HB wrote:...However, I believe the vacuum forms because the water contracts as it cools. Of course some of the suspended water in the saturated steam returns to liquid, but not enough to offset the reduced volume of the cooled water, hence a vacuum.
At the moment, why this situation does not neatly reverse itself upon reheating eludes me. Anyone care to explain why it doesn't?
Actually, it's all about the steam; the water has very little to do with it.
Let's start with a cold boiler. It's not 100% full, so you have some amount of water, with the rest of the space being taken up with air. The air will contain a certain amount of water vapor, but not much.
Now, you turn the machine on, and the boiler heats up. As it heats, two things happen. 1) the air (actually air + a little water vapor) pressure increases, because it's a gas, and in a constant volume gas pressure is proportional to temperature. 2) the vapor pressure of the water increases, meaning basically that more of the water is trying to turn into gaseous water vapor.
The air part of the pressure rise inside the boiler is undesirable, because the boiler is regulated by pressure, not temperature. What's going to happen is that the boiler heater will turn off prematurely because it's seeing a pressure component from the air. (and, if you ran the boiler purely on temperature, you'd over-pressurize it initially, and then after the air is eliminated, the pressure drop would throw the temps off anyway.)
So, you dump the air; either through the vacuum breaker valve, or by cracking the steam valve for a bit. Now, the boiler has nothing in it except water, partly liquid and partly gas, and for a constant pressure (and known volume) it stays at a known temperature. Which is why you can control boiler temp with a pressurestat. (Going back to the point previously, the pressurestat is set to assume the boiler is nothing but water; air in the mix throws the setting off, low.) All nice and predictable, and it doesn't change as you use steam from the boiler. (well, it does, but the heater soon gets things back to equilibrium.)
Now, you turn off the machine, and the boiler cools. Lower temperature, lower pressure; and this time since all you have in the boiler is water, the final gas pressure is basically the cold water's vapor pressure, which is much lower than normal atmospheric. You could obviously build boilers (and steam valves) to stand up to vacuum as well as pressure, but it's harder and not really worth the effort. Instead, air is let back into the boiler, either through the VBV or who-knows, and we're back to the beginning.
(It's been a long time since physics for me too, but I think I have this at least mostly right!)