The Aurelia’s Temperature- now i get it
Nuova Simonelli has done two things really well. They managed to make a “Temperature Stable” heat exchange espresso machine. The Aurelia is so good about hitting a consistent brew temperature that it passed the WBC technical specifications and practical testing. The other thing they have done well has been to keep their web site and marketing material elusive enough about what keeps the Aurelia temperature stable as to make it actually hard work to find out exactly how it functions. From the web site I was able to gather a handful of slightly misleading information (I do not think it was intentionally so), and anecdotal information that I gathered from industry insiders alluded to the finesse of the “heat exchange environment”; double speak.
Tommy Galagher, Counter Culture Coffee’s full time NYC equipment technician, and I spent a little time with Lou Barba, N.S. East region sales rep, and drilled him about the Aurelias plumbing. This was a very illuminating conversation that answered the questions that have been gnawing at me for quite some time.
When I had first blogged about the mysteries of temperature stability in the Aurelia, I had to speculate and extrapolated from the scant information disclosed on the N.S. web site. My misinterpretation lead me to the conclusion that there was a large reservoir of water in the group head itself that served to mix super heated HX water with cooled water at just the right ratios to get a good brewing temp each time. It turn out the is no “reservoir” in the group head. I believe that the web site wording was simply referring to the heat exchange chamber itself. But there is mixing of hot and cold water in the group head.
Here is what happens.
-Cold water from the inlet passes through a brass restrictor (Gicleur valve) before entering the heat exchange. Because it is cold, the valve will not accumulate calcium and lime deposits as readily as hot restrictors of the same material. This slows down the passage of water through the whole system, but maintains the pump pressure. This also accounts for some of the Aurelia’s soft infusion.
-Some of the cold water bypasses the heat exchange and is routed directly to the group head where it passes through another Gicleur valve.
-The rest of the cold water passes through the heat exchange, and it is super heated to a temperature above the boiling point. Set your steam pressure higher and HX water gets hotter, set the steam pressure lower and it goes down.
-The super heated HX water will always be the same temperature so long as you do not loose steam pressure and you do not flush large amounts of water through the group head. The flushing will introduce new cold water into the HX and will take a little time to super heat it again. Tip for the Barista, alternate drawing shots on one group, then the next, to keep each individual HX at it’s hottest at all times.
-Super heated water goes to the group head and passes it’s own Gicleur. The two group Gicleurs can be fitted with the different orifice sizes, ranging from .4-.6mm (if I remember correctly), to mix different amounts of hot and cold. This is how you can set your final brew temperature. N.S. recommends a short two second flush before drawing a shot. This will bring in enough brew temperature water to pre-heat the group head without cooling off the HX.
-The hot and cold water blend is injected at an angle into a small cavity within the dispersion block where it swirls in a circular motion to blend thruroughly before passing through the dispersion screen. Some blend water passes into the pre-infusion chamber. This water is released onto the puck of coffee just before the full pump presure is reached inside the basket.
My first Scace test on an Aurelia, which I wrote about last year, gave me the impression that Aurelias are just like any traditional heat exchange machine. The brew water was super heated and too hot for espresso. It cooled with continuous flushing to achieve the desiered temperature. That particular machine was assembled and shipped to the US long before the competition spec machines had been refined and tweeked. I believe that this older machine had the steam presure set high (raising the super heated temp), and did not have very restrictive Gicleurs. The larger valves would let the water pass much quicker through the system, allowing the HX water to fluctuate greatly in it’s temperature range. With lower steam presure and smaller Gicleurs, I think older machine could be set hit just as stable and acceptable of a brewing temperatures as the competition machines.
I find the same basic configuration of mixing cold and hot water at the group head in my old E-91 Faema. The technology is not that new. The finesse is in mixing very precise quanities of hot and cold, and the ability to control the steam pressure (now using PID algorythms in the new competition spec Aurelias). The Solution is simple and elegant. Sorry if made the Aurelias sound less then crafty in my first post about them.