The latter system has many advantages, but it carries with it the necessity of providing automatic delivery of the material without seriously impairing the suction. In order to accomplish this, some form of “air lock” is required, and a necessary feature of this is a device with close-fitting surfaces or more or less air-tight valves. The simpler plan is undoubtedly to pass the material through the fan, as there is then no question of breaking the suction in order to get the material out of the system. It is common practice to convey for long distances such materials as wood chippings and sawdust, cotton, jute, wool, esparto grass, paper chippings and many other materials. In all cases, however, the designer is confronted with the problem of separating the material from the air, and in many instances to do this satisfactorily is more difficult than the actual conveying; especially is this so with certain sticky materials or materials which will readily “pack” or build up when entering any mechanical discharger at the high velocity necessary in the suction system.
CHAPTER II
DETAILS OF PLANT PUMPS, EXHAUSTERS AND AIR FILTERS
A pneumatic conveying plant of practically any type comprises the following five main components: (1) A pump or exhauster to create a partial vacuum in the pipe line and so induce a high velocity air current in which the material will be conveyed. (2) An air filter in which any light dust carried over beyond the receiver is trapped, to prevent its entering the exhauster where it would quickly damage the piston rings and cylinder walls of reciprocating pumps. (3) A main receiver or discharger, and occasionally one or more subsidiary receivers, in which the conveyed material is extracted from the system and discharged into the receptacle or on to a dump as required. (4) The pipe line, junctions, etc., to connect the point of supply with the desired point of delivery. (5) The suction nozzle through which the material enters the system, together with the “free air” which is to act as the conveying medium.
Pumps and Exhausters. The type of apparatus used for creating the flow of air varies according to the ideas of the numerous makers of the plants. It must be remembered that for a pump working under the conditions required for pneumatic plants it is not a high vacuum that is required; the most important function is to handle very large quantities of air at a comparatively low vacuum.
The most efficient type of plant for dealing with large quantities of air is certainly the reciprocating pump, although several makers are now devoting a lot of attention to the multi-stage turbine type of blower, or exhauster. The probability is that this type may shortly be as efficient as the reciprocating pump, and if so it is almost sure to be used extensively, as it has distinct advantages in other directions compared with the cylinder and piston type. This remark has special reference to the high pressure system for conveying sand, coal, sugar, etc., and does not apply to the “small pipe system” detailed later.
In dealing with ashes, flue dust, crushed iron ore, and similar abrasive materials, it is essential that the air passing through the pump should be filtered thoroughly so that no dust should enter the cylinders and cut the walls and piston rings. The turbo-blower, having no rubbing surfaces, would suffer little or no damage from a small percentage of dust, hence the filtering equipment might be less elaborate and less costly. No lubrication would be necessary in the rotary pump (except, of course, at the bearings), hence no difficulties would arise owing to oil acting as a dust trap, as is sometimes the case in reciprocating pumps with defective air filters.
Fig. 1.—King’s Pneumatic System. Steam-driven Air Pump.
Key to Fig. 1.
A = cylinder of double-acting steam engine driving the whole machine.