The first objective of an air relief/assist system is to increase productivity per kilowatt hour to reduce operating costs and increase profits.
Another objective is to keep the temperature of the product being ground 5 deg C or less above ambient temperature. This will help productivity, eliminate condensation of the ground product in equipment such as holding bins, screw conveyors and elevator legs, and eliminate buildup and bridging.
The third objective is better housekeeping, resulting in less cleanup labour, less dust for reduced risk of fire and explosion, and lower insurance rates.
Proper sizing is the most important part of an air relief/assist system. There have been more systems not sized properly than those that have.
How do you know what is the adequate amount of air needed to make your hammermill perform at its best? For example, on the 1,800 RPM hammermill, use a rule of thumb of 1 1/4 CFM per square inch of screen area. 3,600 RPM hammer mills use 1 1/2 CFM per square inch of screen, these are minimums.
You must ensure the hammermill is totally isolated so you do not pull any air from the elevator leg, or whatever method conveying you are using to convey the ground product. If you are using a screw conveyor to discharge the product into the elevator leg, you must put an air lock somewhere.
Where is the air to come from once the hammermill is isolated? It is recommended that 75ï¿½ï¿½C85 percent come in through the feed inlet with the commodity being ground, to pull the product through the screen.
Ensure the air is of ambient temperature or cooler. If heat created by the mill motor (s) is allowed to go up, and enter into the air inlets with the commodity being ground, this hot air will increase heat buildup in the grinding chamber, decreasing productivity and causing a condensation problem. This defeats all the objectives of an air relief/assist system.
You cannot make a plenum chamber too large, and must keep air velocity down to match your product(s). Not more than 12 velocity feet per minute per pound per cubic foot of material you are grinding is needed.
When you are going into a dust collector cyclone, install the fan as close to the plenum chamber as possible. Pulling the air off the side of the plenum chamber is possible. Air going around more corners creates more vortices, allowing more products to drop out and not go through the air relief/assist system. You may also install the fan on the exhaust side of the cyclone.
You must have a cyclone that meets EPA requirements. Too much or too large a cyclone may not work.
It is recommended not to have more than 9.0 CFM per square foot of cloth area; if possible, having less is better.
Your cyclone/filter is most commonly discharged back into the ground product downstream. The most common place is directly after the air lock.
The above suggestions are general and you should consult a qualified person or organisation to ensure the air relief/assist system is properly designed and installed for your application.
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Article made possible through the contribution of CPM Roskamp Champion.