Abstract: Surging of a centrifugal compressor is avoided by ensuring that in operation ##EQU1## WHERE K and k are parameters whose values depend on the characteristics of the compressor, g is the acceleration due to gravity, h.sub.p is the polytropic head produced by the compressor, Vc is the velocity of sound in said inlet gas, and Mn (the Mach Number) is the ratio of the flow velocity V of the gas at the inlet to the compressor to the velocity of sound Vc therein. This is normally effected by arranging that ##EQU2## where .DELTA..sub.p is the differential pressure across a throttling member disposed in an inlet duct of the compressor, P.sub.1 is the compressor inlet pressure, and P.sub.2 is the compressor outlet pressure.

Abstract: Apparatus for determining fluid flow, said apparatus comprising flow obstruction means and two heat-emitting members which are mounted in a geometrically symmetrical arrangement in a conduit, the heat-emitting members being subject to the same heat losses when there is no fluid flow through the conduit, the flow obstruction means causing the two heat-emitting members to be differentially affected by the fluid flow through the conduit, and means responsive to the ratio of the heat losses from the two heat-emitting members for determining the said fluid flow.