Building a 'simple' dual boiler machine

[bsafnuk]

As the title suggests, I am in the process of building a dual boiler espresso machine, and I am hoping that the collective wisdom of home-barista.com can offer some guidance (or talk some sense into me!)

This project started when I managed to score a 2-group Nuova Simonelli machine off of Craigslist for a ridiculously low price. However, the beast is absolutely huge, and when I brought it home my wife took one look at it and said something to the effect of "Nice machine. We're not keeping it." Considering the size of our kitchen, I think she was being more than reasonable

My plan was to clean it up and sell it to a good home, but then I got to thinking (never a good idea ) that I have all the parts in front of me to build a machine with all the features that I want, at a size my kitchen could manage.

My design is (I think) a very simple one. I will have two boilers, each with an inlet near the bottom of the tank and an outlet near the top (plus misc. fittings for heat elements, thermoprobes, safety valves, etc). The brew boiler is connected directly to the water inlet pipe, which, due to the bypass valve on my procon pump, puts it permanently under mains pressure. There will be a manual valve for the steam boiler (to keep things simple, I do not plan to have an autofill circuit). The outlet on the brew boiler goes to a tee, one direction to the group, the other to a hot water tap. The outlet on the steam boiler goes, you guessed it, to the steam wand.

As I see it, the advantages of having the brew boiler pressurized are
1) Simple hot water tap design (just turn the valve knob, no need for pump activation)
and
2) Easy to build in preinfusion by adding a delay circuit to the pump activation

I could not think of any downside, but perhaps I am missing something... i.e. anyone see any safety issues or design flaws with this setup?

The idea of having the brew boiler pressurized is not my own - in fact the heat exchangers on the Nuova (there is one for the groups and one for the hot water tap) feature such a design.

A few comments on the Nuova Simonelli machine:
The grouphead is a saturated type design - it is in direct contact with the boiler and hot water flows through it. This makes it an ideal candidate for a dual boiler setup, but I was a little surprised to see it on an HX machine. This tells me that either this machine would have needed a tonne of flushing to bring it down to temp after sitting (the HX is something like 600ml) or it radiates a lot of heat to the environment to keep it near brew temp. This isn't a big deal to me as I plan on insulating the group anyways. Aesthetics aside, it seems to me that having an uninsulated group head on a dual boiler machine is a design flaw if temp. stability is the absolute goal. For me, the Nuova group is nothing special to look at, unlike the LM or E61 groups, so I am quite happy to cover it up.


As for my boilers, I plan on getting some large diameter stainless steel tubing, and having end plates and pipe nipples welded on. I will also add on a neck and flange for the group. The total cost shouldn't be too high, but this is the part that makes me the most nervous. Am I putting myself at serious risk with such a boiler? i.e. if the joints fail will I have a steam bomb blast or a gentle hiss?

My final question has to do with heating elements. I sourced some nice stainless steel screw in cartridge type elements at mcmaster.com, but they are a little pricey (especially since I need two of them). They also sell screw-in water heater elements for considerably less. However, these elements are zinc plated and I am not sure what the flange is made of. Anyone have any comments on the suitability of these elements? As a partial compromise I was thinking of using the stainless steel element for the brew boiler, and the water heater element for the steam boiler, but I am worried about the flange rusting.

There were also a bunch of appropriate elements made of Incoloy, but I am not familiar with this material. Is it food safe? A cursory web search showed it in use for fat fryers, but that is a totally different environment than hot water.

Anyways, I appreciate any advice (or smacks in the head, if necessary) and I will keep posting my progress (or lack of it)

-Brad

[Bushrod]

I bet you could run the lines to an E61 group that's not connected to the chassis. That is, you could get a group and mount it on the wall or something.

You really have to be careful with fabricating boilers. I remember back when Junkyard Wars had people create steam powered cars. They were very careful to not let them fabricate the boilers, but seeded the junkyard with a couple.

[bsafnuk]

I agree that an E61 setup (like on the Alexia) is a good solution; however, my goal is to use as many parts as possible from my existing Nuova. Unfortunately, E61 groups are quite expensive new (something like $650 from EspressoParts).

I appreciate the warning about boiler fabrication - it is certainly not something I will be doing myself, but rather letting an experienced welder do the work.

-Brad

[JimG]

As you develop the design, be sure and think about heater wattages and power requirements. With two boilers, you will probably have to either to power everything with a 20A circuit, or develop a control strategy that prevents both heaters from being energized simultaneously.

Jim

[iZappa]

A few comments on the Nuova Simonelli machine:
The grouphead is a saturated type design - it is in direct contact with the boiler and hot water flows through it. This makes it an ideal candidate for a dual boiler setup, but I was a little surprised to see it on an HX machine. This tells me that either this machine would have needed a tonne of flushing to bring it down to...

Which NS modell are we talking about here?

[bsafnuk]

Which NS modell are we talking about here?

It is the Nuova Simonelli ISX. It was last manufactured in the early 80's, though the internal components of modern machines are essentially identical.

Here she is in all her glory:

As you develop the design, be sure and think about heater wattages and power requirements. With two boilers, you will probably have to either to power everything with a 20A circuit, or develop a control strategy that prevents both heaters from being energized simultaneously.

Actually, this part is easy since I will be running it off a 240V circuit (all of my components from the commercial machine are 240V)

-Brad

[iZappa]

Which NS modell are we talking about here?

It is the Nuova Simonelli ISX. It was last manufactured in the early 80's, though the internal components of modern machines are essentially identical.

That was a "new" model to me. The reason I asked was that none of the current NS machines have a saturated brew group directly connected to the boiler.

[bsafnuk]

That was a "new" model to me. The reason I asked was that none of the current NS machines have a saturated brew group directly connected to the boiler.

That's interesting - so they kept the group head the same (I can use all the parts from the Mac and similar models) but disconnected it from the boiler. I wonder why.

-Brad

[iZappa]

That's interesting - so they kept the group head the same (I can use all the parts from the Mac and similar models) but disconnected it from the boiler. I wonder why.

-Brad

The group head has been developed since the MAC machines. Most notably on the Aurelia which is another league.

At least on the Aurelia and the Appia, which now are their best machines, the grouphead is connected to the boiler via pipes similar to an E61 home machine. The reason for this, they told me, is that they want them to work as heat sinks since they are HX machines. Also it makes repairing much easier and inserting different gicleurs fast and smooth. The group is still saturated though, but in the same way as an E61 machine via the HX pipes. As far as my experience goes I think the enormous grouphead on the Aurelia does a very good job to keep a flat temperature profile.

[DavidMLewis]

Great project! I'd strongly recommend a feature the Techno has, which is a simple resettable mechanical thermostat mounted between the two boilers that kills the power to everything if it overheats. Also, make sure to check the electrical potential difference between materials you're putting in electrical contact so you don't wind up with galvanic corrosion issues. And given the one-off welding on the boilers, I'd probably insist on having them tested at double the operating pressure or more. Good luck and keep us posted.

Best,
David