George P. Richardson wrote:
>
> EPaul21988@aol.com wrote:
> >
> > In a message dated 97-02-25 00:46:01 EST, johnhaynes@som-uky.campus.mci.net
> > (John Haynes) writes:
> >
> > << The paramount reason for
> > overheating in a TR2,3,4 is a non stock thermostat or no thermostat.
> > TeriAnn"s description of it is exactly correct. However I must disagree
>with
> > her statement "the longer the water is in contact with the radiator
>surface,
> > the more heat can be transfered. Without the restriction of the thermostat
> > slowing down the coolant flow , the coolent just may not have enough
>contact
> > time to transfer much heat." This statement conveys the mistaken idea that
> > by restricting the flow the engine will run cooler. What actually happens
> > is, if the flow is slowed in passing through the radiator, the flow through
> > the engine is slowed, the longer the water stays in the engine, the hotter
> > it gets.
> > Sam Haynes 59 TR3 Original Owner >>
> >
> > Sam;
> > I'm now lost on this one. You agree that one reason for overheating is the
> > absence of a thermostat, but you disagree with the theory that the
>thermostat
> > functions as a physical barrier to slow coolant down and allow more heat
> > transfer ? Why does the absence of a thermostat result in overheating then
> > ??
> >
> > Bob Paul
> > TS45738L"O"
> > TS7280L
>
> Actually, the real reason is the *pressure* difference. The thermostat
> causes a change in pressure. The water in the radiator, before the
> thermostat, is under pressure from the water pump, forcing it to give up
> heat. The water after the thermostat, in the engine block has a lower
> pressure, allowing it to absorb heat. Substitute the appropriate parts
> of an automobile air confitioner for the water pump, radiator and engine
> block and you'll get the idea.
>
> George Richardson
> '57 TR3, TS15559L
George, you nearly got it right, but you are confusing a simple cooling
system with the refrigeration cycle. The thermostat is NOT analogous to
a refrigeration expansion valve. Here is my explanation for why
removing the thermostat can result in overheating.
First, we need to recognize that the boiling point of water changes as
its pressure increases. At atmospheric pressure, the BP is 212 degF, as
we all know. At 7 psig, the pressure of many cooling systems, the BP is
233 degF, according to my steam tables. How is the radiator
pressurized? In two ways. First, the radiator cap is spring loaded to
limit pressure to 7 psig. Second, the water pump develops pressure
within the radiator, this pressure being the result of the resistance to
flow provided by the radiator itself, the block, the hoses etc and the
THERMOSTAT. The water pump is sized to overcome all of this resistance,
and, in so doing, develop pressure between the pump and the thermosat of
around 7 psig. This enables the temperature in the radiator to run as
high as 233 degF without boiling the water. NOW, if the thermostat is
removed, the pump has less resistance to overcome, so does not need to
develop as much pressure. Thus, the boiling point of the fluid in the
radiator is reduced, and the liquid can then boil, reducing the heat
transfer rate between the liquid and the radiator. This means that even
less heat is transfered from the fluid, allowing its temperature to
increase even further and the engine to overheat. I could go into a
long description of film coefficients, etc. but we don't need to use the
bandwidth!
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