AHU Rehab - fan pitch control

[Herman2]

We have some 20 year old AHUnits, each rated about 60,000 cfm. They have chilled water and steam coils. They also have twin supply fans (vane-axial with pitch control). The pitch controls are frozen in random positions, resulting in actual air flow of 60 to 80% of rating. I vote to toss the pitch controls and fix all fans wide open (100% flow). Then just modulate the chilled water and steam valves to hold the discharge air temperature setpoint.

My boss thinks that will result in much higher energy usage because of the increased air flow. I think we should be operating at rated flow.

What's do the experts think? We appreciate the benefit of your experience.

 

[MintJulep]

Herm,

Not nearly enough information here to reach any conclusions. What is the application?

How long have the pitch controls been dead? If you have been operating at reeduced flow for a long time, have things been ok? If so, why do you think you need full rated flow? Why do you think that a constant discharge temperature will work for you? Fixed discharge gives you no ability to load-match.

How about setting the pitch to full and adding VFD's?

 

[Herman2]

This is industrial space conditioning. The controls have been dead for at least 10 years. We just run all 14 AHUs all the time, with chilled water valves wide-open. No complaints from the plant, but we think we are using too much energy.

Our new control will allow us to reset the discharge air temperature for load-matching and modulate the chilled water valve.

We looked at a variable-speed option but the cost of the drives and installation didn't appear to have a good pay-back.

So, do we run the fans at 100% or just leave them alone?

 

[sterl]

Still not nearly enough data to be analytical on the value or payback of VFD's...

We would always consider Variable Air Flow except in cases where we are dumping into one very large cavity of a building with essentially no day by day operational differences in terms of Make Up Air Volumes, exfiltration/ infiltration rates and so on. In effect: if the operating building needs 100% air flow all the time it is operating, run it that way.

The most difficult thing to establish when talking variable air flow is, what parameter is it that air flow should respond to? Internal building pressure? Space Temperatures and temperature distribution?
What is the nature of the process and the characteristics of the building in terms of air consumption and circulation, and what would represent Quality Air for those conditions?