There’s a commercial playing here in the Phoenix area for one of our local electric utilities. In it, while talking about how wonderfully cheap electricity is, they show an electric fan running. The fan has strips of what looks like cloth tied to it which flap nicely as the fan runs. Although I have seen this before, it’s only been on television or in the movies (or this commercial), but never IRL.
So this is what I want to know: Is this just a Hollywood convention, like the ubiquitous loaf of french bread in every grocery bag, or is this somthing that is, or was, done in real life? And if it is, or was, done IRL, why? The only thing that I can think of is that it was done as a primitive form of evaporative cooling, where the strips are tied to the fan then occasionally soaked in water, although it seems as though that might be kind of a pain. Can anybody shed some light on this one?
I think it is to show that the fan is running. If you’re in a store your eyes will be drawn to the movement of the strips. They probably use the technique in commercials for the same reason, as spinning blades don’t convey the impressions of air being blown as well as showing something being blown by the air.
I can’t see how that would work, given that there’s nowhere to sink the heat - you might end up with air being cooled by the strips of cloth having been chilled, but this would be at the expense of the heat extracted from them having already passed through the system (as warmer air coming out).
It may be something to do with the stroboscopic effect that you get from filming rapidly spinning objects - they can appear to be stationary, or moving very slowly/backwards - the flapping ribbons make it patently obvious that the fan is turned on and that there is a flow of air.
I have seen it done in real life in the factory where I work. It just gives a visual cue that the fan is running. Why? Some guesses: It reminds you to turn it off before going home. It reminds you, “Yes, it’s hot, but there’s a breeze, so shut up.” And of course, Rainie Sue always wanted some ribbons on her fan, just like downtown.
Mangetout,
As the name implies, evaporative cooling works by evaporating water into the airstream. This principle is best illustrated in a personal way by that chilling effect you feel when you come out of a swimming pool and a breeze blows across your body! The chilling effect is caused by the evaporation process of the water drying quickly from your body. The same thing happens when you sweat. A “cool breeze” is just air flowing over your body evaporating the sweat. It is natures way of keeping us cool during hot weather or physical exertion.
Using the “kiss” method (keep it simple, stupid), evaporative cooling is the use of this evaporation process to cool the air passing through a wetted cooling media. The cooling media could be as simple as a wet cloth. In fact, this process has been used for hundreds (if not thousands) of years. Hanging a wet cloth over an open window cools the air passing through the cloth. I would think that early civilizations that understood astronomy, built irrigation canals and impressive structures like the pryamids would have used this principle. That being said I think Johhny L.A. is on the money
Evaporative cooling was very popular where I lived in West Texas 40 years ago and still is used but not as much. In a dry climate it can lower a house temperature by as much as 25 degrees F. Much more economical then cooling by refrigeration.
It’s to show that the fan is running and that is blows better than any other fan at any other store. And it catches the eye. Annoying as all get-out, I say.
As far as why the strips are shown in the commercial, I suspect Johnny L.A. is correct. In real life, we use this technique at work (like AskNott’s place of employment) in engine test cells that have air handing systems. It gives a visual cue as to the whether or not the air handling system is running, and a rough indication of what speed it’s running at. We use computer-controlled air handling systems that are not, shall we say, 100% reliable. Since the outlet of the system is ~10ft off the ground, it’s difficult, and a little dangerous, to get direct feedback on air handler status when engines are running normally. In addition, during those occasions when exhaust is released into the test cell in hazardous quantities, it’s nice to be sure the air handler is removing it (without having to enter the cell).
I know how evaporative cooling works, but I don’t believe that it is possible to cool a stream of air by putting a wet cloth in it.
When a breeze blows across your sweat-dampened skin, heat is carried away with the evaporating moisture, so (and this is the really important part) the air blowing across your skin is actually warmer than it was before because it now contains some heat from your body.
So that fan with the dampened cloth strips… if the strips are getting warmer, then it is cooling the air, if the strips are getting cooler, it is warming the air.
Disagree with your conclusion. Reference an earlier thread about swamp coolers. I think that the fact that your body is a heat source is confounding your thinking, so let’s remove that complication.
You probably know that evaporative cooling works because it takes energy to vaporize water. That energy is transfered, in the form of heat, from whatever material surrounds the vaporizing water: more water, your skin, or the air itself. So, in the case of wet cloth strips in front of a fan, both the air and the cloth strips cool down as the water evaporates. I suspect that eventually the temperature in the cloth reaches some equilibrium, and from then on out all the bulk heat transfer comes from the air.
However, I remain doubtful that evaporative cooling is the reason for the cloth strips in the first place.
OK, in specific circumstances such as AskNott or zut mentioned, there are no visual cues from the fan blades themselves, as they are hidden from view and so something extra is required. Fair enough. And I understand that, from a directing standpoint, that extra visual cue can be called for - that’s why I made the comparison to the loaf of bread sticking out of a sack that shows that, yes indeed, this is a bag of groceries, a convention that is used so often that it has become the source of jokes. But, to refine the OP, is there any practical reason for this under the same circumstances as they are depicted in the commercial or occasional film, i.e., just a normal fan sitting in someone’s house where the blades are easily visible.
And I see Mangetout’s response on preview, so I’ll respond to that. Actually, in evaporative cooling the air really does get cooler. A certain amount of energy is required to evaporate the water and when that heat is taken from the air being forced through damp material, the air is cooled (and humidified). That being said, I don’t know if the example from my OP would work as an evaporative cooler, even a crude one. That was just quick theory off the top of my head.
I think it’s like the french bread in a grocery sack. I’ve seen ribbons on fans in store displays. It probably helps draw your attention to the fans.
I’ve heard of people hanging wet cloth in their windows, but I think that the evaporative effect of tying little wet cloth strips to a fan would be so minimal that it would not be worth the effort. The strips would probably dry so quickly that, unless you were constantly turning the fan off, untying them, dipping them in water, retying them, and turning the fan back on, the cooling effect, if any, would not last long at all.
Pooh; think nothing of it. If I had a dime in my pocket for every time I thought I had the whole picture, and found out I didn’t…well, I’d have to walk real carefully, that’s for sure.
Not sure if this is the reason, but growing I came in contact with some older fans that had fan blade guards that were wide enough to easily slip a finger in between. If you weren’t paying attention (and weren’t in the bath of the blowing air) it was a good visual clue that the fan is running, so be cautious.
Not sure if that’s the reason, but that’s what I always assumed.
My great grandma had ribbons on her fans when I was a child to show me that the fan was running and that I better keep my fingers out of the general area.
Thanks for your answers everyone. I don’t think I really found the definitive answer I was looking for, other than the fact that it’s a visual cue that it’s running (that and Rilciam’s contribution), which is something that I figured from the beginning. I guess what I never understood is why you would need a visual cue when you can usually see the blades spinning around fairly easily. Mostly as a safety thing for the kids, I guess.
And to answer KKBattousai, you use evaporative cooling to cool a house by buying a device called an “evaporative cooler” (also sometimes known as “swamp coolers”). They are fairly simple devices that are hooked up to air ducts like regular air conditioning. They use large pads that are hooked up to your house water supply and are soaked with water. A large fan then pulls air through the pads (cooling the air, as was discussed earlier in this thread) and blows it through your house air ducts.
The advantage of this method of cooling is that it is much cheaper than standard air-conditioning and, under the right conditions, produces pretty cold air. There are a couple of disadvantages though. First, it puts a pretty good amount of humidity in the air, more than some people like. Second, while it works quite well in very dry conditions, it can be almost worthless during more humid weather. Here in the Southwest for example, it works great during May, June, and much of July. But during August and September, when the humidity is usually much higher (relatively speaking, that is), it doesn’t do much for you. To take advantage of this, some people have both kinds of units and switch between them as conditions warrent. In fact, there are some commercially produced dual units that switch between methods of cooling as necessary, although I can’t speak for how well they work.
(also sometimes known as “swamp coolers”). They are fairly simple devices that are hooked up to air ducts like regular air conditioning. They use large pads that are hooked up to your house water supply and are soaked with water. A large fan then pulls air through the pads (cooling the air, as was discussed earlier in this thread) and blows it through your house air ducts.