An Elevator Acts Just Like A Piston In A Car Engine. Here’s Why That’s A Problem

Descending in an elevator after a Mother’s Day rooftop brunch, I had a thought I couldn’t shake: “How is this thing getting past the gigantic column of air below?” I stood there, completely still in one reference frame and plummeting in another, my eyes open wide staring fiercely, wondering how I had never thought of this before. I tried remembering if I’d ever felt a gust of wind shoot out from gaps in the lobby door as an elevator descended to pick me up. “I don’t think so?” I wondered if there is a big vent at the top and bottom of the shaft to equalize pressure. But where does all that air shoot out? Wouldn’t it be loud? I struggled to answer these crucial elevator-piston-related questions that you too, dear reader, likely have grappled with on a daily basis, which is why I’ve dug into it a bit so that you and I can finally relax.

I’ll begin by making it clear that this is a car website, and because the metal cubes you stand in while vertically ascending and descending are technically called “elevator cars,” this article is totally justifiable. Plus, in 2016 Jason Torchinsky asserted that an elevator is a vehicle, and when has he ever been wrong?

Anyway, allow me to restate the dilemma: As an elevator moves up or down in a shaft, it has to displace (i.e. move) air. Because it’s acting just like a piston (or plunger), it wants to try to compress air as it moves towards either end of an elevator shaft, creating an area of high pressure in the direction of travel and low pressure in the other direction.

This, as you could imagine, is a problem, because even though air can be compressed, and even though I’m sure the air on the backside of an elevator is happy to pull a vacuum, this doesn’t happen easily. There’s quite a bit of energy needed to try to compress fluids. And in fact, this energy expense is pretty much how shock absorbers work. Basically, as a piston in fluid moves, it either compresses the fluid or it has no choice but to move the fluid. Compressing the fluid would essentially slow the piston to a stop, and moving the fluid takes away energy/slows things down considerably (which is the point):

So in the case of an elevator, obviously the fluid (air) isn’t going to be perfectly sealed into the shaft (we’ve all seen the gaps in elevator doors), but still: The fluid is being forced to move by the elevator, so isn’t there a bunch of energy being wasted like in a shock absorber?

My search for answers revealed that this “piston-effect” issue that was on my mind is actually way more serious than even I initially thought.

The Piston-Effect And The Smoke Problem

John Klote — a world-renowned smoke-control expert — wrote An Analysis of the Influence of Piston Effect on Elevator Smoke Control, a 1988 U.S. Department of Commerce paper that discusses the “feasibility of using elevators for the evacuation of the handicapped during a fire.”

As it turns out, the piston-effect of elevators has been considered a humongous problem in a fire, as it tends to pull smoke into the elevator shaft, where it can then be sent to other floors, as Klote notes:

The transient pressures produced when an elevator car moves in a shaft are a potential problem for elevator smoke control. Such piston effect can pull smoke into a normally pressurized elevator lobby.

Using A Fan To Mitigate The Piston Effect

The paper goes on to mention those giant elevator door caps we all know, and it talks about a potential fan system (shown above) that could help with this smoke issue caused by elevator piston-effect:

Most elevator doors have large gaps around them [5]. Such large leakage areas around the doors result in lobby and shaft pressures that are nearly equal under most conditions. Thus if pressurization air is supplied to the elevator shaft, the lobbies will be pressurized indirectly to almost the same pressure as the shaft. A concern with such systems is that a few open doors might result in significant loss of pressurization. The first paper [2] of this project demonstrates that this problem can be overcome by use of a system with feedback control. The flow rate of air into the shaft is controlled by a differential pressure sensor to maintain a constant pressure difference across the elevator lobby door on the fire floor. One method of varying the flow rate is a fan bypass system.

The plot below is cool because it shows the pressure difference between the elevator shaft and the lobby as a function of elevator car speed, and in this case the results are for a pressurized system that is trying to minimize the piston effect via a fan that force air at 25 PSI below the elevator car so as the car goes up it’s not pulling a vacuum from the lobby.

A Motorized Damper ‘Hoistway Vent’

It seems to me that such a pressurized system is not required by law, though “motorized dampers” used to control an exhaust valve at the top of an elevator shaft aren’t uncommon (the image below also shows a fan for odor control):

To learn more, I read the state of Wisconsin’s document from the Department of Safety and Professional Services. Titled Ventilation Requirements for Elevator Hoistways and Machine Rooms, the paper actually mentions the piston-effect in a positive light, specifically because the airflow around the elevator as it moves can help keep things cool:

The piston effect of the elevator moving in the hoistway and the opening and closing of the hoistway doors is often adequate for conditioned building air to maintain temperature and humidity to meet A17.1, 2.7.9.2.

It also mentioned both pressurized shafts and damper valves (hoistway venting) to control airflow:

The commercial building code no longer requires hoistway venting for control of smoke. Other means of smoke control may be required by the building code such as elevator lobbies or smoke doors or curtains. The DSPS Website has an article similar to this with information about smoke doors and curtains.

Hoistway Pressurization for Smoke Control The commercial building code IBC 909.21 allows hoistway pressurization as a method to control smoke. A pressurization system must be tested according to IBC 909.21.1. The hoistway or an adjacent space must be large enough for the ductwork necessary for pressurization and pressurization may not negatively affect elevator equipment in a hoistway that may be sensitive to air movement per A17.1, 2.1.4.

Consulting-Specifying Engineer (a publication by and for engineers), actually says damper vents were indeed required as of 2005:

Incorporated into elevator design in the 19th century, and well established in the codes and standards for many decades, vents have been placed at the top of hoistways to prevent excessive pressure during car ascent, control odors and vent smoke during a building fire.

The International Building Code (IBC) currently requires hoistway vents. These must be located at the top of the hoistway and either open directly to the outside or be routed to the outside in noncombustible ducts that are fire-rated the same as the hoistway.

Originally, these vents were passive openings. But with the rise of energy conservation, vents now are often equipped with mechanical dampers that open upon smoke-detector activation. IBC Section 3004 requires hoistways of more than three stories to have vents of 3 sq. ft., or 3.5% of the area of the hoistway.

But as of the 2005 publication of the linked article, Change is in the air, there was pushback on the need for such vents, and part of that had to do with the piston effect not actually being that big of a deal, per CSE:

One major step toward remedying the smoke-migration problem is realizing that the reasons given for placing vents at the top of hoistways have lost their validity.

Take, for example, the argument that an ascending car produces pressures that require venting for proper car operation. In fact, the pressures developed by elevator car movement in a shaft—called piston effect —are small. A downward-moving elevator car will force air below the car into the shaft above the car. Additionally, air leakage around elevator doors on each floor is significant.

Moreover, piston effect varies with the number of cars in the hoistway and the hoistway area. For example, for a single elevator car traveling at a velocity of 400 ft. per minute (fpm), there is a pressure differential of 0.08 in. H 2 O. For a double-car shaft, it is only 0.02 in. H2O for a car traveling at 400 fpm. In a double-car shaft with a car traveling at 700 fpm, the pressure differential is only 0.05 in. H2O.

Check that out; it turns out the piston effect isn’t as big of a deal as perhaps it once was, and renowned Swiss elevator company Schindler has spent considerable effort minimizing it.

Schindler Talks About The Piston Effect In-Detail

Noise Is A Major Problem

“Depending on the speed, usually faster than [about 8 mph], the piston effect [of an elevator car in a single shaft] becomes an issue. Airflow and turbulence around the car can also generate unpleasant noise and slight vibrations.” Here’s a nice screenshot from the Schindler video showing the situation:

Fairings To Smooth Airflow, Especially Above 8 MPH

Schindler says it has developed special fairings to “smooth the airflow” to mitigate the piston effect due to airflow struggling to get around the cars. Between that and sealed doors, Schindler says its elevator cars are quite quiet:

With that said, the landing doors — i.e. the doors at each level — can also leak air as the elevator car compresses it, and this can make for some loud landings:

Schindler says it’s sealed those doors, too, to keep the landings quiet.

One Key To Mitigating The Piston Effect: Vents Between Multiple Elevator Shafts

The video goes on to mention something quite interesting, and something I hadn’t really considered. On buildings with multiple elevators (like most really tall buildings with high elevator speeds — which per my research usually peaks at around 25 mph), the elevator hoistways are connected via vents!

So that means, even if the elevator my wife and I were on was trying to squish air below us as we descended, it was almost certainly shooting that air into a neighboring elevator hoistway via a vent:

There are situations where this wouldn’t work out well, like if two elevators in a two-elevator building were parallel and ascending/descending at the same time, but per Schindler, its control system doesn’t allow this.

What’s more, as Schindler notes, car-to-shaft ratio is key. “They provide sufficient space, especially in a single shaft, for the air to flow around the car.”

So What’s The Takeway About The ‘Piston Effect’?

So that was a deep rabbit-hole I just went down, but it was an important one. I needed to know the answer to how an elevator can fight against all that air, and it turns out: There are a few methods. There are exhaust vents, there are vents between parallel elevator shafts, there are fans, there are fairings, there are just nice big gaps between the elevator and the h0istway walls.

Finally I can sleep at night. This rabbit-hole was worth it.

Top Image: Schindler