An experimental jet compressor (ejector) employing gases at subatmospheric pressures is described and data are presented showing efficiencies obtained with air used as both the driving and the driven fluid. A technique is described by means of which efficiency may be determined as a function of outlet pressure and entrainment ratio. Curves are presented showing that efficiency passes through a maximum as outlet pressure is increased. The compression ratio of the driven fluid likewise passes through a maximum as the outlet pressure is increased, but the maximum of efficiency occurs at a slightly higher pressure than the maximum of compression ratio. The highest efficiency achieved was 20.6 per cent, at an entrainment ratio of 1.3 and a compression ratio of 1.14. The ratio of pressure of driven stream to pressure of driving stream at time of contact has been determined. The value of this pressure ratio corresponding to maximum efficiency was found to increase linearly with entrainment ratio, over most of the range of the measurements.

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