VAV System - VFD vs. Inlet Guide Vanes 1

[stephanemunger]

Hi all, HVAC consultant here seeking some wisdom...

I have been mandated by my client to determine the annual cost savings associated with retrofitting air handling units serving a VAV system from inlet guide vanes to VFD's.

The current method of controlling the airflow with the inlet guide vanes is by maintaining static pressure setpoint at the fan discharge.

From what I am putting together at this point, I fail to see any operating savings at all within the throttling range of the inlet guide vanes. Since the VFD will also be controlled to maintain the static pressure setpoint, at any operating point, both the static pressure and airflow will be the same as with the inlet guide vanes, the system curve does not change, therefore BHP will also be the same.

At low airflow conditions, I can see that a VFD could achieve some savings. But my problem here is to determine what is the point where the inlet vanes are fully tilted and can no longer "help" the fan. I have tried to model how the fan curve changes with the throttling of the inlet guide vanes but I have failed with both the Greeheck (CAPS) and Penn Ventilator (FanSizer) software, they do not provide this capability.

For reference, the units are equipped with 16,000 cfm fans, 20-HP motors, and I am estimating design static pressure as 4" WG.

I've looked at this reference from the Trane Engineer's newsletter, you might want to check it out.

http://www.trane.com/commercial/library/EN20-2.pdf

I've also found this very un-scientific energy savings calculator that claims a kind of "fudge factor" of 62% for inlet guide vanes to 28% for VFD's.

http://www.yaskawa.com/site/DMDrive.nsf/(DocID)/MNEN-5JFQPR/$File/AD4006.pdf

Any thoughts are very welcome...Thanks !!!

 

[walkes]

ABB drives had software that compared energy savings of various types of VAV fan control. Try your local rep.

 

[Yorkman]

A couple of things to also consider: A VFD gives you a soft start capability, although at 20 H.P. I'm not sure that would equate to much in the way of energy savings.
If your fan is suppling a DX cooling coil be sure to set your minimum Hz high enough to prevent too low an air flow at the coil. Or be sure there are provisions to keep suction pressures up. I assume those would all ready be in place with the inlet guide vane system.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[quark]

I concur with Yorkman. There won't be much savings in energy if you base the set point on static pressure. You will have definite advantage if you do it on flow control rather. Can I know what is the reason for static pressure control?

It is a general (and easy) notion that if you provide constant upstream pressure to a VAV and as the downstream is always constant, you will get constant air flow from the terminal VAVs. This is a good idea as far as your main AHU filters are not clogged. If this is the case, this issue was discussed extensively in a previous thread.

Just try out this energy savings calculator from Danfoss

http://www.namc.danfoss.com/hvac/energysavings.html

 

[SAK9]

Some fan manuals give comparison of fan curves with IGV control as well as VSD control.Needless to say fan curves stay low with VSD control resulting in greater power savings at low flow rates as compared to IGV control.

As for control logic, you can not use flow rate for control in a vav system as the system flow will vary depending on the room load.Static pressure control is the way to go but the location of it is debatable.The normal practice is to locate it 2/3rd down stream of the main supply duct from the AHU.

There are more intelligent versions which poll all the VAV boxes and control the SP based on the requirement of that box which is 100% open

 

[GMcD]

The comparison needs to be made with the reduced fan total static pressure with the inlet vanes removed. Those inlet vanes are likely using up around 0.5" to 0.75" of fan static and BHP just sitting there at the fan inlet. Once they are removed, that is less BHP that the fan needs to use to maintain the same discharge SP. That's where the energy savings are.

Not sure why a static pressure sensor right at the discharge duct is being used, normally with VAV systems, the intent is to maintain a fixed minimum duct SP at a point around 2/3 to 3/4 of the way down the furthest duct towards the furthest VAV box. Pressure independent controls at all the "upstream VAV boxes" would maintain their local flow at whatever the inlet static pressure is anyway, IF your system is set up this way.

 

[cme]

inlet guide vanes are 80's technology

vfd's will help with fan acoustics also

drives are so cheap these days compared to say 10 years ago

does the unit have final filters?

what position are the vanes in now at full load cooling and dirty filters?