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Old 07-15-2009, 04:45 AM   #1
Pintodave
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Default Water agitation question??

I hope this is the right forum - not a "chemical" question, but definitely scientific in nature...

I see a lot of people adding motorized devices to agitate the water in some of their brew vessels, which sounds like a really good idea. But then I started thinking about it and reading about the heat transfer properties of water and thermal equilibrium and all that stuff.

From what I know and can understand about it, as you heat a container of water, the heat will immediately start to flow to the cooler areas. This means even though you may not notice it in the kettle, the water should already be "mixing and flowing" and it will keep doing that as it seeks equilibrium. If you watch a pot of water on the stove, it is more noticable. This should also be further enhanced by placement of the heating element as close to the bottom as possible to the tank, as the heat source is at the lowest possible point.

If this is true (I am no thermal engineer, but these are pretty standard theories), then why bother adding a device to mix the water? Is it to further speed up the mixing? Does it need additional speeding up?

If you are recirculating through a heat exchanger with a HERMS type set up, you shouldnt have to agitate the heat exchanger water either because not only will the water have this natural "mixing" action, once the mash temp comes up, you will also have a coil of hot liquid circulating through the heat exchanger.

In the HLT, if the thermo probe is located close to the point of exit, that should be the area of critical temp you are worried about. Same with the recirc in the HERMS, as long as the temp probe is measuring the temp at the point where the mash water is re-entering the tun, that is the critical location (as I have read and what makes sense).

Again, I am speaking "in theory" - never done an all grain batch nor have I brewed on a rig with digital temp controls (thus why I am building one) so I guess I am looking for feedback from both ends of the spectrum. If you do use a device to mix anything in your brewing, why? If you don't, why not?

Thanks

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Old 07-15-2009, 01:24 PM   #2
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Mixing reduces hot spots.
Recirculating = mixing
Had a pump hooked up to my mash tun, now I don't bother.

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Old 07-15-2009, 05:13 PM   #3
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This is a very good question. I was recently theorizing about this myself, as it directly concerns the setup of my HERMS.

As you indicated, as long as your temperature controller is monitoring the temperature of the exiting wort, and is adding or not adding heat based upon that temperature, then why should hot-spots within the heat exchanger (HEX) matter? Who cares if the water in the heat exchanger is at varying temperatures as long as the wort in the mash tun is being maintained at a constant temperature?

The only good explanation for stirring, that I can think of, is that it could increase the RESPONSIVITY of your system to changes in the wort temperature. So, for example, suppose that your temperature controller is set to turn your heating device on whenever it senses a 1 degree variance of the exiting wort temperature from your setpoint. Your setpoint is 153. Suppose your heating device is currently off and the temperature of the exiting wort drops to 152. Your temperature controller senses this variance and turns on. But what is the temperature of the water in the HEX at this point? It must be below 153, or else the wort existing from it would not have dropped to 152 (assuming that heat losses from the mash tun are slower than heat losses from the HEX). So, at this point, the wort in your mash tun is at most 152, if not lower. Your goal is now to get the temperature of the wort back to 153. Your heating device adds heat to your HEX, but it takes LONGER for this heat to be transferred to the wort circulating through your coil, because the wort in the coil is exchanging heat with the water directly around it (rather than the hotter water that is in closer contact with your heat source). Therefore, a stirring device helps speed up the transfer of heat from your heating device to the wort flowing through your HEX coil. The end result is that your mash temps become more stable, because there is less of a delay between the time that your temperature controller senses the variance and the time that the heat from your heating device heats the wort back to its setpoint.

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Old 07-16-2009, 01:56 AM   #4
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Quote:
Originally Posted by david_42 View Post
Mixing reduces hot spots.
Recirculating = mixing
Had a pump hooked up to my mash tun, now I don't bother.
But that is my question - what is a "hot spot"? 1/4 deg, 1/2 deg, 1 deg of variance? If, when you start heating the water, it begins flowing to cooler areas due to the above stated principles, it is moving - in a 10 gallon vessel, how much of a temperature variance will you/could you have with an elec. element installed close to the bottom and with a temp controller?
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Old 07-16-2009, 01:04 PM   #5
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The more I think about this, I am inclined to agree that mixing is not necessary if two preconditions are met:

1. You are using a dedicated HEX with a small volume of water (i.e. not using your HLT, which usually holds 10 - 15 gallons, as a HEX).

2. You are not trying to perform step mashes with your HEX (which is a bad idea with a HERMS anyway).

The larger volume of water you are dealing with, the greater potential for temperature variability throughout the HEX. Also, trying to significantly increase the temperature of the wort through a heat exchanger greatly increases the demands on heat transfer, and would be helped by stirring.

But if you are just trying to maintain a temperature, and your HEX contains a small amount of water -- i.e. 5 - 8 quarts -- then it seems that there would be very little possibility for significant temperature variation to occur within the HEX.

If your temperature controller adds heat whenever the exiting wort drops a degree, then the difference between your wort temp and the heat exchanger temp should only ever be one or two degrees. If there is only a 1 degree difference between the average HEX water temperature and the wort temperature, then the cold spots that could develop around the HEX coil should be less than 1 degree, and this should not really make a difference.

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Old 07-17-2009, 03:16 AM   #6
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That is exactly what I was thinking about - the use of a small heat exchanger, like a cooler. This is the route I am going with my brew rig.

The BK is a non issue because you are boiling, the MT is recirculated, and the HLT is usually heated in advance whether its the strike water or sparge water, so it should have plenty of time to reach some sort of equilibrium. If you have a guilty conscious about it, give it a quick stir or two with a sanitized spoon, but unless someone has tested it and can show otherwise, it may not be necessary to go through the trouble to have a mixer.

When my rig is done I will probably do some testing with this for the educational value.....

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Old 08-22-2009, 08:58 AM   #7
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I know that commercially the mid-sized brewhouses don't bother. They have they're heat sources set up asymmetrically to naturally create greater agitation, and leave it at that. The only reason I can think of to go to the extra effort of adding a mixer is if your heat source can't take your wort up to a nice rolling boil. In that case, the agitation should compensate for the slightly lower temps that would result in poor hot break and hop utilisation.

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