Original Revolving Doors
Revolving doors have been around for 150 years. They are often found on the ground floors of very tall buildings which clues you into their purpose. They don’t just function as a way to enter or exit these buildings, but there’s a science behind their purpose. Their original German name, according to their inventor, an H. Bockhacker, roughly translates as “door without air drafts.” Inside tall buildings, during Winter, if you open a door at the entrance, it usually makes cold air rush inside the building.
Purpose of Revolving Doors and the Stack Effect
This draft of cold air is due to gravity, and the way it affects air volume of different temperatures. Cold air is denser than warm air, which changes air pressure, and when there’s a hole made by a traditional door opening at the ground floor of a building, the denser colder air forces inside, pushing up the lighter and warmer air out holes at the top of the building.
This is called the stack effect. You can witness this in construction sites where the majority of the building is covered with a plastic tarp. Next time you see one of these, notice how the plastic tarps get sucked in at the building’s bottom, and pushed out near the top. The stack effect is magnified by height. This is why you never see revolving doors inside low rise buildings. The stack effect also increases as the temperature changes between inside and out, making it stronger in Winter than Summer.
Revolving doors help limit how much air rushes into a building, because unlike regular doors, the outside and inside of the building doesn’t get a direct connection as people enter and exit. Newer buildings have different ways of reducing the air leakages associated with the stack effect; they use compartmentalization, where floors are self isolated and elevator shafts are fully sealed, highly reducing the stack effect.