The selection of components for a pneumatic conveying system is as important as the selection of the type of conveying system for a given duty. Air movers, pipeline feeding devices and gas–solid separation systems, all have to be carefully considered and there are multiple choices for each.
Blowers and compressors
With air movers a positive displacement machine is generally required. If a blower or compressor is incorrectly specified, in terms of either pressure or volumetric flow rate, the pipeline is likely to block, and with a toxic material this will create its own hazards since the pipeline will have to be unblocked by some means. A minimum gas velocity must be maintained throughout the pipeline system to ensure satisfactory conveying, and it must be remembered that all gases are compressible with respect to both pres- sure and temperature when it comes to evaluating flow rates from specified velocities.
The compression process in most air movers is adiabatic and it is far from being reversible. As a result, the temperature of the air leaving a compressor can be very high. If, for example, air at 20°C is compressed to l bar gauge in a positive displacement blower, the minimum temperature after compression, for a reversible process, would be about 84°C, and with an isentropic efficiency of 80 per cent it would be 100°C. If the same air is compressed to 3 bar gauge in a screw compressor it will be delivered at a temperature of about 200°C.
Oil free air
Oil free air is generally recommended for most pneumatic conveying systems and not just those where the material must not be contaminated, such as food products, pharmaceuticals and chemicals. Lubricating oil, if used in an air compressor, can be carried over with the air and can be trapped at bends in the pipeline or obstructions. Most lubricating oils eventually break down into more carbonaceous matter which is prone to spontaneous combustion, particularly in an oxygen rich environment, and where frictional heating may be generated by moving particulate matter.
Although conventional coalescing after-filters can be fitted, which are highly efficient at removing aerosol oil drops, oil in the super-heated phase will pass straight through them. Super-heated oil vapour will turn back to liquid further down the pipe- line if the air cools. Ultimately precipitation may occur, followed by oil breakdown, and eventually a compressed air fire. The only safe solution, where oil injected compressors are used, is to use chemical after-filters such as the carbon absorber type which are capable of removing oil in both liquid droplet and super-heated phases. The solution, however, is expensive and requires continuous maintenance, and replacement of carbon filter cells.
There have been numerous developments in pipeline feeders to meet the demands of different material characteristics, and ever increasing pressure capabilities for long distance and dense phase conveying. Although the majority of systems probably oper- ate with positive displacement blowers at a pressure below one bar gauge, discharging to atmospheric pressure, there is an increasing demand for conveying systems to feed materials into chemical reactors and combustion systems which operate at a pressure of 20 bar or more.
With positive pressure systems the main problem is feeding the material into a pipeline which contains air at pressure. Due to adverse pressure it is almost impossible to prevent air from leaking across the feeding device. This air will almost certainly carry dust with it, and so if this air or dust must be controlled, some means of containment must be incorporated into the conveying system.
The rotary valve is probably the most commonly used device for feeding conveying pipelines. By virtue of the moving parts and a need to maintain clearances between the rotor blades and the casing, air will leak across the feeder when there is a pressure difference. Rotary valves are ideally suited to both positive pressure and vacuum conveying. The rotary valve is a positive displacement device and so feed rate can be controlled fairly precisely by varying the speed of rotation. The situation with regard to screw feeders is very similar, as these are also positive displacement devices. In positive pressure systems air leakage can be minimized by reducing blade tip clearances, increasing the number of rotor blades, and providing seals on the rotor end plates, but it cannot be eliminated.
Air at pressure will always return with the empty pockets, apart from leaking past blade tip clearances. The air leaking across a rotary valve will often restrict the flow of material into a rotary valve from the supply hopper above. To minimize this influence it is usual to vent a rotary valve in some way. A common device is to provide a vent on the return side of the valve as shown earlier in Figure 22.3. Since the vented air will contain some fine material, this is either directed back to the supply hopper, ducted to a separate filter unit, or re-introduced back into the conveying pipeline.
As there will be a carry-over of material any filter used must be regularly cleaned, otherwise it will rapidly block and cease to be effective. If the air is vented into the supply hopper above, or to a separate filter, the pipe connecting the vent to the filter unit must be designed and sized as if it were a miniature pneumatic conveying system, in order to prevent it from getting blocked. With low pressure conveying systems a venturi can be used to feed the dusty gas from the vent directly back into the pipeline.
If the material to be conveyed is potentially explosive, the use of rotary valves will have to be questioned. With metal blades and a metal housing, a shower of sparks would result if the two were to meet, and a single spark would provide an adequate source of ignition for many materials. With positive pressure conveying systems rotary valve blade tip clearances need to be very small and so differential expansion, resulting from the handling of hot material, or bearing wear, could cause the two to meet. Bearing failure on a rotary valve could well result in a surface at a sufficiently high temperature to provide a necessary ignition source, both within and external to the conveying system. In a fault situation dust can leak from a pressurized conveying system and so bearings external to the system are vulnerable.
The use of blow tanks has increased considerably in recent years and there have been many developments with regard to type and configuration. A particular advantage with these systems is that the blow tank also serves as the feeding device, and so many of the problems associated with pressure differentials across the feeder are largely eliminated. Blow tanks vary in size from a few cubic litres to 30 m3 or more, generally depending upon the material flow rate required and the need to maintain a reasonable rate of blow tank cycling.
With single blow tanks, conveying is by way of batches, but with a large blow tank it may take many minutes to convey the batch, and so the material is likely to be conveyed on a semi-continuous basis. Although continuous air leakage does not occur with blow tanks, as it does with rotary valves, consideration does have to be given to the venting of the blow tank at the end of the conveying cycle, as well as on filling. A similar situation exists with regard to gate valve feeders. Blow tanks generally form the basis of mobile conveying systems, such as road and rail vehicles, and so special provision must be made for venting these during filling operations.
If a discharge valve is not employed on a blow tank there will be a considerable surge at the end of the conveying cycle as the pressurized air in the empty blow tank has to be vented through the pipeline. This will represent a considerable loading on the filtration plant and so it must be sized to take this transient situation into account. With a discharge valve the blow tank can be isolated, once the blow tank is empty, and as a consequence the cycling frequency can be increased quite considerably. In this case, however, the pressurized air in the blow tank will have to be vented before the blow tank can be re-filled and this will create a similar surge loading on the filter through which this air passes.