I'm not sure how Bosco assembles their boilers. You can see a photo of their current 1 group boiler here: New Bosco Sorrento

Thanks Thomas. Good to know! There is a red tick on the gauge at 1.5 bar, which I presumed to be the designed safety high limit of the boiler.

I've always been curious about the difference between diagonal and horseshoe HX Auroras so your comment got my ears perked up. Looking forward to your next installment!

As will be seen in an upcoming post, the horseshoe HX is indeed fine beast, BUT, the diagonal HX is even better.

To validate this 1 BAR pressure limit on a coper boiler you should actually site the regulatory authority. Such a restriction does not exist today in many jurisdictions as espresso machine boilers are not regulated due to the low capacity.

JohnB. wrote:I'm not sure how Bosco assembles their boilers. You can see a photo of their current 1 group boiler here

@JohnB - nice! Looks like they nickel plated the entire shebang! From the look of the seam, it would appear to be either soldered or brazed - hard to tell.

@OldNuc

I was merely suggesting a reason for why the operating pressure for the 80's copper boiler Auroras is 1 bar. If the longitudinal joint on the boiler is soldered, not brazed, then running the boiler above the 1 bar operating pressure may have an impact on the longevity of the assembly.

My research on boilers in general is here.
While they may not be a regulatory authority, the specific citation from which I drew my conclusions is from copper.org.

Temperature and pressure are directly proportional for steam. As the pressure in the system is increased, the temperature increases accordingly. Saturated steam, a condition where steam contains as much water as it can and still be a vapor, at 15 psig has an absolute pressure at sea level of 29.7 psia (pounds per square inch absolute). At this pressure it would have a corresponding temperature of approximately 250°F which is the maximum recommended temperature for soldered joints as shown in Table 4 of the Copper Tube Handbook. Therefore, rather than the allowable pressure of the soldered joints controlling the rating, the allowable temperature is the controlling factor, leading to the rating of 15 psig regardless of the solder alloy used.
...
As with any piping system, the pressure rating of the system is controlled by the lowest allowable pressure of the tube, fitting, joint or joining material. For steam systems constructed using copper tube of Types K or L, the maximum allowable pressure at which the system could be designed would be 120 psig. As shown in Tables 3a and 3b of the Copper Tube Handbook, the lowest maximum operating pressure for Type L copper tube is 127 psig (corresponds to 12-inch nominal Type L tube in annealed form). Since this is more than the allowable pressure for the brazed joint, the 120 psig allowable for the joint is the controlling factor, regardless of the fact that smaller diameter tubes have higher allowable pressures. However, to use copper tube and fittings in a steam system at this pressure the joints must be brazed.


From table 14.4a in the Copper Tube Handbook - the working pressure rating for brazed joints on all diameters of tube for use with saturated steam is 120psi gauge. Soldered joints, with all alloys and for all diameters and for use with saturated steam are limited to 15psi gauge.

Have you ever seen a soft soldered coper sheet boiler in an espresso machine?

Although there is a CDA recommendation to not use soft soldered fittings at >15psig steam pressure it is done routinely on small diameter tubing. I have never heard of any failure.

Common soft solder alloys have melting temperatures ranging from ~350F to ~540F depending on the formula.

Common design requirements/recommendations are not regulatory requirements.

Thomas (and others),

Any chance we can tell what material/techniques were used on the joints, just by looking at them? There seems to be two kind of techniques used on vintage Aurora from what I can see? This thread has been so educational thus far, thanks Thomas!

Images from Kaffee-netz after some kind of treatment (sand blasting/polishing?)



Note that the brass neck (brew reservoir) was joint to the boiler using the grey material. That part will see about up to 4 bar max.

The top picture suffers from poor lighting/color balance-temperature but the 2nd show a low silver percentage silver braze process. Lead and to a lesser degree tin solder alloys can be easily gouged with sharp implements and any of the silver braze alloys are much harder and difficult to gouge. A gouge is not the same thing as a scratch.

There are ANSI standards for cooking equipment (which will include coffee machines of all types) nominal operating specifications.

No idea how the gray coloring was achieved, I suspect it was the lighting. The group to boiler neck is usually a brazed joint. I would look for more pictures of this machine as the gray joints just look wrong.

OldNuc wrote:No idea how the gray coloring was achieved, I suspect it was the lighting. The group to boiler neck is usually a brazed joint. I would look for more pictures of this machine as the gray joints just look wrong.

Rich,

Thank you for the comments. I've looked for a few more pictures and they're consistently grey in color. Most boilers were originally plated (mine too) and so there're not that many naked boiler pictures around.

From : http://www.kaffee-netz.de/threads/verka ... ser.85069/


From : http://espresso.cultureforum.net/t10015 ... tti-aurora (change the "espresso" to "expresso" for the correct link)

Looking on the originating site that looks like nickle plating or 50-56% silver braze. I don't think the outer surface was sand or bead blasted but chemically cleaned. In the original uncleaned pictures there is still a difference in the look of the boiler shell and the area around the fittings. In those uncleaned shots the joints look like silver solder and the group neck to the boiler looks brazed yet is also gray.

I would like to know exactly how it was cleaned up and what camera and flash was used to make the photos.

Looking at some of the other shots you will see a slightly yellow/gray tint in what is nominally gray and a more yellow group neck to boiler brazed joint.

None of this looks remotely like a soft solder joint.

Electroless nickle plating is normally used when you are satisfied with a very thin thickness of the plated nickle. Works good to maintain a corrosion free exterior. I think these color differences are the results of the photography. Bronze brazing and low silver content silver solder9Braze) for the smaller boiler fittings.

I have a 62 L59 Gaggia and the construction looks similar down to the concave boiler head looking a mottled gray (it is copper) due to a nickle wash and the boiler fittings are definitely silver brazed. The other head is brass and is a gray color and shows brass where the age patina has been removed around the edge when it was cleaned up.