Fig. 70.—"Paasche" Automatic Electric Controller.
CHAPTER VII.
Supply of Compressed Air—Paint Supply—Exhaust.
The forms of spraying apparatus having been selected, consideration must next be given to the following details, viz.:—(1) The supply and degree of pressure of the compressed air. (2) The form and location of the paint supply. (3) The arrangement of an exhaust installation to carry away the fumes and waste paint. These will be dealt with in their order.
It is clear that attention has to be paid to a convenient arrangement of reducing valves controlling the pressure of air to the sprayer, and also providing, what is in many cases indispensable, a pressure feed of colour to the sprayer, either from a large pot mounted over the hood, or a special type of bowl screwed on to the sprayer.
Some makers still advocate a simple type with small bowl screwed on to the sprayer, and no means of controlling the pressure save by adjusting the safety valve on the main tank, thus necessitating that each operator shall work at the same pressure, arguing that reducing valves, pressure feeds, etc., etc., are unnecessary refinements, but it may be safely claimed that such opinions are made without due consideration of the advantages of the more complete system, and that the consensus of opinion is in favour of such refinements, giving, as they undoubtedly do, a further increase in speed, and, in many cases, making practicable what would otherwise be impossible, as well as ensuring that whatever work is to be done, the most suitable pressure is instantly available.
In the equipment of an efficient compressed air plant for painting, the type of compressor employed, and the arrangement of the air main, should first be decided upon.
The compressors must be designed to give an absolutely pure supply of air free from oil or grit, and the air-main must be so arranged as to avoid any trouble due to condensation. This is overcome, first, by using, at least for high-class work, a horizontal air compressor, water cooled, with a sensitive sight-feed needle valve lubricator to the cylinder, capable of delicate adjustment, so as to avoid excess of lubrication. Needless to say, the machining of the cylinder, piston, etc., of the compressor must be of the highest class to ensure satisfactory running under such conditions. Then a suitable air filter must be fitted to the intake of the compressor. This filter should be of large diameter, and have a gauze screen and wad of cotton wool.