How do revolving doors help control airflow?

1. It is a cold, windy day. Each time someone opens this ordinary door, a gust of cold air blows into the warm lobby. Brr!
2. Seen from above, an open door is simply a hole in the wall. Cold air pushes in, and the warm air you paid to heat slips out.
3. A revolving door has four wings inside a round case. As it turns, at least two wings always touch the curved walls. There is never a clear path through.
4. A person steps into one compartment and pushes. Only the air in that small compartment travels inside with them. The rest of the wind is left behind.
5. Compare them. The swing door lets a big cloud of cold air in each time. The revolving door lets in only a small puff, so the lobby stays warm.
6. In a tall building, warm air rises like smoke in a chimney and pulls cold air in at the bottom. Revolving doors stop that draft. A side door serves wheelchairs and strollers.

You are looking down from the ceiling. Tap each part to see it work.

• Wings. The four glass panels are called wings. You push one, and all four turn together like a pinwheel.
• Curved walls. Two curved walls stand on either side. The wings sweep along them. The gaps between the walls are where you walk in and out.
• Center shaft. The wings are fixed to a post in the middle called the shaft. It turns on bearings at the top and bottom.
• Brush seals. Soft brushes or rubber strips run along the edges of each wing. They brush the walls, floor and ceiling to close the small gaps.
• Compartment. The space between two wings is a compartment. It carries you, and one small pocket of air, from one side to the other.
• Speed control. A device above the door stops it from spinning too fast, even if someone pushes hard. That keeps everyone safe.
• Side door. Next to every revolving door is an ordinary wide door. People using wheelchairs, pushing strollers or carrying big loads use it.

Send visitors through the swing door and through the revolving door. Watch the blue cold air and the lobby thermometer.

Experiment: Which door lets more cold air into the lobby each time someone comes in?

1. Send one visitor through the swing door. Watch the blue air.
2. Send one visitor through the revolving door.
3. Compare the thermometer after each one.

What we found: The swing door lets a big gust of cold air in every time it opens, so the lobby cools quickly. The revolving door swaps only one small pocket of air, so the lobby stays warm.

Turn on JavaScript to try it yourself.

Let every visitor in without letting the lobby drop below the cold line. Visitors on wheels need the side door.

Buildings spend a lot of energy keeping the air inside comfortable: warm in winter and cool in summer. Every time an ordinary door opens, some of that air escapes and outdoor air takes its place. On a windy day you can feel it as a draft that sweeps across the lobby.

A revolving door solves the problem with a simple shape. Four wings turn inside a round case with two curved walls. However the wings are turned, at least two of them are touching the walls. So there is never an open path from outside to inside. Brushes along the edges of the wings close the last small gaps.

When you walk through, you ride in one compartment, the space between two wings. Only the air in that compartment comes in with you, and one compartment of indoor air goes out. That is far less than the gust that comes through an open swing door. Our comparison with a pet flap is not perfect, because a flap still opens a small hole for a moment, while a revolving door never opens at all.

Revolving doors are not right for everyone. They are too tight for wheelchairs, strollers and big suitcases, so there is always a wide ordinary door beside them. Revolving doors also turn at a safe, limited speed. Used together, the two doors keep a building comfortable and welcoming to all.

Key words

Revolving door

A door with wings that turn inside a round case, so it is never fully open.

Wing

One of the turning panels of a revolving door.

Compartment

The space between two wings that carries a person, and one pocket of air, through the door.

Draft

A stream of outdoor air blowing into a room.

Stack effect

Warm air rising inside a tall building and pulling cold air in at the bottom, as in a chimney.

Air exchange

Indoor air leaving a building and outdoor air taking its place.

Check your thinking

1. Which door lets in less cold air each time a person enters?

   Show the answer

   A revolving door. A revolving door only swaps the air in one small compartment.

2. Why is there never a clear path through a revolving door?

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   At least two wings always touch the curved walls. However the wings are turned, some of them are always against the walls, blocking the way.

3. Why is there an ordinary door next to a revolving door?

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   For wheelchairs, strollers and big loads. A revolving door's compartments are too tight for wheels and large loads.

4. On a very windy day, which door's draft gets much worse?

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   The swing door's. Wind can blow straight through an open swing door, but it cannot get past the wings of a revolving door.

5. A swing door stays open for 4 seconds. Air flows through its 2 square meter opening at 1 meter per second. About how much air is exchanged?

   Show the answer

   About 8 cubic meters. Volume is area times speed times time: 2 × 1 × 4 = 8 cubic meters, several compartments' worth.