Problem-Free Air Conveying Part II

Basic pneumatic conveying is a straightforward process. Proper evaluation of performance requirements and product characteristics are vital parts of that process. In addition, careful choice of the type of system and its components help create a pneumatic conveying system that efficiently and reliably contributes to the performance and dependability of the entire process unit.

The system designer and process engineer should carefully consider many factors when choosing a positive pressure, negative pressure or combined system. In addition, key components such as blow tanks, rotary valves and filters have to be well matched to the task at hand.

Positive pressure systems
The requirement for multipoint product feeding stations in many air conveying systems may define system selection. Figure 1 provides a schematic comparison of the three main options.

Multipoint feeding of a positive pressure conveying system generally is not recommended. The air loss from a single feeder subject to leakage can be a significant proportion of that required for conveying the product. The loss from a number of feeders, if they are not isolated by additional valves, could be very high.

The air loss from a number of feeding points could be difficult to estimate accurately. So, the air available for conveying, which is a crucial requirement, could not be guaranteed. Apart from problems associated with having too little or too much air for conveying the product, the loss of a large quantity of air from a number of feeding points also would represent a very significant energy loss for the system.

Negative pressure systems
A common disturbance in "pull-through" systems is vacuum loss, particularly with batch and intermittently operating systems. The cause of the problem often is the failure of the discharge flap to seal at the base of the receiver vessel. Consider a secondary (policeman) filter prior to the exhauster (Roots-type) to safeguard lobes from worn or perforated primary filter elements, etc.

A vacuum conveying system is not the answer if multipoint discharging of product is needed, since such a system requires complex arrangement of pipework and isolating valves. These valves are sometimes found in low-pressure systems where ductwork is used. Valves in the ductwork, however, have to seal effectively, otherwise the air leakage into the system will adversely affect conveying.

If air leaks into a negative pressure system, it will alter the balance of conveying air velocities along the length of the pipeline. If air ingress is not accounted for in the air mover spec, the line is likely to block. If the air mover is over-rated to allow for air leakage, product flow rate will be reduced. Air ingress is likely to occur along a pipeline at flexible sections such as those used in off-loading systems, particularly if the conveyed product is erosive and the joint is hard metal or ceramic material.

Combined systems
The available power for a combined system has to be shared between the two sections. If a Roots-type blower/ exhauster is used, the pressure capability on both the vacuum and blowing sides will be lower than what can be achieved with an equivalent machine used for single duty.

Operating pressures also will differ. With a Roots-type blower, for example, a pressure ratio of 2:1 is generally considered the upper operating limit. Thus for a positive pressure system the maximum delivery pressure is about 1 bar gauge. For a negative pressure system the maximum exhaust pressure is about -0.5 bar gauge. For a combined system, the limit on pressures is approximately 0.4 bar gauge on blowing and -0.3 bar gauge on vacuum.

Since the two parts of the combined system operate at different absolute pressures, the pipelines likely will be different diameters. Figure 2 illustrates this point.

Air also probably will leak across the feeding device on the positive pressure side, so the air flow rates will differ. An imbalance in product flow rates between the two halves of the system necessitates a full assessment of the operating pressures, pipeline bores and air flow rates.

System component problems
Many problems encountered relate to the various system components. The problems generally result from either incorrect specification, or a failure to take account of the conveyed product properties. Not all types of system components are discussed here. Most of the problems associated with screw feeders, for example, are common to rotary valves, so simple representative components are considered.