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Why not a PID-controlled valve on an HX thermosyphon to regulate grouphead temp?
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eero
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Posted Mon May 27, 2013, 4:13pm
Subject: Why not a PID-controlled valve on an HX thermosyphon to regulate grouphead temp?
 

I've read quite a few posts about the merits (or lack) of PID-ing the boiler on an HX machine.   But since the brew temperature is primarily determined by the grouphead temperature, and this is regulated by the passive flow of hot water through a thermosyphon, why not instead aim to directly control this water flow?   A PID could take input from a grouphead thermocouple, and use it to adjust a variable-flow valve on the thermosyphon, so as to attain the desired brew temperature.  

This seems like it would offer the temperature stability of a PID-ed double boiler machine, but with only slightly higher cost and complexity than a standard HX machine.

Am I missing something that would prevent this from working?

Thanks,
Eero.
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CoffeeRon
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Posted Mon May 27, 2013, 6:13pm
Subject: Why not a PID-controlled valve on an HX thermosyphon to regulate grouphead temp?
 

That's actually an interesting idea. Unfortunately the temperature of the grouphead is only a smaller part of the brew temp. The bigger part is the flow of water through the HX that is the actual brew water. Regulating the flow/temperature of that is an interesting idea, I'm not so sure it would be do-able though even with a complex system. Who knows though, maybe no one's ever given it a try?
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dana_leighton
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Posted Mon May 27, 2013, 6:34pm
Subject: Re: Why not a PID-controlled valve on an HX thermosyphon to regulate grouphead t
 

This is sortof similar to the idea Bill Crossland had for profiling brew water temperatures using the mixture of hot and cold water to achieve a specific temperature... see this post

 
Dana Leighton - Espresso hack and CoffeeGeek moderator
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andys
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Posted Tue May 28, 2013, 6:09pm
Subject: Why not a PID-controlled valve on an HX thermosyphon to regulate grouphead temp?
 

eero Said:

A PID could take input from a grouphead thermocouple, and use it to adjust a variable-flow valve on the thermosyphon, so as to attain the desired brew temperature.  

This seems like it would offer the temperature stability of a PID-ed double boiler machine, but with only slightly higher cost and complexity than a standard HX machine.

Am I missing something that would prevent this from working?

Posted May 27, 2013 link

A "affordable" variable flow valve that can operate at 230F and above, for years, with minimal maintenance, probably doesn't exist. Or do you have such a valve in your possession?

This is a geek-type project that no manufacturer is likely to touch, both from the expense point of view and for the warranty issues -- which means I've been contemplating doing it on my Londinium....  :-)

I haven't spent much time researching the valve choices, though. If you have done so, please share.

 
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eero
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Posted Tue May 28, 2013, 8:08pm
Subject: Re: Why not a PID-controlled valve on an HX thermosyphon to regulate group temp?
 

andys Said:

I haven't spent much time researching the valve choices, though. If you have done so, please share.

Posted May 28, 2013 link

I do have an engineering background, but I don't know if I'm qualified to spec the parts....

In the interrim, however, I found this old post from Home-Barista, in which another_jim proposes using a solenoid valve to divert thermosyphon flow:  idea-for-simple-temperature-control-in-e61-machines

In fact, you (AndyS) responded to the original post :)   And a few posts later, EricS responds that getting  the group temperature right would completely eliminate the need for a cooling flush, which he backs up with data from a Vibiemme DS.  And Godshot suggests a potential solenoid valve to do the job...

As far as I can tell, there's been no followup on this, so should I assume that it doesn't work, or that nobody's gotten around to trying it?
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andys
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Posted Tue May 28, 2013, 8:36pm
Subject: Re: Why not a PID-controlled valve on an HX thermosyphon to regulate group temp?
 

eero Said:

As far as I can tell, there's been no followup on this, so should I assume that it doesn't work, or that nobody's gotten around to trying it?

Posted May 28, 2013 link

Since there is typically flow towards the group through both thermosiphon pipes when one pulls a shot, adding a solenoid valve that may or may not be open at the time might complicate things. But it may work fine.

I don't recall reading an online report from someone who had done it.

 
-AndyS
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Freddo58
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Posted Wed May 29, 2013, 3:00am
Subject: Re: Why not a PID-controlled valve on an HX thermosyphon to regulate group temp?
 

andys Said:

Since there is typically flow towards the group through both thermosiphon pipes when one pulls a shot, adding a solenoid valve that may or may not be open at the time might complicate things. But it may work fine.

Posted May 28, 2013 link

^^^
This.  Also the temperature differential between group and HX coil will be at it's lowest when flushing/pulling a shot, a variable flow-restrictor would shut off flow, recovery of boiler temp/pressure is fast, and over-heating of the water in the HX brew coil may be too rapid.  So perhaps you'd end up with an HX machine which is still "quirky", but less manageable than it was beforehand.
OTOH, during the warm-up cycle, at least on my machine, more flow through the thermosyphon may speed up the warming up process.  Within a few minutes of switching on, boiler pressure (and hence boiler temp) is up, and it settles in to normal cycle of about 22 seconds from bottom of pressurestat dead zone to the top, then about 1 minute 20 seconds to fall back to the bottom of the dead zone (boiler has insulation).  Perhaps a PID (or even simple thermostat) to actuate a solenoid high volume bypass of the thermosyphon flow restrictor could mean considerably faster warm up time.
What may be worthwhile is to not use the pressurestat to directly switch power to the element, but to switch only a low voltage/current to control a SSR which would switch element power.  That way (if a suitable low dead-band pressurestat can be obtained), wear on the pressurestat contacts can be eliminated from the much more rapid switching, and the typical temperature variation from top to bottom of pressurestat dead band (usually 0.15 - 0.2 bar = 2-3 deg C) can be all but eliminated, but the advantage of pressurestat over PID for HX boiler temperature control can be retained.  It should also be simple to retrofit.  But where can a very low dead-band (but accurate and reliable) pressurestat be obtained?
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calblacksmith
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Posted Wed May 29, 2013, 7:31am
Subject: Re: Why not a PID-controlled valve on an HX thermosyphon to regulate group temp?
 

That thread was from '05, I would tend to think that if the idea panned out, we would have seen something by now.
On another thought, the HX is independent of the GH temp so by regulating the GH temp, you will not reduce the over temp water in the HX system, which is the need for the flush in the first place. If the GH was cold enough to offset the over temp water in the HX then when you went to pull a second shot, the water would be too cold.

I really don't know why some people get their panties in a knot over the flush, really it isn't a big deal, EVERY machine needs a flush if only to remove spent grounds from the GH so what is the big deal? Some machines need the flush to warm the GH so flushing is with us any way you go as far as I can see.

Just flush the water into your cup to pre warm it and all is well with the world, at least my world anyway.

AS ALWAYS, YMMV!

 
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Wayne P.
Anything I post is personal opinion and is only worth as much as anyone else's personal opinion. YMMV!

Feed the newbs, starve the trolls and above all enjoy what you drink!
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DavecUK
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Posted Wed May 29, 2013, 9:57am
Subject: 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!
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Iluvdabean
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Posted Wed May 29, 2013, 10:03am
Subject: Re: Why not a PID-controlled valve on an HX thermosyphon to regulate group temp?
 

calblacksmith Said:

That thread was from '05, I would tend to think that if the idea panned out, we would have seen something by now.
On another thought, the HX is independent of the GH temp so by regulating the GH temp, you will not reduce the over temp water in the HX system, which is the need for the flush in the first place. If the GH was cold enough to offset the over temp water in the HX then when you went to pull a second shot, the water would be too cold.

I really don't know why some people get their panties in a knot over the flush, really it isn't a big deal, EVERY machine needs a flush if only to remove spent grounds from the GH so what is the big deal? Some machines need the flush to warm the GH so flushing is with us any way you go as far as I can see.

Just flush the water into your cup to pre warm it and all is well with the world, at least my world anyway.

AS ALWAYS, YMMV!

Posted May 29, 2013 link

Man I agree with this statement " I really don't know why some people get their panties in a knot over the flush, really it isn't a big deal" I think its just part of the process and enjoy it. I have a bowl nearby and use it for that puropse.
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