From: J. Forster (jfor@quik.com)
Date: Thu Jul 08 2004 - 23:48:04 PDT
Ryan Gill wrote:
> The problem with the ones on the Ferrets was the resistive
> load could quickly drain the battery so they were
> all pulled from use. The larger vehicles with
> more battery power seemed to do ok.
A typical electric 4 to 6 cup coffee pot or tea kettle draws 1250 to 1500 watts for
5 to 10 minutes. That's 52 to 63 amps at 24 volts, which is a lot to expect of any
battery.
More scientifically: heating water from 20 C to 100 C requires 80 cals/gm = 80
cals/cc. The mechanical equivalent of heat is 4.18 J/cal = 4.18 Watt-Seconds/Cal.
So it takes 80 x 4.18 = 334 W-S/gm The actual power required will be higher.
So to raise 1 cup, roughly 250 cc, of water to the boiling point, it takes 250 x
334 = 83,500 watt-seconds which is roughly 24 Volts @ 1Amp for 1 hour or 24 Volts @
30 Amps for 2 minutes. That's for 1 cup.
> > > Post war you started seeing a boiling
> >> vessel that would run off vehicle power but was
> >> portable away from the vehicle.
> >
> >I assume this was electric?
>
> Initially yes. The US seems to like using fuel
> fired boiling vessels with all of the associated
> issues of poisonous gases and such.. I'm not sure
> what they keep thinking with that...when they add
> them at all.
Basically, fuel is a lot more efficient way of storing energy than batteries. I
posted a comparison some weeks ago and a pint of gasoline has about the same energy
content as a car battery a 1/40 or less of the weight.
> Yep! It's less time spent doing comfort/survival
> things by the troops and if you can make hot food
> while on the move, it's even better for your
> crews.
True. Also, the IR signature of a vehicle may be lower than a stove in the open.
> >If you've got waste heat, it could easily be used to distill water.
>
> Give it time. Modern Military engines certainly
> have lots of waste heat, however, I'm not certain
> how easy it is to clean such devices...perhaps a
> disposable distilling element?
If you distill with vacuum at near room temperature, rather than with a heater, the
container with the concentrated and dry or nearly dry waste could be discarded.
IMO, that's what NASA does.
> When one thinks about it, it seems clear that the
> brits often have the best ideas when it comes to
> tanks. After all, they did invent the beasts.
Perhaps it's because they follow the dictum that simple is better than complicated.
There are two ways to look at bells and whistles: the optimistic and the cynical
(that's something MORE to go wrong). The Brits seem to tend toward the latter.
-John
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