Van Kannel Revolving Door

A revolving door has moving wings closing on fixed jambs — a genuine crushing hazard at full size. Build this at model scale and keep fingers clear of the closing edges.

Van Kannel writes that "persons can pass both in and out at the same time" while weather is excluded. Two problems solved by one geometry.

Warm a tall box and hold a smoke source at a low opening. Air is pulled IN at the bottom. That inward draught is the force fighting an ordinary door.

Make a circular enclosure open on opposite sides — one facing the street, one the lobby. The curved walls are the sealing surfaces.

Keep the two apertures narrower than the gap between wings. If both openings can ever be clear at once, you have built a hole in the wall with extra steps.

Fit four leaves at right angles to a vertical shaft. Four gives two enclosed compartments at all times; three gives more capacity and a poorer seal.

Line the vertical and top edges so they wipe the drum. The seal, not the wing, is what excludes the weather — a wing with a 5 mm gap is a 5 mm slot all the way up.

Turn the assembly a full revolution and look for any angle where you can see straight through. There must be none — check every position, not just the obvious ones.

Carry the wings' weight on a bearing at the base with a light guide at the top. It must turn under a child's push while supporting the whole assembly.

Run the stack-effect test again with the drum in place, turning. Almost no air crosses — you have kept the pressure boundary while people pass.

Fit friction or a fluid damper to cap rotation speed. An unrestrained revolving door can be spun fast enough to injure whoever is inside it.

Hinge the wings so they can fold flat under pressure from inside. This is the book-fold or breakout position, and it is a legal requirement for escape routes.

Block the rotation and push a wing from inside. It must fold and give a clear opening. A door that cannot fail open is not acceptable on an exit.

Time counters passing through both. The revolving door carries people in both directions at once and never has to be held — capacity is continuous, not batched.

The patent. US 387,571, "Storm-door structure", granted 7 August 1888 to Theophilus Van Kannel of Philadelphia. The specification describes a door of pivoted wings turning on a central post, admitting people in both directions while excluding wind, rain, snow, dust and street noise. Van Kannel won the Franklin Institute's John Scott Medal for it in 1889, and the first installation is generally given as Rector's restaurant in Times Square around 1899. A German patent by H. Bockhacker of Berlin predates it by seven years, so "first revolving door" is a claim worth qualifying.

Stack effect is the problem it actually solves. A heated multi-storey building has warm buoyant air inside; it rises and leaks out at the top, drawing cold air in at street level. The resulting pressure difference at the entrance can be large enough to make a hinged door hard to open and to slam it shut behind people, and every opening dumps conditioned air. A revolving door keeps at least one wing sealed at all times, so the pressure boundary is never broken — it is a continuously operating airlock. Modern energy studies still show meaningful heating savings from using the revolving door rather than the side door beside it.

Two problems, one geometry. The pressure seal and the two-way pedestrian flow come from the same arrangement. Each compartment carries its occupants independently, so inbound and outbound traffic never conflict and nobody has to hold a door for anybody. That is a genuinely elegant piece of design — but it comes with a hard constraint, which is that the aperture width must always be less than the angular spacing of the wings. Get that wrong and the seal is lost at some rotation angle, and the whole benefit disappears.

The failure mode had to be added later. A door that always seals is, by construction, a door that can jam shut — which is unacceptable on an escape route. Modern revolving doors have collapsible or breakout wings that fold flat under push from inside, and building codes limit how much of a required exit width a revolving door may provide. This is the same reasoning as the fail-safe air brake and the fusible-link sprinkler: decide in advance which way the device should fail, and build that in rather than hoping.