Considerations of dialysate port design

A design in which the inflowing “jet” of dialysate hits the fiber bundle is obviously not very suitable. This “jet” would tend to push the outer fibers inwards and on the opposite side of the bundle a “flow shadow” area would form, which has a low dialysate flow and hence locally a low dialysis performance.

It is desirable to arrange for a distribution of the dialysate flow before it reaches the bundle. This can be done in somewhat different ways. A basic principle is to provide for an annular channel in which the dialysate flow can first spread around the bundle and from there flow in a peripheral direction to then enter the fiber bundle, as indicated in the following figure.

The dialysate inflow to the annular distribution space could also be arranged tangentially to produce a more or less rotating flow in the distribution space. The space could also be shaped to follow the curvature of the inner surface of the potting plug, so that the width of the axial gap above the distribution space, in which the flow enters the bundle end, is everywhere nearly equal. Furthermore, the radial width of the annular distribution channel can be varied so as to keep a nearly constant flow velocity in that channel (cf. shaping of an annular distribution space in certain header designs here).

The inner bore of the dialysate entrance and exit should not be perfectly circular in the cross section, but have suitable ridges or grooves to allow for air escape during potting, if this is done in a centrifuge with polyurethane feed tubes stuck into the dialysate ports.

In this case it is useful to arrange for a stop-point of the insertion of the resin feed tubing so that the end will not touch the wall of the annular channel shown above. If no such channel is used, the tubing should not touch the fiber bundle. It is important to prevent that liquid resin hits the fiber bundle at locations, which at the end will be inside the cured potting plug, since otherwise the “varnishing” effect would increase (see here).

The external shape of the dialysate entrance and exit tubes – with an arrangement for connecting the tube from the dialysate pump and the return tube at its other end – is defined in standards, which though some are national in fact are international, since they are basically all the same.

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