matic pressure regulators are fitted to maintain the air pressure in the receiver nearly constant. These compressors are only used for moderate air pressures up to about 500 lb. per square inch. For higher pressures many varieties of three and four-stage types are made. The Brotherhood compressors are tested from the quantity of air actually pumped into a reservoir of predetermined capacity, a method which gives the net results after all losses due to the clearance volume, imperfect action of the air valves, leakage past the piston, or loss due to increase of temperature during compression. The "Sentinel" air compressors by Messrs Alley & Maclellan, Ltd., of Glasgow, are of style; that is, a piston valve controls the air inlet, and the discharge is actuated by valves which are practically unbreakable. The governor gear is of simple but effective design, and not liable to get out of order easily, the action of the same being; that on a rise of 2 lb. above the working pressure, air is admitted to the controller, which shuts an equilibrium valve situated on the air inlet. Thus the air supply can be practically cut off, and the compressor run on a partial vacuum, which is economical as regards power absorbed when the machine is running light. An intercooler of compact design, but of large surface, is arranged in the base plate, and the air, which is cooled to a certain extent by the INLET OUTLET Fig. 23.-Enlarged Transverse Section through Air Cylinder of Brotherhood Compressor, to show Valves. vertical type. These machines, which are the outcome of many years' experience, rank high in both mechanical and volumetric efficiency. The special features of the design, Fig. 24, are the following: The complete machine is enclosed, and thus no dirt or grit can possibly get at the bearings to cause trouble through heating. The lubrication is of the forced type, the oil pump being driven from one end of the crank shaft, and a filter is fitted which is of easy access, and can be cleaned in two minutes; this, with bearings of exceptionally large surface, ensures good running under all conditions. The valve gear is of the combined mechanical and automatic water jackets that are round the cylinder, passes from the LP. cylinder (in compound types) down a pipe to the cooler, where the pipes are arranged so that the air must pass backwards and forwards a number of times before reaching the HP. cylinder for final compression. The floor space occupied has been reduced to a minimum, and the designs have been so arranged that either belt, motor, or steam can be used for driving purposes. In the latter the steam cylinders are arranged on top of the air cylinders, and when a machine of larger capacity is required, Messrs Alley & Maclellan have special crank cases to take two, or three cylinders as may be required. Thus, for a 400 cubic feet Fig. 24.-Vertical Single Cylinder Two-Stage Air Compressor. Alley & Maclellan, Ltd., Glasgow. jacketed, by means of which four deliveries of air are given during each revolution, the effect being that of four separate single-acting air compressors. Each cylinder has its own trunk piston, and the four connecting rods are driven by a common crank pin. The delivery is practically continuous, since there are no reciproca tions, and the speed is high. Portable air compressors are made for the purpose of supplying pneumatic tools and pressor piston is driven by the tail rod of the engine piston, which brings the air cylinder directly over the fire-box. The engine and air cylinder are flanked by two horizontal air receivers having connecting pipes coming from the delivery valves above the cylinder. The circulation of the cooling water in the air cylinder jacket is maintained by a plunger pump operated by an eccentric on the crank shaft. Other portable compressors driven by steam |