fan drive for brarny

thread carried over from "can VFD's on motors do magic?"

The first question that needs to be answered on the combustion air fan system is whether variable speed is useful or not. Resizing the motors and throttles down to the actual demand will save slightly more energy than doing it with inverters.

On the other hand, for very little extra energy (about 4% of the load kw), you reduce get the benefit of precise control and, if you retain the existing motors, the easy increase of future system capacity. If you downsized the motors AND added drives, you would reduce the inverter losses to 2% of load kw but you would not have the ability to expand easily.

That's the fundamentals of it as I see it. Its your call!

Dick,
VFD's are used in combustion control systems that use a fuel demand jackshaft. This allows an Oxygen trim to be applied to the combustion without the movement of the jackshaft. With most boiler or combustion systems there is a desire to increase the efficiency of the fire. The jackshaft that ties the fuel valve and windbox dampers together cannot be utilized for this. So with any demand setting the Oxygen content in the flue gas may be trimmed by changing the velocity of the forced draft at the windbox damper. This is accomplished by slightly reducing the forced draft fan speed via VFD.

Bitmore

Thanks, bitmore. I'm not an expert on boiler or kiln burners or their control requirements. In fact, I'm almost entirely dependent on purchasers describing their needs to me accurately on most systems. Of course, it helps to ask a few of the right questions, too.

From brarny's original post, it sounded to me like the variable speed could be optional. It sounds from your post like it really isn't.

I have no way of knowing so I try to structure my responses a bit broad in case I missed something.

Thanks again.

Thanks DickDV and Bitmore for your replies so far, much appreciated.

Bit more info on the combustion air system on my kilns for you to digest.

The air demand is pretty much fixed for each rate of throughput, adjusted manually, and throughput is very stable (barring catastrophies), gas input is fixed on all zones except the final firing zone where temperature is mainrained by modulating gas input only. A bit archaic as far as fine control of each zone is concerned but we've taken control gear off to elliminate the cycling we used to get. Natural variation is small enough for the final firing zone to cope quite well (barring catastrophies again).

The question is, therefore, can the fan speed be reduced and by opening the restrictions up still achieve the same volume input of air. If so are there energy savings to be had by slowing them by fitting VFD's.

Unfortunately I'm relying on my scoolboy knowledge of physics and my experience in the 'black art' of kiln firing as a basis for my assumptions regarding this project.

Whether you can back my assumptions up or shoot me down in flames I look forward to your input.

It is nice to approach these things from different angles sometimes, I am glad I fell upon this little group and look forward to reading other issues that arise.

Thanks again

Brarny

In any common fan situation, you can expect to save worthwhile amounts of energy by eliminating flow restrictors and using VFD's to modulate fan speed and air flow.

There is nothing "state of the art" about this and the results with drives are very predictable.

I don't think anyone could give you responsible savings estimates without a thorough study of your system altho, it can safely be assumed that it would be worthwhile, usually on the basis of mechanical maintenance savings alone.

I just thought some insight on VFD usage in a combustion system was warrented. Yes you are right, some systems use a variable damper on the suction side of the forced draft fan to increase or decrease the amount of "ether" being consumed by the fire. This usually results in some added physical stress on the motor and fan, not to mention the added cost of electricity!

Bitmore