This article was originally published in Practical Engineering 1940 Vol1 No15. Information within this article is therefore correct as of 1940. The publication of this material aims to provide historical insight on the subject and its place in industry.
For a wide range of applications in the engineering industries, and many other applications, the high-speed rotary compressor, which acts equally well as an air compressor or a vacuum pump, has many advantages. It is small, light in weight, smooth and noiseless in operation, without vibration or pulsation, and highly efficient, with negligible wear and tear because of the absence of rapidly operating valves. Another advantage also is the easy direct drive by electric motor steam turbine or petrol engine on the same baseplate. The latter is a convenient method when no electric power is available, and typical of the latest practice is a small, neat, and compact set recently designed by Hick Hargreaves and Co., Ltd., Bolton.
Compressor or Vacuum Pump
This has a single-stage water-cooled rotary compressor which, when operating as a vacuum pump, has a capacity of 102 cubic feet of air per minute from 28in. mercury, when evacuating a closed vessel of 160 cubic feet capacity in 10 minutes. In the case of compressor operation the duty is a maximum,, at, say, 10lb. per square inch pressure, of 88 cubic feet of air per minute for the same power.
With small single-acting sets of this description air pressure up to 601b. per square inch can be obtained. The small direct-coupled petrol engine is of the vertical, single cylinder, 4-stroke type, 31/2in. bore and 4in. stroke, with water cooling assisted by a radiator and fan. It runs at 1,450 r.p.m. and develops 5.5. b.h.p., whilst the total weight of the combined set is approximately 6 cwt.
Features of the Design
The design of these rotary compressors and vacuum pumps, both on the one-stage and two-stage principle, consists essentially in the use of a close-grained cast-iron rotor, carried on a spindle, and running eccentrically in an outer cast-iron cylinder, which is either air- or water-cooled. This rotor, carried by a heavy steel shaft running in roller bearings, is provided with a number of loose, thin, hardened steel blades, contained loosely in a series of deep radial, very narrow slots, the blades being thrown outwards by the centrifugal force so that they always make an airtight connection as they sweep round. To avoid wear and tear, also, the tips of the blades are carried on loose floating restraining cast-iron rings, free to revolve with the rotor.
Some other features are self-contained, mechanically operated lubrication, with separate feeds to each point; flexible coupling between the compressor and the driving units; and an automatic governor operating on the principle of closing the inlet when the compressed air begins to exceed the desired pressure.
The single-stage compressor, type R.C., for example, delivers compressed air at 5-60lb. per square inch and runs at speeds of 485-2,850 r.p.m., and the smallest size, with a speed of 2,850 r.p.m. delivering air at 5lb. pressure, takes 0.5 h.p. with 8.7 cubic feet of free air per minute and 3.0 h.p. for 53 cubic feet per minute. At the other end of the scale a very large compressor delivers air at 30lb. per square inch and runs at 485 revs., taking 200 h.p., with 1,810 cubic feet of free air per minute. With regard to the double-stage compressors, type R.C.C., the normal range is compressed air at 70-150lbs. per square inch and speeds of 485-2,850 r.p.m., while the largest standard sizes when running at 485 r.p.m. and taking 425 h.p., delivers air at, say, l00lbs. per square inch, equal to 2,000 cubic feet of free air per minute.