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The flush on a heat exchanger

October 28, 2011

Controlling temperature on an HX is the subject of much fear.   How do you control temperature on a heat exchanger machine?  You cannot just set the temperature with a PID like you can on a double boiler, or even on a single boiler.  It requires doing a flush, or “water dance.”  It must be hard, right?  Wrong.  It is not hard at all.  It does take a few tries to get used to it and get in the rhythm.  It probably even took me a few weeks to totally master it, but after that it is truly second nature.


The key fact is that if you can count you can control the temperature on a heat exchanger!  As we said before the design of the HX machine is that there is a steam boiler and the water for coffee is taken inside a tube through the boiler on its way to the grouphead, borrowing its heat from the steam boiler.  The initial water is too hot and comes out boiling.  It then cools the longer you flush it. You can watch the water dance in this video created by Dan Kehn and see the way the water sputters and spurts until the 24 second mark and then settles down

http://www.home-barista.com/tips/cant-tell-when-hx-water-dance-ends-t3754.html


Even easier though is to flush through a bottomless portafilter or with the portafilter out and you will not even have to look.  The water hisses until the flush is over and then it settles down.  


In either event once the hissing is over you are ready to start counting.  On my machine if I count six seconds and cut the machine off, put the coffee/portafilter into the machine and start it up again six seconds later I am right in the middle of the range for espresso that is desirable.  If I want to raise the temperature a degree I wait one extra second before starting the machine after putting in the portafilter.  If I want to lower the temperature one degree I flush for one extra second.  (Obviously and analogously if I want to lower it 5 degrees I just do so for 5 seconds).  


Now it may not be exactly one degree per second, but that does not matter.  I do not need to know the temperature down to the last degree – all I really want to know is that if I find a temperature I like I can reproduce it nearly exactly whatever temperature that is and that if I decide I want to lower the temperature form shot one to shot two, I can do that, too.  The flush certainly suffices for that.  


Once I have a favorite temperature for a shot I write down the seconds from the flush (for example the shot I am drinking now is 8s,6s) and I know I have a reproducible temperature profile.




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