Wikipedia:Reference desk/Archives/Science/2020 September 12

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September 12

Ventilation fan

I'm wondering why typical household ventilation fans are as small as they are. My reasoning is as follows (tell me if I'm wrong somewhere). Ventilation requirements are basically in air changes per hour which means for a given room size you need a given cfm (cubic feet per minute) of air, and fans are rated in CFM. 1000 cfm would be a typical small indoor fan. Basically the fan pushes out a cylinder of air the same diameter as the fan blades, and its velocity (per minute) is the cfm divided by the area swept by the blades. From the molar weight of air and the ideal gas law, you can find the mass of 1000 cfm of air. The kinetic energy per minute is 1/2·m·v² which after some unit conversions gives you the electric power in watts needed to run the fan (there is a small loss factor but this wattage really does seem to be reflected in the fan specs). The thing here is, that the cfm stays constant but the airspeed (and therefore power) decreases with the square of the diameter. So to minimize your electric bill, you really should want the largest diameter fan you can get.

This shows up in big ceiling fans, but right now I'm using a 20 inch box fan, it is pretty noisy and power hungry, and I'm wondering why I couldn't get one that was say 4 feet in diameter. Any idea? Am I missing something? Thanks. 2602:24A:DE47:BB20:50DE:F402:42A6:A17D (talk) 02:31, 12 September 2020 (UTC)

Additional question: I have the 20" box fan blowing through a furnace filter to help clean the air coming out.[1] The filter slows down the airflow somewhat, so that would be one advantage of a smaller (higher velocity) fan. Is there a reasonable low-tech way to gauge the airspeed change? Say, by seeing how much a piece of paper (maybe weighted by a penny) hung in front of it gets deflected? I could try to concoct something like this. I have a voltmeter so in principle I could measure the electrical power going into the fan, but I'm not really concerned about the power bill right now. I mostly want to compute the cfm getting through the filter. Thanks. 2602:24A:DE47:BB20:50DE:F402:42A6:A17D (talk) 02:38, 12 September 2020 (UTC)

The something that you're missing is practicality. A large fan on the ceiling is acceptable, but consumers are concerned with factors in addition to efficiency, such as:  How would a 4-foot fan fit with my decor; and where can I find space for such a thing? -- They don't make 4 ft. residential fans because nobody would buy them (with one exception?); but, if you really want to, you can buy one: [2] —2606:A000:1126:28D:FDD2:BED1:91BF:2ACB (talk) 05:32, 12 September 2020 (UTC)

That fan you linked is a huge, expensive, noisy, 1.5hp (1.1kw) industrial fan. I was imagining something that ran at much lower speed. very quiet, using little energy. If not 48", then 30" or so seems practical for many settings such as in a wide window. Right now my 20" fan is sitting on a chair in the middle of the room, which is terrible decor by any stretch, but it's helping the air and a 30" would fit about as well. I also have to wonder why anyone uses 12" or 16" fans when they can use 20". Finally I'm wondering whether this relationship between diameter and power efficiency (watts per cfm) is well known--or alternatively, whether it is mistaken. It does make me more interested in getting a large ceiling fan for hot days. Home Depot has some residential ones up to 7 feet across, and industrial ones (appropriately called "big ass fan"[3]) up to 14 feet (750 watts, they don't even tell you the cfm but I think I can calculate it from the diameter and power). Anyway, thanks. 2602:24A:DE47:BB20:50DE:F402:42A6:A17D (talk) 08:15, 12 September 2020 (UTC)

I would refer you to Big Ass Fans for all your Big Ass Fan needs.... - Nunh-huh 05:15, 14 September 2020 (UTC)

A larger box fan might also run into safety concerns, especially as regards children and small animals around it. --Khajidha (talk) 14:16, 13 September 2020 (UTC)

Optically hide a big fan in new construction with a dropped ceiling, pitch black tallest open-ended honeycomb that's worth it ceiling grate for the fake lower ceiling, pitch black entire ventilation system and largest gap between top of grate and bottom of fan that's worth it. You can paint the bottom of the grate any color you want, in fact a light color might camo the fan more. Put as many lights as you can that still seems homey, all under the grate with caps to prevent direct grate illumination and evenly distributed in area and brightness, summing to a typical total room illumination level in all. Well that's the best I could come up with. Sagittarian Milky Way (talk) 13:16, 12 September 2020 (UTC)

Or slowly and slightly turn the top half or more of a very tall but otherwise standard square subway grate-type grate, you can make it geometrically impossible to see the fan no matter where you put your eyeball. The top of the grate cells should be pointing up and to the left or right or front or back, whichever is downwind in the exhaust system. Sagittarian Milky Way (talk) 17:32, 12 September 2020 (UTC)

If I got a ceiling fan I wouldn't want it to be hidden. They look cool as hell imho. But, part of that is they look a bit retro since the modern era generally uses a/c instead of fans. 2602:24A:DE47:BB20:50DE:F402:42A6:A17D (talk) 20:12, 12 September 2020 (UTC)

I may be mistaken, but I think most of the energy expenditure goes into the mechanical friction of the motor plus electrical losses because electrical motors are not 100% efficient even disregarding friction. The energy saved by using a fan with a large diameter may be disappointing. Noise production costs energy, so go for the less noisy fans. Commercial ceiling fans have anywhere from two to ten blades, sometimes curved like the wooden propellors of early planes, sometimes plain and flat. I think the curved ones are that way because it looks slick, and the flat ones because it is cheaper to manufacture. I doubt the areodynamic properties are a serious factor entering the considerations of the manufacturers.  --Lambiam 20:22, 12 September 2020 (UTC)

I haven't done the math for very large or small fans, but for midsized ones (and for vacuum cleaners), the energy really does seem to mostly go into moving the air. See air watt. I believe my 20" box fan uses around 100W (haven't measured) and it definitely makes noise, but think of how loud a 100W stereo would be. 2602:24A:DE47:BB20:50DE:F402:42A6:A17D (talk) 20:45, 12 September 2020 (UTC)

The Dyson C47 has reportedly a suction power of 180 airwatts, but an input power of 1210 W.^[4], which is an efficiency of 15%.  --Lambiam 16:25, 13 September 2020 (UTC)

What makes radar so valuable?

I read about radars, and it seems that except for special beyond-horizon radars; most radars can detect only objects which are in a sight line with the object. Meaning that theoretically, can been seen with a naked eye(with binoculars I guess?). So why then the radar makes such a different? why nobody uses a visible light for example?--Exx8 (talk) 18:47, 12 September 2020 (UTC)

A fighter jet at radar limit is too small for binoculars. In daytime. Sagittarian Milky Way (talk) 19:16, 12 September 2020 (UTC)

See Chain Home for the first early warning radar, which had a detection range of 160km (100 miles) back in 1936. Mikenorton (talk) 19:29, 12 September 2020 (UTC)

Another factor is that radar can "see through" clouds and other meteorological phenomena. See Radar#Principles. AndrewWTaylor (talk) 19:34, 12 September 2020 (UTC)

In addition to the points above, even in fair weather you'd need to keep scanning around in a 360° sweep to notice vehicles, possibly several, approaching from unpredictable directions. With radar you need to look merely at the screen.  --Lambiam 19:57, 12 September 2020 (UTC)

@Sagittarian Milky Way, why? what makes radar able to detect thing in a distance which can't be observed with visible light?--Exx8 (talk) 20:25, 12 September 2020 (UTC)

Well, at night it is dark, so you can't observe anything with visible light unless it has lights on or you light it up somehow, in which case black paint would do a good job of stopping reflections. During the day your sensors will be flooded with sunlight because it is daytime so you have to somehow resolve a tiny dark speck. Maybe you could do that with a high resolution wide angle camera these days but that takes fairly modern tech. With radar, you illuminate the target and it's enough to notice a very faint reflection since there is not much background noise at radar frequencies. It's like being able to see a distant star at night because of the faint light reaching you. Finally, you can measure the doppler shift in the reflected radar signal and that tells you the velocity of a moving target. 2602:24A:DE47:BB20:50DE:F402:42A6:A17D (talk) 20:51, 12 September 2020 (UTC)

At night, if you don't have too much light pollution, you can look for artificial satellites passing overhead. Except, you don't get to look at any one for very long, because they'll pass through your line of sight in tens of seconds. Things in orbit are moving quite a bit faster than aircraft, but still, this gives you an idea. Now imagine trying to pinpoint something in the daytime moving fast at 50,000 feet, and keep your bearing on it while it moves, and trying to identify it. And then do that for possibly dozens of aircraft at a time. --47.146.63.87 (talk) 23:31, 12 September 2020 (UTC)

RADAR = "RAdio Detection And Ranging". In addition to the detection and determination of azimuth, which radar does better than visual sighting as described above, it also gives accurate ranging to aid in tracking or targeting. -- ToE 00:16, 13 September 2020 (UTC)

I read an article recently about Joan Curran who invented Radar chaff. Was very interesting to learn about her work. Zindor (talk) 12:53, 13 September 2020 (UTC)

This book (which made its author famous) may be of interest [5]. 2A02:C7F:D63F:AF00:48A1:AB9F:4B06:6D65 (talk) 18:04, 14 September 2020 (UTC)