The experiment setup

This whole area just smack of DIY and I think that is a great part of the appeal. In addition the science behind is just on the cusp of understandable for me and lends itself well to garage experiments.

The first thing that I wanted to do was just to see one of the key affects in action. Namely, chilling water by simply dropping the pressure above it. This just blows me away. Even more out of step with everyday experience is that if you keep dropping the pressure do extremely low pressure the boiling point of water approaches and then goes below zero. There pressure has to be very low: at 0.3 psi its around 17 degrees, drop that to 0.09 and boiling temperature is .. wait for it zero degrees!!! The boiling point is very sensitive to the pressure level at pressure below 1 psi. At 1 psi the boiling point is around 40 degrees and then at 0.09 its zero degrees.

In my experimental set up in the garage, although the pump I have says that it can go to 5pa which is about 0.0007 psi I have my serious doubts about it being able to achieve and maintain that. Time will tell what is needed to get that low.

As a side note its important to make a distinction here between evaporation and boiling. Evaporation is a phase change from liquid to gas that occurs just at the surface of the water. Boiling is a phase change from liquid to gas that happens in the bulk of the liquid and not just at the surface. Boiling would clearly liberate a far greater number of molecules than evaporation alone and would be very desirable but I have no idea about the practicality of achieving and maintaining that pressure in a set up that bumps around off road in Australia. Again, time will tell.

So here's my garage level understanding of the process of chilling by evacuating the system.

When we measure the temperature of the water we are measuring the average energy of the water molecules that are colliding with the temperature probe. However, the energy of the water molecules throughout the water has a distribution of energies. I'd hazard a guess and say that like so many naturally occurring distributions the distribution of energy is a Gaussian distribution / Bell Curve.

Anyhow, so as you drop the pressure above the surface by removing air the higher energy water molecules have a greater chance of escaping the surface of the water as there is less air to get in the way.  As a result the molecules escape i.e the water evaporates or changes state from liquid to gas.
As a result of the higher energy molecules escaping

The energy required to vapourise water turns out to be 2260 KJ/kg. This is a lot of energy and apparently 5 times the amount of energy needed to heat water to 100 C.

I was very keen to see this process in action so I made a simple set up my garage with a pressure and temperature controller that I made for some work that I do with epoxy. It lets be see the temperatures and pressure real time and log them.

So here is the very simple set up. A vacuum pump (bought on ebay and used for evacuating air conditioning systems to recharge them) and the 'evapourator' is just a wine bottle with a small about of water in the bottom. The black tube and wire you see are the pressure probe and a thermocouple wire for measuring the pressure and the temperature. 

Here is the whole set up. The keyboard is sitting on top of the pressure/temp data logger/controller which is made using a great little kit called a Maximite which uses a PIC 32 microcontrol but has a BASIC interpreter built in so you can program it and read from its 20 I/O pins. Great project in its own right.

So here is the cool part.The screen shot below shots the cooling affect due to the pressure reduction. The horizontal line is ambient temperature of about 22 C and the falling line is the temperature of the water in the evacuated bottle. In this case the time axis shows about 1/2 hour.

The bottle was just standing in the open air so when the temperature his about 10 degrees is stopped getting cooler.At this point it was taking in  heat from the surrondings at the same rate I was taking out via the maintained vacuum. The following evening I but the bottle in an insulated bag and it got down to 5 degrees. 

A couple of days later I filled the bottle up with a lot more water (about 1/3 full) and logged the data. after 83 minutes the temperature fell to around 13 C from its initial 27 degrees. After that its rate of cooling was very very slow.

As can be seen the rate of cooling is much slower as there is a great deal more energy is needed to be removed to drop the temperature. However, the cooling was much slow that I expected and one possibility is that the surface area of the water exposed to the vacuum is too small to allow a sufficient number of molecules to escape. I'll play around with this next time.