casing hooks 27 and 29 and by extending longitudinal notches 30 and 33 rabbets 50 have several advantages including the reduction of contact surface between the spacer and casing hence reducing casing overheating due to conduction better control of flow section of the air circulating in circumference because manufacturing dispersions are lower for rabbets 50 than for the bottom of the groove of the casing hooks better control of peripheral speed of air flow and convective exchange coefficients and greater convective heat transferring surface on casing 1 and thus better control over the flow of heat and its homogeneity heat exchanges are produced in spaces 31 and 34 they are regulated by the surface casing 1 wet by gas of the speed of air flow in circumferential direction the number of longitudinal notches 30 and 33 and therefore the length of circumferential paths a communication between chamber 18 and 19 is thus established the ventilation gases flowing through longitudinal notches 30 then through the space 31 where they disperse and finally through radial notches 32 notches 30 and 32 that weaken structures and concentrate stress are established only on the hooks of junction 11 i e on portions of edges not likely to produce high stress concentration the movement of dispersion of the flow through space 31 contributes to regulate the flow of gas on the circumference of the machine and thus the ventilation effect the changes of direction to which the flow is subjected result in loss of load beneficial to the effectiveness of ventilation finally gases are discharged in centripetal direction towards rings 3 it should be obvious at this.