Air-lift Pumping
The disadvantages are, that the mechanical efficiency is low; that a considerable amount of air is entrained in the water, and aerated water is very unsuitable for boiler feed purposes; and that means must be provided to allow air to escape by passing the discharge from the pump over a weir or similar contrivance. It is necessary to have some reliable form of oil trap between the compressor and the well to prevent contamination of the water by oil carried over by the air from the cylinders of the compressor; this is difficult, because the oil is not only “atomized” but is actually vaporized while in the compressor cylinders and as a gas it is difficult to reclaim. The air must be kept as low in temperature as possible, and it is usually passed through a cooler before being delivered down the well.
At times, air-lifts are installed for conveying other liquids to a height, and when these can be treated at a high temperature it is advisable, as the efficiency is then much improved. Even under these conditions it is advisable to cool the air to the lowest feasible temperature, before using it as a lifting medium.
When starting up, the column of water in the rising main has to be moved as a solid column, and consequently a higher pressure of air is required at starting than when the column has been set in motion, as the water and air then pass up in alternative “pellets.”
In chemical works and allied industries this pneumatic method is frequently used for pumping acids, and other corrosive liquids from one place to another. Compressed air is a very handy medium for this class of work as ordinary mechanical methods are ruled out, due to the impossibility of introducing corrosive liquids into the pumps and syphons unless great expense is incurred by the use of acid-proof materials.
The air-lift is also very advantageous for pumping water which contains a large amount of sand or similar gritty material which would cut and score the walls of an ordinary piston pump. Air-lift pumping is frequently used, therefore, on new bore-holes until the sand, etc., has been eliminated, after which the final pump can be installed without fear of damage.
The question of submergence will frequently make it impossible to use air-lift without boring many feet deeper than would otherwise be necessary, but when the water bearing strata is low this form of pumping is frequently very convenient.
Miscellaneous Applications of Pneumatic Conveying. Several other interesting applications of pneumatic conveying may be enumerated but, being somewhat outside the primary scope of this book, they will not be discussed in detail. The main object of the author is to raise interest in the handling of solid materials in a manner practically unknown to the general reader.
The housing problem has developed the pneumatic handling of cement in a liquid form, and houses are now being built of reinforced cement in the following manner. An expanded steel frame is supported between concrete or brick piers, and on wood sheeting where necessary, and liquid cement is blown on to the metal in the form of a liquid spray: the first coat dries quickly and leaves a certain amount of cement covering the framework. Then follows another coat, and again another and so on, until the whole of the framework has been covered to an appreciable thickness. The result is a thin wall or slab of cement reinforced with the steel and of great combined strength. Slightly domed roofs constructed in this manner have proved very strong and durable.
The Aerograph is an instrument working on the same principle for the application of paint, and it is used a great deal in the art world, in the manufacture of Christmas cards, in panel painting, and in interior decoration generally. Excellent “tones” and shades are obtained by the simple method of varying the thickness of the colour or the number of coats applied. It is usual to convey the surplus colour and fumes away from the operator by means of a stream of air through a special hood placed at the back of the work, thus maintaining clean pure air for the operator.