Ask the Experts about Industrial Equipment

I was recently copied on email that sums up nicely the Dec. 10th entry. Some of it is paraphrased to shrink it down.

Q: I have a technical question about the combustion stove fan that we have purchased recently from your company in 2004.

Due to the current situation, we need to operate at lower production and as a result we require less combustion air.

As the combustion fans were sized larger at present the louvers are 100% closed. There is still small gaps between the louvres causing some air to push through.

1. Are the fans intentionally designed to leave some gaps/openings, even though the louvers are 100% closed? If so, what is the reasoning behind it?
2. Is it possible for us to reduce the open area in order to reduce the air flow? Is there any concern to the integrity of the fans (vacuum effect, build up of pressure, etc) if we close up the gaps?
3. What happens if the fan is running and we completely seal all the gaps?
4. What other options can we do to reduce the air flow while maintaining control?

A: The questions you raise are all very good ones. Let me first start off by clarifying the purpose or function of the inlet damper. The inlet damper’s role in the system is to modulate the volume or flow through the fan with turn down ratios of up to 10:1 dependent on fan design and selection criteria.

Standard dampers have a leakage rate of 4-10% and low leakage dampers are designed for 2-5% leakage at the fully closed position.

In your specific case, the fans were not designed to run at very low (less than 10:1 turn down). Non-optimal flow conditions and excessive vibration can develop at ultra low flows that can lead to mechanical damage if left alone without any supervision. At the very least, the heat generated from re-circulation of air inside the fan casing could lead to over-heating of the impeller as the kinetic energy is being added by the impeller but there is nowhere for the heat to escape.

I suspect that this is not the only fan at your mill facing these challenges as production is cut back to contend with current market supply and demand. Which brings me back to the primary design intent of the damper and it’s operating range vs. its efficiency window.

As mentioned above the damper is capable of modulating flow very well between approximately 10% and 100% flow. A damper on the inlet side is designed to reduce the power consumption or improve the efficiency of the fan as well as modulate flow, compared to an outlet damper that simply adds additional system pressure to the outlet of the fan to control the discharge volume of the fan.

The “efficiency window” of an inlet damper is in the neighborhood of about 60-100% open (0-40% closed) depending on fan type or design. Once the inlet damper is closed more than this the inlet damper begins to mimic the characteristics of an outlet damper. In other words it provides only artificial static pressure to the fan system and as a result the fan efficiency plummets. When a fan is running with dampers fully closed the efficiency is less than 15%, and likely less than 10%.

At approx. $60,000 per year for each 100 hp used, inefficiency costs can add up very quickly. Please also remember that fans and pumps are the two largest power consumers on the grid in the industrial market and should be the first place to look to save money.

We can help you with either a mechanical or electrical solution to your current operational requirements. These range from the various variable speed or reduced speed technologies to re-sizing the impeller and/or motor to match you current needs.

We would be happy to meet with you in improving the efficiency of your fans for your reduced production requirements of today and your record-breaking production runs of tomorrow.

Sincerely,

Mark Glover
Vice President
Canadian Buffalo

Note: Do you have any questions related to heavy duty industrial equipment? Email us , and we’ll answer your question in our next “Ask the Expert” segment.