Hot Porridge, Hannah Clarke Preston MacGoun, 1910

This post is specifically for Samuel Law because I know that Sam just loves porridge. If you aren't Sam, please stop reading.

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.



Results



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

Hello:

Truly amazing job ... =-)
Can't wait to see it finished.

bidoowee wrote: ... decided to switch to a terminal block rather than trying to accomplish everything inside the controller box.

+1

Cheers,
CIV

You noted that the brew reservoir is now bronze. As bronze varies am I to assume you used silicon bronze. Recall some mention of silicon in brass and bronze previously in this thread.

Congratulations! That's really impressive temperature stability.

Thank you civ and arcus!

redbone wrote:am I to assume you used silicon bronze

Yes indeed - this is a silicon bronze alloy that I have developed with the foundry to replace the arsenic bronze used in the antique group. The same alloy is used for all of the castings, even the cam which doesn't come into contact with the water. The silicon acts as a corrosion inhibitor as well as increasing the alloy's machinability.

Yum..can I have a second serving please?

But seriously nice work, I am jealous of that temperature stability! And after all that work, I think you are at least allowed to say that the thermodynamics was designed and calculated carefully , with trial and error.

Plus, I think it will be a fantastic steamer! Are you running at about 1.4 bar there?

Illustration: George Alfred Williams, Ten Boys from Dickens, K.D. Sweetser, 1901

samuellaw178 wrote:can I have a second serving please?

You want MORE?

Well, to satisfy my own curiosity, I wanted to know what would happen with a shorter injector. The testing method is exactly the same as the last trial except that I cheated the group up to temperature with a series of pulls and then left the machine to idle for half an hour before performing the tests.



I can only assume that the first pull which peaks at 83 C is the result of cold water being drawn back into the chamber from the waste pipe. I can't think of any other plausible scenario that would explain the data. The results are similar to the performance of the antique Aurora tested in May in that there is a gradual per-shot heat-gain in the group except that the peak puck temperatures are much more tightly grouped.

samuellaw178 wrote:the thermodynamics was designed and calculated carefully, with trial and error

I actually did start down the avenue of creating a fluid dynamic model of the system for thermodynamic design simulation purposes. However, I concluded that, at least with my admittedly novice skill level coupled with the limitations of the software package that I was using, there was simply no way it was going to deliver useful results. This approach would be very useful for comparing design concepts, but I find it hard to believe that it will get close enough to real world to make those comparisons definitive.

As for steaming capacity, yes 1.4 bar and yes, a four liter boiler delivers the goods

Any day now...

LObin wrote:I can't believe you went on vacation and left us hanging...

Lol. Not on vacation. Just doing a lot more or what I've been doing for nearly a year: waiting for suppliers to deliver.



They did, yesterday, at least a first attempt at a finish...





The cams will do, but the group has to go back for another go. Hard to tell from the photo but the chrome also looks suspiciously yellow - unlike the sample they sent me before. Anyhow, wont know until I receive them and that ain't gonna be by Christmas. And I really wanted to hang them on the tree!