Posted Wed May 29, 2013, 10:22am Subject: Re: PID-controlled valve on HX thermosyphon to regulate grouphead temp?
You would need a variable flow type valve which would operate in small discrete increments and be of the size, temperature tolerance, price and duty cycle where it would be suitable for the task in an espresso machine. I know of no such valve commonly available. In addition the thermal lag of such a system, plus the specific heat capacity of the group, variable flows of the thermosyphon as the valve operates, would make it extremely difficult to control the system using a PID. As other people have pointed out the temperature issues are complex and regulation of group temperature really won't do it. In fact the easier way would be what some other manufacturers do on specialist machines (dual boilers) and electrically heat a proprietary group (under crude PID control) to reduce the offset temp in a dual boiler system.
It would also be a troublesome component complexity that really would not add anything but unreliability to an HX machine.
For the big manufacturers, it's hard enough to get them to do the simple things, let alone "blue sky" ideas....but then they have a lot invested and stand to loose a lot of money if things go wrong!
The electrically heated group is what is done on the (relatively cheap) Bezzera BZ07 and BZ10 HX machines. This regulated by a simple thermostat, but could be converted to PID (there are some posts on doing this to a BZ99 I think around the forum somewhere). This gives about a 6F temperature swing in the group when idling, and about a 1-2F temperature swing in the starting temperature after flushing. The group idles below brew temperature. Flushing the HX is still required, and this pulls the group temperature up to brewing temperature.
One advantage of the electrical heater is that it does start heating the group immediately at switch on, so the warm up time of the machine is probably somewhat improved over an equivalent thermosiphon machine. The other thing is that as the group idles on the cold side, pulling a shot without a flush doesn't result in a completely burnt shot, although it isn't as stable, reliable or good as a post-flush shot. Flushing the BZ07 is very simple, and doesn't use a whole lot of water, so there's very little reason not to do it. But y'know, sometimes you forget in your caffeine deprived state.
Bezzera do sell versions of the BZ07 and 10 with PID control, but on the boiler, not the group head. It's not clear to me what the point of this is, beyond selling to the "must have a PID" crowd.
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