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How revolving doors are more cost-effective than traditional doors (mit.edu)
45 points by ilamont on Jan 4, 2010 | hide | past | web | favorite | 31 comments

This is closely related to my field of work, so I feel the need to be pedantic.

Please note my criticisms should not be taken to knock the thrust of the work itself. The research question had to do with social effects, while I am criticizing the modeling behind the infiltration estimates.

First of all, the "stack effect" is not the most important driver of flow in modern buildings (especially not ones that have revolving doors). Mechanical systems are designed to dominate stack and wind-driven flows, precisely because engineers want to control the flows in the building. The analysis presented on the web page and in the research report discussed only stack and wind effects.

Second, the equations shown in the figure on the web page are from a steady-state "multizone" modeling approach. Steady-state models don't represent swinging doors well, because of several important transient effects: (1) momentum effects associated with the intermittent nature of the flow (makes airflow smaller than a steady-state model would suggest; and (2) active pumping by the door (makes airflow larger than a passive transient model would suggest).

Third, I think the blanket statement that "heating is less efficient than cooling" is just flat-out wrong. Heating is pretty easy-- just plug in a lightbulb, and 100% of the electricity it burns gets converted to heat. For this reason, many buildings have core zones that require cooling year-round.

Fortunately, I don't think any of these criticisms affect the broad validity of the work. Their modeling seems to have been driven largely by experimental studies, not by the material described on their web page and in their paper. Therefore one can assume they got the right order of magnitude (one might also note that the cited papers are from 1958 and 1961-- in other words, the basic technical part here is not new, and it's unlikely they botched it too badly).

One final criticism. MIT has a very good faculty that specializes in building energy issues. Any of them could have pointed out these problems, and helped guide a much stronger technical analysis. Unfortunately, none of that faculty seems to have been consulted for this study (the acknowledgments list a bunch of urban studies and management faculty, plus people from the facilities department). I know this was just a class exercise, but it seems to me representative of the sorts of analyses that can result when management people start mucking about in technical matters...

At a number of commerical buildings I frequently, I... frequently see people use the swinging doors located to the side of the revolving door (for ADA compliance, if nothing else). They do this even when there is no traffic at the revolving door.

Often, they will hit the button to automatically open the doors. This is even worse than using the door manually, as the timer that controls the door motion usually leaves the door open for many seconds -- presumably to accommodate a slower user or one with a wider configuration, e.g. a wheelchair -- before beginning to close. Mind you, these are people who appear to be fully mobile and not unduly hindered or handicapped.

Quite often, I'll see people use the swinging doors to exit, where they just hit the crash bar and proceed. Presumably, they find this quicker (by a matter of a second or three) and more convenient.

(The cynic in me says: And after all, if they are even aware of the difference in efficiency, they are not personally paying the heating bill and cooling bills.)

I wonder whether a brief, one page "flyer" version of the linked information could be effective in altering behavior. I wouldn't ask permission to post them; I'd just engage in a guerrilla campaign of slapping them up when and where I can get away with it. Preferably, the flyer would have a professional appearance and the salient information would be encapsulated in an easily scanned sentence or three in large font. There might be more detail below, and hopefully also an eye-catching graphic.

I'm no graphics artist, but maybe I'll give it a go.

I use the side doors - opening them by hand - because revolving doors move more slowly than I like, and it irritates me. It's the equivalent of a "loading..." bar for a website's flash intro - I don't want to deal with it, just get me into the building please.

I use the revolving door whenever possible, because as a kid I used to love going around and around and around in them until my parents got mad and told me to stop dawdling.

I feel like there should be an XKCD for this...

I used to do that too (and I think I was a kid much earlier than you). There was no seal, iirc, they moved much much faster. Somebody along the way figured out the energy saving potential.

I believe there is a speed limiter on modern revolving doors. If I recall correctly it is a "paddle in oil" arrangement whose resistance force is a 4th power of speed.

I do not know why I think I know this, but it seems to be true: http://www.internationalrevolvingdoors.com/IrdManualDoors.ht...

As for "why", I'm sure fast doors both scare people and attract lawyers.

Very often I'll see two sets of doors, which form a small room which gets cold/warm, isolating the rest of the building from the outside. While they may not be as efficient as revolving doors, I wonder where they sit on the curve? Somewhere in the middle?

There are other considerations - swinging doors are less complicated (possibly), take up less room and are easier to lock (again, possibly). They are probably cheaper to construct and maintain as well.

An air lock -- or vestibule.

There are forms of these that are temporary. Some restaurants and bars will put one up in the winter; typically made of heavy canvas rigged across a metal frame, often with clear plastic window inserts to keep users aware of traffic coming in the opposite direction.

Most often, the primary motivation seems to be to increase guest comfort by preventing cold blasts. But they also aid energy efficiency.

I just dined at a nice restaurant that had a set of heavy velvet curtains in a semi-circle around the inside of front door. Not only did it prevent cold blasts to the tables close to the door (I always hate that) it made for a really elegant transitional experience between the dull outside space to the exquisite interior.

My (very) first draft ended up being what I stuck with. I was going to position the text, maybe add a border, etc. But when I printed this out, it seemed a good combination of minimal and to the point.

I realize the value I quote is specific to the MIT environment studied, but I'd rather not complicate things. It gets the point across, and the URL is given for those who want to investigate further.


(Not that I expect much credit for this simple approach. But now I have something I can put in my bag, ready for the next opportunity.)

EDIT: Also, the term "use" might be considered vague or otherwise non-specific. But it is brief and fits the intended tone of KISS (Keep It Simple Stupid). In my mind, "use" doesn't create the mental 'stutter' that "consume" or another verb might. And perhaps its repetition in the second sentence provides a subliminal reinforcement. (While the use of "do" avoids repetition within the same sentence to negative effect. Call the literary criticism police; I'm out of control.)

You don't want people to "consider using the revolving door", you want people to actually use it, "please use the revolving door" (add an arrow indicating which side it is).

Also your opener "swing doors use 8 times the energy" suggests that in some way the door itself directly transforms the energy. I'd go with just the above 5 words and then a para in 12pt (or so) giving details for those who will inevitably wonder why they should change their door usage habits. Depending on your demographic you might want something more like a CTA "Go Green! Use the revolving door".

I think "Swing doors waste ..." is more accurate. And perhaps easier to grasp. Doors don't _use_ energy, unless they're motorized.

I was attempting to address this in my "literary criticism moment". "The average Joe" is, in my thinking, unlikely to be this specific in their analysis. "Use" gets the point across without invoking more complex topics (what does "consume" or "waste" mean) and/or words that may already be politically and emotionally loaded.

As for the "Please consider" part (responding to the other response in this branch of the discussion tree), I didn't want to be "telling" anyone what to do. I thought it would be more productive / invoke a more positive response to ask them to think things through for themselves. Also, since I'm not the building owner nor manager, it's not really my place to dictate policy. I'm making a suggestion to the individual reading the sign.

I don't go into more detail on the sign, because detail -- or the impression of same -- tends to put people off. If anyone's really interested, the cited URL is pretty simple and "MIT Energy Initiative" is pretty easy to remember and google. (Also/instead, maybe I was just lazy.)

Thanks for the feedback, though. Valid points. I'm just explaining my reasoning, not seeing them as invalid.

I know of a university cafeteria that simply puts up a “Use the revolving door” sign on the swing door. It’s pretty effective. Nobody will stop you if you use the swing door, but hardly anybody does use it.

My office building has the same set-up (2 pairs of swinging doors, 2 revolving door, 1 button-triggered door) and the same problems. I see a lot of people coming in through the powered door and exiting through the swinging doors. There are signs already posted saying something to the effect of "For energy efficiency, please use revolving doors." That apparently doesn't work well enough.

How about a gentle, relatively quiet but discouraging audio tone that activates when the swinging door opens?

Something like "uh uh", "uh oh", or "wah wah ~waaaah~" that is audible within, say, a x=20 foot radius. Maybe triggered only on subsequent usage within a y=10 minute window.

One might argue it discriminates against the handicapped, or causes them unnecessary discomfort. I would draw an analogy to seatbelt reminder tones in cars. They cause unnecessary discomfort to everybody but are so mild as to be worth it, overall.

Everyone can stop the seat belt reminder by putting on their seat belt. Handicapped people can't use a different door.

I never thought of revolving doors as a way to save energy through heating. I wonder if this is due to the fact that revolving doors have significantly larger 'footprint' that means the actual door mechanism doesn't interfere with the natural convection currents of a building.

Simply think about how convection works (in the winter) in a building it circles from 'warm' interior walls to 'cold' external walls (especially if they're glass), which at ground level causes cold air to move along the ground towards the interior of the building. Now what happens when you open a door to the frigid outside? The cold external air will hijack the convection current and be drawn inside. The opposite holds true in summer when the interior is colder than the exterior, however with the convection currents reversed the system is hijacked again and warm air rises from the door and up the interior wall, similarly cooled air from the buildings interior now flows through the open door, further enhancing the cycle.

I've seen these convection patterns many times inside buildings, especially with steam during cooking. For instance, boil a kettle in front of a window (especially a large, or poorly insulated window), if it's summer the steam will roll up the wall and billow across the roof, in winter the steam will get dragged down over any counter tops and to the floor (if you're lucky, you can sometimes see the steam rise in the middle of the room, or conversely in summer it may sink at the centre of the roof).

Incidentally, I never thought being an avid tea drinker would actually contribute to my scientific knowledge.

I think you're making the analysis more complicated than it needs to be. Revolving doors are more energy efficient because less inside air is exposed to the outside. Or, from the article:

The revolving door stops conditioned air from moving freely. An open swing door is like letting go of a balloon- the air rushes out of the opening. A revolving door is never open- seals remain in contact with the walls of the door at all times. Only the air in the chamber with the person going through the door is transferred.

I often refer to revolving doors as parallel to serial converters. I my last employer's building, when walking out in a group we had to form into a two by two column to get out of the building through the two revolving doors.

I wonder about this every time I go through a revolving door. I'm glad to see a webpage speak to it.

You can also speak to anyone who works near both revolving and regular doors, such as building receptionists. They will quickly tell you the revolving door lets far less cold in, and will probably tell you with a good deal of frustration how much they hate the regular doors.

Revolving doors scare me...


That looks like a fine example of sound safety engineering at work. All doors are risky in highly windy environments. My neighbor is missing several finger tips due to a wind-slammed door. I've also witnessed a sliding door dislodged from its tracks and tumbling down the street. In this video, the safety glass shattered as designed, leaving the man startled, but unharmed.

From what I understand, it's actually a safety system malfunctioning. During a fire, the 4 doors are supposed to collapse together, to let people go straight through. For some reason, it tried to switch to fire mode with the guy still inside.

Interesting... why would they collapse with that amount of force? It seems like it is collapsing awfully fast to be by design, not wind.

A fun thing to do (if you're not inconveniencing anyone) with automated swinging doors (that swing out). Because of the finite state machine in the door controller, if you step on the pad on the outside (where the door swings to), you can block your friend inside.

Would save even more energy if they were attached to a generator...

You're probably just joking, but in case you aren't, no it would just cause all the users of the door to have to consume that much more in calories.

The door must be hinged, it also has a braking system to prevent it from rotating too quickly. Adding resistance to drive a generator in place of heat/sound generation seems not unreasonable. In cars I think they call it regenerative braking.

Of course an analysis of the heat loss path will help to determine if this is worthwhile or if the heat can be used more directly to aid heating the building.

A regenerative system on a door that has brushes that cause friction with the sides isn't going to make more power than it will cost to create a regenerative braking system in the first place. It adds a whole layer of complexity that will affect reliability and in the end it's going to 'save' piddly little bits of power.

If you take the cost of an energy storage system you have to divide the price of the unit by the amount of power that you can buy for that money discounted with what you think power will cost over the lifetime of the unit.

This will most likely come out negative.

If there are enough children entering your building, it might just work.

On a more serious note, people do actually use this to improve the supply with drinking water in developing countries by attaching a water pump to a roundabout, search for "playpump".

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