A heat exchanger, comprising: an elongated pressure vessel shell having an axial dimension with a removable closure at one end of the axial dimension, a primary fluid inlet, a primary fluid outlet, a secondary fluid inlet (42,44, 46), a drain outlet (48, 50) and a heat transfer assembly comprising: a primary fluid inlet conduit extending into the pressure vessel from the primary fluid inlet; a primary fluid outlet conduit extending into the pressure vessel from the primary fluid outlet; a plurality of pairs of heat transfer plates supported in tandem with each of the pairs of plates sealed around the periphery to define a primary fluid flow channel in between a first and second heat transfer plate (12, 14) of each pair, with each pair having a heat transfer plate inlet opening fluidly connected either directly or indirectly to the primary fluid inlet conduit and a heat transfer plate outlet opening fluidly connected either directly or indirectly to the primary fluid outlet conduit to form a parallel flow path with flow in the same direction through each of the pairs of heat transfer plates in a direction orthogonal to the axial dimension of the pressure vessel shell; means for expanding a heat transfer capacity of the heat transfer assembly over an original heat transfer capacity the heat exchanger has when the heat exchanger is first placed into service; and wherein the plurality of pairs of heat transfer plates are arranged in modules with at least one of the modules, including at least one of the pairs of heat transfer plates, connected in tandem with an adjacent module or the primary fluid inlet or the primary fluid outlet with a nondestructively removable mechanical coupling and characterized in that the means for expanding the heat transfer capacity of the heat transfer assembly includes a spacer module having substantially less heat transfer capacity than the modules of pairs of heat transfer plates, with the spacer module connected in tandem with the modules of pairs of heat transfer plates, the spacer module being at least as long in the axial dimension as the modules of the pairs of heat transfer plates and having an inlet duct passing axially therethrough, fluid connected either directly or indirectly to the primary fluid inlet conduit and an outlet duct passing axially therethrough, fluidly connected either directly or indirectly to the primary fluid outlet conduit. a primary fluid inlet conduit extending into the pressure vessel from the primary fluid inlet; a primary fluid outlet conduit extending into the pressure vessel from the primary fluid outlet; a plurality of pairs of heat transfer plates supported in tandem with each of the pairs of plates sealed around the periphery to define a primary fluid flow channel in between a first and second heat transfer plate (12, 14) of each pair, with each pair having a heat transfer plate inlet opening fluidly connected either directly or indirectly to the primary fluid inlet conduit and a heat transfer plate outlet opening fluidly connected either directly or indirectly to the primary fluid outlet conduit to form a parallel flow path with flow in the same direction through each of the pairs of heat transfer plates in a direction orthogonal to the axial dimension of the pressure vessel shell; means for expanding a heat transfer capacity of the heat transfer assembly over an original heat transfer capacity the heat exchanger has when the heat exchanger is first placed into service; and wherein the plurality of pairs of heat transfer plates are arranged in modules with at least one of the modules, including at least one of the pairs of heat transfer plates, connected in tandem with an adjacent module or the primary fluid inlet or the primary fluid outlet with a nondestructively removable mechanical coupling and characterized in that the means for expanding the heat transfer capacity of the heat transfer assembly includes a spacer module having substantially less heat transfer capacity than the modules of pairs of heat transfer plates, with the spacer module connected in tandem with the modules of pairs of heat transfer plates, the spacer module being at least as long in the axial dimension as the modules of the pairs of heat transfer plates and having an inlet duct passing axially therethrough, fluid connected either directly or indirectly to the primary fluid inlet conduit and an outlet duct passing axially therethrough, fluidly connected either directly or indirectly to the primary fluid outlet conduit.