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Kettle Evaporation -

Discussion in 'All Grain Brewing' started by Thirsty Boy, 31/7/10.

 

  1. Thirsty Boy

    ICB - tight shorts and poor attitude. Fuck yeah!

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    Posted 31/7/10
    Ok, so while merrily arguing with Nick in another thread with evaporation in the title, I have also been having grand chicken cacciatore that my lovely wife made for dinner and annoying her in the kitchen while doing a couple of benchtop experiments on evaporation rates.

    Just to be clear though - so as not to make it seem like I am carving off a separate thread in which to continue arguing with Nick in a more concentrated fashion, I have not and don't really intend to cover the particular evaporation topic we have been bickering about about.

    What I am interested in is this.

    Can Evaporation Rate be changed by adjusting your boil vigour via increasing or reducing your applied heat??

    One that's always annoyed me. I am often assured that the primary factor in evaporation rate is the diameter and thus surface area of your brew kettle. Boiling harder or softer will either not or only very minimally change the amount that you boil off. I am sure that this is rubbish - its plain to me that if you boil harder, more will boil away. So here's my little test - take it with the grain of salt that I am trying to confirm my pre-conception rather than simply seeking raw data.

    Start with 2 x 1L Schott Duran beakers, obviously not identical, but pretty damn similar - filled to the 900ml line. Bring to a hard boil on separate burners of the stove.

    [​IMG]

    Once boiling, turn off heat momentarily and adjust back to the 900ml line with boiling water from the kettle

    [​IMG]

    Bring back to full boil very briefly, then adjust heat down on one burner so the water is boiling actively but "softly" - make the other beaker boil "hard"

    [​IMG]

    Photos are a hard way to judge boil intensity, so here's a video too if you are willing to waste the bandwidth.

    [​IMG]

    Boil for a decent period and compare the amount boiled off. I boiled for about 40mins and here's what came out. Soft boil on the left, hard boil on the right.

    [​IMG]

    And its pretty obvious that I got the result I was expecting. Now its not brewing, its in little beakers on the stove, things would of course be different in a real world situation. But nonetheless, the only significant difference between those two beakers was the intensity of the boil... and the difference in boil off rates was I think you'll agree, quite large.

    So I'd conclude from that experiment that while kettle diameter is one of and maybe the most important factor in determining evaporation rates, its certainly possible to at least make an impact on your kettle boil off by adjusting the amount of flame you shove into the bottom of your pot.

    Another little experiment on the way shortly

    TB

    PS - SWMBO will probably kill me if she finds out I posted something to the web that includes a picture of how dirty out stovetop currently is. We did just finish cooking dinner though, and I beat the shit out of that stove with brewing stuff :)
     
  2. Thirsty Boy

    ICB - tight shorts and poor attitude. Fuck yeah!

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    Posted 31/7/10
    I'm also interested in this one.

    Does putting a lid on or partially on your kettle change the evaporation rate??

    I discussed this with PP a couple of times. I thought it obvious that this would be true - and think that I find it to be true in my brewery. But PP is dead sure and has done a bit of testing to the effect that it either doesn't or only fractionally changes things. So I thought I'd better test it too while I was bugging the wife.

    My contention is that the lid makes the boil more intense for a given flame/heat setting, so you can apply less heat to get the same boil vigour, and the result of both the lid being on and the reduced heat flux, will be that you boil off less liquid in a given time.

    So - same as last time. Two beakers, up to the 900ml mark, up to a hard boil, adjusted to 900ml with boiling water.

    This time one got a lid - as you can see, it goes over the whole beaker, but there is a nice big spout for steam to escape from, and its pretty obvious that this lid isn't going to be holding in any significant amounts of pressure etc.

    [​IMG]

    Now comes the bit where its nowhere near as quantitative as the last test.... Bring both beakers back to a medium boil. They naturally boil at different intensities because of the lid, but I try to adjust them so that to my eye both are boiling with the same amount of vigour. I have my pre-conceptions about this test... so I ask my wife who doesn't know what its about to check, she makes me adjust the non-lid beaker down a little.

    [​IMG]

    Boil both beakers for the sameperiod, checking periodically to make sure the boil intensities don't drift. 30-40min or so. Compare amounts boiled off. Here's what I got.

    [​IMG]

    Less dramatic than the boil intensity expt.. but still a significant difference. Of course, in this case there is a large element of "judgement" which muddies the waters a chunk. And also... I boiled off 78% of the (unlidded) sample to get that large a difference. No one s boiling off that much of their wort!

    What if it was less... well too hard to measure with any accuracy at home, so lets make a wild assumption or two. Say for examples sake that the unlidded volume is 200ml and the lidded volume is 400ml. So that means a 100% difference... but over a 70% total boil off. If the boil off is linear over time/volume (and I have no idea) then that's 50% difference in a 35% boil off & 25% diff in a 17.5% boil off. So those inferences being true (they probably aren't) it'd make really, really roughly 25% difference to have a lid almost completely on your kettle if you were "normally" boiling off 17% or thereabouts of your kettle volume.

    Which should suffice to drop a 17% boil off to a 13ish% boil off.

    Now I don't for a second pretend that that is going to actually translate directly into what would happen in a full size boil, and it says virtually nothing about what might happen when a lid is covering a lesser proportion of the kettle opening. But it does point to the possibility that covering or partially covering your kettle with a lid can make a small, but noticeable difference to the amount of liquid that boils away in a given pot at similar boil vigour.

    That's enough for tonight, if I think of more and get bored enough I'll post em. Feel free to add your experiments too.

    TB
     
  3. marksfish

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    Posted 31/7/10
    great job if i had the gear i would try some wort just to see if it made any difference :icon_cheers:
     
  4. clarkey7

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    Posted 31/7/10
    That's great Thirsty. :icon_cheers:

    I brew outside and notice a huge difference in boil off volumes with windy days.

    Building barriers from bricks, kids toys, sandpit covers and BBQ setting chairs helps.. :rolleyes:

    PB
     
  5. Tyred

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    Posted 31/7/10
    Simple experiments but it shows the point quite well. This is my take on it.

    The first example simply shows what should be known by common sense, that the more vigorous boil drives of more liquid. This could be due to the surface tension of the liquid being broken more often allow more vapor to be driven off.

    The second example is a little trickier and would probably be due to a couple of factors. It make sense that a lidded boil will loose less liquid as the lid provides a point for condensation and allows for the return of some liquid to the solution. The other factor is that the lid also allows for the increase of humidity in the local area above the liquid making it slightly harder to push the liquid into gaseous form. Pocket Beers noted this when boiling outside on windy days. The wind drives off the vapor off the surface of the liquid allowing for more to be put into the surrounding area. Blocking the wind lessens this effect.
     
  6. Nick JD

    Blah Blah Blah

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    Posted 1/8/10
    How do you stop the condensed vapour (containing all the stuff you are boiling to be rid of) from refluxing?
     
  7. Thirsty Boy

    ICB - tight shorts and poor attitude. Fuck yeah!

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    Posted 1/8/10
    You don't particularly.

    Reflux off a lid is an issue related to re-introducing unwanted volatiles. That will obviously happen, but they will re-boil and re-evaporate. If you were on the lower limit of the evaporation range and barely getting rid of the required volatiles in the first place, I imagine it would be an issue. But seeing as the reason you would want to put a lid on in the first place would be to reduce your excessive evaporation.... You have a bit of spare capacity in your system for volatile stripping which will take care of it.
     
  8. PistolPatch

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    Posted 1/8/10
    Good on you Thirsty though I am a bit shocked at the state of your stovetop :).

    You've probably had enough but if you get bored again today, can you try the following please ;).

    Try the difference between a simmer and a gentle boil to see if the difference is less marked than the difference between a gentle boil and a hard boil.

    The other one that would be interesting is the second experiment but only using half-filled beakers and 3/4 covering one of them. (Use gentle boil on both.) I suspect the increased headspace and larger opening could make a bit of difference.

    Good on you once again for getting the beakers out and taking the time to do the above :super:.
    Pat
     
  9. Thirsty Boy

    ICB - tight shorts and poor attitude. Fuck yeah!

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    Posted 1/8/10
    I can give the simmer vs gentle boil a go... But only for you Pat :) i was reluctant to use a "simmer" as part of the experiment... I wanted to keep it to more brewing like parameters and I wouldn't ever simmer a wort. But as an experiment I guess it would tell us something. What about the difference between two boils that I would consider the low and high extremes of "acceptable" boil vigor?

    The second one I don't think I could meaningfully do... Limited by the vessels I have available. Those beakers are the only two almost identical, fairly well graduated containers I have... if I half filled them and went for an experiment that was trying to measure a smaller effect.... I think any difference would get lost in observational error. I'll maybe give it a go, but I don't hold out much hope that anything useful will come out.

    Next time I'm bored and there is nothing on the Telly.
     
  10. PistolPatch

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    Posted 1/8/10
    You de man!

    Those beakers make for nice illustrations. I'm about to watch a movie so maybe I'll stop being lazy and try the second thing above in a 20 lt pot I've got. Will be a bit inaccurate though as I won't be able to do a side by side. Anyway, I'll have a crack :).

    Donya,
    Pat
     
  11. grod5

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    Posted 1/8/10
    Only one problem I can see is that the Chef Consul infra-ray had some issues with gas pressure distribution with its two front burners. So the boil vigor (heat intensity) may not be equal. Maybe this experiment should be undertaken by volunteers from AHB to increase the sample size for a better educated opinion.

    Sadly, I will be doing my hair that day and cant assist with the research group.

    daniel
     
  12. Nick JD

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    Posted 1/8/10
    Daniel! Don't mess with the "science". :lol: Can I even be bother to suggest the same experiment should be repeated with the beakers swapped and then the results averaged? No I can not because this shit ain't ever gonna make it into the Journal of Brewing & Distilling.
     
  13. PistolPatch

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    Posted 1/8/10
    Okay gave 5 boils a go in a pot 23cm high with a diameter of 27.8cm. All boils started with 10cm of water and then boiled for 40 minutes.

    First Three Boils - Simmer versus Boil plus 3/4 Lid Cover

    1. Gentle Boil* with Lid 3/4 On - 7.4 cm remaining after boil so 2.6 cm evaporated.
    2. Smallest Simmer Ever with Lid 3/4 On - 8.2 cm remaining after boil so 1.8 cm evaporated.
    3. Gentle Boil* with No Lid - 7.3 cm remaining after boil so 2.7cm evaporated.

    * This was the hardest boil I could manage to get on my stovetop.

    Here's a pic of how much the lid was on and the lid sloped back into the pot...

    Lid_On.JPG

    So, initial conclusions are:...

    A. Simmer versus Gentle Boil: A significant difference occurred here although the simmer could hardly be called a simmer as there were only bubbles on the bottom of the pot - none on the top. You certainly wouldn't want to boil like this. However reducing the enrgy source makes an obvious difference to evap rate.

    B. Lid 3/4's On: Having the lid 3/4 on on a half-full pot makes very little difference to the evaporation rate (probably because the pressure above the surface is not significantly reduced.) The vigour of the boil did not visibly change if the lid was taken away. Of course, if the lid was fully on, there would be a difference due to increased pressure (like we all know from completely covering a boiling saucepan with a lid.)

    Last Two Boils - Using a Floatie

    Thirsty has suggested floating something on the surface of the wort to decrease evaporation rate and energy required for the boil. In this case I used a saucepan to float inside the pot. The saucepan had a diameter of 19.5 cm and therefore a surface area of 29.9 cm2. The pot had a surface are of 60.7 cm2 and so the effective surface area of the pot was reduced by about 50%.

    4. Floatie with Gentle Boil - 8.4 cm remaining after boil so 1.6cm evaporated.
    5. Floatie with Maximum Heat Source* Applied - 7.3 cm remaining after the boil so 2.7 cm evaporated. (This produced a very hard boil).

    *In other words, the same heat energy was used as in 1 and 3 above.

    The floatie looked like this during Boil 5...

    Floatie.JPG

    Conclusions from the Floatie are...

    C. Floatie can be used to assist achieving a rolling boil in situations where the energy source is weak.
    D. Floatie can be used to significantly reduce evaporation rates whilst maintaining a rolling boil.
    E. Floatie can be used to significantly reduce energy consumption. (I am estimating that the flame used in 4 above was about 60% of that used in 1, 4 and 5.

    So, Thirsty, I am liking your floatie idea a heap and will give it a go on my next single batch though I always seem to do doubles lately.

    Spot ya,
    Pat
     
  14. Thirsty Boy

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    Posted 1/8/10
    I actually considered this, and also swapping burners - but neither of the experiments I did requires the applied heat to be the same. The first obviously had different heat intensities... And in the second I deliberately changed the heat settings to get a consistent boil vigor. The flame settings were noticeably different.

    So the energy output of the burners was irrelevant to the experiments and the old chef stove was more than sufficient form the job.

    Pat - interesting, i'll have to give it a crack now to see what results I get.
     
  15. Wolfy

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    Posted 1/8/10
    Interesting, informative, well done, and amusing.
    However, are you not just wasting water, energy (both your own and the gas) and time to prove what is logical and common sense?

    Boil harder and get more evaporation losses.
    Cover the pot and lose less to evaporation.
    Have less surface area and so you have less evaporation losses.

    The first idea/concept, I use all the time when cooking and need to 'reduce' the liquid more quickly (usually by that stage I'm getting hungry and want to eat so speed the process up as much as possible).
    The second idea/concept is again used all the time when cooking to retain as much liquid as possible (I'd boil dry my potatoes all the time if I didn't put the lid on).
    And the last idea/concept is once again often used when cooking, since a large surface frying pan will allow you to 'reduce' the liquid much more easily than the same in a smaller diameter saucepan.

    If anyone argues the logical conclusions of your experiment, maybe they should stop arguing about pedantic brewing theories online and get in the kitchen and do some (more) cooking. :)
     
  16. yardy

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    Posted 2/8/10
    too much time on your hands you blokes :ph34r:
     
  17. Thirsty Boy

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    Posted 2/8/10
    I don't think so, I mean I'm still just having a bit of fun, but I'm not so sure that common sense necessarily prevails in all these situations.

    Boil harder - how many times i've had people tell me it won't make any, difference to boil off rate? Enough so i needed to prove it one way or the other.

    Cover the pot and lose less - seems logical, seems to work at kitchen level, but Pat has done some work measuring this stuff and was finding that lids partially on made no difference. And really, look at the physics of vaporization and it shouldn't make much difference. but I was sure it would... So a test. my gut feel is thee is going to be a large difference between lid partly on and lid totally on. Plus I think that the difference in surface area to volume ratio in a kitchen sized boil to a brewing sized boil is going to make a difference. My pre-conception is that the lid even partially on will make a noticeable difference, but I still think it's worth a little testing over a beer or two while there's nothing on TV.

    Less surface area - once again, I know that reducing it works, and that large surface area make a big difference to boils, boil vigor, energy source you can use etc etc. But surface area really should only matter to evaporation rather than vaporization (I have been ruthlessly misusing the terms which is probably driving any real scientist silly enough to read this thread to distraction) .... But I'm not sure that both aren't at play, so tests with barely simmering vs boiling water might help a little to clear up the picture.

    I like to be right.... And if I'm going to talk about this stuff and be arrogant enough to advise other people what I think they should do... I want to be sure what I am saying is true, not just that it makes common sense. I'm also arrogant enough to think that some people will be interested in watching me do it. Besides, playing with fire and boiling water is a lot of fun... Might make mud pies next :)

    TB
     
  18. Fourstar

    doG reeB

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    Posted 2/8/10
    It sounds like your little experiment was as amusing as watching water boil. :p
     
  19. Nick JD

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    Posted 2/8/10
    I'm gonna say it one last time, reducing the surface area of the top of a boil by half, and seeing a "rolling boil" in half of the surface doesn't mean you've increased the vigour of the entire boil - just half of it. The net vigour of the boil is exactly the same as without the floatie.

    Ask yourself this question: what kind of boil is happening in the hidden half of the pot?

    Which has always been my point. How this is confounding the intelligent people here is beyond me. What you are thinking of as a rolling boil is actually a gentle boil in entirety - you're only looking at the rolling part.
     
  20. Fourstar

    doG reeB

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    Posted 2/8/10

    Now im no expert in thermo/fluid dynamics but as you noted, you may not have incrased the 'vigour' (intensity) if the entire boil but by channelling the evaporation via a concentrated point you may have increased the convection/rolling of the boil, right? Isn't that the primary purpose of popping the lid on anyway? To increase the convection? An explosive boil jumping out of the kettle is just as pointless as one that is shimmering.

    The only way i could believe this to not be the case is to visually observe it in a glass beaker.

    Thirsty, i throw the mantle to you! :p
     

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