A tubular pressure vessel for cross-flow filtration having two centrally located side ports that penetrate the sidewall and communicate with a central chamber that divides the pressure vessel bore into two axially separated sections that each contain a plurality of cylindrical, cross-flow filtration cartridges. The pair of diametrically opposed, central side ports are provided through the use of a short length of tubing that provides structural stability to the overall pressure vessel arrangement. A feedstream may enter the pressure vessel centrally and flow in both axial directions, creating two permeate streams and two concentrate streams, the latter of which might exit through horizontal side ports located near each end of the pressure vessel. Interconnecting such tubular side port structures in adjacent pressure vessels in a vertical stack provides a load-bearing vertical column and allows a single feed line to supply multiple pressure vessels. When an array of such vertical stacks is used, horizontal side outlet ports for the concentrate streams can be likewise interconnected to provide combined concentrate outlet streams. Alternatively, two feed substreams may be pumped into each vessel through side ports near each end, and a combined concentrate stream removed through the central tubular side port structures.