Horizontal pipe sections are normally making most of many pneumatic conveying pipe layout and the point of inlet of the solids to be transported in always horizontal. Therefore, to better understand the behavior and possible upsets of a pneumatic conveying line, it is essential to have in mind how the bi-phasic flow of air and solids behave in horizontal pipes. This page is explaining how the flow of product and air establishes in straight horizontal sections and to which parameters and phenomena the Engineer must be attentive.
In dilute phase conveying, the solids must be in suspension in the air. Therefore the air velocity must be high enough to avoid that the particles settle at the bottom of the pipe and create a plug. This phenomena of particles depositing because of a too low air conveying velocity is called saltation. Each powder or bulk solids can be characterized by a saltation velocity which is minimum air conveying speed above which the particles will be kept in suspension and transported. It is interesting to remark that the saltation velocity - which prevails for a «side» air flow - is usually much higher than the terminal velocity - which prevails for a vertical air flow - which happens in straight vertical section. Thus, the horizontal conveying in dilute phase is actually requiring higher speed than vertical conveying to stay sustainable.
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In dense phase, the dynamic is totally different than in dilute phase. The powder circulates in dense plugs, thus the difficulty here is not to keep the solids in suspension but rather to compensate the friction in between the product and the pipe walls. To be noted that in vertical sections, the friction is much less as the powder does not lie on the piping, but the weight of the column of product must be compensated.
Note : this page is under development and review, it will be updated soon.