Due to the fact that there is a direct contact between both fluid flows there is a difficulty in calculating efficiencies. The air flow entering via inlet line 3 must not necessarily exit completely at outlet line 4. Depending on the boundary working conditions which are pressure ratios as well as rotational speed there can be a leakage mass flow rate from inlet 3 to outlet 2 or from inlet 1 to outlet 4 (ref. Fig. 6). For this reason all different mass