The Plate Heat Exchanger: Thin Plates, Turbulence, and a Frame You Can Open

Cut three copper sheets 150 × 100 mm. The middle one is the heat-transfer plate; the outer two are blanks. Print two gasket frames in TPU, 3 mm thick, each the outline of the plate with a 10 mm border, so each frame encloses a flow gap. Stack: acrylic end plate / copper blank / gasket frame / copper plate / gasket frame / copper blank / acrylic end plate. Drill four corner bolt holes through everything and clamp with threaded rod and wing nuts, tightening evenly. Drill and fit hose tails so that gap 1 is fed at the top-left and drains at the bottom-right, and gap 2 is fed at the bottom-right and drains at the top-left — counterflow across the plate. Add ridges to the gasket frames or press shallow chevrons into the copper with a blunt punch to break up the flow.

Run hot water through one gap and cold through the other at equal, timed flows, and read the four temperatures with the thermometer (or reuse the four-probe logger from the counterflow rung). Work out the heat carried by each side. Now the reason the design exists: undo the wing nuts, lift the stack apart and look. Any sediment, scale or film is on the copper where you can wipe it. Wipe, restack and tighten. An exchanger you can open is an exchanger you can keep clean — which in a dairy is the difference between a safe product and a spoiled one. Retighten evenly: a gasket squeezed harder on one side leaks on the other.

**Richard Seligman, 1923** — the first commercially successful plate heat exchanger, made by the Aluminium Plate & Vessel Company he had founded in London in 1910, for the high-temperature short-time pasteurisation of milk. No patent number is asserted here: it could not be read from a primary source, and the design is fully documented. Plate exchangers now run in food, brewing, district heating, heat pumps and chemical plant; brazed versions with no gaskets at all are the small cube in many domestic boilers. **Honest limits.** Gaskets limit the temperature and pressure, and the chemicals they can meet. Narrow gaps clog on fibrous or particle-laden liquids, where shell-and-tube exchangers still win. And the high pressure drop of those turbulent gaps costs pumping power.