high discharge air velocity operating range say 4500 to 2500 FPM. Include discharge head loss through the nozzle in calculating fan performance. Look also at Strobic Fans.
RE: Easy Fan Question
Locking them in position as you mention is a good quick fix, and you're right - restricting the discharge will cause fan inlet pressure to become less negative, causing your VFD to increase speed.
-CB
RE: Easy Fan Question