Thermodynamics of espresso

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Rufus T.F.

#1: Post by Rufus T.F. »

There is always air above the basket, sometimes quite a lot of it eg. when you like brewing in a bigger basket than your dose requires (I do). When hot water hits that air at 9 bar or so, the pressure of the gas increases rapidly and so should the temperature (pV=nRT). Is the change in gas temperature so small that it does not matter and water immediately stabilises it to its own temperature, or do we have a momentary jump in the temperature of water hitting the puck?

I'm wondering if this is, perhaps, an answer to why my 15g of espresso taste different brewed in a 15g basket vs an 18g basket.

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another_jim
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#2: Post by another_jim »

If you have a pressure meter on the group, you will see that the air is not compressed. It is forced out of the puck as the water displaces the air. Until that happens, the pressure does not rise. The more the headspace, the longer the time interval for the pressure rises above the puck

Suppose you have a machine with no air space in the group and a very large group jet, like for instance an old 1990s LM Linea. Then leaving a lot of airspace in the basket, like filling an old LM basket with 14 grams, will get you 5 seconds of low pressure preinfusion and a decent shot. Fill the same basket with a Seattle style 18 gram dose and no head space, and you get about 1 second of dwell time and, unless you go through a long list of blending, roasting, and shot prep contortions, a nasty tasting shot.
Jim Schulman

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Bluecold
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#3: Post by Bluecold »

On a lever machine (assuming a typical lever group), there will be a lot of air above the puck, just before the cylinder gets filled with water. When the cylinder fills up, a lot of the air rises above the water volume. This air comes out in a foamy fashion after the shot is done and you leave the pf in the group.
What fraction of air rises above the water column and what fraction is expelled out of the puck I do not know.

As far as temperature increase by the compressed gas, I do not know, but the extra heat energy will be small, since the work required to compress a bit of gas is rather small, in the order of 4.5bar*surface of 6cm circle*1cm stroke. This is gives 13 joules, and the heat energy of water is 4 joules per gram/c. A column of water 6cm diameter and 1cm high will need 113 joules to rise 1 deg C in temperature. This gives a temperature increase of <0.2 deg C, in a rough calculation where the force is taken as linearly increasing from 0 to 9 bar, instead of a pv curve.
LMWDP #232
"Though I Fly Through the Valley of Death I Shall Fear No Evil For I am at 80,000 Feet and Climbing."

Rufus T.F.

#4: Post by Rufus T.F. »

I understand the puck is soaked by water flowing down through gravitational pull for a few seconds. But how does the machine know when the air is displaced by the water? Or is it just the matter of density? Air goes out through the dry puck and the much higher density of the water filling the space causes the pressure increase?
another_jim wrote:If you have a pressure meter on the group, you will see that the air is not compressed. It is forced out of the puck as the water displaces the air. Until that happens, the pressure does not rise. The more the headspace, the longer the time interval for the pressure rises above the puck

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Jake_G
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#5: Post by Jake_G »

Rufus T.F. wrote:I understand the puck is soaked by water flowing down through gravitational pull for a few seconds. But how does the machine know when the air is displaced by the water? Or is it just the matter of density? Air goes out through the dry puck and the much higher density of the water filling the space causes the pressure increase?
This took me a little while to figure out, myself. I had always envisioned rain drops falling from the shower screen and landing on the puck while the air had to rise to the top, due to buoyancy. Makes sense until you realize that the second a drop of water enters the chamber, it has decreased the volume of the chamber. While there may be some pressure raise in the headspace on the order of a few millibars, what seems to be observed is that the reduction in volume results in air moving through the puck until sufficient water has landed on the surface of the puck and rendered this path of exit unavailable. The machine has no idea any of this is happening (not being sentient, and all :wink:), but furthermore this just happens. A cc of water comes in, a cc of air leaves. In a lever machine, this would be as soon as the water enters the empty cylinder above the shower screen. Eventually a pool of water collects on top of the puck and it begins to grow faster than the puck can wick it away and pull the water through.

At this point, when the puck is sealed with some water above it, the remaining air is trapped above the water level and is indeed compressed. This is the charge that evacuates the portafilter when the 3 way valve opens at the end of a shot.

With respect to pV=nRT, as Roeland points out, the heat implications of compressing air to 9 bar are insignificant. I'm sure it happens, and that energy is in fact provided by the hot group and hot water entering the group under pressure, but it is the equivalent of factoring in the effect hitting flying insects on your fuel economy when figuring out how much fuel you will need for a cross country road trip. Yes, the effects are real. No, they don't actually change the end result.

Cheers!

- Jake
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John49

#6: Post by John49 »

With the Strietman open boiler and the lever left in the raised position, all of the remaining air can escape up through the cylinder opening.

Rufus T.F.

#7: Post by Rufus T.F. »

It seems various espresso machine designs deal with the air above the puck in different ways (kinda obvious, given how they vary in eg. pressure application profiles). That would mean though, that the mix of water mass and amount of air in various designs differs. As a result, the amount of oxygen reaching the puck differs. Sometimes the puck is attacked with a mixture of water and air, other times it is mostly water. I wonder if there have been any scientific data on how different levels of oxidisation influence the result in the cup.