Boiler pressure setting - influence on the cup?

[chelya]

How does pressure setting influence the temperature range on HX machines? How does it influence the cup (other then the temperature)? Why would one choose to adjust the pressure?
Abe mentioned it in his review - to allow a different range of temperatures on HX machines, pressure can be adjusted. That made me curious.

[cannonfodder]

Boiler pressure is directly relational to temp. I can not quote the formula off the top of my head but I am sure someone can post it. The increased pressure=increased boiler temp. That temp affects how hot your grouphead gets, very important with an E61 machine. It also affects the length of the cooling flush needed prior to pulling a shot. Your recovery time (reheat between flush and over temp or shot to shot temp) is also shortened.

On my millennium, at 1.4 on the top of the cycle, I have about 12 seconds between flush and brew before my water starts to overheat. I also make a lot of milk drinks. That increased temp gives me lots of steam. I believe most folks run at about .9 to 1.1 to avoid the overheating and long cooling flush. I have plumbed the water supply into my machine so water usage is not an issue for me, but waste water is.

Found the table. Check out Dan's 'How I Stopped Worrying and Learned to Love HX's', that is where the chart came from.


Boiler pressure (bar) Water temp (Fahrenheit)
1.0 253
1.1 255
1.2 257
1.3 259

[Walter]

Boiler pressure is directly relational to temp. I can not quote the formula off the top of my head but I am sure someone can post it. The increased pressure=increased boiler temp.

If memory serves me well for an ideal gas the formula is

pV=nRT

p...Pressure
V...Volume
n...mol (basically the number of molecules)
R...a constant
T...Temperature

with steam you are not really way off the mark.

How does pressure setting influence the temperature range on HX machines? How does it influence the cup (other then the temperature)? Why would one choose to adjust the pressure?
Abe mentioned it in his review - to allow a different range of temperatures on HX machines, pressure can be adjusted. That made me curious.

The process is quite simple, the boiling water evaporates and the air/steam mixture above the water gets heated, the space in the boiler is fix and limited (boiler size - amount of water) which means the gaseous phase cannot expand, thus the pressure rises (roughly according to the formula above; for a proper evaluation you would have to apply the Clapeyron/Clausius-Clapeyron equations, IIRC). And since the steam is in equilibrium with the boiling water the temperature of the liquid and gaseous phase is the same...

The pressurestat is an adjustable switch which turns boiler heating on if the pressure drops below a certain value (e.g. 1.0 bar) and turns it off again as soon as the pressure reaches a certain value (e.g. 1.2 bar). Hence increasing the pressure (with the pressurestat setting) means rising the temperature in the boiler and - as a consequence - brewing temperature...

If the brewing temperature is too high the coffee will taste bitter and burnt...

P.S.: All this was about steam pressure in the boiler, not to be confused with brewing pressure as provided by the pump, which is an entirely different story...

[HB]

Why would one choose to adjust the pressure?
Abe mentioned it in his review - to allow a different range of temperatures on HX machines, pressure can be adjusted. That made me curious.

Abe's review described brew temperatures ranging from 91C (195.8F) to 96C (204.8F), a range which his Giotto wasn't able to deliver at the same boiler pressure by flushing. Practically all the coffees that I work with are brewed in the narrower range of 198F (92.2C) to 204F (95.6C), with the majority in the ~201-203F range. A four degree range is well within the sweet spot of most HXs simply by adapting the flushing amount. You can add a couple more degrees to that range by switching to the flush-and-go technique ("pro's method"), but as Abe pointed out, it requires more skill to nail a precise temperature consistently, especially for the smaller HX machines.

Keep in mind though when reading these discussions that boiler pressure is only one variable that determines an HX machine's thermodynamics. The size of the HX, the percentage immersed in the water versus steam, the deliver tube lengths, group design... all of these contribute their part, so it's folly to draw precise instructions from general advice about HX machine operation. For example, the article How I Stopped Worrying and Learned to Love HXs really speaks to E61 / HX machines; the steps its advocates apply vaguely to the Cimbali Junior and not at all to the Elektra A3, despite that they are both HX machines. There's differences among prosumer HX machines too. The flush routine for the Andreja Premium would apply in principle to the Expobar Lever, but fails miserably in the specifics (the Lever has a much longer flush and it rebounds much faster at the same boiler pressure setting).

Although I'm never one to shy away from applying technology, my advice to many intermediate baristas is to put the thermometers, measuring cups, timers and the like in the drawer for a month and focus on developing your intuition. While I have the tools to meter and profile an extraction, more times than not I already know what the readout will say before I look. The sight and sounds of the machine, the way the extraction develops, and most importantly the taste tell me everything I need to know; of course, the readouts are helpful for tough diagnosis situations and communicating results to others.

Finally, the above advice applies best when you're working with a machine capable of delivering consistency without extraordinary machinations. Ironically the entry-level machines like the Rancilio Silvia require far more rigor to pull the same quality shot as upper prosumer / commercial machines, a fact that is often lost on newbies and pros alike.

[cannonfodder]

Teacher, may I be excused, my brain is full, any more and it might explode. You know, just when I thought I was getting a good handle on thing I realize how little I know.

[chelya]

Teacher, may I be excused, my brain is full, any more and it might explode. You know, just when I thought I was getting a good handle on thing I realize how little I know.

Tell me about it...

Thanks for such detailed answers!

[gscace]

Boiler pressure is directly relational to temp. I can not quote the formula off the top of my head but I am sure someone can post it. The increased pressure=increased boiler temp.

If memory serves me well for an ideal gas the formula is

pV=nRT

p...Pressure
V...Volume
n...mol (basically the number of molecules)
R...a constant
T...Temperature

with steam you are not really way off the mark.

How does pressure setting influence the temperature range on HX machines? How does it influence the cup (other then the temperature)? Why would one choose to adjust the pressure?
Abe mentioned it in his review - to allow a different range of temperatures on HX machines, pressure can be adjusted. That made me curious.

The process is quite simple, the boiling water evaporates and the air/steam mixture above the water gets heated, the space in the boiler is fix and limited (boiler size - amount of water) which means the gaseous phase cannot expand, thus the pressure rises (roughly according to the formula above; for a proper evaluation you would have to apply the Clapeyron/Clausius-Clapeyron equations, IIRC). And since the steam is in equilibrium with the boiling water the temperature of the liquid and gaseous phase is the same...

The pressurestat is an adjustable switch which turns boiler heating on if the pressure drops below a certain value (e.g. 1.0 bar) and turns it off again as soon as the pressure reaches a certain value (e.g. 1.2 bar). Hence increasing the pressure (with the pressurestat setting) means rising the temperature in the boiler and - as a consequence - brewing temperature...

If the brewing temperature is too high the coffee will taste bitter and burnt...

P.S.: All this was about steam pressure in the boiler, not to be confused with brewing pressure as provided by the pump, which is an entirely different story...

Sorry. This is a munge that is pretty much wrong, and a common misconception it seems. Ideal gas law doesn't have anything to do with it. The temperature is purely related to the vapor pressure of water, which is an exponential function of temperature. You can't use the ideal gas law because the volume isn't closed. The volume becomes open as soon as you open the steam valve, or when the vacuum breaker is open to the atmosphere, as when heating the system up to the water vapor boiling point.

The temperature / pressure relationship can be obtained from steam tables, which you can find them on the internet if you do a google search.

-Greg

[Walter]

Sorry.

No problem

This is a munge that is pretty much wrong, and a common misconception it seems.

Chemical/Physical laws when dealing with "ideal substances" are often not correct to the point, but I wouldn't call that a "misconception". Exactness is often sacrificed for simplicity, in order to make the maths not too complicated, but if the approach is ok it works for me (and for 99% of scientists 99% of the time)

Ideal gas law doesn't have anything to do with it. The temperature is purely related to the vapor pressure of water, which is an exponential function of temperature.

Stating that ideal gas law doesn't have anything to do with it when the matter concerns a gaseous phase, suggests little familiarity with the basics of physics and chemistry and the underlying theories. Did you check how far off the mark we are with that formula? To be honest I didn't bother to do the maths. However, I did mention that Clapeyron resp. Clausius Clapeyron should be closer to the point. Or are those equations also pure rubbish in your opinion?

You can't use the ideal gas law because the volume isn't closed. The volume becomes open as soon as you open the steam valve, or when the vacuum breaker is open to the atmosphere, as when heating the system up to the water vapor boiling point.

Talk about a misconception! How does the pressure increase when the system is open? What happens with the pressure as soon as you open the steam valve? Are you really saying your boiler is open most of the time and reaches e.g. 1.2bar while the steam valve is open? How does the pressurestat work if the system is open?

The temperature / pressure relationship can be obtained from steam tables, which you can find them on the internet if you do a google search.

Yes, I know. But knowledge management by Google also has its pitfalls it appears. I still prefer my old books about thermodynamics. Science has moved on since Mach and no longer are descriptions or listings of facts the only way to approach the observed reality. If a theory can be established and if it holds water, I always prefer a formula over a table....

Cheers

[gscace]

Sorry.

No problem

This is a munge that is pretty much wrong, and a common misconception it seems.

Chemical/Physical laws when dealing with "ideal substances" are often not correct to the point, but I wouldn't call that a "misconception". Exactness is often sacrificed for simplicity, in order to make the maths not too complicated, but if the approach is ok it works for me (and for 99% of scientists 99% of the time)

Ideal gas law doesn't have anything to do with it. The temperature is purely related to the vapor pressure of water, which is an exponential function of temperature.

Stating that ideal gas law doesn't have anything to do with it when the matter concerns a gaseous phase, suggests little familiarity with the basics of physics and chemistry and the underlying theories. Did you check how far off the mark we are with that formula? To be honest I didn't bother to do the maths. However, I did mention that Clapeyron resp. Clausius Clapeyron should be closer to the point. Or are those equations also pure rubbish in your opinion?

You can't use the ideal gas law because the volume isn't closed. The volume becomes open as soon as you open the steam valve, or when the vacuum breaker is open to the atmosphere, as when heating the system up to the water vapor boiling point.

Talk about a misconception! How does the pressure increase when the system is open? What happens with the pressure as soon as you open the steam valve? Are you really saying your boiler is open most of the time and reaches e.g. 1.2bar while the steam valve is open? How does the pressurestat work if the system is open?

The temperature / pressure relationship can be obtained from steam tables, which you can find them on the internet if you do a google search.

Yes, I know. But knowledge management by Google also has its pitfalls it appears. I still prefer my old books about thermodynamics. Science has moved on since Mach and no longer are descriptions or listings of facts the only way to approach the observed reality. If a theory can be established and if it holds water, I always prefer a formula over a table....

Cheers

Walter:

If you wanna use the ideal gas law, you have to have a closed system. You can then relate temperature and pressure as you know. But in order to do so, the number of moles of gas in the system has to remain constant, which is not the case in espresso machines.

Since liquid exists in an espresso machine boiler, the number of moles of water vapor in the gas phase varies. The amount of water in the vapor phase is a function of temperature only, with a weak interaction between the water vapor and the other constituents in the gas above the liquid. As far as the pressure contribution of other gasses is concerned, they would contribute to the total pressure based on the ideal gas law because the number of molecules is conserved in the system, unless they are forced out by some mechanism. Mechanisms that seem reasonable and likely to me are increase in pressure due to increasing temperature when the vacuum breaker is open and the system is heating up, and anytime the steam valve or hot water valve are opened after the machine is up to temperature. There is no air generator inside the boiler. But there's plenty of liquid water that will flash to vapor as soon as the pressure drops below the saturation vapor pressure, such as occurs when opening valves. So it's a very reasonable assumption that water is the predominant species in the vapor phase in the boiler, and therefore the pressure is pretty much due to water vapor pressure and nothing else.

You mention Clausius / Clapeyron, which is the basis for the water vapor pressure correlations developed by various folks. Take a look at Arnold Wexler's vapor pressure formulation for water from 0 to 100C. That's what we use here, corrected to ITS-90. It does not look anything like the ideal gas law. And use of the ideal gas law does not get you anywhere close to the right answer. A simple check would be to attempt to calculate the difference in temperature with respect to pressure using the ideal gas law, and then to do it with the steam tables. They give very different answers.

-Greg

[Walter]

Walter:

If you wanna use the ideal gas law, you have to have a closed system. You can then relate temperature and pressure as you know. But in order to do so, the number of moles of gas in the system has to remain constant, which is not the case in espresso machines.

Since liquid exists in an espresso machine boiler, the number of moles of water vapor in the gas phase varies. The amount of water in the vapor phase is a function of temperature only, with a weak interaction between the water vapor and the other constituents in the gas above the liquid. As far as the pressure contribution of other gasses is concerned, they would contribute to the total pressure based on the ideal gas law because the number of molecules is conserved in the system, unless they are forced out by some mechanism. Mechanisms that seem reasonable and likely to me are increase in pressure due to increasing temperature when the vacuum breaker is open and the system is heating up, and anytime the steam valve or hot water valve are opened after the machine is up to temperature. There is no air generator inside the boiler. But there's plenty of liquid water that will flash to vapor as soon as the pressure drops below the saturation vapor pressure, such as occurs when opening valves. So it's a very reasonable assumption that water is the predominant species in the vapor phase in the boiler, and therefore the pressure is pretty much due to water vapor pressure and nothing else.

I do not want to use the ideal gas law for an exact description of the gaseous phase above the water in the boiler, and more to the point, I never claimed to (which I tried to make clear in my first post). But the ideal gas law serves well enough to describe the mutual dependencies of pressure and temperature. And exactly these dependencies are the reason why we can regulate the brewing temperature of our machines by regulating the boiler pressure.

And this, I believe, was the question raised in the first post of this thread. The question was answered well enough by cannonfodder, but since he mentioned a formula, I thought that was the one he meant.

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At this point we might as well end our little exchange, because the rest is pretty much off topic here, nonetheless I will share my thoughts with you:

While I agree with you, that the ideal gas law does not describe the system exactly, I still think it is a reasonable first level approach. At any given moment when your machine is idling, the boiling water is in perfect equilibrium with the gaseous phase above it. We have a closed system at a certain temperature with a certain pressure and a certain number of molecules in the gaseous phase. And with regard to the pressure it is IMO entirely irrelevant whether the gaseous molecules are N2, O2, CO2, H2o or whatever, they all behave very similar if not equally in that aspect.

Of course this equilibrium is disturbed as soon as you interrupt the system e.g. by opening the steam valve. Then our little quasi-static (notabene: it is not really static, but in equilibrium, thus a static description can describe it reasonably well) system becomes dynamic all of a sudden. And if we leave the steam valve open long enough our pressure drops - roughly - to the atmospheric value, our little pressure cooker has become an ordinary pot with boiling water in it, and the pressurestat is of little value to regulate the temperature there...

Most of your considerations above address the disturbed system, where no equilibrium exists and dynamic processes are predominant in the system. But there the description becomes rather complex...

You mention Clausius / Clapeyron, which is the basis for the water vapor pressure correlations developed by various folks. Take a look at Arnold Wexler's vapor pressure formulation for water from 0 to 100C. That's what we use here, corrected to ITS-90. It does not look anything like the ideal gas law. And use of the ideal gas law does not get you anywhere close to the right answer. A simple check would be to attempt to calculate the difference in temperature with respect to pressure using the ideal gas law, and then to do it with the steam tables. They give very different answers.

-Greg

You certainly must be aware what the temperature range in the boiler is, but for those here who aren't, we should be fair enough to mention that the temperature is well beyond 100°C, thus I seriously doubt, that a vapor pressure formulation for water from 0 to 100C - corrected by the Triple-point of water at 0°C does the trick of describing a closed, overheated, pressurized system exactly, whereas the Clausius-Clapeyron equations dont...

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P.S.: Edited as per request of moderator to tone down the rhetoric. And hereby i humbly bow out of this discussion