|Thermosyphon system with E-61 group|
The E61 Thermosyphon (the process that keeps the group hot) is quite simple, but perhaps not always clearly understood. It is however, important to ensure your shot is extracted at the right temperature to make the perfect espresso. The graphical picture above shows where hot water enters/leaves the chamber at the top of the group. This hot water can be taken from high up on the boiler of a single boiler (non-HX machine) or directly from the top of the heat exchanger on a HX machine.
The water returns to either a lower point of the boiler or heat exchanger and because it has given up some heat to the group - it sets up a convection current which starts water slowly circulating from boiler/HX to group and back.
Only the portion of the group shown above the brew group area - actually has water circulating within it. The heat travels to the rest of the group by simple conduction through the brass of which the group is conducted.
I think that a general mistaken impression could be that the space within the group, where the hot water circulates, is much larger than it actually is - another point to remember is that this chamber is far from empty, because it also has the E61 "mushroom" assembly inside it.
The far point of the group (the bit you lock your portafilter into) reaches about 90 degrees when brew-able in a HX machine and will be a little cooler in a non-HX machine. Any machine whether single boiler, HX or dual boiler, should always have the group (where the portafilter locks in), at a lower temperature than the brew water.
NOTE: the temperature of the E61 group can be further managed by the use of various "thermosyphon restrictors", really little plastic washers with different sized holes in them placed in the pipes leading to and from the group.