If it is not a Single boiler or a double boiler, what exactly is a heat exchanger?

Okay so an HX machine is not a dual boiler and it is not a single boiler.  What is it a triple boiler?  A zero boiler?  Some fractional boiler?  No it is in some sense just a very smart single boiler.  Here is the clever idea.  You have a large boiler for steam.  You have a reservoir of water that is separate from the boiler.  It has a tube (a heat exchanger if you will) that coils through the center of the boiler, but never opens into the boiler.  The boiler is kept super charged at above boiling temperatures to function ideally for steam (yes, water will go over the temperature of boiling if it is under pressure).  The water in the tube is affected by the boiler and comes up close to its temperature.  If you flush the water out it starts out too hot, but then as the new water comes through it ends up cooler than the boiler and hotter than the reservoir, by flushing for just the right time you can attain whatever temperature you desire.

The thing that intimidates people is the flush.  You have to understand how to do it in order to attain the temperature you want and you cannot just set the temperature on a digital readout.  

This means that most people (including me) when they start looking for the ultimate espresso machine start out thinking that it will be a double boiler and do not consider an HX as seriously.  I think this is unfortunate, and once you get to play with one and compare the espresso on great HX machines with that on great DB machines, I think it turns out that it is not at all clear which one is better.  

The flush is not very hard to master (and the next post will probably be about just that) and once you have mastered it you hardly even recognize you are doing it.

As a postscript, I will mention again that I have not used a fourth class of machines – the thermoblock machines like the CC1 and the Silvano that are the new guys on the scene and which I hope may be great machines in their own rights (but I long ago learned that I have to try a machine myself before jumping to conclusions positive or negative, regardless of price, rumor, etc).

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