My shorthand for these distinctions are:
- Dragon - key characteristics are lots of flash boiling, fast recovery, nearly zero thermal memory, and slowly rising brew temperature profile. Simply stated, after the cooling flush, the heat exchanger output is the brew temperature. Examples include the Elektra Semiautomatica, Bezzera BZ07, and the Olympia Maximatic.
- Mixer - key characteristics are modest flush, medium to slow recovery, considerable thermal memory, and initial rising then falling brew profile. Unlike the Dragon, the Mixer's brew temperature isn't determined solely by the output of the heat exchanger. Other factors, such as cool water mixing via an heat exchanger injector, backflow from a thermosyphon, and the attenuating effect of a heavy grouphead temper the final brew temperature. Examples include HX E61 espresso machines like the Vibiemme Domobar Super and Quickmill Vetrano.
- Agnostic - key characteristics are small, fixed volume flush or none at all, and long thermal memory. Careful tuning of a Mixer with tweaks in the design can produce an espresso machine that is heat exchanger in name only. Examples include the Cimbali Junior and Nuova Simonelli Aurelia.
The practical benefit of recognizing the characteristics of heat exchangers is the time saved learning the correct brew temperature management scheme. For example, I recognized the Olympia Maximatic as a Dragon by flushing the group until the water stopped flash boiling, waiting a minute or so, then repeating; it was fully recovered. The Elektra Semiautomatica is also a Dragon with a slightly heavier grouphead, but they share the same flush-n-go technique for targeting the brew temperature.
The video below demonstrates this point:
Hint: Don't watch the video, LISTEN to the video!
Note: The above is a modified version of my comments from Olympia Maximatic - Second Look, reposted for easier reference.