It's Friday night. You've had a long week and now, finally, you get to STFO. You rock up at your favourite weekly venue, swap a smiling hi with your buddy on the door, and push through into a sweaty blast furnace of a venue that melts that smile half way down your chin. And you only brought two spare shirts. Happy dancing, chump.
It's been a while but last weekend inspired me to post here again. There's more to say yet about dance dynamics but this post will steer a little wide of the beaten track, into a different branch of physics: thermodynamics. We're taking this detour because recently, while sweating up a storm at a dance, I realised that I've been quietly complaining to myself about irrational ventilation and cooling set-ups in dance venues for a long time. And it occurred to me that the world might be a happier place if I stopped being lame and tried to actually do something about it.
So, here goes: A nerd's guide to reducing the spare-shirt requirements of your venue, with only the equipment you already have.
Dancers get hot. Just in case, you know, you haven't noticed. How hot? Well, while running, a human typically outputs 700-1400W of power (Smil, 2008). I figure that dancing at medium-to-high tempos is about as tiring as running, but slow tempos considerably less so. So, let's take 500W as a typical power output for a reasonably energetic dancer. That power is quickly converted to heat because dancers don't go anywhere except round and round and up and down a bit, before ending where they started.
How much is 500W of heat? Well, you've seen one of these things before, right?
A small, radiant heater usually outputs about 2000W on its highest setting. So, get 2 couples swinging out in a room, and you might as well have switched on your bathroom heater. Get 50 couples and you've got 25 heaters all running at the same time. Now, that might be just the ticket if you're dancing in in a Siberian winter with the windows open. But otherwise, you'll know it's usually not long before even the people sitting by the bar are sweating.
When it comes to keeping people comfortable, how much heat they produce is less interesting than where it goes. The first place it goes is into the air around them. For practical purposes, there are really only two ways to then get the heat out of the room. One option is to use a heat pump (air conditioner) to extract the heat to the air outside the room, while leaving the inside air where it is (switch on the A/C and keep the windows closed). The other option is to remove the hot air itself and replace it with cooler air from outside (assuming it's actually cooler outside) by opening the windows and using fans. There is a third option too: Temporarily remove the heater from the room (e.g. to a balcony) and let him/her radiate into the outside air for a while before going back inside. But that's not really cooling the room, it's just not heating it up in the first place.
With the right set-up these different methods can all be effective at cooling the room but there's something else to add here, in the interests of sustainability. An air conditioning unit uses a lot more energy than a fan does. Like, about 10 times more. A hall that takes 3 A/C units to cool effectively usually won't need anywhere near 30 fans if the A/C is left off! So, the first option to consider for cooling a hot room is always smart ventilation, assuming there are enough windows/doors for adequate air flow, and assuming it's actually cooler outside than inside.
But what do I mean by 'smart' ventilation?
The picture below shows what is, in my experience, the usual set-up for trying to keep people cool at a dance if there's no A/C (and often, even if there is):
Unfortunately, many venues were designed without good ventilation in mind. Often, there are windows only on some walls and they don't always open as they should. Anyway, let's assume that our windows, on two walls in this example, open ok. There's also a door in another wall, and one corner of the room with no natural ventilation at all. There are three fans, all aimed at the dance floor, and two of them have hot, sweaty dancers standing in front of them.
Is this the best way to keep people cool?
Remember, to cool this room through ventilation, one-way airflow is needed, to and from the outside world. That is, hot air leaving and staying out, and cool air entering and staying in, until it warms up and finds its way out again through another window. Fans can help with this but in the set-up here, they're doing something very different.
Presumably, the idea is that it's refreshing for any given dancer who manages to stand/dance in front of a fan. The trouble is that all the heat blown off that dancer is simply distributed to the rest of the room, making it even hotter for everyone else. The fan and dancer together are functioning precisely as a fan-forced heater. That's, well, not cool.
The fact that all the fans are pointing in different directions makes matters worse because it prevents consistent air flow, in through one set of windows and out through another ('cross-ventilation'). Instead, flow is complex and turbulent, preventing quick heat extraction to the outside world.
Now, let's consider what might be a better way to use the same equipment in the same venue:
Here, the fans are not being used with individuals in mind but rather, the whole room. Two of them are now aimed out the windows. This isn't as silly as it seems. The point is to blow hot air out of the room on one side so that cool air is sucked in on the other side, rather than just recycling hot air. Note that this is usually more effective than trying to suck cool air in, by standing a fan next to a window, facing inwards.
The third fan is still aimed at the dance floor, but this is done in such a way as to promote cross-ventilation from the 'intake' windows, across the hot dance floor, to the 'exhaust' windows (indicated by the long, grey arrow). Individual dancers who want to be cooled down by the fans can stand behind them (shown), rather than in front, or they could possibly squeeze between the fans and the windows they're aimed at. Either way, the point is to immediately extract their heat from the room, rather than blowing it all over everyone else.
Finally, let's take a look at what an 'ideally' ventilated venue might look like (Well, indoor venue, anyway; ultimately, there's no substitute for having no walls at all!).
Firstly, this venue has more windows and, importantly, some of them are in opposing walls, allowing for direct cross-ventilation. Two of the fans are directed out of the windows in one wall, close to the dance floor, sucking cool air in through the other windows, over the dancers and out the exhaust windows. The third fan is placed to assist this process. Conveniently, the fan-equipped windows open onto a balcony, where sweaty dancers can stand in the airflow as it's ejected from the room. Voila!
Of course, this description and the others above are over-simplifications. Air flow in these rooms would in reality be complicated by many factors, depending on the venue design, the number of people, the heights and sizes of the windows, etc.
Nonetheless, I hope the basic ideas sketched here: choosing smart ventilation over A/C where possible, promoting cross-ventilation and extracting hot air rather than recycling it, might be useful to venue operators and dancers everywhere!
Reference
Smil, V. Energy in Nature and Society: General energetics of complex systems. MIT Press, 2008.
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