Easy Fan Question

[BronYrAur]

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?

 

[wfn217]

It will increase.

 

[BronYrAur]

Do you mean that it will become more negative?

 

[wfn217]

I meant it will become less negative.

 

[BronYrAur]

So to be clear, let's say I am measuring +1.0" on the discharge side and -2.0" on the suction side. If I partially close a discharge damper, the suction pressure will come up to, let's say -1.5". Agree?

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?

 

[MintJulep]

You really need the system curve and fan curve to answer this.

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.

 

[wfn217]

My reasoning is that if you reduce the flow by closing down on the discharge damper, you will decrease the pressure drop across the inlet pipe.

 

[zekeman]

Not true.
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.

 

[wfn217]

zekeman,

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.

 

[quark]

Conceptually, wfn217's answer is more precise. The centrifugal devices just overcome the system resistance and they can't create pressure in the system, like positive displacement devices.

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.

 

[BronYrAur]

I'm still confused. This was supposed to be an "easy question". So as quark said, closing the discharge damper will reduce flow and cause the suction pressure to increase (move toward atmosphere), correct?

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.

 

[willard3]

If fan outlet volume decreases (close damper), inlet volume decreases by law of conservation of mass through the fan.

As velocity and pressure are a function of flow volume, when you close the outlet damper, inlet pressure becomes less negative.

 

[lilliput1]

Instead of a damper at the discharge, consider adding a OA intake with automatic damper at the exhaust fan intake. On drop in exhaust air requirement as sensed by increase in static pressure at the duct static pressure sensor at the most remote ductwork branch, reduce fan speed to maintain design static pressure set point. Limit the minimum speed to produce the design minimum discharge flow and velocity. On furter reduction in exhaust air requirement, modulate open the OA intake damper, maintaing the duct static pressure sensor set point.

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.

 

[ChasBean1]

This sounds like a misapplication. I believe the Iris dampers try to maintain a fixed flow to maintain the stack velocity. The only trouble is that the demand from your hoods already determines what flow should be. The Iris dampers are creating a second flow criterion in series with the more important first. The result is a battle for control. For example if hoods demand 7,500 cfm and the Iris dampers control to maintain 10,000 cfm to keep the right stack velocity, the Iris dampers will be wide open when hoods are in a low flow condition. If hood use increases to a 12,000 cfm demand, the Iris damper will close down and only maintain 10,000 cfm. The result is low flow and improper control at the hoods.

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

 

[lilliput1]

The control application is for variable volume fume hoods or application where exhaust pick up points can be shut down locally. If application is constant volume then all you have to do is to taper the end of the discharge stack for a fixed nozzle size that would attain the design discharge velocity, Typically minimum 3000 FPM.