The flexible hose to which the cleaning nozzle is attached is usually made of rubber, having a smooth bore and strengthened by embedded steel wire or strip which ensures sufficient strength combined with lightness and flexibility. If used with the positive displacement pump, it is generally of i in. internal diameter, or of II- in. internal diameter if used with the multi-stage fan.
In central installations the diameter of the fixed iron pipes forming the mains which carry the dust-laden air to the filter is usually such as to allow an air velocity of between 4o ft. and 90 ft. per second.
Domestic Form.—The advantages of such a convenient and sanitary method of keeping homes free from accumulations of dust were widely appreciated and led in the course of a few years to the production of small cleaners in a variety of forms suitable for daily domestic use and driven by hand or by small electric motors which could be connected with the lighting circuit. Such machines are now common domestic appliances and have done much to displace the broom and duster, the employment of which is both inefficient and insanitary, owing to the unavoidable dispersal of dust in the atmosphere. It is well known that dust, when disturbed and caused to float about in the air, acts as a carrier of germs, and accumulations liable to be disturbed may be a considerable danger to health.
A bacteriological analysis of the dust in any public building gives a sufficient proof of this and observations made over a series of years in a large printing works showed a marked improve ment in the health of the workpeople after the installation and regular use of a vacuum cleaner.
During the World War many factories making high explosives were equipped with central installations of the vacuum cleaner for the purpose of minimizing the dangers of explosion and T.N.T. poisoning, and in one factory, fulminate of mercury was successfully dealt with.
Modified forms of the apparatus are now applied to many industrial processes in which the removal and conveying of dust or powdered material can be carried out economically and con veniently by pneumatic means. One of the more important recent developments is its application to the sooting or removal of fine ash from boiler casings, flues and economizer chambers, a laborious and unhealthy process when carried out by hand with brushes and shovels. In many cases the plant is arranged so that
the operation can take place while the boilers are at work, thus rendering it unnecessary to close down, or for men to enter the casings or flues.
In such a plant a positive acting pump is generally used, its capacity depending upon the number of tons of dust which it is required to remove per hour.
The air suction main leading therefrom passes to an air washer and thence to a cylindrical dust collecting chamber, provided with a dust discharge valve at its base, and usually mounted on a staging, so that it can be emptied into wagons or barges. From this collecting chamber, a main pipe which conveys the dust-laden air is led to the boilers and economizers, etc., and branch pipes fitted with valves enter the boiler casings and flues, and terminate in one or more suction nozzles fixed at points where the dust accumulates. These nozzles are so designed as to admit a sufficient inrush of flue gases with the dust being sucked away in order to ensure its conveyance along the mains to the dust collecting chamber.
In other cases, where it is necessary to enter the flues for the purpose of cleaning, the operator uses a similar suction nozzle connected by a flexible metallic hose to a valve situated at a convenient point in the dust main, and in this manner one man can remove the dust, without inconvenience or disturbance at the rate of two tons per hour. Cement dusts and arsenical pow ders have also been dealt with by similar plants.
Lampblack is collected by suction nozzles from rotating tables, and aluminium and bronze powders used in paints are transferred from the pulverizing stamps to collecting chambers. In these cases the filtration of the conveying air is carried out by means of fabric filters. (H. C. Bo.) Some modern electric cleaners made in the United States clean and polish linoleum and varnished or waxed floors. One cleaner is equipped with a detachable handle for conversion into a portable unit for cleaning automobile upholstery, stair carpets, crevices and corners.