Fan Brake Horsepower at Dead Head 1

[dR3w]

Hi,
I am doing a dynamic simulation of a fan for a power plant simulator and am trying to calculate the required power at a dead head or run out condition.

Given the general power equation:

HP = (CFM * Head * K )/(K2 * Eff)

Where
CFM is fan flow
Head if fan pressure differential or head
K is a compressibility factor
K2 is a conversion constant?
Eff is fan efficiency.

It appears that when the flow is zero, and the curve is at the dead head position that the power goes to zero. This obviously is not the case, and my guess is that the efficiency also goes to zero in this case. Thus the equation is no longer valid.

Does anyone know of any other equations that may better describe the power at or near this point?

Also my best guess is that he efficiency in this formula is the variable efficiency given on the fan curve and not an optimal efficiency. Is that correct?

Thanks for any help you can supply on this matter.

 

[GregLocock]

Yes, the efficiency is zero when it is shifting no air. That equation is no longer any help.

Yes, the efficiency in that equation will vary with operating condition.

The power required at the zero flow point will be largely due to friction. If you plot the wasted power for your fan as a function of flow you should be able to identify an empirical relationship for the wasted power, which can be usefully extrapolated to the zero flow condition.

You should also be able to get an estimate by working out the actual friction forces on the blades, from a skin friction coefficient.

Cheers

Greg Locock

 

[pmover]

suggest obtaining the fan performance (predicted or tested) curve data from the mfg. you may then determine hp at dead-head conditions or otherwise.

or contact a reputable fan mfg and request/obtain a technique for determining hp or otherwise at dead head conditions. of course, the fan type impacts the results as well.

good luck!
-pmover

 

[fredt]

pmover's suggestion is good. The actual fan curve should give you power at zero flow. Although it varies depending on fan type you can expect somewhere between one third to one half the rated power. Because there is no flow with a true total shut-off this power is converted to heat and high pressure fans especially can quickly become very hot if there is no flow.

 

[unclesyd]

fredt has made a very good point about the heat. On a high HP fan you would be surprised how fast you can wreck the inboard bearing of fan when dead headed.

It happened twice at our site on a very large fan (1000 HP). The first time was a control problem where the damper didn't have a stop to prevent complete shut off. The second was during the investigation as to what happened the first time. The damper linkage went over center and couldn't be pulled back manually and by the time the local disconnect could be tripped there went another bearing and a lot of red faces.

 

[ShaunE]

for information:

Centrifugal fans:

backward curved bladeower rises to max at the middle of flow range then falls at highest flow rates, this is known as a non-overloading power characteristic.

Paddle bladed: power rises continuously with flow

forward curved blades: power rises continuously with flow

Axial flow fan:

power rises to max at the middle of flow range then falls at highest flow rates, this is known as a non-overloading power characteristic.

Mixed flowower rises to max at the middle of flow range then falls at highest flow rates, this is known as a non-overloading power characteristic: non overloading as per backward curved blade centrifugal & axial (obvious really!)

Reference - Air conditioning systems design commissioning and maintenance, Roger Colby Legg ISBN 0 7134 5644 2. I think its out of print now.