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Radiator questions

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Posted by nicksanspam on August 24, 2007, 5:46 pm
I'm thinking about using a recent Taurus auto radiator or a Ford or GMC
pickup radiator to heat water with hot sunspace air, but I'm concerned that
a) it's hard to find specs for radiator performance, and b) a mechanic says
an aluminum core radiator might crud up with corrosion in less than a year
with oxygenated water and no antifreeze.

How many Btu/h can an auto radiator move from 150 F air to 140 F water,
using its electric fan, with no wind? An engine that makes 200 HP at 25%
efficiency (unlikely with no motion :-) would burn 800 HP of gas. If 25%
of the heat leaves from 200 F water to 100 F air via the radiator, the air-
water thermal conductance is 0.25x800x746x3.41/(200-100) = 5100 Btu/h-F,
good compared to a 2'x2' all-copper $00 MagicAire 2347 duct heat exchanger
that moves 45K Btu/h from 125 F water to 1400 cfm of 68 F air with a 0.1
"H20 pressure drop.

Antifreeze would be expensive for a 1000 gallon heat storage tank, and
an antifreeze heat exchange loop would add to the cost and lower efficiency.
How much antifreeze do we need just to prevent corrosion? Is there some
corrosion inhibitor we can use at a low concentration?


Posted by stu on August 24, 2007, 11:12 pm



Why use Aluminium? I wouldn't have thought it would be hard to find an old
copper one which will.
A. Be a little more efficient
B. Won't have the corrosion problems Aluminium may have.
As you wont be running it under pressure even a leaky one should be easy
enough to patch up.

As for Aluminium corroding, if it's the only metal in the system I don't
think you will have a problem. If I was making something like this (and
someday I'd like to) and using second hand stuff I think I'd suck it and see
what happened.

Posted by beemerwacker on August 24, 2007, 11:24 pm
Be patient Nick, George will answer all.

Until then, consider the following. The radiator is based on the
constant flow of the water pump and the thermostat, which opens and
closes on a regular basis. If one were to take the engine, measure the
block and head temperatures, as well as the radiator fluid
temperatures, one would find the relationship between all of them.
Now, we know that a automobile engine with a failed thermostat will
malfunction in some way. If the thermostat is stuck open, the engine
will never ever warm up, thus giving us an indication that a typical
auto radiator is over designed for the application with a wide open
flow. If stuck closed, it will overheat.

Another similar way would be to take say, 200 degree water heated at
an even rate and flow it through the radiator and measure the
difference between the inlet and outlet at a given flow of fluid and
air through the radiator. I would suspect that you will need a much
lower flow than you expect to extract a given BTU without flow

Feel free to take all of that out of context in any way you see fit.

Posted by daestrom on August 25, 2007, 2:16 pm

One quick google suggests at least 25% ethylene glycol...

But that's still pretty high if you're talking about the whole storage tank

Another suggests that it isn't (corrosion) very bad if you can be sure to
use water that doesn't have any halides in it.  But filling a whole storage
tank with demineralized water is probably about as expensive as doping it
with EG.


A general rule to reduce corrosion is to use water that has a low electrical
conductivity and not connect dissimilar metals.  So if you use an Al
radiator, you might consider using short sections of hose to connect it to
copper piping.  This helps minimize forming a galvanic cell that will
corrode the metal with a more negative potential.


Posted by nicksanspam on August 27, 2007, 11:31 am

We can do that, but mechanics say an electrically-isolated aluminum radiator
with rubber hoses and insulating brackets can still corrode (oxidize?) soon,
with nothing but water inside. My old boy scout canteen ended up heavily
pitted with white crud on the inside. I think this was oxidation vs mineral
deposits or galvanic corrosion, since there were no other metals involved.


So far, I've only found Gunk anti-rust and water pump lube for car radiators,
$ for 11 oz to protect 5 gallons or so, ie about $00 for 1000 gallons, and
a KPR VCI powder that costs about $0/lb and needs 0.25% concentration, ie
about $00 for 1000 gal. The tank will have a pressurized single-wall copper
pipe coil inside to make hot water for showers, so it would be nice to avoid
toxic tank water.

How long would it last? I gather it needn't be "fitted," just touching
the water and electrically connected to the aluminum. The 170 F tank water
won't change, so it won't have new minerals. It can absorb fresh oxygen,
esp since the radiator will drain down every day for freeze protection.
But it can't hold much oxygen, since it is hot. Then again, the corrosion
rate probably increases with temperature. The Farwest site mentions a 500
Wh/lb capacity for Mg rods used in pipelines, ie so many coulombs. How can
we turn that into a lifetime in a water heater with fresh O2 but no new
minerals? I've read that the oxide layer slows the corrosion rate, in air.


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