and anti stall casing treatment the casing treatment includes an annular recess in the casing for removing low momentum flow adjacent the tips of the rotor blades and returning the flow to the generally cylindrical flow passage upstream of the point of removal a plurality of curved guide vanes are located within the annular recess so as to define an annular inlet downstream of the vanes and or an annular outlet upstream of the vanes each guide vane projects radially inwardly from the casing towards a free end which is exposed at or near the mouth of the recess to define a series of curved channels within the recess adjacent the annular inlet and or the annular outlet application_number US 50597103 A description fig1 of the drawings illustrates a portion of a casing 10 of a multi stage axial flow turbo compressor and one of a series of rotor blades 12 on a rotor shaft not illustrated extending centrally through the casing a series of stator blades 14 and 16 are secured to the casing upstream and downstream of the rotor blades respectively as shown to delay the onset of stall conditions at the tips of the rotor blades the casing 10 includes an anti stall casing treatment arrangement designated generally with the reference numeral 18 in this embodiment of the invention the arrangement 18 comprises an annular recess 20 in the casing 10 and a plurality of spaced apart guide vanes 22 within the recess with reference also to fig2 and 3 of the accompanying drawings the recess 20 is formed by a rear wall 26 a front wall 28 which together with the rear wall defines a mouth 30 leading into the recess 20 and an outer wall 32 between the rear wall and.