Vacuum conveying systems, mostly operating within an absolute pressure ranging from 400 to 800 mbar, are widely used for short distance conveying where dust contamination of production facilities has to be prevented. Depending on the vacuum pump used, realistically the lowest absolute pressure which might be operated in industrial conveying systems might be as low as 200 mbar. In vacuum conveying the task which has to be fulfilled is often to convey products from several distribution points to one collecting point which could be a scale hopper as shown in the picture below.
Due to the restriction of a maximal Δp which will in most of the cases not exceed 800 mbar the conveying distance is limited for many products to 120 m pipeline or less. Furthermore, in vacuum conveying the operator of a system is facing the fact that receiver volumes which are evacuated are limited to 8 m3 or less. Bigger volumes would result in too costly equipment.
Almost all kind of raw materials can be conveyed using dilute phase conveying. In many cases it offers a simple and cost effective solution. Due to the relatively high conveying velocities in the product pipe, resulting in an even product distribution, there is a minor risk of product sedimentation. Ideally dilute phase conveying can be applied for abrasion-resistant granular materials which itself are not causing abrasion on pipelines. Since dilute phase conveying is suspending materials evenly even in dense phase conveying pipe empting is achieved using increased gas velocities, respectively by decreasing the solid mass load.
Dilute phase conveying may be applied for any uniform bulk materials with an even particle size distribution where there is no risk of segregation. This would be the case e.g. for sugar, salt, flour, raw PVC, pellets etc. as long as we don’t mix it with other components with different material properties.
The disadvantage of dilute phase conveying can be found in the wear of pipes and instruments. For sensitive granular or agglomerated products it is not suitable either due particle destruction. Furthermore, in terms of energy consumption dilute phase conveying is less interesting since the air or gas volume needed is much higher than in dense phase conveying. As a result of this bigger vacuum pumps with higher energy consumption are needed.
Semi-dense phase vacuum conveying allows a gentle product flow down to the material saltation point where particles are moved away from sediments by the pure force of the gas velocity. It is recommendable in terms of energy saving and further to treat sensitive products gently, and to protect pipelines from abrasive wear. Densephase conveying needs in certain cases more complex control algorithms than a simple dilute phase conveying since pipeline pressure drop has to be kept in a certain range to keep the solids to air ratio stable. The typical application area is when fragile materials like tablets, chips or extruded rice needs to be vacuum conveyed.
Dense phase conveying is particularly recommendable for light materials as e.g. styrofoam or very abrasive materials as glass-fiber filled plastic pellets with fiberglass contents over 40%. But also greasy products as high fat containing cocoa may be advisably conveyed by dense phase at low speeds. A clear advantage is that high product flows can be achieved with a minimum air load, respectively with minimized energy consumption.
When it comes to segregation free conveying of mixtures with raw materials of a wide spectrum of particle density and sizes it is almost mandatory to go for a dense phase vacuum system if a batch cannot be remixed before packaging.
A further improvement can be achieved using pulsed air or gas injection which is creating a plug flow from the distribution to the receiving point. Product mixes conveyed this way will be transported absolutely segregation free. AZO developed its own system to control dense phase plug flow. With the vacuum plus conveying system, one of AZO’s top systems, almost any conveying task can be achieved. This high-end system is not recommendable for simple conveying tasks.