Temperature drop when water in vacuum

[Peter_1]

BAsic question but answer seems not that easy... when placing water in a high vacuum chamber, boiling point drops due to lower pressure. But temperature also drops and what is the physical explanation for this? Is it right to say that the heat needed to evaporate comes from the internal heat of the water because the surrounding heat of let's say 20°C can't heat fast enough the water to supply the necessary heat to evaporate?
This said, if we should vacuum very slow or add a lot of heat to the water, temperature will not drop? Or, if we supply +/- 2400kJ/kg (evaporating energy of water), temperature will stay then stable?

[NH3LVR]

I had always assumed that it cooled just as a gas we would used for refrigeration.

[Peter_1]

I had always assumed that it cooled just as a gas we would used for refrigeration.

Sure, but what is the physical explanation behind this phenomena?
Why drops the temperature of the water...?

And to go on further ... this questions arose when explaining log pH to students... It was a simple question and I couldn't give him a satisfying answer...
. why condensing pressure stops rising further once it starts condensing? What determines the condensing pressure, why it stops at that specific point at a certain DT from outside temperature (air cooled)... what physical phenomena lowers the condensing pressure when outside drops and why stabilizes it again at a lower temperature?

https://highschoolenergy.acs.org/content/hsef/en/how-can-energy-change/energy-of-evaporation.html

During evaporation, energetic molecules leave the liquid phase, which lowers the average energy of the remaining liquid molecules. The remaining liquid molecules can then absorb energy from their surroundings. This process can take place at any temperature because some of the molecules in a liquid will always have enough energy to enter the gas phase.
Phase changes release or use energy because they bring particles closer together or cause them to move farther apart. To understand why these processes release or use energy, recall that all atoms or ions have at least some attraction for one another. Overcoming these attractions, as particles move farther apart, requires energy. When particles come back together, energy is released.


remains my questions regarding condensing equilibrium point

[NH3LVR]

The Physics are beyond me. Let us know if you find a simple answer to your question.

[cricri]

Water is simply a refrigerant, R718, you can find it in the Danfoss slide.
often used to cool vegetables, it's named Vacuum cooling.

[seanf]

And to go on further ... this questions arose when explaining log pH to students... It was a simple question and I couldn't give him a satisfying answer...
. why condensing pressure stops rising further once it starts condensing? What determines the condensing pressure, why it stops at that specific point at a certain DT from outside temperature (air cooled)... what physical phenomena lowers the condensing pressure when outside drops and why stabilizes it again at a lower temperature?


What about looking at it in terms of the liquid volume and vapour volume is changing as energy is removed?

[RANGER1]

Posted wrong subject

[Peter_1]

I had always assumed that it cooled just as a gas we would used for refrigeration.
This was a good one

https://highschoolenergy.acs.org/content/hsef/en/how-can-energy-change/energy-of-evaporation.html

[skarrs71]

The reason fluids boil and condense at set temperatures are to do with simple and latent heat characteristics. Water boils at 100 degrees Celsius at atmospheric pressure. If you increase or decrease the pressure of the vessel you increase and decrease the boiling point. The reason the state change is so crucial in refrigeration is that a fluid absorbs or rejects much more energy during latent heat transfer required to boil or condense the fluid than in simple heat transfer required to raise or lower the temperature.

If heating any pure fluid at any constant pressure the temperature rises at a "constant" rate (simple heat) until the boiling point at said pressure is reached. Once this point is reached all energy absorbed (latent heat) by the fluid is used to change the state at a constant temperature, only after all the fluid has changed state will the temperature start to increase again (simple heat). It is quite easy to demonstrate with a thermometer in a pan of water being heated but the exact thermodynamics of it and reasons for it are a bit fuzzy to me, I don't really need to know why only that it happens.

Hope that helps, I know it is hard to get ones head around, I had trouble explaining to my old engineering manager why the temperature in a steam boiler delivering steam at 6 bar(g) was higher than 100 degrees C!

EDIT

If I remember this right, the reason fluids don't change state instantaneously is because individual molecules have differing energy levels within the fluid therefore the molecules with higher/lower energy levels will change state first. I know it is harder to pull a good vacuum in a freezer due to the lower temperature, ie the moisture molecules have a lower energy level to begin with and require more "persuasion" (heat or vacuum) to enable pulling a decent vac. Put the evaporators on defrost whilst evacuating system.