Fan Efficiency??? 1

[Thealanator]

A colleague just told me that a contractor changed out some fans on a couple of cooling towers at our facility. He said that the amp draw dropped from 40 to 18 and asked if we might be able to calculate energy savings. Supposedly the blades are of a new design, with a honeycomb structure.

My first impression is that there are too many variables at play (power factor, speed, air pressure, humidity, etc.). Can the mass of a blade make that much difference?

 

[DRWeig]

I don't think blade mass can make that much difference, but let's see what the others in the forum have to say.

Are you sure you got the same blade geometry? Is it possible to measure air velocity versus previous?

Goober Dave

 

[MintJulep]

A better than 50% gain in efficiency within the same physical envelope is highly unlikely.

That big a drop in amps can probably only come with a big drop in the air moved.

 

[HVAC68]

Such huge difference could be due to

(a) The existing fan is so highly inefficient !
(b) The new fan is running at lower speed ? VFD ?




HVAC68

 

[KiwiMace]

Steady state power consumption is proportional to airflow, dp and efficiency. Assuming the airflow and dp are constant, this would necessitate a massive improvement in efficiency.

The mass of the blades would affect transient energy consumption, if the blades speed changed a lot, but again I doubt this much.

There are real world scenarios you could imagine, most of which involving a VFD, but I lean towards the contractor cleaning the tower fill at the same time as replacing the fan blades.

 

[imok2]

For those of you interested in this subject please read this very interisting

http://www.ishrae.in/journals/2001oct/article05.html

 

[Zesti]

Was the fan-change part of a complete overhaul of the coolingtowers?

Maybe the coils/fins were changed or cleaned and this resulted in less resistance, lowering the power of the fan.

As for the energy-saving: calculate fan-power from the measured amps and then have an educated guess of the number of hours the fans run annually. Multiply and you have kWh.

 

[MintJulep]

resulted in less resistance, lowering the power of the fan.

In general, less resistance results in more air flow which in turn increases fan amps.

 

[MrFreeze42]

MintJulep is right. Generally speaking, less resistance often does increase both air flow and fan amps, but it does seem a bit counter-intuitive.

http://www.hvacknoxville.org

Repair Services.

 

[KiwiMace]

I guess that is a result of the actual fan press and hp curve for a prop fan versus the fan laws, and where the fan was running before the load was dropped.

Link is to an old presentation online with the various curves.

 

[ParadigmEnergy]

If you want to calculate the energy savings and simply ignore what has happened (not good engineering ethics but adequate to fulfill your boss' request ;-) ) you can:

(1) strap a power logger to the MCC fan feed and measure actual consumption,

(2) if your electrical vault has a power quality analyzer (Power Logic panel, for example), then go in afterhours when all the systems are off and the cleaners are gone, find the power meter and record the kW, go to the DDC and turn on the fan, and return to the panel to take the kW reading. Multiply the kW by the hours of operation and you have your kWh,

(3) multiply the new Amps x Volts = Watts x hours = kWh as a ballpark estimate.