E61 thermosyphon flow during extraction question
I've been reading everything I could find online about the typical E61 thermosyphon setup as I find on my la Scala Butterfly. I understand the actual thermosyphon cycle and I understand the application of the restrictors that are sometimes installed to control the group temperature while idling. I understand how the discharge temperature of the heat exchanger changes with changes of boiler pressure, and the purpose of the cooling flush
what I don't understand (and haven't been able to get any info on) is what happens when the pump turns on...
on my machine (and it appears typical give other schematics I've see online) there is nothing in the thermosyphon return line to prevent backflow. So when the pump comes on the water is going to be pumped through the parallel tubes of the circuit with some water going up through the boiler (the shorter route with lower flow induced pressure losses) and some will go up to the group head via the other line. This dual flow would then recombine within the first chamber of the group head before going on it's merry way into my coffee cup and then on a grand adventure.
So how do people understand this??
To me it seems that the discharge temperature (under pumping conditions) out of the heat exchanger must be significantly above desired temperature so that, when cooled by the 2nd flow, the resultant temperature would be correct (assuming the design is done well in the first place & the boiler is setup correctly). By "correct" I mean "above desired brew temperature to allow for heat loss through the group head on the way to the coffee puck".
Does this sound right?
Has anyone seen a check valve in the thermosyphon return line to prevent water being pumped up in reverse to the brew head?
And doesn't this mean that, if you pump long enough / draw enough shots, you'll eventually get cold water up the return line combining with the hot water out of the Hx?
... curious ...
A quick google of "e61 thermosyphon" images came up with this:
So perhaps there is an NRV after all. Also looks as if the cold water is introduced into the HX itself anyway.
Alternatively, is there not something in the group mechanism to prevent flow from one half of the thermosyphon loop?
Thanks for that "Mr Jack". That image is very interesting to me, but for all the wrong reasons! First thing I noticed is there's a " short circuit from the pump discharge to the tank return. As drawn that setup can't do anything other than pump from the tank straight back into the tank. Any other flow path would have a significantly higher pressure and wouldn't see any flow. The thermosyphon part of it seems right though (as set up on my actual machine) but there's still nothing stopping parallel flow through and around the boiler Hx.
When I first started looking I also thought perhaps there was something in the group head to stop this reverse flow up the "cold" line but there really doesn't seem to be.
As you have probably deduced, most HX machines include a (adjustable ?) pressure relief valve in the pump return line.
Actually, if you look at that "return" line into the tank whilst running a shot, you will see that much of the pump output does infact go straight back to the tank..... Because the pump capacity greatly exceeds the flow rate required for the shot.
Really, the pump is doing little more than pressurising the HX circuit, the thermal syphon circulation is unlikely to be interrupted.
All systems vary in detail, but most are designed to "mix" the incoming cold feed from the pump within the actual HX tube, and since , A) the HX is well above brew temperature, ..
And , B) the shot "make up" cold water volume is relatively small (30/60ml into a 1.5 L boiler )
....then the risk of a under temp situation in the HX group feed is very unlikely.
This is where good thermal design , boiler capacity, thermal recovery time, heater sizing, HX design, restrictor sizing, etc .....shows the difference between different manufacturers expertise.
Also, as a final comment, remember the fundamental advantage of the E61 group is that it's large thermal mass is actually what sets the shot temperature.... The water in the HX only needs to be close to the required temp, not exactly precise.
Last edited by blend52; 26th March 2015 at 01:33 PM.
Thanks for that description. The concept of the pump just pressurizing the thermosyphon circuit without (necessarily) interrupting it is fascinating and makes a lot of sense (now that it's pointed out to me!). Obviously that circuit will be upset somewhat by the removal at one end of the shot and injection at the other of the make up water ... Would be interesting to model that to see how it works.
Looking at the size of the pump discharge pipe vs the thermosyphon lines it is apparent the make up volume is small in relation to the volume of the thermosyphon depending on how long you run the pump I guess.