Fan Surge and VFD Control

[sfxf]

We designed a central cold air system, in which a radial fan with VFD is used to deliver 900 cfm at 50 inch static pressure. The air is to serve five outlets with 180 cfm each at 45 inch constant static pressure. Any of these five outlet can be closed by a butterfly damper any time, in other words, the system's load varies from 0 to 900 cfm any time. We installed a 4" manual relief damper at the end of the main duct, however, when all the five outlets are closed, the fan got problem even we open the 4" relief damper. The surging happened and the fan was making huge noise. We were trying to fix the problem by adding a bypss between the supply (discharge side of fan) and return air duct. I am thinking we need a heavy duty control valve at bypass that can modulate corresponding to the variable system load. However, what is the best way to control the valve, by pressure or by flow rate? Currently the static pressure at the end of the main duct is controlled by VFD... Thanks in advance for any suggestion and help.

 

[trashcanman]

Look carefully at the fan curve. Confirm that it can actually produce the cfm you want in the design pressure range. It sounds like the fan is operating to the left of normal operations. If there is a point where the fan can be at 2 points on the curve at the same cfm, that is where the surge is probably coming from.

50" static is enormous, and probably requires a BIG motor. Are you that tight on duct space? Also, a centrifugal fan will generally provide more static pressure than a radial fan. Why radial?

 

[sfxf]

Thanks! Trashcanman(???).

The AHU manufacturer selected the radial fan, and I am not quite sure the reason. You've got a good point, and I will ask them this question.

My situation is: The fan has been installed, and has no problem to provide 900 cfm at 50" static pressure. The problem occurs when the system reduced cfm by closing all dampers, and it could not prevent the fan from surging by opening the 4" relief damper. We look at the option of bypass for energy saving purpose also. Anyone has experience with the control of modulating the bypass valve?

 

[willard3]

I will ask the same question.....why a radial fan?

Radial, foreward curved and axial fans all have a characteristic fan curve where the fan can surge and operate at two different volumes at the same pressure....causing surge.

Sounds like surge to me.

 

[sfxf]

Let me clarify: The fan is radial blade pressure blower, and it is a centrifugal fan. The fan selection should be all right.

My situation now is that I want to add the bypass modulating valve to relieve certain amount of air that the system does not need, in order to avoid surging in case all dampers at end users are closed. The static pressure at the end of the main supply duct has to be maintained at 45" min, no matter how much cfm is needed in the system. Anyone can advise what is the best way to control the bypass valve? Thanks.

 

[Warpspeed]

To maintain the required minimum operating pressure, slowing the fan Rpm below a certain speed is not an option.

Unfortunately, at any particular fan operating speed there will be a flow volume below which surging is going to occur. The only real way around that problem is to bypass sufficient air to ensure fan flow can never fall low enough to get into the surge region, even with all the dampers closed.

A larger bypass volume around the fan should fix it, but that may be a very inefficient and wasteful solution.

Attempting to modulate the bypass somehow may introduce further instabilities. It may be possible, but it is a far from simple problem.

 

[willard3]

To solve the problem, you must get a fan curve to see how much you must bypass to get it out of surge.

Get a fan curve from the supplier (it may be on the web).