Paul W9AC Writes:
"As I recall, water can never exceed the boiling point temperature
under normal atmospheric pressure.
That's true at any pressure, only the boiling point varies with the
pressure.
Additional heat does not raise water
temperature but causes steam and steam too never increases beyond the
boiling temperature at normal atmospheric pressures.
While there is water present at the heat source, but once all water is
evaporated temperature certainly increases in proportion to what is now
sensible heat.
So Steam by itself when having its temperature raised is sometimes referred
to as superheated, i.e. beyond the liquid/gas state and just a gas state.
The same term is used in refrigeration to describe superheat as the
temperature above saturation point of gas leaving an evaporator.
But under pressure,
it's a completely different ball game. Added pressure raises the boiling
point and the temperature of steam can get extremely high."
The water and steam are at the same high temperature until the material
state transition is complete. You are looking at the principle which made
steam engines work so well, i.e. a pressurised boiling container. The
pressure controlled by the latent heat present in a fixed volume.
Hi Paul, you're correct about the boiling point temperature of water, but I
believe steam acts somewhat differently from what you described. As I
understand it, here's the explanation: At 100C and atmospheric pressure,
water undergoes a liquid-vapor phase transition to steam. This transition
requires a huge amount of absorbed energy,
This is called latent heat
about 540 Calories per gram of water, and this energy (in the form of heat)
is absorbed from the surroundings. That's why a teakettle will rapidly warm
up to the boiling point of water, and then just sit there for a relatively
long time, absorbing heat from the stove, until enough energy is absorbed to
turn the water into steam. It's also why a steam burn is so much worse than
a water burn, since when steam recondenses back into water it dumps that
huge amount of heat into whatever it is in contact with.
By comparison, water is a puny coolant. One gram of ice at 0C will only
absorb 180 calories when its temperature is raised to the boiling point - 80
calories to melt the ice (the solid-liquid phase transition), and another
100 calories to heat it up to the boiling point.
Once steam is formed, then (in principle) it can take on any higher
temperature above the phase transition temperature, until the water
molecules disassociate. There is no further phase transition above
the liquid-vapor (steam) transition.
Yes now we have sensible heat (heat which causes a change in temperature)
increasing the temperature of steam once all water has evaporated.
The bottom line is that so-called vapor phase cooling is a fabulously
efficient form of cooling, much more efficient than water cooling.
Furthermore, a circulating closed VPC system, such as you described, will
operate at a constant 100C, which makes it ideal for cooling semiconductors.
Of course, the heat of vaporization has to go somewhere, so a pretty beefy
radiator and fan is required for larger amplifiers if, say, 1500W of heat
are being dissipated.
The heat of vaporisation is latent heat absorbed by the material in changing
state from liquid to gas. If you refer to the dissipation heat of
condensation, then you describe it better.
vk4tux
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