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hot porridge

Hot Porridge, Hannah Clarke Preston MacGoun, 1910

[Ed: re-post from Nov 17, 2018]

The new results in my ongoing quest for Goldilocks porridge (aka boiler-group thermodynamic interaction and stability, but porridge sounds much better) are in and I have to say that I’m rather pleased.

What are we testing?
This is temperature profile of the new boiler with diagonal HX and injector. The boiler and the HX chamber are both made of stainless steel but, unlike the previous Horseshoe HX prototype, the brew reservoir is now bronze (for reference, stainless is roughly 20 times less thermally conductive than copper and copper-based alloys). The diagonal HX configuration eliminates the separation between the HX chamber and the brew reservoir and they both form one single volume of hot water at a lower average temperate than the boiler water. Cold water is injected directly into this volume and the resulting mix, now at a lower temperature, moves on into the group during a shot.

Methodology
Methodology is similar to previous tests: the machine was turned on several hours in advance to make sure that everything is at its ultimate idle temperature. The probes are K-type thermocouples placed in the same spots as prior tests – the only difference being that the brew reservoir now has a dedicated threaded thermocouple socket – no more tape coming unstuck or clamps falling off. Shots are simulated by using a flow restricting valve placed on the outlet of the portafilter.

Shot simulation procedure is:

  • Pull
  • Pre-infusion 7 seconds (lever in down position) –
  • Shot 20-25 seconds for lever to return to the cam inflection point (lever just past straight up and down)
  • Post-flow 10-30 seconds (lever returns to rest position)

Various timings between the shots are tried: 5 minutes, 4 minutes, 3 minutes, 2 minutes, 3 minutes.

Comments
The pseudoScace™ device (puck temperature readings) has too large a thermal mass to give meaningful results for peak puck temperatures when inserted cold. I therefore left it in place, before, during and after the test to minimize its impact. The one second sampling time period is too long to give reliable readings at the moment of the pull. On a few of the shots there is a significant drop seen at the puck at the moment of the pull. I believe that this is due to the piston creating a vacuum as it is raised and drawing cold water back up through the pseudoScace from the waste line. A change in equipment would be required to eliminate this if this hypothesis is correct.

Conclusions and observations
The original Aurora diagonal HX I profiled back in May demonstrated uncanny thermal stability at the brew reservoir, but the group suffered per-shot heat-gain and was slow to return to its baseline idle temperature. These results show that the brew reservoir temperature is dipping significantly but the group and the puck temperatures are, by comparison to the antique machine, rock steady. The maximum overall delta at the puck is 3.1 C (between the walk-up and the third shots) but the inter-shot maximum delta is 1.8 C (the minimum inter-shot delta is 0.5 C for 2 minutes between shots).

Summary
So, to summarize: best performance at 2 minute intervals, significantly lower puck temperature fluctuation than the antique machine and little to no group heat-gain. This, I think, may be a slightly better mouse trap – though not really by design, rather by accident of the thermal interaction of the materials. I’m not going to complain.

If you will permit me, and at the risk of tooting my own horn:

Courtesy of UC Davis, Special Collections
Title: Magazine ad for Bank of America: hammer and nail montage.
Creator/Contributor: Halberstadt, Milton, Photographer

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Goldilocks part 3

The set of the bears. Plate 7, 1664, by Marcus de Bye, after Marcus Gheeraerts I, 1559. Gift of Bishop Monrad, 1869. Te Papa (1869-0001-67)

Though I’ve run out of three bears analogies, I’ve stuck with the story’s structure: first the porridge was too hot, then it was too cold, and finally Goldilocks found one that was just right. This iteration of the HX caused me to take my hat off, yet again, to the Italians. Many months ago, Dr. Pootoogoo brought a boiler from a later-model Brugnetti to my studio. The flange bolts were so badly rusted that it wasn’t ever going to go back into service without replacing them. It happened to be one with a diagonal HX that I hadn’t examined before and it inspired my to try a similar concept with the horseshoe HX prototype. I was quite surprised that the 60ml HX (baby bear) didn’t deliver water that was cooler than the brew reservoir temperature even though the HX volume was close to the 50ml shot volume. I also started thinking about what the ‘correct’ temperature for the brew reservoir should be. It occurred to me it might not be the best thing for it always to be the same. For example: if the group is at 75 C and the water coming in is at 102 C, the resultant water temperature at the puck is 92 C (these are roughly the numbers for both vintage machines) and we know that the group gains heat after a shot, let’s say for the sake of argument it gains 3 degrees and requires about 2 minutes per degree to recover i.e. 6 minutes. It follows then that for the next shot, if it is to be pulled [i]before the end of the recovery time,[/i] it would be preferable to have the brew reservoir water at a lower temperature than 102 degrees so that when it reaches the puck it will be at same magic 92 degrees. Because of the difference in thermal properties of the materials (i.e. the brass group and the water) and their relative volumes (i.e. big thermal mass of brass vs 50ml of water) it isn’t a one to one relationship. But it is linear – i.e. it will be a constant times the temperature rise of the group. So at any given time during group recovery, the required brew reservoir temperature is the reservoir idle temperature minus the group temperature rise times some constant. In math not English:

Tbr = Tbr_idle – K(Tgroup – Tgroup_idle)

In other words if the heat gain curve of the group could be inversely mirrored by the brew reservoir, then water will be at the right temperature when it reaches the puck [i]no matter when it is pulled[/i]. This is really just destructive wave interference:

If the brew reservoir temperature curve is positive (i.e. there is heat gain), then it will compound the problem of heat gain at the group. But if the brew reservoir temperature drops after a shot, then it will compensate.

The diagonal HX design consists of a large diameter pipe which connects directly to the back of the brew reservoir – essentially increasing the volume of the brew reservoir four-fold. In fact, the concept of the brew reservoir is pretty much gone altogether in this design – the group flange actually becomes one end of the heat-exchanger. A small diameter injector tube runs through the middle of the large diagonal pipe. Line water comes in through the injector too fast for the surrounding water to heat it to boiler temperature and mixes with the water behind the group to get the really stable results that we saw in the earlier testing.

I replicated the basic principal minus the diagonal tube and in so doing figured out why the diagonal design ended up that way i.e. diagonal.

The last kink in the 6mm tubing was only way to thread all the larger diameter fittings onto the injector. And this is a one-way operation: once it is brazed together you can’t take it apart again.

Brazed and (sort-of) cleaned.

And here are the results:

Blue – Boiler
Red – Brew reservoir
Purple – Group neck

Now, although the results aren’t perfect, it shows that the concept works. The group heat gain for successive (unnaturally) rapid shots has been significantly diminished and the recovery time for the group is less than half of what it was (less than 3 minutes). The length of the injector plays an important role in how much boiler temperature HX water mixes with the line water and consequently in the temperature of the water that reaches the brew reservoir. But, as I said, this is a one-shot fabrication and is too much trouble to alter. It would be much easier to change and/or maintain if the injector tube screwed into a straight length of larger diameter tube that maybe ran directly to the group right through the boiler, maybe on a diagonal…

Wait – someone already thought of that.

Enough porridge already.

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Goldilocks part 2

Illustration by Pervprude

It is high time for more porridge!

Several days after I did the first tests, it occurred to me that the horseshoe tube that I had made when I built the boiler was not the same as the one in the Brugnetti boiler. I wasn’t really thinking that hard about how a heat-exchanger actual works while I was making it. So I took apart the boiler (again). This is the original tube, nicely blackened after a few months of service:

I measured the interior volume of the ‘Papa Bear’ version and compared it with the volume of the vintage HX horseshoe (modeled in CAD): 115ml for the vintage and 135ml for Papa Bear. So I put Papa Bear to the bandsaw, and cut him down to make Mama Bear.

Blue – Boiler
Red – Brew reservoir
Purple – Group neck

(The data for the neck was very noisy for some reason that day – the rapid deviations do not reflect what is actually going on.)

Despite having the same volume as the vintage tube, ‘Mama Bear’ doesn’t behave the same way. The peaks in the brew reservoir temperature are similar to Papa Bear, i.e. around 5 degrees, and recovery times are about the same. It should be noted however, that with adequate recovery time, the neck temperature shows a high degree of stability.

I then modified the horseshoe again, lowering the volume to around 100ml (Auntie Bear??) – but the results didn’t change significantly. So I decided that drastic measures were called for: ‘Baby Bear’ – roughly 60ml volume which is about half of the vintage.

Though the results are better, there are still spikes in the brew reservoir temperature.

Time to alter the Goldilocks plot line.

Introducing ‘No Bear’! Partly to make sure that I wasn’t entirely out to lunch but also to measure the other extreme, I connected the supply to the brew reservoir directly to the mains, bypassing the HX and injecting room temperature water into the reservoir. Unsurprisingly the results are dramatic!

Very cold porridge indeed.

Interestingly however, while the neck temperature rises by a few degrees initially, it recovers quickly (less than a minute) and then drops to 2.5 degrees below idle.

Where does this leave us? The concept of the HX is of course to inject (fresh i.e. non-boiler) cold water through the hot water in the boiler in order to raise it to the ‘correct’ temperature to feed the reservoir. If the machine has been idle for any length of time, the water in the HX will be at the same temperature as the boiler. Subsequent shots will draw cold water into the HX, which, depending on how it is designed, will consistently bring room temperature water up to a specific temperature (either boiler temperature or slightly lower), as long as the heating element can keep up with the demand. So some not very earth-shattering conclusions:

  • Changing the design of the HX will determine the temperature ‘profile’ of the water delivered to the brew reservoir.
  • Somewhere between 60ml and 0ml of HX volume, the water delivered to the reservoir will offset the heat gain and result in equilibrium.

There is one additional question that results from the three bears test: why do identical horseshoes in the vintage and new boilers not exhibit the same thermodynamic behavior? My hypotheses is that materials used for the brew reservoir and boiler are playing a much bigger role than I first thought. Both of these parts on the prototype are made from stainless which is roughly 20 times less thermally conductive than copper and bronze. To test my theory, I put the prototype group onto the vintage machine: compared to the all-stainless boiler assembly, the new group runs around 12 degrees hotter on the copper/bronze boiler. The stainless brew reservoir is slow to acquire heat from the boiler and the water in the reservoir and reluctant to relinquish it to the air or pass it on to the group.

SO…. two rather more significant conclusions:
1 – There should be (or rather, spoiler alert, there is, as will be seen in an upcoming episode) an HX design that meets the requirements of stainless boiler and group combination.
2 – It is time to make a new boiler – using the bronze brew reservoirs that I received a few months ago.

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Goldilocks part 1

Illustration: Arthur Rackham

[Ed: Re-post from May 12 2018]

I imagine everyone remembers the story of Goldilocks and the Three Bears?

I love the painterly references on the wall – a Degas ballerina on the left, an old master on the right, an enameled reproduction of a Greek vase in blue? But I digress. Apart from carrying a wee bit of Freudian baggage, it is also perhaps not the most pertinent of fairy tales for our times (how would you react if you came home and found someone sleeping in your kid’s bed?). Anyways, what I wanted from it for today’s post is the porridge. You know, the three kinds of porridge that are too hot, too cold and just right? You see where I’m going with this… Except that in my story there are four bears, or rather three bears and no bear, but neither of those versions scan very well, so we are going to stick with the original title. First up is Papa bear’s porridge.

This is one of the first temperature profiles that I took of the prototype machine. The methodology is very similar to the tests on the Brugnetti [Auroras] that I posted previously. As always, the Scace-values are not terribly useful except for comparison.
A reminder of the colors:
Blue – boiler wall
Red – brew reservoir
Purple – group neck
Green – PF receiver on the group
Yellow(ish) – Scace puck

Observations:

  • The boiler temperature curve is sinusoidal as opposed to saw-tooth and has a delta of 0.7 C between minima and maxima. This performance was obtained using only the P term of the [PID] algorithm. It could probably be improved with some additional tuning, but I haven’t bothered because it is already pretty good.
  • Brew reservoir temperature is stable at idle and shows no trace of the boiler temperature variation.
  • Recovery times for a shot are about 2 minutes per degree for the group and 1 minute per degree for the brew reservoir.
  • Group temperature at idle (~70 C) is quite a bit lower at idle than the Auroras (~80 C and ~83 C for the horseshoe and diagonal versions respectively).
  • Don’t touch the probes.

Conclusions:

  • Job (pretty much) done on the PID boiler control.
  • Plenty of room for improvement on the brew reservoir temperature stability which is gaining too much heat after a pull.
  • The whole shebang has to be hotter.

Next up: Mama bear and No bear.

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temperature profile diagonal HX

1987 Porsche 911 Turbo Cabriolet

[ED: Re-post from May 4 2018]

This is a longer test (over an hour) on a 1987 diagonal HX machine.

Methodology:

The same methods as were used for the horseshoe HX machine were used with a few minor differences.

  • This machine is on 24/7 so there is no warm-up period.
  • It had been at idle for at least a couple of hours.
  • The same pseudo-Scace device and needle-valve was used to simulate shots.
  • The period between simulated shots is generally longer and I did not repeat the faster than commercial usage of the previous test, rather, I waited for the group to recover between shots.
  • At minute 3 and 32 there are cleaning flushes – i.e. pulls that were significantly larger in volume than an actual shot.
  • At minute 23 a PF filled with real coffee was locked into the group and a shot pulled.
  • At minute 25, the steam wand was used to foam milk for a cappuccino.
  • The additional orange line that starts at minute 46 is another K-type thermocouple that was placed on top of a puck of coffee before the PF was locked into the group.

Observations:

  • This machine has the same pressurestat as the horseshoe HX machine from the previous test and consequently we see the same saw-tooth wave for the boiler temperature.
  • With the exception of the cleaning flushes, the brew reservoir is [b]remarkably[/b] stable – exhibiting almost no discernable trace of the fluctuation in boiler temperature. The temperature drop for the outside of the reservoir after a shot is pulled is less than one degree and the recovery time is between two and three minutes.
  • The group as a whole exhibits the same tendency as the horseshoe HX machine to gain heat with each shot.
  • The recovery time for the group is a little over a minute per degree C of heat gain i.e. essentially the same as the horseshoe machine. As the groups are identical this is unsurprising.
  • The shot test with real coffee plus the additional thermocouple starting at minute 46 shows a 90 C peak shot temperature and is quite likely to be accurate.
  • At minute 40 I removed the PF to prepare for the next shot and the preparation time is longer than usual because I was fussing with the thermocouple. Removing the PF has a fairly significant effect on overall group temperature as the neck falls to the same 83 C that it was when the machine was at idle.

Conclusions:

  • The diagonal HX is about as good as it gets as far as temperature stability of the brew water reservoir goes. The reservoir recovers from what little variation there is in less than half the time that the group takes to recover between shots.
  • The slower, more realistic, pace of shot pulls in this test is illuminating. The actual temperature gain seen at the group is between 4 and 5 C per shot. This translates to a group recovery time of five to six minutes – possibly a little slow for a commercial setting, but definitely fast enough for home use and entirely manageable with a cooling strategy such as a cooling flush of a known volume.
  • 1987 was better than 1982.