What is the purpose of a vacuum breaker valve - Page 3

#21: Post by **Endo** » July 9th, 2009, 9:25 pm

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."

Can you explain how someone can steam without steam pressure?

#22: Post by HB » July 9th, 2009, 9:28 pm

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.

Please, speak for yourself. I am still hoping for an answer to the original question based on science, not restatements of observable behavior. Karl's seems the closest so far.

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.

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?

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.

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

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...

Well, I guess it's technically a vacuum breaker valve and air purge valve (?).

#23: Post by **kschendel** » July 9th, 2009, 10:09 pm

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?

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.

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

I assume that those machines either have some sort of deliberately leaky air return path, or their boilers are built to sustain prolonged vacuum as well as greater than atmospheric pressure. (or even some combination of the two.) I'm arguing from theory here, not sure what the specifics are (and I'd be interested to learn.) "I'm a doctor, er I mean a computer geek, not a boiler designer..."

July 9th, 2009, 10:11 pm

Edit: Bad math removed to avoid confusion.

#25: Post by **networkcrasher** » July 9th, 2009, 10:17 pm

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.

So I'm thinking that in order to reach P1, one has to have the intermediate stage of P1+P2 as the air is displaced by water vapor (i.e. steam). Would that be the reason false pressure is an issue? Maybe water won't heat up enough to go to steam in a container under a vacuum with no air.

#26: Post by **networkcrasher** » July 9th, 2009, 10:19 pm

Yep, that's it. Found this post on the web:

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).

#27: Post by **Psyd** » July 9th, 2009, 10:58 pm

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

While my physics courses are behind me as well, water doesn't contract, or expand, does it? I do know that it cannot be compressed, and I just made that leap. Water, when heated beyond what it can sustain as a liquid can only expand by becoming a gas, er... right? So the only 'contraction' it can do is to change state from gas to liquid again. Or did I intuit myself into another physical corner?

So, I'm following this thread with interest, as I just asked on the lever forum about turning off my Gaggia Factory at temperature, and walking away, when I'm done. Should I, with this conversation, intuit (there it is again...) that this means that I should NOT be doing that?

#28: Post by **kschendel** » July 9th, 2009, 11:00 pm

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

You got the right equation, but the T's need to be Kelvin, not degrees Celsius. T1 = 295 and T2 = 395. That gives me a P2 of 1.3, but remember that's just from the air that you started with. You have to add the vapor pressure from the water at 395K; I have no idea what that is, but it's definitely more than 1 bar (vapor pressure of water at 373K is 1 bar and it's a nonlinear function.) Given your statement about the water-alone pressure, I'd guess that the vapor pressure is also near 1.3 bar, giving a total of 2.6 bar if I'm doing this right.

#29: Post by HB » July 9th, 2009, 11:07 pm

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

The expansion valve on dedicated brew boilers opens as the machine is heating up and goes drip, drip, drip into the driptray. Either the water is expanding, or the boiler is getting smaller.

#30: Post by **Endo** » July 9th, 2009, 11:15 pm

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

Yes it does. The coefficient of thermal expansion of water is roughly 6 times that of a steel boiler.