Date: Wed, 17 May 2000 21:43:38 -0400
Reply-To: ed <edevinney@ANENT.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: ed <edevinney@ANENT.COM>
Organization: Pismo Beach Institute for Advanced Leisure Studies
Subject: Re: Heat in an air-cooled Syncro (long, boring, self-involved)
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Wow! Now that's analysis.
I think it misses one point in the warm-up equation, though. The water-cooled
motor dumps (almost) all its waste heat into the heater loop. The air cooled
911 motor has an 11-bladed BHF (Big Honkin' Fan) which takes away enough heat
that the 2L and maybe 2.2L sixes are OK without an oil cooler. The larger
motors need progressively more oil cooling, since the fan apparently still
only carries away 2L worth of heat...
So I'm suspecting that the oil carries >5400BTU/hr of heat away.
But the overall point is a good one, indeed. Lots of current to electrically
heat, too. And it seems such a shame to waste all that heat coming off the
exhaust :-)
Cheers -
ed
David Beierl wrote:
> At 21:50 5/16/2000, ed wrote:
> >Some folks have apparently used an oil-water heat exchanger to drive the
> >stock system. Good idea, but the oil temp in my own 911 takes a
> >donkey's year to get up, especially when the weather is cold - 11 quarts
> >of oil is a lot to heat up. One could electrically heat the water til
> >the oil comes up to temp, but seems klugey and in any case retaining
> >water heat requires pumps, controls and other cruft. MSDS recommends
>
> Some numbers of possible interest:
>
> Specific heat of water: 1 (1 BTU per Lb per Degree F)
> Density of water: 1 (8 lb per US Gallon)
> Specific heat of oil: a bit under 1/2
> Density of oil: somewhere around 0.8
>
> Capacity of Vanagon cooling system: about 36 lb.
> *Disregarding losses* which means that this number is pretty low...
> Heat input needed to raise cooling system from 60F to 160F in ten minutes:
> ~22,000 BTU/hr*
>
> *Of course the heater circuit gets warm much earlier because at first the
> system is warming only the water in the engine and heater loop. Also of
> course the radiator loop might never get to 160 -- but it has a bloody
> great radiator in it throwing away heat by the bushel. Also none of this
> addresses how much heat the heater actually passes into the cabin, only
> what it takes to get the operating fluid up to temp, hence an idea of what
> is available from the motor. To figure the capacity of the cabin heating
> system you'd need to ask VW engineers or analyze/measure the heat exchanger
> capacity.
>
> Weight of 11 qt oil: ~18 lb.
> Heat input as above: ~5,400 BTU/hr*
>
> *This is a bit more than the output from an electric room heater, which we
> all know is utterly insufficient to heat a Vanagon standing still, let
> alone whizzing down the road. Which suggests that if the oil takes a long
> time to warm up, the net heat transfer into it, and hence the amount
> available eventually for cabin heating, is better than waving matches in
> the air, but not all that much better.
>
> Hypothetical capacity of the water in an oil/water loop: 6 lb.
> Heat input as above: ~3,600 BTU/hr
> Amps @12v to do this electrically, not counting heat going back into the
> oil: ~75
>
> Formula: t [minutes] / 60 * Hin [BTU/hr] = Wt [lb] * delta-T [deg F] *
> Specific heat [where water = 1]
> Rearrange terms as desired or use an equation solver like the one on my
> calculator...
>
> david
>
> David Beierl - Providence, RI
> http://pws.prserv.net/synergy/Vanagon/
> '84 Westy "Dutiful Passage"
> '85 GL "Poor Relation"
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