Pneumatic conveying is an efficient and secure method for conveying raw materials and finished products around plants, from arrival bays to shipping out. Pneumatic systems are versatile, but there are design considerations for particular kinds of material. A major variable is a relative density between the transported material and the volume of flowing air. The options are referred to as “dense phase”, “dilute phase”, and “semi-dense phase”.
In dense phase, conveyors move air at a lower speed, resulting in a lower volume of air in proportion to the material it pushes. Factors like pipe bore and material density also have some bearing on the ratio achieved. The aim is to roll material through the conveyor, rather than making it airborne.
In dilute phase, air is pumped at high enough velocity to keep materials fully and continuously suspended. To achieve this, air-speed is usually between 5000-8000 feet per minute.
The aim of a semi-dense approach is to achieve a balance between rolling and airborne material. While a system might be run in a semi-dense state to control the rate that materials are delivered into other processes, this method should not be looked on merely as a compromise between higher and lower powered systems. For certain materials, semi-dense conveying is often more suitable than either alternative, regardless of delivery rate considerations, which can be adjusted using other parameters and devices.
Whilst it’s true that lighter materials are easier to make airborne, materials have many other chemical and physical characteristics beside weight to consider. Some are prone to packing or tangling, some are sticky, and others are mixtures or consist of granules of different sizes not intended to separate.
The necessary air ratio depends on these numerous characteristics, as will your choice between a pressure-driven or vacuum conveyor. Specialist installers, like http://www.aptech.uk.com/pneumatic-conveying-systems/vacuum-conveying/, draw on real-world experience of how materials behave in practice, to adjust your conveyor’s design. Safety considerations are also a factor.
Dense phase is best for abrasive, fragile or hygroscopic materials, including sugar and sand. Systems can still be designed to deliver materials at a high rate if required. Semi-dense conveying is best suited for aeratable, abrasive and friable materials like cement and ash, and dilute phase is typically used for light materials that are neither fragile nor abrasive, such as flour or cornstarch.