The ordinary pressure blowers are merely another form of fan blowers. They were orig inally designed for use with cupola furnaces, and forges, but are also extensively used for producing mechanical draught for the furnaces of boilers, for use with mechanical stokers, sand blast machines, pneumatic tube delivery systems, etc., or for any work requiring pressures as high as 16 ounces per square inch.
The fan wheels are usually made of thin galvanized steel, and are enclosed in a shell of cast iron. The larger sizes are usually provided with two driving pulleys, and are usually made to discharge horizontally at the bottom, but they can be built in any other of the four right angles. See Fig. 1.
Positive These machines are a more recent form of the rotary blowers, and are very extensively used for operating high pressure forges, blow-torches, gas furnaces, low pressure sand blasts, small pneumatic tube systems, and similar purposes requiring a capacity ranging from 25 to 150 cubic feet of air per minute at a pressure ranging from four to eight ounces. They can be driven by direct connection engines and electric motors, or by power driven belting, and the principle of rotary motion applied to air under high pressure ap pears to eliminate a great deal of the incidental friction, so that when high efficiencies can be obtained they are preferable to fans and blow ing engines.
These machines consist of an outside casing or cylinder of cast iron provided with massive head plates which carry the journal boxes as shown by Fig. 2, which illustrates a horizontal blower geared to an electric motor on the same bed plate. Within this casing, two impellers revolve on horizontal shafts which are con nected by gear wheels outside the casing, thus giving them an invariable relative motion.
The contact surfaces of the impellers are formed on mathematically correct lines, and they revolve together with uniform clearance and without metallic contact either with each other or with the enclosing casing, thus pre venting internal friction. There are no waste spaces between the impellers to cause the formation of air pockets, and no sharp corners or edges to produce sound vibrations, and as the impellers are symmetrical relatively to their shaft centres, they remain perfectly balanced at all speeds.
High Pressure Positive In this type, where the heat of compression constitutes an element of danger, the entire casing and journal boxes are water jacketed, and complete water circulation is maintained through the im pellers and the casing during operation in order to prevent expansion under the temperatures due to the high pressures of compression. The water is forced into the impellers at one end through a hole drilled into the centre of the shaft, and out at the other through a similar passage.
These machines will maintain a pressure ranging from seven to ten pounds per square inch. In some designs a spray of water is used to pack the machines so as to prevent the air from escaping backward, and also for the pur pose of absorbing the heat due to compression. This water is carried over into a separating tank and drained off automatically. This method enables the attainment of an efficiency under high pressures fully as high as that at tained under pressures ranging from three to fourpounds. See HEATING AND VENTILATING; Ant COMPRESSOR.