Designing Controls

In this project we will be designing controls associated with equipment designed to take a coarse product, grind, pulverize and classify the material to the proper consistency (like that of face powder), dry it and then transport the material to a receiving process. If the material being processed is combustible, the control of the mixed material/air temperature cannot exceed the combustion point of the product. The coarse material also contains 15 to 30 percent moisture so the incoming air used for transport must be hot enough to help drive the moisture from the product and allow the material/air mixture be warm enough at the discharge of the pulverizer to prevent the mixture from sticking to the discharge piping.

designing controls 1

The coarse material is fed into the grinder/pulverizer using a gravimetric feeder, a feeder that weighs the material passing over the belt, where the material is ground, dried, classified and separated, and transported to the receiving process.

We also must consider the limits for the transport of two-phase (air and solids) material transport. Two phase material transport falls within one of 4 transport domains.

designing controls 2

When the particles are small enough and the flow velocities are high enough, the solid material and the gas (air) move and act as a single entity. This Homogenous region is the ideal relationship of particle size and fluid velocity to safely and properly transport the material through the piping. All of the flowing material behaves as if it is a single uniform fluid.

When the particles are larger and/or the fluid velocity is less, the individual particles begin to respond to gravity and display characteristics different from the transporting fluid. This is the Heterogeneous Region of the chart. However, all of the particles still remain in suspension and are properly transported by the fluid. The transporting fluid can still properly move the solids in this region of the chart.

As the particles become larger or the velocity decreases more, the particles can now reach the bottom of horizontal piping during the transport process. This can result in “Saltation Flow” where the solid material forms a moving bed of material – or drifting of the material in the piping. All efforts are made to prevent the velocity/particle size relationship from entering this region since this can cause problems with particle transport.

The final region is where the velocities are low and/or the particles are large. In this region the transport piping will become partially plugged with a non-moving bed of solids. If there is enough force behind the transport medium this will result in equilibrium between the remaining area in the pipe and the resulting velocity of the product or “plug” flow will occur. Plug flow is when “plugs” of solids material are transported through the piping by the pressure of the transport medium. However, since the source pressure is not generally unlimited, the piping will become plugged and it will require mechanical means to clear the piping.

So the first thing we do as control engineers is to determine the minimum velocities through the transport piping. For materials with specific weights between 50 and 100 pounds per cubic foot and particle sizes around 100 to 200 mesh, air transport velocities of 50 feet/second are the required minimums. Desired velocities for transporting this two phase mixture with 30% to 40% by weight of the solid material would be approximately 70 feet/second. Velocities above 90 feet/second only serve to erode the piping with no additional transport benefits.

We also consult the grinder/pulverizer manufacturer information to determine the minimum and desired mass air flows for proper grinding, separation and classification within the pulverizer. Presumably the original system designer properly sized and mated the pulverizer and transport piping size. But it never hurts to double-check their calculations to determine the limiting point in the system.

Once we have determined the velocity constraints for the system, we can now turn our attention to the measurement of the flow into the system. Maintaining and controlling this air flow measurement is important to preventing problems in the grinding and transport process. We need now to determine how sophisticated of flow measurement is needed to maintain the process.

Next time – Analysis of the flow measurement. Grinding, drying and transport project – Phase 2

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