It must be remembered that in low pressure systems handling shavings, dust, etc., the problem is quite different from that in high pressure systems handling wheat, etc. In the case of removing dust or shavings from a machine, the material is already in motion, and only requires drawing forward and into the pipe system, but in the case of conveyors for wheat, coal, etc., and in the case of suction cleaners, the material to be moved is heavy and stationary and has to be lifted and started in motion before it can be carried away. This necessitates a much higher air velocity through the collecting nozzles.
Idle Nozzles should be Closed. It is perhaps advisable to draw attention at this point to the disadvantages of using more than one suction nozzle on one receiver at one and the same time. The reader is asked to recall the fact that the material is not lifted by vacuum, but that the production of a partial vacuum causes a stream of air to pass up the pipe at high velocity. The material to be conveyed is entrained with the air, and due to the frictional contact between the particles of air and the particles of material, the latter is lifted and carried forward.
If the conveying plant is to be efficient and of reasonable capacity, the pipes must be relatively large, and in order that the desired partial vacuum may be maintained in them (establishing a vigorous air current) without the use of an unduly large pump, it is important that air be admitted only through those nozzles which are actually in use. Also, when more than one nozzle is in use at the same time, it is necessary to keep each nozzle covered with material to such an extent that the same amount of air passes into each pipe. Unless this is done a large quantity of air will pass up one pipe, and a small quantity up the other, and the amount of material taken in at each nozzle will vary as the quantity of air varies. To consider an extreme case, suppose that the man operating at one of the pipes allows his nozzle to become exposed. Air will rush in at this nozzle to the full capacity of the pump, with the result that little or no air will pass up the second pipe, and consequently no material either. Thus, if one man is sufficiently neglectful to leave his nozzle idle and open, he renders practically useless the other nozzle or nozzles on the same main.
Even with care this is bound to occur to a certain extent, as is shown by the figures given by makers for the estimated power consumption, viz., about 1 h.p. per ton on single-nozzle plants, and 1½ h.p. on double-nozzle plants.
Under these conditions it should be considered whether it is more advisable to install one large plant with two nozzles, or two small plants, each with only one nozzle. The decision depends upon the extra cost of power for the double-nozzle plant compared with the higher capital charges on the two single-nozzle plants.
CHAPTER IV
TYPICAL INSTALLATIONS FOR GRAIN
The pneumatic principle has been applied to the handling of grain in bulk to a much greater extent than to any other material. As previously stated, the original successful plant invented and designed by Mr. Frederic Duckham was for the handling of wheat for the Millwall Docks Co., and the success of this plant was such that it was imitated by engineers in this country and abroad with equally satisfactory results.
Grain lends itself admirably to pneumatic transport because it is easy of flow, regular in size, and practically self-feeding. Also, the removal of dust, which is incidental to pneumatic conveying, is a special advantage where grain is concerned. Figs. 17 and 18 show clearly the advantages of pneumatic conveying in point of simplicity and labour saving.
Typical Quayside Plant. Fig. 19 shows a typical lay-out for a comparatively small plant handling 50 tons per hour. The diagram is almost self explanatory.
A represents the special suction nozzle through which the grain enters the system, together with the “free air” which acts as the conveying medium.