FBA Issue 31: March / April 2010

 

 

 

 

The number one objective of an air relief and air assist systems is to increase productivity per kilowatt hour while reducing operating costs and increasing profits. The number two objective is keeping the temperature of the product being ground 5OC or less above ambient temperature.  This will also help productivity; eliminate condensation of the ground product in the holding bins, screw conveyors, elevator legs, etc., which also eliminates buildup and bridging.  The number three objective is better housekeeping, resulting in less cleanup labor, less dust for reduced risk of fire and explosion, and lower insurance rates.  It also looks better when management shows visitors through the plant.

 

 

The most important part of an air relief/assist system is proper sizing.  There have been more systems not sized properly, than have.

 

How do you know what is the adequate amount of air needed to make your Hammermill perform at its best? As an example, on the 1800 RPM Hammermill, use a rule of thumb of 1 ¼ CFM per square inch of screen area.  3600 RPM hammer mills use 11/2 CFM per square inch of screen is, these are minimums.

 

For example, if you were installing an 1800 RPM Hammermill with a 38" diameter and 15" wide rotor of any manufacturer, the rotor will make from 900 to 1000 CFM of air by itself under normal conditions.  Let refer to this air circulation within the Hammermill as a small storm.  Now we install an air relief/assist system to pull 1200 CFM of air through the mill.  You now have an air system that moves 100-300 CFM more air than the mill is making, but the performance is no better and with more heat buildup is worse than before, much of the time. 

 

Why?  You do not have an air system large enough to overcome the small storm, so it becomes large storm and allows the product being ground to circulate inside your Hammermill at a greater rate.  You are actually chasing the product trying to catch it before you grind it.

 

By putting an air relief/assist system on the Hammermill that will provide 1875 CFM at 5 WCI  of vacuum at the mill outlet, you should have enough air to overcome the large storm and make the Hammermill perform as intended, providing all the following factors are correct. 
 

 

First, you must make sure 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.  The most common method is putting a shroud on the last 3 to 5 feet of the screw conveyor, starting at the beginning of the discharge and go back toward the Hammermill. 

 

Then cut off fighting in the amount of 1 ½ times the diameter of the flighting, back from the beginning of the discharge. This shroud forms a tube, the lack of flighting forms an air lock with the ground product. When you make an air lock of this type you must add 50% more horsepower to the screw conveyor to allow for the extra horsepower requirements needed to puts the ground product those last few inches through the air lock.  If you have the height, you can install a rotary air lock on the discharge of the screw conveyor, "in place of cutting off the flighting", before the ground product enters the elevator leg. However, this is one more piece of equipment to maintain.