As has often been said, "There isn't anything new under the sun." Well then what is this new idea of dense phase conveying? For over 60 years, materials have been handled in dense phase, it just wasn't publicized as a special type of conveying. Well then, what is dense phase?

There are so many ideas and definitions for dense phase, it is easier to define what it is not. Dense phase is not dilute phase. Turning this around, anything that isn't dilute phase conveying is some form of dense phase. Well then, lets define dilute phase conveying.

In dilute phase conveying the conveying gas volume and velocity is sufficient to keep the material that is being transported in suspension. This means that the material is being conveyed in a continuous manner, and is not accumulating on the bottom of the conveying line at any point.  This is also referred to as stream flow.

To have dilute phase conveying the gas velocity must be high enough and the solids loading low enough to be above the saltation velocity. The saltation velocity is the point where the solids concentration for the gas volume and velocity is too high to remain in suspension and it starts to drop out and deposit on the bottom of the pipe. Any conveying with material settling and accumulating on the bottom of the horizontal conveying line would then be dense phase conveying.

Dense phase conveying takes several different forms, depending on the conveying equipment supplied and the physical characteristics of the material being conveyed. In the simplest and oldest form, if a fluidizable material deposits on the bottom of the conveying line it is like a deposit of a liquid. With the normal stream flow conveying taking place in the center of the conveying line, there is a drag affect on the "liquid" (fluidized) material on the bottom of the pipe.  This fluidized material is dragged along, at a slower velocity than the conveying gas, and may flow in intermittent surges or waves, depending on loading and gas velocity.

This type of conveying is referred to as "two phase" because there is both dilute phase and dense phase in the conveying line at the same time. Of course the words "two phase" are not to be confused with two phase in an engineering sense where we talk about two phase meaning solids and gas. A typical material that has been conveyed in this manner for over 60 years is Portland cement.

If the material is not fluidizable and we drop below the saltation velocity, the deposited material forms random waves which are not fluidized, and if these waves fill the pipe, it frequently results in a line plug. A material of this nature would be plastic pellets.  But if I desire to convey plastic pellets in a slow velocity conveying system, I can use another form of dense phase.

Because of the porosity of a bed of pellets, I can fill the pipe with pellets, and supply a small volume, higher pressure air supply. This will move slugs or plugs of the pellets along the conveying line, much like separate trains on a common track. There is a gap between these plugs, keeping the length of each plug rather uniform and the pressure drop across each plug similar. In this dense phase conveying the velocity of the gas is no longer important.  The differential pressure across the plug, the line pressure before and after the plug, pushes the piston through the conveying line. As the piston gets longer, the differential pressure rises, and if the piston is too long, this results in a line plug.

Under many conditions I may desire to move a material that is not as coarse as pellets, but not fluidizable in a slow velocity conveying system. In these cases the material is discharged from a pressure vessel (blow tank) through a converging discharge bend, which compacts the material into an extrusion like discharge. This extrusion is then cut into small lengths by the introduction of booster air or intermittent cycling of the air to the pressure tank and then to the conveying line.

As mentioned above, if the length of the piston is too long, the pressure is so high that a line plug forms. So we must take care that these pistons do not catch up to each other and form a piston of double length. As a result when conveying these types of materials (sugar, sand, salt) the use of boosters is incorporated.   Boosters can be looked at with two functions. First, while the system is running the introduction of air between the pistons helps to keep them separated.

Second, if for some reason two pistons do connect and form a plug, if a booster is located near the middle of this plug it will cut the piston in two. Moving the front piston away, and when they are separated the second piston will follow. So boosters can be looked on as equipment to prevent the formation of plugs in the first place, but if they form then boosters help to remove the plugs.

This article was written by Paul Solt, who is an expert in pneumatic conveying and a consultant for Pneumatic Conveying Consultants . Thanks to Paul for a great article!