Pneumatic conveying systems are classified by their operating principle into two types: dilute phase and dense phase. Either can run under pressure or vacuum.
Dilute-phase (also called stream-flow) pneumatic conveying is best compared to what happens in a parking lot on a blustery day. While walking to your car after work, you’re blasted with dust and dirt that’s been picked up off the ground by fast-moving wind. The faster the wind, the more material hits you. As the wind speed picks up, larger dirt particles also become entrained in the wind. Picture the debris carried by a tornado or hurricane: These storms are pneumatically transferring enormous “particles” in dilute phase. Just like the wind picks up the dust, the dilute-phase conveying system relies on the airstream’s velocity to pick up and entrain each particle, keeping the particles in suspension throughout the conveying line.
A typical dilute-phase pneumatic conveying system is shown in Figure 1. They pick up velocity at the system’s start (that is, the airstream velocity at which material is picked up and entrained at the material feed point) is generally considered the system’s most critical area, because the air is at its lowest speed in the entire system at this point. Because the material is dropping from a static state into the airstream below it, the material must immediately become entrained. The air speed required to pick up the material depends on each particle’s size and density, but can range from 3,000 to 9,000 fpm / 15,2 m/s to 45,7 m/s. The air mover must also be able to overcome the flow resistance caused by the frictional loss of the air and material against the conveying line’s inside wall.
A simple way to think of a dilute-phase conveying system is that it operates at a relatively high velocity at a relatively low pressure differential. To design a dilute-phase pneumatic conveying system with the air volume to convey your material, you must use mass calculations (that is, pounds of material per pound of air) while considering your installation location’s ambient air temperature, humidity, and altitude. Then to achieve the proper mixture of air and material in the system, you must meter the material into it at a controlled rate.
Dense-phase pneumatic conveying is also best described with an analogy: It’s similar to what happens in making sausage, when you use high pressure to force ground meat into a casing. An ideal dense-phase conveying system would extrude material with enough pressure to transfer it in one long, continuous piece through the pipeline’s entire length, just like a continuous length of ground meat inside a sausage casing. But we need to keep in mind that there are a lot materials who have high frictional resistance against the conveying line’s inside wall. Instead, air and material flows through the line in any of several patterns.
While various dense-phase conveying system types are available, all use a relatively high pressure differential with a relatively low air velocity. The most common dense-phase type of bulk material handling systems is delivered on board of maritime and offshore related ships and applications. An example of a dense-phase conveying system is shown in Figure 2. Figure 2 provides batch transfer using a Sender tank (also called a blow tank or pressure tank). In this Sender tank system, material from a storage vessel is loaded by gravity into the Sender tank. After the Sender tank is full, its material inlet valve and vent valve are closed and compressed air is metered into the Sender tank. The compressed air extrudes the material from the Sender tank into the conveying line and to the destination. Once the Sender tank and conveying line are empty, the compressed air is turned off and the Sender tank is reloaded. This cycle continues until all the material required for the process has been transferred or the atmospheric hopper above the Sender tank is completely emptied.
Some dense-phase systems have supplemental air injectors (also called air boosters or air assists) located along the conveying line (Figure 2). An air injector works by injecting compressed air (or an- other gas, such as nitrogen, to match the conveying gas) into the conveying line. The added air can clear any plugs caused by low air volume or pressure, eliminating the chore of dismantling the line to remove plugs. Using the injectors reduces the system’s air volume safety factor by reducing the air volume required for reliably conveying the material. (An air volume safety factor is typically built into a pneumatic conveying system’s design to ensure that the system has slightly more air volume than the application requires; however, while this extra air volume helps the system reliably convey material without plugging, it also increases the system’s energy consumption.)
Air injectors along the conveying line can also be used to gently restart flow when material is left in the line after the conveying cycle. This is an advantage for a system handling an abrasive or friable material or a material blend. When the system is restarted without supplementary air, the higher-speed material flow can cause an abrasive material to produce excessive and premature wear on the conveying line and other material-contact components. This higher-speed flow can also damage a friable material, resulting in unacceptably high amounts of particle attrition. It can also deblended a mixture of materials with different particle sizes and bulk densities. Using supplementary air in the line can not only prevent wear, attrition, and deblending problems when a power outage or other event abruptly shuts down the conveying system when it’s full of material, but can provide more system design flexibility for an application where you want to intentionally leave material in the line between cycles.
Air injectors must be designed as fail-safe check valves to prevent the conveyed material from intruding into the injectors compressed-air supply. Such intrusion can occur when material slugs (also called pistons) form inside the dense-phase conveying line (Figure 2); because of the slugs’ changing velocity in the line, usually before line bends, the air in the pockets between the slugs can become pressurized to a level higher than that of the compressed air injected into the line. If this overpressure condition occurs at an air-injection point and the air injector doesn’t have a check valve, some particles can enter the compressed-air supply.
Is there a Standard Way of Categorizing When a Pneumatic Conveying System is Operating in Dilute Phase or Dense Phase?
Unfortunately, there’s no industry standard for measuring these operating phases. So just because a pneumatic conveying system has a rotary airlock valve, it’s not necessarily operating in dilute phase, and just because a system has a Sender tank, it’s not necessarily operating in dense phase.
However, you can use these rules of thumb for determining a pneumatic conveying system’s operating phase:
- Most dilute-phase pressure systems operate below 1 bar, while most dense-phase pressure systems run above 1 bar.
- Most dilute-phase vacuum systems operate below 400 mbar, while most dense-phase vacuum systems run above 400 mbar.
- Depending on the conveyed material, most pressure and vacuum dilute-phase systems have an air velocity between 17 meter per second and 45 meter er second and most pressure and vacuum dense-phase systems have a 15 meter per second or lower air velocity.
- In a dilute-phase system, the material velocity is nearly the same as the air velocity. In a dense-phase system, especially one with slug flow, the average material velocity is much slower than the air velocity. In either system, the material can’t move faster than the air.
Check out the following table, which can show you the main differences.
|Lion Bulk Handling||Imperial||Metric|
|System Type||Dilute Phase||Dense Phase||Dilute Phase||Dense Phase|
|Pressure||4 psi -15 psi||15 psi – 100 psi||0,27 – 1,03 bar||1,03 – 6,9 bar|
|Vacuum||8 in Hg – 12 in Hg||12 in Hg – 20,7 in Hg||270 – 400 mbar||400 mbar – 700 mbar|
|Air Velocity||3500 fpm – 9000 fpm||3000 fpm or lower||17,8 m/s – 45,7 m/s||15,2 m/s or lower|
One caution: When you’re talking to a dense-phase system supplier about selecting a new system, make sure that the material velocity numbers the supplier is using are clearly defined. Some suppliers use air velocity and material velocity numbers interchangeably. Make sure you know what numbers the supplier is talking about before you accept the supplier’s material velocity claims.