Pneumatic Transfer: Which Phase is Best?

By: Hose Master | On: January 9, 2020

Bulk material transfer is a discipline that is employed in a variety of industries including agriculture, petrochemicals, food production, and steel making. When it comes to bulk material transfer, the primary goal is moving materials from point “A” to point “B.” This sounds relatively simple, but there are some crucial choices that determine a bulk transfer system’s success. Bulk material transfer is often done utilizing a pneumatic system, which moves bulk materials through a stripwound hose using positive or negative pressure. Pneumatic systems offer several benefits for conveying materials and the design of these systems is largely dependent on the type of material being transferred. Let’s examine the different types of pneumatic transfer systems and the benefits they provide to bulk transfer applications.

Dilute Phase

Stripwound hoses are common components of pneumatic transfer systems and can be used to convey a variety of dry bulk materials.

Dilute phase conveying is the most commonly used method for pneumatically transporting bulk materials. It works by pushing or pulling materials (using positive or negative pressure) from one location to another by suspending them in a high velocity air stream traveling between 3,200 to 8,000 feet per minute. A key principle involved in dilute phase transfer is minimum suspension velocity- the rate at which transferred materials become airborne. In order to travel in a suspended state, materials must be transferred above their respective minimum suspension velocity. Below this rate, materials may stall and clog the system.

There are several benefits to utilizing a dilute phase conveying system. First and foremost is the high speed at which the dilute phase conveys materials, which allows materials to be moved quickly over long distances without additional air streams. The use of high velocity air in the dilute phase also allows the system to operate continuously. Once the system has begun loading or unloading, the material is held constantly and consistently in the air stream. This makes the flow rates and loading times very predictable and controllable.

While the dilute phase’s high speeds create efficiencies for a conveying system, they also can work against it: the material can hit the conveyor wall and break, fragments can remain in the hoses and cause mixing, and the continuous impact can cause pipes and hoses to wear out and fail if the material is abrasive. If a dilute phase pneumatic system experiences any of these problems it may be a sign that a liner should be added to the stripwound hoses or that the system would potentially benefit from utilizing dense phase transfer instead.

Dense Phase

While dilute phase transfer relies on raw airspeed, dense phase relies on a combination of lower velocity and high pressure. Under these conditions, the conveyed material groups together and the even pressure throughout the system causes it to move in a rolling, wave-like fashion. This behavior is called fluidizing– when groups of solid material moves together and act like a liquid. It is the presence of this phenomenon that distinguishes dense phase from dilute phase.

Abrasive media can wear away the interior of a stripwound hose, resulting in premature failure of the assembly.

Although dense phase transfer is less common than dilute phase, it has some distinct advantages. For example, if the material being conveyed is delicate or friable, then it will incur far less damage traveling in the dense phase because of the natural cushioning effect that occurs during dense phase transfer. Also, because the dense phase moves materials in slow rolling waves, the system’s components (such as stripwound hoses) suffer less impact-related damage, which reduces maintenance costs and increases the longevity of the system.

There are, of course, disadvantages to using dense phase systems as well. First, dense phase systems are generally more expensive to maintain and operate because they move materials less easily over long distances than dilute phase systems. Secondly, dense phase systems have a crucial limiting factor in that not all bulk materials can be fluidized. Being able to move in a fluidized state is a requirement for materials conveyed in the dense phase and, and without that property, dense phase is not possible. Finally, dense phase systems can become plugged if the right considerations and calculations are not made. Because the material is more concentrated, the system is more susceptible to clogging.

Choosing a Transfer Phase

Utilizing the right transfer phase for a bulk material transfer application can have a huge impact on the efficacy of a pneumatic system. Whether you are designing a new system or troubleshooting an existing one, Hose Master can supply stripwound assemblies to help you get the best efficiency and longevity from your pneumatic transfer systems. For more information, please contact our Customer Service team and we’d be happy to help.

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