WELCOME
Mirage technical manual
page 6 of 65
Abridged machine performance
The Mirage functions according to the so-called single boiler system with heat exchanger and thermo-
siphon system. The water in the hot water/steam boiler is heated to approximately 125 °C to achieve
lasting steam power. For each group, a wide tube (the heat exchanger) runs through the hot water
boiler in which the water for brewing espresso is heated up without direct contact with the water inside
the boiler.
Within this heat exchanger the coffee brewing water is also heated up to approximately 125 °C, which
is, of course, too hot for brewing espresso. A wide and long tube running outside the boiler connects
the top of the heat exchanger to the group head and then from the group head back to the bottom of
the boiler. The group head is a modified E-61 which is made from brass, a metal with high heat
conductivity. This high heat conductivity means the group head easily takes up heat from the (warmer)
water and sheds heat to the (colder) environment, thus cooling down the water within. Since cold
water has a higher density than warm water, it wil
l “sink” through the wide tube towards the bottom of
the heat exchanger. Warm water from the heat exchanger will replace the cooled water and a circular
flow is established. This is the so-called thermo siphon system.
At idle
At idle (no shot being pulled) the temperature in the group head is mainly affected by 3 parameters:
the flow rate of the water, the temperature of the water coming into the group head and the heat loss
to the environment (see Figure W 2). Wide conduits will lead to high flow rate and thus high
temperature of the group head, narrow conduits will lead to low flow rate and thus low temperature of
the group head. Flow rate in the
wide conduits of the Mirage’s thermo-siphon system is controlled by a
flow restrictor (or jet). The restrictor is a fitting with a small opening which is either fixed or (optional)
adjustable. The conditions of the environment (the location of your Mirage) may change slightly due to
room temperature, draught and air humidity. As the temperature difference between the group head
and the environment is large (about 70 °C), a difference in room temperature hardly influences the
temperature of the group head although a high humidity or large air flow past the group head will lead
to a larger heat flow from the group head and thus lower temperature of the group head.
The flow restrictor
installed in your Mirage is optimized for “normal” conditions, e.g. boiler temperature
of 125 °C, a room temperature of 18-22 °C, air humidity of 70-90% and little draught, brew water
temperature of 91-93 °C.
The first espresso
When a shot is being pulled, water is drawn from the thermo siphon system just before it enters the
group head. The water just coming from the heat exchanger is too hot but, since the flow restrictor is
located at the top of the heat exchanger and conduits are otherwise wide, flow from the lower conduit
is larger than from the upper conduit and nearly all water drawn from the system comes from within
the group head which has the right temperature. After passing through the group valve, which is
initially a little colder, the brewing water is led once more through a chamber in the group head with
large area surface such that it will adopt the group head temperature before reaching the coffee bed.
A second espresso
There is only little water contained within the group head (about 30 ml) and water from the large tubing
running from the group head down to the heat exchanger is a little bit too cold for brewing coffee. This
lower tubing contains about 30 ml as well. However, the large mass of the group head (approximately
4 kg) is capable of storing a lot of heat and since the conductivity and surface area are high, the cooler
water from the lower tubing which flows through the group head first will rapidly heat up to the right
temperature. The group valve is already heated up by the first espresso and water flows through the
group head a second time to establish the proper temperature before it reaches the coffee bed.
