Most experiments on flow induced vibration have been conducted in wind tunnel sections. In these experiments the mean velocity in all gaps is the same and can be calculated by the pitch ratio. The nonuniform velocity distribution in real heat-exchangers is taken into account by the extended Connors equation, but there is no evidence of the validity of this equation (Weaver and Parrondo, 1990). Until now it has been impossible to calculate the flow distribution for complex geometries with sufficient accuracy (Mulcay et al., 1986). A real heat exchanger bundle with two baffles has been modelled by a fluid dynamic program. With the calculated gap velocity distribution Ψ and the tube vibration mode shape function Φ, stability criteria values K′ can be calculated by equation (3) for all tubes and for different stream values. Experiments were conducted in a real heat exchanger to get the excited tubes in the bundle and the critical volume streams (Leyh, 1993). The K′-values of the critical tubes calculated with the critical volume streams agree well with the stability constant K drawn from the well known stability map (Chen, 1987). The calculated velocity distribution is a usefull aid to improve the prediction of the vibration excitation in bundles with baffles.