Paul remarked that the above suggests that preinfusion can be amply explained as "the hot water ... getting in touch with the ground coffee." I like the last line about "exploding dregs" too.As already written, the delivery group is kept at the right temperature for the delivery of the coffee by means of a thermosiphon circulation. Inside of the group there are three valves:
1) Delivery (72); closed in the rest position.
2) Infusion (73); open in the rest position.
3) Drain (74); open in the rest position.
The group is controlled by means of a little lever, which must be brought from the rest position to the working one (in the semiautomatic machine) or by means of a push button (3) in the case of the automatic one. In this moment, the delivery valve (72) will be closed and the infusion (73) as well as the drain (74) valves will be closed.
During the first 5-6", if the grinding is correct, no coffee is getting out from the spouts, in as much as the hot water is getting in touch with the ground coffee in the filter, but can't flow through because at the beginning the pressure isn't sufficient to win its resistance. The raise of the pressure on the ground coffee is delayed, because of the fact that there is a 0,7mm nozzle, which allows a flow of 10-14cm cubed/sec. of water with a pressure of 9Atm. and an infusion valve together with the infusion chamber below.
At the beginning, just after the group has been commanded, the spaces over the ground coffee are empty and this gives a delay in the pressure raise; as soon as these spaces are filled, the infusion valve opens automatically (roughly at 1,5 bar), giving thus a further delay, necessary for the filing of the infusion chamber, below the valve.
A good infusion can be obtained only with the right grinding of the coffee.
Once the infusion period has been finished (5-6"), begins the delivery of the coffee, which in the half-automatic will be finished by the manual return of the lever to the rest position and in the automatic by the automatic unhooking of the push button. At the end of the delivery, the opening of the infusion and of the drain valves, assures the release of the pressure from the filter holder, so that it can be unhooked without the peril that the dregs explode and hurt the operator.
Faema "no stop" group, circa 1988