Abstract: A liquid electrolyte fuel cell stack (13) includes a plurality of fuel cells (19) disposed in groups between a plurality of cooler plates (18-20), the cooler plates being connected by tubing (29) to a vertical coolant outlet manifold (27). The coolant outlet manifold has a coolant cross sectional flow area which increases from near the bottom to near the top, either by virtue of an increasing internal dimension (34-38) of the manifold or by virtue of an insert (41) which is larger at the bottom than at the top. The insert may be either a linear or rotund trianguloid, cone, conoid, pyramid or pyramoid. The internal dimension of the coolant outlet manifold or the dimension of the insert may be stepped or continuous, linear or non-linear.