Radiator Cap

[Tom Cloud]

>     Newton tells us that the rate of heat flow from one region into 
>     another is proportional to the difference in temperature between the 
>     two regions (in this case, the block and the water).  Therefore, given 
>     an engine block of a certain temperature, cooler water will remove 
>     more thermal energy per unit time than warmer water.  A 
>     lower-temperature thermostat will ensure a cooler supply of water to 
>     remove the thermal energy from the block, and so cooler water is in 
>     this sense more efficient.
>     
>     Each volume-unit of water is in contact with the block for less time 
>     if the water is flowing quickly, as compared to water that is flowing 
>     slowly.  Therefore each volume-unit of water carries away less thermal 
>     energy if the water is moving quickly.  But in a quickly-flowing 
>     cooling system, more volume-units of water are flowing through the 
>     cooling passages per unit time, so the amount of thermal energy 
>     carried away per unit time will be the approximately the same 
>     regardless of flow rate, the only difference being the change in the 
>     rate of thermal exchange with temperature difference as described 
>     above.
>     
>     On the other hand, the question is somewhat irrelevant to automotive 
>     cooling systems.  To my knowledge a higher-temp thermostat flows at 
>     least as much as a lower-temp thermostat for a given engine, once both 
>     have reached the fully-open position.  The only difference is the 
>     temperature at which the valve fully opens, not in how much it opens.  
>     
>     mike skolones


my problem is not with the cfm rating of the thermostat but rather
with the claim of (aluminum) radiator manufacturers that slower flowing
water transfers more heat out of the engine.  Doesn't seem too
difficult to see that slower flowing coolant gets warmer, lessening
the delta-T and therefore the heat transfer.  Yes, that water may
carry more actual "heat", but it's not the amount of heat per unit
of water you're interested in but rather the amount of cooling -- or
heat removed -- for your engine .... and I cannot see how very rapidly
moving water that might absorb only 1/2 calorie of energy per gram
yet disposes of 2500 calories total per unit time would be preferred
over a system where the water absorbs 10 calories per gram, traveling
slower, yet only disposes of 2200 calories total per unit time.

This is a little off the diy-efi subject, but it keeps bouncing back
up on this and other lists ... and I've never seen anything to substantiate
the claim that -- in the same system -- slowing the water flow removes
more heat.  I can see that it might remove heat "more efficiently",
but I doubt the "efficiency" gain, if any, is of any consequence in
the total heat removed -- which is what we're after.  The implication
in the ads, as I understand them, is that the system would work better
if the flow were slowed -- and, I suppose this would be true -- if you
got a larger radiator!

Tom Cloud

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