Date: Thu, 6 Jan 2000 15:01:55 -0500
Reply-To: David Beierl <dbeierl@IBM.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@IBM.NET>
Subject: Re: (long) EVERYTHING you want to know about heat,
was Porsche 6 cyl heating van
In-Reply-To: <01BF582E.62F7A540@ip26.raleigh13.nc.pub-ip.psi.net>
Content-Type: text/plain; charset="us-ascii"; format=flowed
Hey Bulley -- This is great, but I think you oversimplified a few things,
so I'd like to add a few comments...
At 10:11 1/6/00 , Bulley wrote:
>system gets far hotter. The size of the motor dictates how "much" heat is
>available from either source. A 2.7 liter Porsche motor theoretically will
>have far more heat available (through the exhaust tubes) than the 2.0 air
>or 2.1 water VW motors. Great. Onward.
Either motor will have roughly equal waste heat available *at a given power
level,* assuming roughly equal efficiency. The big motor can deliver more
power (i.e. burn more fuel) and when it is doing so it will waste more heat
(i.e. have it available for cabin heating).
There is another thing which affects the situation, though -- each motor is
shedding a certain amount of heat as engine cooling, and a certain amount
as exhaust heat. The water-cooled motors use engine cooling for cabin
heat, and the air-cooled use exhaust heat.
And just to stir things up a little -- the amount of heat available from a
given transfer fluid is equivalent to the temperature times the mass times
the specific heat of the fluid (i.e. how much heat it holds per mass per
degree of temperature rise). Water masses in the neighborhood of one ounce
per fluid ounce. Sea-level air is in the neighborhood of one ounce per 30
gallons. The specific heat of water (arbitrarily assigned as 1) is
extraordinarily larger than almost anything else: Aluminum has a specific
heat of 0.2, most other metals and materials about 0.1. Air is similar to
aluminum. This large heat capacity is just another of the many properties
of water which make it so astonishing (my favorite is its expansion on
freezing -- if this were not the case, most of our lakes and I suspect
oceans would be frozen year-round). All of this is to say that air-heating
and water-heating are vastly different in practice. *For a given
temperature difference,* you have to move roughly 20,000 times the volume
of air as of water to achieve the same heat transfer. It is simplistic
-- false, even -- to compare the source temperatures as a primary measure
of the heat available from the system. The heat available is that heat
shed from the engine as cooling or as exhaust. Either one (at the sort of
engine efficiencies available in Vanagons) should be more than
adequate. It is in the implementation of the system that success or
failure lies (as you point out).
>The CARRIER can be electricity, oil, air, water, or something else. Each
>TRANSFER of state from the ELEMENT to the final destination causes loss of
>heat, inefficiency.
Agreed -- almost. Each state-change causes loss of *energy* -- as *HEAT*
into the surrounding system. *However* here we are talking about starting
with heat, ending with heat, heat in the middle. All we're doing is giving
heat a chance to flow downhill, and the process is completely efficient
because heat is already the most degraded form of energy. The
inefficiencies from our standpoint, i.e. warming the cabin, arise from 1)
failure to capture available heat from the input stream, i.e. inefficient
heat exchanger from engine to carrier stream to output stream, 2) failure
to move enough mass of carrier to achieve the desired heat transfer, and 3)
heat loss from the carrier stream before reaching the destination, i.e.
heater core or cabin interior. An auxiliary reason 4) would be excessive
heat loss from the cabin. This is huge, and the reason is that there is so
much input heat available already paid for that there is no economic reason
to spend money and weight on cabin insulation. Your practical experiments
have shown that #1 is not an issue -- there is plenty of heat available at
the heat exchanger. However, both numbers 2 and 3 come into play, and you
have solved them appropriately, i.e. insulate the ducts and increase the
flow. I am curious from an ergonomics standpoint whether the increased
flow is enough to be annoying in the cabin, but I imagine not. The V'gon
heat distribution system is IMHO a blunt instrument already.
>home. Through all of these 'transfers', you have lost a 95% of the heat
>from the original coal fire. Inefficient, (that is why it is so expensive
>to heat with electricity).
Hyperbole. It's about 2/3 loss, and the vast bulk of that goes up the
chimney at the power station. Now that people are working on ways to
capture that energy, the net system efficiency of the best systems is maybe
closer to 80%.
>Air-cooled motors have much more efficient and direct heat transfer. The
>air that will warm you passes through exhaust tube heat exchangers (tran
>sfer 1). Fiery-hot air moves directly to the vehicle to heat you (transfer
>2). So why do air-cooled VW heaters have crappy performance? Read on.
Irrelevant, as I said before.
>The limiting factors however, are the CARRIER, and the CONTAINMENT. Water
>(a carrier) has greater "thermal capacity" than air; once you get it hot,
>it is less apt to cool down. Therefore, water encounters less loss on the
>way to the heater core in the car.
Many factors here: Water has higher specific heat, so less mass
required. Also higher specific gravity, so less volume per mass. Result
is *much* smaller surface area to lose heat. OTOH, water transfers heat to
the tubing wall about 300 times as fast as air (per equal wall
area). OTOOH the transfer to the tubing wall goes up with the square of
the temperature difference, which is greater (initially) with the air
system. Also, the result of the area difference in tubing is doubled since
the tubing is being forced-air-cooled from the outside. Net result is as
you say, that the air duct needs insulation a lot more than the water tube.
>downside however, water does not transfer ALL of its heat to the exchanger,
>it departs the exchanger warm, still carrying heat you could use. Not so
>great.
No big deal. You haven't lost the heat. If you want to capture more on
the first pass, use a bigger -- or better -- heat exchanger.
>Air (as the carrier) directly warms your body without additional transfers.
>It isn't leaving lukewarm for another go around in the motor, it is here to
>stay, to warm you, so it is a more efficient choice.
Irrelevant.
>Air as a carrier has another deficiency. It is far more apt to loose
>VELOCITY than water.
Disagree. You put X amount of fluid in the pipe, you're going to get X
amount back out. Air is a lot springier than water, so until you reach
steady-state there will be more going in than coming out, but that's
strictly temporary. Of course if you make your duct leaky because you
can't be bothered, you'll lose that way. It's an open system, so you're
not forced to eliminate leaks to avoid replenishing the fluid.... Either
way you have to expend energy in pumping to overcome friction and
turbulence losses. Water doesn't like to go around square corners any
better than air. The major deficiency of air as a carrier IMHO is the
sheer physical size of the installation required to move the necessary
volume of air without resorting to high pressures with attendant pumping
losses.
>Finally, we come to the CONTAINMENT. Water is routed through rubber tubes,
>and plastic valves; high-temp threshold of about 320 degrees. Your metal
>ducting and paper tubes bear 400 degrees with a grin. I've never tested VW
>ducting to see at what temperature it scorches, but I guarantee, if we put
>VW heater hose, and VW ducting in the oven, and turned up the heat until
>one failed, you'd be scraping rubber off the oven floor. Paper can handle
>much hotter heat.
Irrelevant. Rubber can hold water with a grin. <g> And it doesn't leak
working fluid into the outside world.
Anyway, my picking your theory apart doesn't change the fact that your
practical methods are 100% solid-gold Bulley, the genuine article accept no
other none valid without our founder's signature reproduced on the
bottle. On the religious issue of air vs. water I'm afraid I'm an
ecumenist. Implementation is all.
Best regards,
david
David Beierl - Providence, RI
http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
