What's a heat exchanger (HX) & how's it work?

[Tex]

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.

Cooling flushes

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.

[Ascholten]

Also, as you open the spigot, that water that is at 250 degrees or whatever, will instantly flash to steam.  One thing to keep in mind, one can get severely burned if they play around and are not careful.  I wonder how much water hammer is in the spigot 

Aaron

[Tex]

Also, as you open the spigot, that water that is at 250 degrees or whatever, will instantly flash to steam.  One thing to keep in mind, one can get severely burned if they play around and are not careful.  I wonder how much water hammer is in the spigot 

Aaron

That's what David's video so vividly demonstrates; and I believe he still has the scar on his forehead to prove just how hot these machines can get. (Caution: If you're bending over to get a good look at the bottomless portafilter watch out for the grouphead sticking out - it's very hot!)