Manufacturers of plate and frame heat exchangers nowadays mainly offer plates with chevron (or herringbone) corrugation patterns. The inclination angle phi of the crests and furrows of that sinusoidal pattern relative to the main flow direction has been shown to be the most important design parameter with respect to fluid friction and heat transfer. Two kinds of flow may exist in the gap between two plates (pressed together with the chevron pattern of the second plate turned into the opposite direction) : the crossing flow of small substreams following the furrows of the first and the second plate, respectively, over the whole width of the corrugation pattern, dominating at lower inclination angles Glower pressure drop); and the wavy longitudinal flow between a two vertical rows of contact points, prevailing at high phi angles (high pressure drop). The combined effects of the longer flow paths along the furrows, the crossing of the substreams, flow reversal at the edges of the chevron pattern, and the competition between crossing and longitudinal flow are taken into account to derive a relatively simple but physically reasonable equation for the friction factor xi as a function of the angle phi and the Reynolds number Re. Heat-transfer coefficients are then obtained from a theoretical equation for developing thermal boundary layers in fully developed laminar or turbulent channel flow - the generalized Leveque equation - predicting heat-transfer coefficients as being proportional to (xi . Re-2)(1/3). It is shown, by comparison, that this prediction is in good agreement with experimental observations quoted in the literature.