Pneumatic Conveying Rates and Filtration Surface Area
The amount of filtration surface area available for separating the solids from the conveying gas can considerably impact transfer rates and energy consumption when choosing a pneumatic conveying system.
Pneumatic conveying systems are classified as dense phase (Tanglian ‘ Powder Pump systems) or dilute phase. They use dramatically different volumes of conveying gas. Dilute phase systems generally operate under positive pressure, with the gas stream exceeding the solid particles’ “saltation” velocity. As a result, they require vastly different amounts of filter surface area to function correctly.
This is the absolute minimum gas velocity required to keep solids suspended in the gas stream and flowing through the conveying line.
When the solids arrive at their destination, they must be separated from the gas stream using centrifugal cyclones and an appropriate filtering receiver. Given the high volumetric flow rates of gas required to achieve saltation velocities in transfer lines, the filter surface area required to obtain a good face velocity on the medium can be enormous.
Solids will contact the media excessively if the surface area is insufficient, causing premature degradation of the filter’s surface membrane or the underlying substrate media. An inadequate filter area increases pressure drop through the system, increasing energy usage. If the pressure drop is too significant, the gas stream velocity will drop below the saltation point, and solids will fall out of it in the conveying line.
On the other hand, dense phase pneumatic conveying systems operate at much lower gas velocities since the material’s saltation velocity is no longer required. In dense phase systems, the solids-to-gas ratio is substantially larger than in dilute phase systems. This means that a given quantity of transported materials requires less gas separation. Furthermore, suppose the dense phase system is under negative pressure (as in Tanglian ‘s Powder Pump systems). In that case, gas’s low mass flow rate can easily attain the velocity required for dense phase conveying because the gas expands due to the vacuum’s reduced pressure. Another feature of the Tanglian Powder Pump is the tangential solids entry nozzle, which allows for cyclonic solids separation before the gas stream passes through the internal filter element. This reduces the physical effect of solids on the filter and their loading. Furthermore, the Powder Pump cyclically fills and discharges materials. The filter element is back-pulsed and cleaned with each discharge cycle.
The physical area available for the filter media is inevitably limited due to the Powder Pump system’s compact form. Early system designs only used a flat membrane, which meant that its surface area was limited (and proportional to) the system receiver’s diameter (4″, 6″, 8″, or 12″).
Following Tanglian design upgrades, the flat filter was replaced by a new, proprietary, purpose-designed, and built sock filter. As seen in the table below, this design improvement improved the filter surface area for each system size.
Some system operators reported considerable transfer rate gains (25 to 50 percent) after modifying their previously installed systems due to the increased surface area.
Since then, Tanglian has improved its design to allow three and four filter socks to be added in more extensive Powder Pump systems. The table below shows the enhanced filtration surface area of multiple vs. single sock filters.
After installing the additional filters in the test system, the resultant increase in filter area significantly reduced pressure drops across those systems, allowing material that would not otherwise transfer to be delivered during customer-seen tests.
Reduced pressure drop across the system also enables greater distances to be covered with the same motive force. Because less pressure is necessary to overcome the pressure drop across the filter medium, more of the total differential pressure supplied to the gas stream in the system is now available to overcome resistance in the transfer line.
When choosing an appropriate system design to handle specific solids conveying problems, there are numerous factors to consider. The available filter area is one of them, affecting transfer rates and distances as indicated above. To give the optimum performance for our dense phase Powder Pump systems, Tanglian has worked to maximize this particular design element. Please read our new bulletin or contact us for more information on our filter design update choices.
