The Rotary Regenerator: Store Heat in a Wheel and Carry It Across

US 1,746,598, 'Regenerative Heat Transmission Apparatus'. The regenerative mass is made of thin plates — *"preferably, of corrugated plates"* — packed in sectors of a cylinder, which moves between conduits carrying the two fluids, *"one of which constitutes a heating fluid"*. The drawings show the rotor, the housing that divides the gas side from the air side, and the seals at the division. Three things to find on the sheets: the corrugated packing (surface), the division of the housing (the two streams must not mix), and the seals (where they would).

**The wheel:** press a strip of aluminium foil over corrugated cardboard to crimp it, lay it on a flat strip of foil, and roll the pair tightly round a 10 mm hub until the roll is 150 mm across. Tape the outside. Air can pass through the little channels lengthwise; the foil is the matrix. **The housing:** cut two parallel ducts in XPS foam board, side by side, each a little narrower than half the wheel, with a slot between them that the wheel spans. The wheel sits across both ducts on its hub, driven by the geared motor at a few revolutions per minute. Seal the gaps round the wheel with strips of foam. Blow warm air from the heat gun (low setting, at a distance) down one duct and room air from a fan up the other. Put a temperature probe in each outlet.

With the wheel **stopped**, run both air streams. The cold-side outlet warms briefly — from heat already in the foil — then falls back to room temperature: a stationary matrix saturates and passes nothing. Start the wheel. The cold-side outlet warms and stays warm, and the warm side's outlet cools. Measure both at several motor speeds. At low speed the effect is small; it grows and then levels off — the two regimes in the notebook. Sniff the cold-side outlet at the highest speed with a smoky source (a snuffed match) in the warm side: that faint smell is carryover.

**US 1,746,598, 'Regenerative Heat Transmission Apparatus', Fredrik Ljungström of Lidingö-Brevik, Sweden, assignor to Aktiebolaget Ljungströms Ångturbin; filed 27 November 1925 (Serial 71,871), claiming a Swedish filing of 28 November 1924; granted 11 February 1930.** Ljungström and his brother Birger had already designed a radial-flow steam turbine; the air preheater recovered heat that power-station boilers were sending up the chimney, alongside the economiser. The Stirling engine's regenerator in this catalogue is the same idea in a single passage, and the rotating 'heat wheel' in building ventilation is the same wheel run between exhaust and fresh air. **Honest limits.** The streams leak into each other at the seals and by carryover, so a regenerator is used only where a little mixing does not matter. The matrix fouls and needs soot-blowing. And if the flue gas is cooled below its dew point, the cold end of the matrix corrodes.