Paul W9AC Writes:
"As I recall, water can never exceed the boiling point temperature
under normal atmospheric pressure. Additional heat does not raise water
temperature but causes steam and steam too never increases beyond the
boiling temperature at normal atmospheric pressures. 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."
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, 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.
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.
73,
Jim W8ZR
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