If water didn't expand with temperature then how would a thermosyphon work? I'm pretty sure it expands, but by a very small amount

kschendel, thanks for correcting my math. I knew it seem to easy, I'll edit my post to avoid confusion.

jlunavtgrad wrote:If water didn't expand with temperature then how would a thermosyphon work? I'm pretty sure it expands, but by a very small amount

convection causes currents if i'm not mistaken.

convection is caused by thermal expansion.

That's my point - a thermosyphon works via natural convection via thermal expansion.

Sorry, I thought you were asking a question.

Convection simply means the movement of molecules in a fluid. The mechanisms are diffusion and advection. A better way to put it might be that the circulation is caused by buoyancy changes coming about from the heating and expansion of the water.

But this has nothing to do with vacuum breakers.

Here's some more food for thought. Do you know boiling can be brought on by a vacuum? For boiling to occur the external pressure only needs to be lower than the vapour pressure of the liquid. You don't even need heat. As the pressure in the boiler rises, the boiling point actually goes up.

I'm a Rankine type of guy but that little bit doesn't matter as you can use degrees R or degrees K. What does matter is to remember that you are writing in terms of the absolute pressure, i.e. psia (pounds per square inch absolute). What we see on the pressure gage is psig (pounds per square inch, gage) or, on the great majority of these espresso machines, bar.

As regards the heating of a sealed boiler having about 70% water, the pressures developed would be, as Karl said, the sum of the partial pressure due to air and the corresponding vapor pressure of water at that particular temperature.

As regards water changing volume with changes in temperature, say from 22 C to 122 C (71.6 F to 251.6 F), the density goes from ~ 62.2 lbs/cu ft to ~58.8 lbs/cu ft.

As regards the vapor pressure of water at 395 K / 122 C / 251.6 F, that's ~1.10 bar gage pressure at sea level and the partial pressure due to air at that temperature would be ~ 0.34 bar gage.

another_jim wrote:I have no clue where the air comes from if there is no vacuum breaker and the system stays sealed; but if the pressure isn't from water, it has to be from air.

One little idea: even if the system is sealed when the boiler is in the operating temperature, it is not necessarily it when cooled down. Metal in valves contract etc.

erics wrote: As regards water changing volume with changes in temperature, say from 22 C to 122 C (71.6 F to 251.6 F), the density goes from ~ 62.2 lbs/cu ft to ~58.8 lbs/cu ft.

And, if I recall my basic-er physics, if the same volume of material weighs less at a different temperature, that means that there is less material occupying the same volume, or that the same amount of material occupies more space, meaning that water, indeed, does expand.
Thanks for solving that one! I did make an incorrect intuitive leap from the incompressibility of water to it neither contracting nor expanding due to temperature.

With that in mind, does that mean that my Gaggia Factory is built to withstand the contraction of the water, both as liquid and as vapor, that occurs when I turn him off; or that I should be allowing the pressure to escape and opening the fill cap before I walk away?
There are a few folk wondering the same thing of the Lever Forum, and I'd be happy to pass the info on, if anyone has a definitive answer.

OTOH, couldn't I just get a Vacuum Breaker Valve that's threaded the same as the fill top on my Tin Man, and solve my problem?

Endo wrote:Can you explain how someone can steam without steam pressure?

If your steam boiler cooled down (element fails to heat, somehow turns off, whatever) & the steam & water cool & contract the pressure can drop below atmospheric pressure (if you don't have a vac breaker valve.) If you plunge a wand into milk & open the valve you suck it right into the boiler.