Extra cost of bulk deliveries offset by manpower saving
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DROBLEMS arising from the use of vehicles for handling materials in bulk. in place of sacks or containers were assessed at a conference on "The Transport, Storage and Use of Bulk Powders and Granular Materials" at the Institution of Mechanical Engineers in London on Tuesday.
Mr. A. Ratcliffe, an engineering consultant and the conference chairman, emphasized that very little had been published on the economics of bulk systems and that design data available were often inadequate.
Sacks could be delivered to a number of points and the measured contents could provide a basis for quantity control; to the problems of distribution, measurement and feed control on customers' premises if bulk storage were used could be added a tendency of the material to cake and segregate.
In total the problems and difficulties at the receiving end could be formidable, but the availability of bulk transport had sometimes enabled salesmen to persuade customers to buy standard products in place of a multitude of varieties, to the benefit of the producer. The willingness of small customers to invest in storage equipment had made a great impact on commercial and sales policies.
Mr. J. W. Finals, chief tank haulage manager of Pickfords Ltd., in a contribution, "Problems of Discharging. Bulk Vehicles to Storage", stressed the common failure of producers and users of powders to appreciate that the higher cost of bulk deliveries was more than offset by the saving in manpower provided.
Powders cited by Mr. Finnis as "giving the most trouble" included hygroscopic materials that readily coagulated or were loaded hot, flyash and lime being notable examples of the latter.
Experiments described by Mr. Finals included the development of articulated tankers of 1,050
cu.ft. capacity for the movement of powders to and from the Continent, the vehicles being equipped with auxiliary power units to operate the blower and tipping gear. These units had been run on propane gas which had created difficulties particularly in cold weather.
Bulk powder traffic had not been expanded to the same extent as the delivery of liquids to the Continent mainly because it did not afford the same saving in the weight and cost of packaging.
Recent legislation had enabled 1,500 cull. vehicles to be used, comprising two tanks of 800 cu.ft. and 700 cu.ft., which gave a total payload of 19/20 tons. As before, the tanks were discharged by low-pressure air assisted by tipping. Consideration was being given to the development of suitable tanks for liner-train movement, but difficulty was being experienced in producing tanks of sufficient capacity, in the dimension allowable, to be economic.
Dealing specifically with the flowability of powders, Mr. J. C. Williams, School of Powder Technology, University of Bradford, emphasized that it was necessary to take account of the effect of pressure on the strength or compactness of the material being carried or stored, which was a problem that also applied to soil machines.
In a general review of "Extraction and Control of Flow from Bunkers", Mr. R. G. S. Jonston, of the Central Electricity Generating Board,, said that both drag-line and belt feeders could be designed to ensure uniform withdrawal from a bunker. Screw feeders discharging beneath a slot outlet could assist uniform discharge if the screw pitch was increased at the delivery end.
While vibrating feeders could be applied to a wide range of materials, wet, sticky or aerated powders could cause trouble. A weighing unit was essential for accurate feed control.
In a contribution, "What Makes Materials Arch in Bunkers", Mr. J. C. Richards, British Coal Utilization Research Association, pointed out that cohesion could result from interlocking of lumps of irregular shape (and that this could cause bridging over an opening or by surface attraction) or from stickiness that allowed fine particles to adhere together. A hanging flexible plate could promote flow and the use of a lowfrequency vibrating system could be successful as a means of breaking arches.
The problem of cohesion in shallow bins could be overcome by using a mechanical stirrer, a high-frequency vibrator or an aeration system. Research was still needed to match arch-breaking devices to the shear properties of various materials.