Ever wondered what
they were referring to when discussing HX espresso machines? Here's a very simplified look at a heat exchanger equipped espresso machine boiler. Put simply, the HX is a tube through the boiler. The water in the HX is flash-heated by the super-hot water in the surrounding boiler.
In this image the fresh water pumped into the HX at the top pressurizes the contents of the HX to ~9 bar (131 psi), forcing the hot water out of the top & bottom of the HX and into the group.
The longer the water remains in the HX the hotter it'll get, until eventually it's at the same temp as the surrounding boiler (~1.2 bar or 251°F) - way too hot for brewing coffee. This is why you'll hear HX users referring to cooling flushes or water-dances. They're used to cool the HX & group to proper brewing temps (~200°F) before a shot is pulled.
remove the super-heated water from the HX, replacing it with cooler fresh water, that's then heated to the correct brewing temps in the HX.