Variable Freq Drive on a Fan?

[new2hvac]

Is there a significant energy savings when installing a VFD on a variable pitch axial fan? The folks at the VFD company tell me this would be similar to having a fixed blade fan and inlet guide vanes, but I suspect they're opinion is a little slanted.

Is there an easy way to calculate this?

Thanks!

 

[okidataman]

I don't have a quick and easy answer, but I did find some interesting links.

I am reading them myself.

http://www.saftronics.com/pdf_fils/CaseStudy/CS_VFD_001.pdf

http://www.greenbusinesscentre.com/...11-Installation of VFD in place of VFC in.pdf

http://www.drives.com/industry/default.asp?np=/industry/HVAC/ESP.htm

My company (Siemens Building Technologies) recently evaluated a customer site for a Variable Volume, Variable Pressure program. If you email me, robert.okamura@siemens.com I will put you in contact with an engineer. When properly implemented, the savings equates to about $50 per horsepower each month.

 

[Yeldud]

The answer to this question depends more on the expected operating profile of the fan system and so there is no one answer that is correct. I don't know a lot about adjustable pitch axial fans, but do know they are highly efficient. If your fan system spends much time at 75-100 percent of design capacity, then a VFD may cost you more in electricicy than it saves, because the VFD has inherent operating power losses, that you could otherwise avoid. If your system spends much time at high turndown, then economics might start to favor vfd's, coupled with adjustable pitch. IF you know your expected operating profile, the best expertise to answer some of these questions lies not with the vfd manufacturer, but with the adjustable pitch axial fan manufacturer. Let the forum know what they tell you. good luck

 

[CB2]

I see alot of VFD retrofits on the type of fan you have. Primary reason is failure of the "adjustable pitch" controls due to lack of quality and maintenance. Lock the vains to full open and integrate a VFD to the system.

Energy savings for fan and pump applications using VFDs are very well documented for HVAC applications. Generally, energy savings will pay for the installed cost of the VFD within one year.

Suggest you deal with a vendor that has a significant presence in your local HVAC market, local support is very important.

 

[ChasBean1]

There could be some misinterpretation here. Variable pitch could be 1) blades or volume vanes that modulate based on an input parameter (e.g., duct static pressure). It could also mean 2) you have to open a panel on the fan casing, loosen some screws, manually change blade angles, and re-tighten the screws.

In general, I would prefer to use VSDs wherever possible. There are drive losses as mentioned above, but the basic fan laws say that if you start with a 50 HP fan and cut the speed in half, the power required will reduce to 6.25 HP (a cubic relationship).

Without diving too deep on this and if I were to make a decision based on current knowlege, I would first fix the blade pitch to the optimum efficiency angle and set the motor sheave to give the optimum RPM at maximum anticipated system flow. Then I would use a VSD to control the fan to maintain the right static pressure in the duct to account for varying conditions.

Gut feel is that variable pitch is a lower level application that could cover up design sloppiness - VSD does likewise, but with more first cost and a more substantial energy savings potential.