pdx wrote:Its not just about pressurestats & false pressure.

Synessos (with pid boilers) have them. From the Synesso repair manual:

"In the event that your steam tank runs out of all pressure the vacuum breaker opens to allow air into the tank. If it fails to open and milk steaming is in progress the vacuum created by running out of steam will allow milk to be drawn through the steam wand and into the steam tank. This requires a thorough cleaning process."

Endo wrote:I think we moved on from the basic functioning of a VBV and p-stat on post #2 and are now discussing VBV when a PID is used.

kschendel wrote: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.

kschendel wrote: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.

cbrucecampbell wrote:We should stop calling it a "vacuum breaker" valve. Technically it may be that, but that name misleads people who are trying to understand its purpose in espresso machines...

HB wrote:So the pressure exerted by water + air + water vapor is higher at the same temperature than water + water vapor, thereby "fooling" the pressurestat into turning off prematurely?

Er, not all espresso machine steam boilers have vacuum breaker valves (e.g., Olympia Maximatic, Ponte Vecchio Lusso).

kschendel wrote:Yes. One way to look at it is that at temperature T, the water has some vapor pressure P1, and the heated air has some pressure P2. Total pressure is P1+P2. If we take the air out of the equation, the total pressure at that temperature is just P1. Another way to look at it (that gives the same answer, necessarily) is that at temperature T, the water is pumping X molecules into vapor, adding to the Y air molecules already there; this creates a higher total pressure than just X alone. Same thing, two ways of saying it.

In principle, a liquid in a sealed container can never boil in an equilibrium situation (uniform temperature, and slow increase of temperature so that the vapor pressure is in equilibrium with the liquid). You get boiling when the pressure in the liquid is smaller than the vapor pressure for that temperature.

If the temperature is uniform, then the total pressure will be equal to the vapor pressure PLUS the partial pressure of the initial air included in the vessel, and will hence always be larger than the vapor pressure, so no boiling.

However, if:
- the liquid is heated locally, locally the vapor pressure can be larger than the overall pressure, so it can start boiling *there*.
- if the heat is applied quickly, so that there has not yet been an equilibrium between the vapor pressure and the new temperature, the pressure might be lower than the vapor pressure for a while (until enough vapor is produced to reach equilibrium).

HB wrote:My days of college physics are long behind me. However, I believe the vacuum forms because the water contracts as it cools.

jlunavtgrad wrote:Yes, by my calculations a sealed boiler with a fixed amount of water and air going from room temperature to 253 degrees would build over 5 bars of pressure (interestingly, at this pressure and temperature the water would not boil). If the boiler had only water + water vapor we know that the pressure would be about 1.3 bars. The reason that water + vapor alone produces less pressure for the same volume and temperature is that it has to fight the vapor pressure of water.

If anyone wants to check my math I used this equation: p1/t1 = p2/t2
P1 = 1.0
T1 = 22
P2 = 5.5
T2 = 122

Psyd wrote:While my physics courses are behind me as well, water doesn't contract, or expand, does it?

Psyd wrote:While my physics courses are behind me as well, water doesn't contract, or expand, does it?