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West of I-95 in Virginia: Joe Polito - joe@vaheat.com (804-350-8829)
East of I-95 in Virginia, Maryland and DC: Chris Haslego - chris@vaheat.com (804-370-3855)

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Dense and Dilute Conveying

Although conveying materials through a piping system seems simple enough that the same solution should be applied to many applications, the reality is that materials which appear similar may act very differently when trying to fluidize and convey them. The saltation point, or a condition where the material will either clump together and sink, or have the top layers break off and flow with the stream is dependant on the stream velocity, particle size of the material and design of the system (a pull or a push system). When a system is considered "dense phase" then the particles will settle on the bottom, and a "dilute phase" has the material fluidized to the point that it acts like a liquid instead of a group of solids.  

Dense phase conveying – The material saltation point is hit and material forms at the bottom of the piping. Materials that are composed of large mono-sized particles, such as polyethylene and nylon pellets, peanuts, and certain grains and seeds, convey very well in plug flow. This is a more gentle process than dilute phase conveying where the high velocities required to fluidize may cause damage to nylons and polymers, or grains and seeds might not subsequently germinate as a consequence of rough handling.
For: Dense, abrasive and fragile materials

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 method needs high gas velocity paired with low enough solids loading to be successful. This method has higher power requirements to maintian the increased velocity. Coarse granular materials such as sand that have very poor air retention and permeability are generally only capable of being conveyed in conventional dilute-phase.
For: Pellets, powders, dusts

Vacuum Systems
The system operated at a low pressure, which is normally 0.4-0.5 atm below atmosphere, and is utilized mainly in conveying light free flowing materials. This method is efficient at conveying material from many points to a single unloading point.

Pressure Systems
This system is ideal for conveying material from one loading point to a number of unloading points. It operates at a pressure of 6 atm and upwards.

More information on vacuum and pressure systems