[Stoves] Using a differential windlass as motive power for stove

[AJH]

On Sun, 10 Dec 2006 07:46:55 +0200, Crispin Pemberton-Pigott wrote:

>Dear Steve and All
>
>>...Now, if we use Drew's weight system, all THAT energy is stored in a
>>lifted weight. The energy required to lift a weight of mass M, through
>>height h = Mgh, where g is the gravitational acceleration constant, g =
>>9.81 m/s/s
>>Lets assume we lift it
>>
>>Energy = Mgh or  1800J = M x 9.81 x 2
>>
>>Rearranging for M, 1800/2/9.81 ~ 90kg,

Much the same calculation we did some years back on the list, it
struck me as to onerous and I looked into a flash steam coil, with
some input from Ken Boak. The biggest problem I had with that was the
parasitic losses of the steam coil seemed unacceptable.

I think the windlass being described is what I was taught was the
"Weston differential pulley" when used as an engine hoist this device
does not need a brake. That suggests to me that its efficiency will be
below 50%, I can see friction losses in the windlass and  the pulley
and flexing of the rope but otherwise cannot see where losses occur.
>
>OK so far.  I think 3 watts is unnecessary.  One watt would be enough.

As has been pointed out 3 Watts input will not result in that amount
of energy being in the moving air. I think you'll be lucky to get 70%
conversion of shaft power into air and as Drew has said small
motors/generators have bigger losses than bigger ones. Having said
that the small electronically commutated fan is ubiquitous with PCs
and as you pointed out a laptop fan is often a centrifugal one, 3W
centrifugal fan new here with a MTBF of 15000 hours is GBP15 new.

>As a design exercise this is really good.  Perhaps a spring is a better 
>store of energy (wooden or plastic) than a lifted weight.  The fam might be 
>quite slow, radial and thin to prouce the air volume/pressure required. 

As discussed previously the fact that a fan absorbs power at the cube
rate of its rpm means it may be self regulating in running down a
clockwork mechanism, at the time I noted that Swiss clockwork musical
boxes used a centrifugal fan as a speed regulator.

AJH