Does a spring mechanism exist for a ventilator?

[John Haine]

On John Haine Said:

Please just do some basic sums.  10kg weight exerts about 200 Newtons.  Lift it 2m, you’ve stored 400 joules.  If it drops in 100 seconds that’s 4 watts.  Frankly you’d get warmer just weight lifting.

Alas, doing sums is one of the great divides in Model Engineering.   Some practical men go so far as despise maths, science and theory, and instead rely entirely on experience. They are foolish!  Experience is essential, but it’s limited compared with understanding mother nature’s rules, which become vital when designing anything new.  Unfortunately, some university trained engineers despise practical men just because they can’t to the maths, which is equally foolish.   They need each other!

Unfortunately natures rules are best expressed mathematically, and many of them aren’t simple.  Though Stovemaker’s project could be approached experimentally, using rule of thumb methods,  that’s an extremely expensive way of approaching a design problem where the physics is already well-known.

Stovemaker gives more detail of his project in post #768731.   My previous guesses about his device were all wrong, me assuming that the spring driven fan part was only required to circulate air around a room and the heat source was something else.  Not so!

Stovemaker’s description is of a heater in which a spring provides all the energy.   It’s a fan-heater in which heat is obtained by coupling inefficiently with magnets so that a fan is heated by induction currents.  The method isn’t crazy – it’s analogous to why an overloaded electric motor gets hot.   As far as I can tell Stovemaker’s device isn’t a perpetual motion machine, therefore it can be built.   Several problems to solve though:

• Efficiency. Gut feel is that inducing current in the fan by slipping magnetic fields and heating the fan blades inductively W= I²R will be inefficient compared with fitting the fan with a slipping mechanical brake.  Magnets wouldn’t wear out though, so he scores by reducing maintenance, maybe…
• Adjustment.  The device will have to be adjusted to slip ‘just so’.   Have to build one to be sure, but I think  getting the slip right will be tricky.   There’s a balance: no slip means the fan will move air without heating it whilst excessive slip won’t produce much heat because the induced currents are low, and the fan won’t spin either.   Plus, the fan needs to spin quickly – faster the better, and 500rpm is slow.
• Power.   This is the killer!  John’s sum shows that a 10kg weight dropping 2metres over 100 seconds produces about 4W.    That’s way too small to heat a room. In comparison, my bathroom has a 1.5kW fan-heater which takes about 10 minutes to raise about 14 cubic metres of air from 16°C to 20°C.  That’s 0.25kWh,  compared with John’s dropping weight example, which I calculate only stores 0.0001kWh:  no need to build one, the calculation shows the weight has to deliver 2500 times more energy.  A much bigger weight and/or falling from a great height is needed.  (Usual warning about my dodgy maths, but I think the magnitude of the problem is clear!)
• Space.  Can’t find the comparative table in my books this morning but from memory a falling weight has a higher energy density than a spring.  That means a giant spring would be needed to heat my bathroom.  Not having the numbers to hand stops me estimating how big the spring needs to be, but anyone who has the figures can do the sum.
• Ergonomics.  Assuming the purpose of a spring powered heater is to go off grid, then the spring will have to be wound by muscle power.  An average fit man can output about 200W continuously, so that’s about 75 minutes hard work winding the spring before it does any heating.  (I’m far from being averagely fit!)

Boils down to an interesting idea, loads of fun to had developing it and finding out how well it works, but theory strongly suggests the result is unlikely to be practical.

Plenty of clever history and science behind the “Mechanical Equivalent of Heat“.  James Watt was a pioneer, his need being to explain to customers who only understood horses, just how much better his steam engine was than muscle power.  By measuring the work done by a team of horses lifting water from a deep well, he derived the Horse Power, a unit that survives to this day even though hardly anyone knows how hard or for how long a horse can be worked. (A James Watt horse is about 750W, roughly 4 average men.)

The equivalence of mechanical and other forms of stored energy was proven by Joule about 60 years after Watt.    Thereafter the Mechanical Equivalent of Heat was expressed in Joules/Calorie, but the modern metric system eventually rationalised the whole concept into the Watt.   British and US Heating engineers still seem to use BTU though, which is the amount of energy needed to raise one pound of water through 1°Fahrenheit:  about 1055 watt-seconds aka Joules, in real money!  The Centigrade Heat Unit (one pound of water through 1°C) looks to have disappeared though.

Dave

 

 

[duncan webster 1Participant @duncanwebster1]

Some time ago the journal of one of the UK engineering institutions published an article about an energy storage method based on dangling a big weight down a mineshaft. Some simple sums showed that it’s capacity would be trivial compared to the hydro storage schemes in North Wales. It was disappointing that the journalist didn’t understand this, not Fleet Street, supposed to be tech savvy.

[noel shelleyParticipant @noelshelley55608]

When one starts to do the sums, one soon realises that any sort of human powered device will not produce any useful heat. I seem to think that the human body will add 60 watts to a room by just being there ! Whilst Stovemakers idea could result in some interesting experiments but would prove of very little of practical help in reaching his goal. As for a manpower being 200W I was under the impression it was about half that figure ? The man powered dynamo for battery charging in WW2 was I believe 80 watts. Noel.

[FulmenParticipant @fulmen]

A quick web search suggests that an average person can output less than 100W continuously (8hours manual work) while an athlete can provide perhaps 2kW in short bursts (like a sprinter).

[stovemakerParticipant @stovemaker]

Hello everyone, thank you again for all your helpful replies. Indeed, no weight driven or spring driven mechanism will do the trick. I realize this now.. i will start exploring other avenues.

[SillyOldDuffer]

On SillyOldDuffer Said:

[…] Stovemaker gives more detail of his project in post #768731.   My previous guesses about his device were all wrong, me assuming that the spring driven fan part was only required to circulate air around a room and the heat source was something else.  Not so!

Stovemaker’s description is of a heater in which a spring provides all the energy. […]

 

With the greatest respect, Dave … I’m almost certain that your CURRENT guess is the one that’s wrong !

Please humour me by re-reading that post in conjunction with Stovemaker’s other comments:

I am quite sure that he wants a device to distribute the hot air from a wood-burning stove.

MichaelG.

.

Edit: __ I posted that ^^^ before I saw stovemaker’s latest

Now I am really bewildered !

[stovemakerParticipant @stovemaker]

Dave is right, my idea was to rotate a fan in front of permanent magnets, positioned in a circle with alternating polarity . This, by induction, wil heat the fan (which should be metal, preferably aluminium,), which then at the same time is heated, and disperses its heat. But the power… You simply cant get enough energy for this. It works for lighting some leds, but not this..

[John HaineParticipant @johnhaine32865]

When air is heated it’s molecules move faster.  When it’s pushed around by a fan it’s molecules move faster too but the extra momentum soon becomes randomised and just makes the air slightly warmer.  No need for anything else to generate heat.

[BazyleParticipant @bazyle]

No offence to Stovemaker but sadly this reminds me of the current wave of adverts for ‘ceramic heaters’ trying to give the impression they are somehow more efficient than other electric heaters. I think such advertising should be banned as it is clearly intended to con pensioners and the gullible. When this crap started a few years ago they even conned a Member of Parliament to give them a mention in the House.

[stovemakerParticipant @stovemaker]

Not quite seeing your point John. Are you saying that a warm fan would not dissipate much more energy than a cold one?

[stovemaker]

On stovemaker Said:

Dave is right, my idea was to rotate a fan in front of permanent magnets, positioned in a circle with alternating polarity . This, by induction, wil heat the fan (which should be metal, preferably aluminium,), which then at the same time is heated, and disperses its heat. But the power… You simply cant get enough energy for this. It works for lighting some leds, but not this..

Sincere apologies for my misinterpretation

I will crawl back under my stone

MichaelG.

[Les Jones 1Participant @lesjones1]

Have you considered how much power a human can produce. One reference I found online claimes that an elite Tour de France cyclist can produce about 350 watts and an average weekend cyclist can produce about 1/16 horsepower (= 746/16 = 47 watts) So if he used that power for 6 minutes every hour  to store that energy (Winding up a spring or rising a weight etc) that would only give 4.7 watts for an hour. 4.7 watts would not heat a room by any significant amount. Even is he produced 47 watts continously it would not heat a room very much. A typical small fan heater produces 1000 watts of heat.

Les.

[John HaineParticipant @johnhaine32865]

No, I’m saying that there’s no point in using anything but a fan, all the energy that goes into it will end up eventually heating the air.

[FulmenParticipant @fulmen]

True, but at some point that stops being useful. Try dumping a kW or two as wind in your living room and tell us if it feels warmer or not 🙂

[noel shelleyParticipant @noelshelley55608]

A few simple points ! Why do we wear clothes ? Isn’t It to trap air round us that the body will warm, reducing heat loss ? The human body being largely water/ fluid the latent heat of evaporation will cause cooling. Isn’t that why we use fans in the summer when it’s hot ? Noel.

[Howard LewisParticipant @howardlewis46836]

As Noel says, our clothes trap a layer, or more, of air around us, insulating us, reducing our heat loss to the air around us..

It used to be said that the average human being in a room was equivalent to 0.75 Kw.

Which is why a room full of people soon gets hot.

It is beginning to sound as if the objective is generate eddy currents in the metallic fan so that it gets hot and distributes its heat

If that is the case, I would imagine that it would have to rotate pretty fast to generate enough heat to actually heat its environment.

And then I wonder about the efficiency; energy in vs energy out.

If I am wrong, please correct me

Howard

[duncan webster 1Participant @duncanwebster1]

<p style=”text-align: left;”>Energy out will always equal energy in. The only way to get rid of energy would be to create mass, and I don’t think anyone has done that yet. We’ve done it the other way in nuclear fission,</p>
E=M*C^2 and all that

[BazyleParticipant @bazyle]

People definitely don’t put out 0.75KW, more like 75W at rest and maybe 200W if vigorously sawing through a 3in steel bar by hand. (edit after a bit of googling)

One of the few lessons I can still directly remember from Primary school is an mage of Mrs Luxmore telling us that two thin jumpers were better than one thick one. Typical ’50s curriculum for 7yr olds then. Probably now its about badgering your parents to replace the gas boiler with solar panels.

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