Problem-Free Air Conveying Part II

Productive and reliable system and component choices depend on knowing about the potential problems each of them can cause

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Rotary valves:

It is essential to feed a conveying line at the correct rate. If the feed rate is too low, the pipeline will be under-used; if too high, the pipeline could block. Varying the rotational speed of the rotor can provide flow control. There is an upper limit for any given size of valve, however, for the pocket filling efficiency will decrease with increase in speed.

If a variable-speed drive is used, the flow rate will be infinitely variable, as it is with a blow tank, up to its maximum capability with a product. If some form of gearing is provided, only step changes will be possible. Many rotary valves are dedicated to a single product and duty, with no speed control incorporated.

Conveying over a different distance requires a corresponding change in feed rate. If a different product is to be conveyed, it is quite likely that both the pipeline and rotary valve characteristics for the product will be different.

Granular products tend to shear in a normal drop-through rotary valve. The product might prompt considerable vibration, which could shorten the life of the rotary valve and the drive motor. Cohesive products can cause a problem with the proper discharge of product from the rotor pockets. In this case, a blow-through-type rotary valve would be recommended.

Air leakage across a rotary valve primarily depends upon the rotor-tip clearance and the pressure drop across the valve. Leakage also varies with the product being fed. A cohesive product, for example, will help to seal the various clearances and reduce leaks.

Air leaking across a rotary valve means less air is available to convey the product. In specifying the air requirements for the air mover, leakage must be considered. This leakage represents a total loss of energy from the system.

The air leaking across the valve may interfere with the feeding of product into the rotary valve. In this case, venting might solve the problem. A certain amount of product could be carried over with the vented air, so the vent line must be kept clear. Air leakage will also increase in tandem with valve size. A larger-than-necessary valve probably will generate unnecessarily high air leakage as a result.

Rotary valves are not generally recommended for handling abrasive products. Apart from abrasive wear of the sliding surfaces, erosive wear will be severe due to the very high velocities of air leaking through the valve. Wear will increase rotor-tip clearances and boost air leakage. This, in turn, causes air loss in the conveying line that could ultimately result in pipeline blockage.

Filters:

Be aware that the sample of product you supply to a filter manufacturer for selection and sizing could differ significantly from that to be handled by the plant filter. If it is a friable material and the conveying air velocity is high, the product at the end of the conveying line could be very different.

Cloth filters will gradually block with fine product that cannot be shaken free, and performance will be less effective. Filter bags, therefore, require periodic replacement.

Operators can monitor performance of the filters to a certain extent by noting the empty-line pressure-drop values. A pressure gauge in the air supply or extraction lines enables checking of the empty line pressure drop. This pressure drop represents the combined resistances of the pipeline and filtration unit. If the pipeline is purged of product, any changes in pressure drop can generally be attributed to the filter. An increase in this pressure drop would indicate that filter cleaning is not effective and should be checked.

Alternatively, an additional pressure gauge could be positioned in the receiving hopper. A positive pressure system exhausting to atmosphere will give a direct reading of the pressure drop across the filtration unit. It also will let operators record an on-load assessment of the pressure drop on a regular basis. In a negative pressure system, the difference in pressure between the gauge in the receiving hopper and the gauge in the air extraction line will have to be taken.

With reverse-air-jet filters, ensure that the air supply for the filter bag is correctly connected and of adequate capacity, and the cleaning cycle timer is set and operating correctly.

The surface area of filter cloth required is largely based on the volumetric air flow to be handled. The ratio of flow rate to area provides an approximate face velocity. However, if the filter is in a negative pressure system, the volumetric flow rate will be significantly higher, necessitating a much greater cloth area to maintain the same face velocity as an equivalent positive pressure system exhausting to atmospheric pressure.

A check on the empty line and filter-unit pressure drop is important during commissioning and before any product is conveyed to baseline the pressure drop across the filter against the design specification.

In batch conveying cycles, air flow rate per unit time is not uniform. At the end of a cycle, when the blow tank is just empty, a large volume of air is stored under pressure in the blow tank and pipeline. Venting this air, together with the compressor output for conveying, generates a significantly higher filter duty. Take this high air flow rate into account in the specification of the filter. Isolating the blow tank from the conveying line when it is empty, and venting it separately can reduce this surge.

Elizabeth Knight is a senior consultant and Dr. Don McGlinchey is a consulting engineer at Glasgow Caledonian University's (GCU) Center for Industrial Bulk Solids Handling, Cowcaddens Road, Glasgow G40BA, U.K. Tel: (440) 141-331-3715. The authors wish to recognize the contribution made to this article by their esteemed colleague, Dr. Pedrag Marjanovic, who, the authors note with sadness, has since passed.

Most problems with filters generally result from incorrect specification of either the air flow rate or the expected particle-size distribution. Filter cloths and screens will rapidly block if they have to cope with unexpectedly high flow rates of fine, degradated powder. The net result usually is an increase in pressure drop across the filter, meaning a reduction in the pressure drop available to convey the product.Rotary valves come in a wide range of sizes and types, and probably are the most commonly used devices for feeding pipelines in low-pressure systems. The mechanism of feeding, however, gives rise to a number of problems, and in positive pressure systems allowance must be made for air leakage.Of all system components, blow tanks probably are least understood when it comes to operation and control (Figure 3). Single blow tank systems operate on a batch-wise mode and conveying is not continuous. Figure 4 depicts the transient nature of the flow.
The rotary lobes in blowers are machined to close tolerances. Any ingress of dust or product into the machine will have a serious effect on the performance of the blower. Downstream of the blower, or any other air mover, non-return valves should be included in the air supply lines to prevent the possibility of back-flushing of products.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.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.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. 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.

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