A stacked assembly of plates is provided, a first portion of the length of the assembly being formed of one or more first perforated plates, with a central open region enclosed by inner annulus and limbs from the outer diameter of the inner annulus to the inner diameter of an outer peripheral annulus, whereby the first plate has a central passageway for the first fluid inside the inner annulus and a plurality of outer frusto-annular passageways for the second fluid, and, at one end of the first portion, a second portion of the length of the assembly formed of one or more second perforated plates of similar construction to the first plates, each second plate differing from the first plate in that its outer peripheral annulus has one or more discontinuities to provide an inlet or an outlet into the outer frusto-annular passageways of the second portion.