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djmcmath
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Posted Fri Feb 11, 2005, 12:31pm
Subject: HX/E-61 Temperature Control
 

I know this is a topic that's been discussed, but my search-fu is weak.  All I can find are threads advocating the water dance -- not something I like.  The big weakness in the water dance is (well, ok, there are several) the excessive water use, the approximation of temperatures, and the general inaccuracy.  Even after gathering a lot of experimental data, and even if I get all the details right, I can't guarantee a shot temperature inside half a degree.  For many, I'm sure this would be acceptable, but I tire of doing a complex procedure to produce mediocre results.

So I'm looking at some alternatives.  Here are my thoughts -- please let me know what you think.  There are two flows that go to the grouphead:
1 - E-61 thermo-siphon flow.  The obvious control method would be to take temperature off the grouphead and cycle a cooling fan.  That's obviously silly, however, because the flow is controlled by natural circulation: more cooling means more flow, which just means I'd draw more heat from the boiler (to within the limits of the flow capabilities of the siphon, presumably) without actually making the steady-state grouphead temperature any more controlled.  Therefore, The Right Answer seems to be a flow control valve.  If the grouphead is too cold, the valve goes open.  Grouphead too hot, the valve goes shut.  Ideally, it would be a mixing-type valve that would actually control the temperature of water going to the grouphead, rather than just controlling flow.  But that's hard -- it seems like a simple open/shut valve would keep the grouphead within a couple of degrees of the target temperature.

2 - Brew water flow.  Comes out of the boiler at about 120C, needs to drop about 25C before it gets to the grouphead.  This is only true for the first shot, really.  Subsequent shots, if timed correctly, will require only a degree or two of cooling to be correct.  The Best Answer seems like a cooling heat exchanger, but that would be complex and expensive.  The next step down would be to use an air-medium rather than a gas-medium cooling exchanger -- wind the boiler-to-grouphead pipe into a circle, then set up a pair of computer-case cooling fans.  Wire them into a controller that takes inputs from somewhere near the grouphead.  


Ok, I can already see that this plan is full of bad ideas.  But I think it's a start.  Brainstorm?  Throw stuff at me?  Point me to the place where somebody else already invented this wheel?  Forgive me if it seems like overkill -- I'm a nuclear engineer in my other life.  :)  Thanks for the help,

Dan




PS -- in case anyone's curious, the machine in this particular case is an Isomac Relax.  The conceptualization of The Right Way to control HX/E-61 temperature should be nearly universal.
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Rick
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Rick
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Posted Fri Feb 11, 2005, 2:54pm
Subject: Re: HX/E-61 Temperature Control
 

djmcmath Said:

I know this is a topic that's been discussed, but my search-fu is weak.  All I can find are threads advocating the water dance -- not something I like.  The big weakness in the water dance is (well, ok, there are several) the excessive water use, the approximation of temperatures, and the general inaccuracy.  Even after gathering a lot of experimental data, and even if I get all the details right, I can't guarantee a shot temperature inside half a degree.  For many, I'm sure this would be acceptable, but I tire of doing a complex procedure to produce mediocre results.

So I'm looking at some alternatives.  Here are my thoughts -- please let me know what you think.  There are two flows that go to the grouphead:
1 - E-61 thermo-siphon flow.  The obvious control method would be to take temperature off the grouphead and cycle a cooling fan.  That's obviously silly, however, because the flow is controlled by natural circulation: more cooling means more flow, which just means I'd draw more heat from the boiler (to within the limits of the flow capabilities of the siphon, presumably) without actually making the steady-state grouphead temperature any more controlled.  Therefore, The Right Answer seems to be a flow control valve.  If the grouphead is too cold, the valve goes open.  Grouphead too hot, the valve goes shut.  Ideally, it would be a mixing-type valve that would actually control the temperature of water going to the grouphead, rather than just controlling flow.  But that's hard -- it seems like a simple open/shut valve would keep the grouphead within a couple of degrees of the target temperature.

2 - Brew water flow.  Comes out of the boiler at about 120C, needs to drop about 25C before it gets to the grouphead.  This is only true for the first shot, really.  Subsequent shots, if timed correctly, will require only a degree or two of cooling to be correct.  The Best Answer seems like a cooling heat exchanger, but that would be complex and expensive.  The next step down would be to use an air-medium rather than a gas-medium cooling exchanger -- wind the boiler-to-grouphead pipe into a circle, then set up a pair of computer-case cooling fans.  Wire them into a controller that takes inputs from somewhere near the grouphead.  


Ok, I can already see that this plan is full of bad ideas.  But I think it's a start.  Brainstorm?  Throw stuff at me?  Point me to the place where somebody else already invented this wheel?  Forgive me if it seems like overkill -- I'm a nuclear engineer in my other life.  :)

Posted February 11, 2005 link

The problem with temperature control on the HX machines is that for home-use machines, most shots require you to maintain boiler temperatures high enough for successful steaming, while cooling off an overheated HX and heating up a cool brewhead.  Of course, the last two change substantially on the second and subsequent consecutive shots.  

Since all the heat comes from the boiler, you are presented with a complex thermodynamic system with varying amounts of lag in it.  Temperature control in such a system will be, er, "problematic".  

It sounds like the kind of fun people have building "Rube Goldberg" machines.

http://www.rube-goldberg.com/html/bio.htm

I love this quote:

"Rube's drawings depict absurdly-connected machines functioning in extremely complex and roundabout ways to produce a simple end result; because of this RUBE GOLDBERG has become associated with any convoluted system of achieving a basic task."

It sounds like you need a two-boiler machine. However, I look forward to reading the responses.  This could be fun!  :-)

Rick
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gscace
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Posted Sat Feb 12, 2005, 7:07am
Subject: Re: HX/E-61 Temperature Control
 

Rick:

That's pretty much my response as well.  If you wanna get half degree stability, you're gonna have to start from a different system than an e-61 unless you pretty much don't wanna steam at all or are willing to cycle between modes.  And a hx machine turned down that cold may prove to have an undersized hx for that temperature when used in continuous duty.  Plus the plumbing for the thermosyphon loop will be incorrectly sized.  Best to start with a more appropriate platform.  

-Greg
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djmcmath
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Posted Sat Feb 12, 2005, 1:09pm
Subject: Re: HX/E-61 Temperature Control
 

This is very unfortunate.  I just bought this machine in December, so I'm not exactly liking the idea of retiring it so soon.  It just seems like it ought to be simpler than all that -- the temperature is _close_, but not quite right, and it seems like the equipment to correct the closeness to precision shouldn't be that complicated.  I mean, I won't disagree with you Rick -- you've definitely laid out the thermodynamic elements of the problem correctly.  It just seems like the actual engineering shouldn't be that complicated.

Maybe I'll just start experimenting ...

Dan
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Rick
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Rick
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Posted Sat Feb 12, 2005, 5:03pm
Subject: Re: HX/E-61 Temperature Control
 

djmcmath Said:

This is very unfortunate.  I just bought this machine in December, so I'm not exactly liking the idea of retiring it so soon.  It just seems like it ought to be simpler than all that -- the temperature is _close_, but not quite right, and it seems like the equipment to correct the closeness to precision shouldn't be that complicated.  I mean, I won't disagree with you Rick -- you've definitely laid out the thermodynamic elements of the problem correctly.  It just seems like the actual engineering shouldn't be that complicated.

Maybe I'll just start experimenting

Posted February 12, 2005 link

I don't envy you having a HX machine but wanting more precise temperature control.  I think the HX is well suited to someone like me, who drinks mostly cappas, isn't interested (or, frankly, capable of differentiating) in the subtle differences a degree or two can make.  I'm more a "close counts in horseshoes" kind of guy, and for me and my ilk, the HX is just great.  

Nevertheless, I think it is worth experimenting if you have the yen.  I initially told champignon that his idea of PIDing a Cimbali Junior was a waste of time.  Now I would like to do the same to my Wega to take the pressurestat out of the equation.  I don't think it will make a big difference in the cup, but I hate "overpressure events" like the one I nearly had a few weeks ago when my pressurestat stuck.  So, who knows what benefits can come from experimenting?  Clearly not I!

Your idea of using active control of some element NOT attached to the boiler is right, I think.  I doubt anyone can model the system so that they control HX temperature by controlling the heating element.  There's just too much lag in the system for that.  

Of all the possibilities I can think of, your idea for water mixing seems the most promising to me.  How about a water-mixer that sits on the line from the HX to the grouphead?  It could mix the too-hot water from the HX with cool water from the inlet water line?  The mixing valve could be controlled through a device that instruments the temperature of the HX water and the grouphead, and uses an algorithm that models the thermodynamic behaviour of the two as they affect brew water temperature?  I guess it would be like a manifold with two parallel inlets -- one from the HX, the other on a line from the pump that bypasses the boiler.  ??

Rick
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djmcmath
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Posted Sat Feb 12, 2005, 5:27pm
Subject: Re: HX/E-61 Temperature Control
 

Rick, I like your thinking; thanks for the help.

Some thoughts in response -- the pressurestat is really "close enough" for the boiler.  Mine cycles about .1 bar, which works out to about 1.5C.  Given the other inaccuracies in the system, the pressurestat is definitely not the weak point.  In any case, it's far tighter than the silly thermoswitches that come stock on so many other machines.  :)

Your mixing valve is an excellent idea.  If I could find one, the implementation would be relatively straightforward, really.  Output from the pump would branch to the boiler and to the mixing valve.  A thermocouple would take readings just downstream of the mixing valve.  If too cold, close off the cold side some.  If too hot, close off the hot side some.  Too easy.

Only problem is if the boiler water comes out too cold -- nothing there to heat it.  I suppose I could just keep the boiler so hot that shot recycling times are easily in the 5-10 second range -- far faster than I can reload.  

The other problem is that it really doesn't control grouphead temperature at all.  I suppose a quick flush before the first shot would get GH temps at least close.  Subsequent shots would all get a similar temperature bump from the GH, which I'd factor in to the target temps, I suppose.   ????

Any idea where to actually _get_ a valve like that?  PIDs, thermocouples, and SSRs are easy.  3-way mixture control valves?  Hmmm....   I suppose I could use two separate open-shut valves, rather than a single 3-way valve.  Servo controlled valves shouldn't be that hard to find.  Controlling a pair of valves might require some interesting software work, though ...

Alright, thinking down a new and better line!  Thanks again.  :)

Dan
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JeremyR
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Posted Sun Feb 13, 2005, 10:15am
Subject: Re: HX/E-61 Temperature Control
 

I doubt you'd need two valves, one controlling the cold water should do it. The problem is finding one small enough, it's easy enough getting a 22mm one, but a 6-12mm one? good luck. You could of course add a r/c servomotor or a small stepper to a ball valve. You could also take the temp of the grouphead and put one of these in the thermosiphon to control the grouphead temp. I did a lot of research into this and eventually concluded that to get the accuracy I wanted, another boiler or two was the way to go. All solid state, no extra moving parts PIDs on all boilers, way to go.
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djmcmath
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Posted Sun Feb 13, 2005, 12:00pm
Subject: Re: HX/E-61 Temperature Control
 

Yeah, I realized late last night while I was thinking through a control algorithm that a second valve would be utterly redundant.  Good catch, cubastreet.  The other problem you raise is also one I've been concerned about -- how long will a valve last?  I hate moving parts in machinery that I'm fond of ...

Did you end up adding boilers to an existing HX machine, or building a dual boiler machine?  I'm hesitant to plumb in another boiler... been down that path, and it's a steep and slippery slope.

Dan
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JeremyR
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JeremyR
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Posted Sun Feb 13, 2005, 4:20pm
Subject: Re: HX/E-61 Temperature Control
 

Not done yet. I have a 2 group HEX machine I'm going to add brew boilers to for work, and a Silvia I'm going to add a steam boiler for at home. In the HEX machine I've added a modified central heating thermostat to the thermosyphon which seems to work OK, but it still needed a good mixer for the brew water. I decided that in the long term a brew boiler would be more reliable and probably more accurate.

j
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kaanage
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Posted Sun Feb 13, 2005, 5:25pm
Subject: Re: HX/E-61 Temperature Control
 

djmcmath Said:

Your mixing valve is an excellent idea.  If I could find one, the implementation would be relatively straightforward, really.  Output from the pump would branch to the boiler and to the mixing valve.  A thermocouple would take readings just downstream of the mixing valve.  If too cold, close off the cold side some.  If too hot, close off the hot side some.  Too easy.

Only problem is if the boiler water comes out too cold -- nothing there to heat it.  I suppose I could just keep the boiler so hot that shot recycling times are easily in the 5-10 second range -- far faster than I can reload.  

Posted February 12, 2005 link

If you're going to do mixing to control temp, then why not eliminate the HX and mix water from the boiler with cold water? In between shots, a thermostat could control the syphon flow to prevent the grouphead from overheating.

Greg

 
do'in it on the cheap
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