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PID control of coolant temperature for dephlegmators

These are various style condensers for many different applications.

PID control of coolant temperature for dephlegmators

Postby jake_leg » Mon Mar 12, 2012 5:38 am

The first time I heard of this idea was from Joe (Rednose). It seems to me that adjusting the temperature of coolant is far easier than adjusting coolant flow. Does anyone have any experience or comments?
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Re: PID control of coolant temperature for dephlegmators

Postby vb » Tue Mar 13, 2012 8:22 pm

I was working with Swede (well Swede was working on it and I was around for moral support) on a PID system that pulsed flow on and off to maintain dephlegmator temperatures. it worked well, but had to have consistent input water temperatures.
using a water circulation system screwed things right up.
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Re: PID control of coolant temperature for dephlegmators

Postby kiwi » Tue Mar 13, 2012 8:47 pm

Wouldn't it be much easier to use the other side of the coin, power management?
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Re: PID control of coolant temperature for dephlegmators

Postby Seb » Tue Mar 13, 2012 11:18 pm

What you would need to maintain temps is a 3 way mixing valve there used on HVAC systems keeping the exchange tempered to a controlled level
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Re: PID control of coolant temperature for dephlegmators

Postby jake_leg » Wed Mar 14, 2012 2:25 am

kiwi wrote:Wouldn't it be much easier to use the other side of the coin, power management?


Seems pretty sensible. I don't have an opinion really, I'm just thinking out loud about how to get a repeatable CM run.

The fact that Joe mentioned this method (regulate the reflux coolant to a fixed flow and vary the temperature) suggests to me that it is probably used in a commercial setting. It sounds like Swede's approach of holding the temperature of the coolant constant and pulsing the flow could work equally well.
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Missunderstanding

Postby Rednose » Wed Mar 14, 2012 6:24 am

A PID is not the right way to control condenser temperature, I use them to shut off the gas power and activate an alarm if the temperature rise more than normal.

I also have one in the boiler as temp and pressure are related in an event of clogging the still head which never happened.

I have 4 inch shotgun condensers which carry few water and can overheat pretty fast in case that the pump stop working for any reason.

To regulate the dephlamator we use Danfoss valves like the one in the pic, the German stills use the same valve.

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Re: PID control of coolant temperature for dephlegmators

Postby jake_leg » Wed Mar 14, 2012 6:35 am

Thanks for clarifying. I misunderstood the purpose of the PID.
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Re: PID control of coolant temperature for dephlegmators

Postby zedzedtop » Thu Apr 05, 2012 4:14 am

I don't see why a pid wouldn't work for dephlegm output control. This is assuming you have a way to control coolant flow to the dephegm via the output of the pid. Think it was harry or pint, but there is some sort of ideal output temp of the dephlegm coolant... 82C iirc.
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Re: PID control of coolant temperature for dephlegmators

Postby jake_leg » Thu Apr 05, 2012 4:28 am

82 Fahrenheit? :)
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Re: PID control of coolant temperature for dephlegmators

Postby zedzedtop » Thu Apr 05, 2012 4:33 am

We wish!
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Re: PID control of coolant temperature for dephlegmators

Postby jake_leg » Thu Apr 05, 2012 5:04 am

Was Harry talking about a continuous still using wash as the coolant?

It seems to me that the important parameter is the amount of vapour knocked down. You can knock the same amount of vapour down by running coolant fast in a dephlegmator with a small surface area (giving a low output temperature) as you can running coolant slowly in a dephlegmator with a large surface area (giving a high output temperature).
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Re: PID control of coolant temperature for dephlegmators

Postby zedzedtop » Thu Apr 05, 2012 5:57 am

No, it wasn't in reference to a continuous distillation. I never got it figgerd out xactly. The dephlegm's job is to dump a portion of the vap back into the column, allowing a % to pass. iirc, having the constant output temp, throughout the run, somehow adjusted for the change in eth% in the boiler. With constant power input, one would get less output over time as the batch ran.... having a constant output temp of the dephelgm cooling water would mean, assuming the input water temp was constant, that the power extracted by the dephlegm is constant. I'm not sure why you'd want this... seems that you'd want it adjusted one way or the other, but there you go. Maybe i should dig up the old posts...
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Re: PID control of coolant temperature for dephlegmators

Postby jake_leg » Thu Apr 05, 2012 6:17 am

OK, if the idea is "Let's fix an output temperature for the coolant, and keep it there for the entire run using a PID" then I can see why you would want that output temperature to be near the vapour temperature of the desired %ABV product, assuming that the dephlegmator is big enough to knock down any amount of vapour.

That way, the PID will act to stabilize the %ABV of the product. If the coolant flow is too low, more product comes off, the product %ABV drops, its vapour temperature rises above the set temperature, and you get higher output coolant temperature. In response the coolant flow should increase so that more vapour is knocked down, less product comes off, and the %ABV of the product falls back down to the desired value.

But this seems more relevant for CM control of tall packed columns at equilibrium than 3-6 tray columns run hard.
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Re: PID control of coolant temperature for dephlegmators

Postby zedzedtop » Thu Apr 05, 2012 2:52 pm

It might have more relevance to plate stills than packed neutral columns. If we consider plate stills to be hybrids between pots and packed, then the temp of the uppermost vapour will tend to stay fairly constant over the duration of the run, rising towards the end, but not as abruptly as a packed column run for neutral. As the temp rises at the top, a lower percentage of vapour will be returned to the plates, assuming that the dephelgm work remains the same. It seems to me that the still is then running away from us... not what you'd want for neutral, but maybe for these types of stills. On the flipside, having constant flow through the dephlegm would mean that it will be doing more work towards the end, if the input water temp was the same, which it wouldn't be if the dephlegm and pc are run in series, as they often are.

I guess I'm just not clear on what the optimum role of the dephlegm is, in terms of how it should be adjusted (or not) as the run progresses. Of course, 'optimum' is subjective, but there must be some consensus on these things, being that most commercial artisan stills are run this way. I'll poke around to see if I can find the references to the dephlegm output coolant temp.
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Re: PID control of coolant temperature for dephlegmators

Postby zedzedtop » Thu Apr 05, 2012 4:06 pm

here:

viewtopic.php?f=61&t=4036

but I know I saw it explained better somewhere else. Has to do with not overcooling the returned reflux as much as setting the amount of returned reflux. I can see that having the temp of the returned reflux, at or very near the temp of the top plate, a good idea. But how is it that having the coolant flow adjusted for this purpose, also the optimum flow for setting the reflux ratio?
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