The challenge of the thermo-syphon system is to get water of low nineties temperature to the coffee, provided by a HX that stores water at 125 degrees. The extra heat is needed to keep the group at its perfect temperature, the remaining heat is needed to deliver brewing water of the correct temperature. To start the brew process the group solenoid valve opens the path from the thermo-syphon system towards the coffee in the filter holder. At the same moment the pump starts to run, keeping the pressure at a steady 9 bar, everywhere between pump and coffee.
The sudden exit, provided by the open solenoid valve, has a drastic result. All water will now flow to that exit, pushed out by the pump pressure. The flow in the lower tube changes direction, it flows upwards to the group, instead of downwards to the bottom of the HX. The flow in the upper tube remains the same. The crucially important second function of the restrictor now becomes clear. The flow through the upper tube towards the group is severely restricted by the1.5mm orifice of the restrictor. The lower tube does not have any restriction, it has an inside diameter of 8mm. This full-flow 8mm, compared to the 1.5mm, means 99% of the water reaching the group arrives via the lower tube.
This suits the requirements very well, since this lower tube contains the cooler water (generally 88-92 degrees, compared to the upper tube 115-125 degrees). To get the most out of this, we install a very long lower tube in the Mirage, containing 30cc. The remaining water comes directly from the HX. All water leaving the HX is replaced simultaneously by fresh cold water. The hot water leaves the HX through the lower tube, from bottom of HX. The cold water enters the HX also through the bottom. The cross flow of outgoing hot and incoming cold water is used to help each other. As there is just a 1mm copper wall between these 2 flows, the outgoing hot water is cooled down considerably, losing its heat to the incoming cold which starts to heat up right away. The cold water is forced to travel to the top of the HX via a long injection tube.