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Pneumatic conveying in vertical pipe sections

Specificity of vertical pneumatic conveying

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Section summary
1. Definition
2. Impact of vertical conveying on pressure drop

1. Definition

In a typical pneumatic conveying pipe layout, most of the pipe sections are horizontal. However, in certain situations, the elevation may not be negligible. It is specifically the case for factories having dry-mix towers which can reach 30 or 50 m height. This is all the more important to understand the impact of vertical conveying that it is very seldom to find a test center able to simulate such elevations, with maximum of 5 to 10 m generally.

2. Impact of vertical conveying on pressure drop

Dilute phase pneumatic transport

In conveying vertically a power or bulk solids in a vertical pipe, the interaction in between the pipe, the air and the solid transported will roughly be similar to the conveying in horizontal pipe, as the solids are dispersed in the carrying air.

The pressure drop will be slightly higher than the pressure drop in an horizontal line (horizontal pipe section with already well established flow, without considering the acceleration of gas and solids at the beginning of the line) as the weight of the air and the weight of the solid in the vertical pipe needs to be compensated.

In shortcut methods, this is taken into account by having an equivalent length of 2 times the length of the vertical section : Le(vertical section) = 2*Lv. In a test plant for example, the Engineer may compensate the lack of elevation by adding more horizontal distance.

In more accurate model, the pressure drop due to the weight of gas and solid can be calculated.

Dense phase pneumatic transport

Manufacturers of pneumatic transport system are generally not too afraid of vertical section they cannot modify in their test line. The reason behind this is that friction of the solids with the pipe is much more intense in dense phase conveying versus dilute phase conveying, and more important in horizontal pipe compared to vertical pipe where the powder is not lying on the pipe.

3. Inclined pipe in pneumatic conveying

Inclined pipes should be avoided when designing a pneumatic conveying pipe layout. Indeed, the saltation velocity in inclined pipe is actually higher than in horizontal ones [1], or the choking velocity in vertical pipe. Thus, even if the air conveying velocity is correct for vertical and horizontal section, it may be lower than the saltation velocity in inclined pipe, leading to deposition of product, reflux, pressure instability and maybe pipe blockage. If it happens, the air conveying velocity must be increased, which will be costly and may also affect the solids breakage / build-up along the pipe.


Source

[1] Powder Bulk Engineering, Pneumatic Points to Ponder, March 2017