Date: Sun, 25 Apr 2010 20:58:48 -0500
Reply-To: Tom Hargrave <thargrav@HIWAAY.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Tom Hargrave <thargrav@HIWAAY.NET>
Subject: Re: Water Cooler System Design Flaw Workaround?
In-Reply-To: <4bd4a0be.9753f10a.4118.61da@mx.google.com>
Content-Type: text/plain; charset="us-ascii"
David,
Your logic is flawed - you are considering instantaneous and not continuous
heat transfer. The coolant is circulating and heat is being continuously
pulled off through the radiator.
The real question should be - if you could figure out how to push 20% more
air through the radiator than you are now, how much additional heat can you
continuously pull out of the system? Would the answer be 20% more or
something less? I can't answer that question but maybe someone else can.
BTW, this is exactly what Mercedes did in the 1987 300SDL I own. There is a
cooling fan mounted in front of the engine then there is an auxiliary fan
mounted in front of the radiator. The aux fan comes on under 2 conditions
when the A/C compressor is on or when a high temp sensor screwed into the
engine coolant circuit comes on.
And I can say that the aux fan works very well. I've been idling in 100F+
temperatures, watched the engine temperature climb to 110C then drop back
down to 95C after the aux fan came on. And no, my fan coupling is not bad. I
originally though so and replaced it with a new one. I also checked to make
sure the fan was pulling plenty of air through the radiator and it was. The
aux fan pushed even more air through the radiator.
They've used this system since at least the late 60's, maybe earlier. At
least everyone I've owned except for the 50 had a aux cooling fan that was
switched on the same way.
Tom
www.towercooler.com
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@GERRY.VANAGON.COM] On Behalf Of
David Beierl
Sent: Sunday, April 25, 2010 3:01 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Water Cooler System Design Flaw Workaround?
At 01:22 PM 4/25/2010, Jake de Villiers wrote:
>When approaching the long (15 - 30 minute) second and third gear grades
>through Eastern Washington and Idaho you'd turn on the fan before the
>rad got too hot.
Ok, I see the idea. But do the numbers make sense?
Lessee... at 75 horsepower output and 1/3 efficiency the engine's going to
be putting out something like 200 kw of waste heat, or 3.4 kWh per minute.
18 quarts of 50/50 coolant weighs around 36 lb and has a specific heat
capacity of roughly 0.85 BTU per lb per deg F at working temperature. So
changing the entire coolant by 20F would get you
0.85 x 20 x 36 = ~600 BTU, about what a sedentary human throws off in an
hour.
600 BTU = ~0.18 kWh. 3.4 kWh x 0.18 x 60 = ~32
So a twenty-degree change in the entire coolant would absorb the waste heat
of the engine for about thirty two seconds. I think this is conservative
since the coolant inside the engine won't be participating in this, and
because 75 hp may be conservative for climbing a steep hill.
Somebody check my numbers. If this number is anywhere near correct I don't
think it's worth the hassle.
Yours,
David
