Vaneless diffuser

Modern eompressors ineorporate a parallel wall, vaneless diffuser that yields exeellent off-design performanee. On these maehines, the... 

The calculation of the overall stage efficiency must also include losses encountered in the diffuser. Thus, the overall actual adiabatic head attained will be the actual adiabatic head of the impeller minus the head losses encountered in the diffuser from wake caused by the impeller blade the loss of part of the kinetic head at the exit of the diffuser (A(/ed), and the loss of head from frictional forces (A(/osf) encountered in the vaned or vaneless diffuser space... 

Vaneless diffuser ioss. This loss is experienced in the vaneless diffuser and results from friction and the absolute flow angle. 

Whether surge is eaused by a deerease in flow veloeity or an inerease in rotational speeds, either the indueer or vaneless diffuser ean stall. Whieh stalls first is diffieult to determine, but eonsiderable testing has shown that for a low-pressure-ratio eompressor, the surge initiates in the diffuser seetion. For units with single-stage pressure ratios above 3 1, surge is probably initiated in the indueer. 

Most centrifugal compressors in service in petroleum or petrochemical processing plants use vane less diffusers. A vaneless diffuser is generally a simple flow channel with parallel walls and does not have any elements inside to guide the flow. 

Johnston, R. and Dean, R., 1966, Losses in Vaneless Diffusers of Centrifugal Compressors and Pumps, ASME Journal of Basic Engineering, Vol. 88, pp. 49-60. 

Choice of vaned or vaneless diffusers, providing optimum curve shape and high efficiencies. 

Matching the flow between the impeller and the diffuser is complex because the flow path changes from a rotating system into a stationary one. This complex, unsteady flow is strongly affected by the jet-wake of the flow leaving the impeller, as seen in Figure 6-29. The three-dimensional boundary layers, the secondary flows in the vaneless region, and the flow separation at the blades also affects the overall flow in the diffuser. 

Wake-mixing loss. This loss is from the impeller blades, and it causes a wake in the vaneless space behind the rotor. It is minimized in a diffuser, which is symmetric around the axis of rotation. 

Vaned diffuser ioss. Vaned diffuser losses are based on conical diffuser test results. They are a function of the impeller blade loading and the vaneless space radius ratio. They also take into account the blade incidence angle and skin friction from the vanes. 

---