Kettle Evaporation -

[Wolfy]

... needless to say the nucleation points created by adding the yeast resulted in a geyser of very hot wort all over the kitchen!!).

I did the same last time I made agar slants, the liquid was so close to the boil when I took it out of the microwave, that by putting in a spoon to stir it the whole lot bubbled up the spoon, over the top of the jug and onto my fingers.

You might be right about the flasks shape, I don't know how to explain the 'floaty' and what people have noticed when trying it, I just keep thinking of something like less surface area hence less evaporation, and more 'energy release' in a smaller surface area hence the boil looks more rapid.

 

[bum]

The use of pils malts and the residual DMS is kinda desirable for some lagers.

Of course. It wasn't my intention to make a sweeping generalisation in regard to the use of pils malts.

DMS is produced by
thermal decomposition of S-methylmethionine (Fig. 4.34), the half-life of which is
reported to be 35 min. at 100C.

The use of the term term 'half-life' leads me to assume that 35 minutes is not actually the answer to the question I asked. Would that be correct?

The other stuff is all interesting but I don't have the equipment required to measure my beer down to parts per million. A tongue is about as scientific as my lab gear gets.

So in terms of Rude's post - is his assumption that the majority of all volatiles should be gone in the first half of the boil a safe one? Of course I understand that it is easy to partially cover the kettle without lid condensation dripping back into the wort re-introducing volatiles - just wondering, is all.

 

[Supra-Jim]

Here you go Bum:

Half-life : The time required for one-half of a specified substance to degrade or become inert.

Cheers SJ

 

[Nick JD]

So in terms of Rude's post - is his assumption that the majority of all volatiles should be gone in the first half of the boil a safe one?

As far as I understand it, it takes 35 minutes for half the SMM to turn into DMS at 100C. It all depends on how much SMM your malt had.

So if you boil for 30 minutes and then cover (completely) you'll leave about 1/2 more of the DMS in your wort than not covering. How much this is depends on your malt.

If you no-chill then this is even more complicated because SMM is still decomposing into DMS while the wort is slowly cooling. However, it can't decompose in your no-chill if it's (nearly) all gone.

If you had 100ppm at 0 minutes you'd have 50ppm at 35, 25ppm at 70 and 12.5ppm at 105 minutes. Putting the lid on after 30 minutes might give you corn - might not.

 

[bum]

That's a few too many ifs for my liking so I'll stick with my current practice for now. Thanks for the explanation.

 

[Thirsty Boy]

Wolfy, Pat - I'll repeat what I have said a few times before. The floaty does not just increase the appearance of vigor in the boil, it increases that actual rate at which the liquid boils. I have in fact tested it quite thoroughly.

And its not about pressure... pressure would increase the boiling point and require you to add more energy to get to it, but after that why would it change the boil vigor or the amount of liquid that is boiled off. Although local vapour pressure will also effect evaporation and that's probably playing a part. Wolfy did get the actual reason though.. it is about reducing evaporation (but that's nothing to do with vaporization by boiling) by reducing surface area.

I'll just finish this post then I'll take a while to post up the latest experiments I have been doing. More of the same, sorry - but I'm not particularly interested in the DMS thing. I'm happy I know the answer to that one. You control DMS with length of boil rather than increased vigor.. as long as you are getting the 8-10% per hour, that's enough to strip DMS; and after that its about half life as Nick posted. More DMS calls for a longer not a harder boil.

You will find in them a number of things that are relevant to why the floatie thing works, mainly though I am looking at lids, boil off losses and some of the things I have discussed with Pat.

TB

 

[RdeVjun]

I did the same last time I made agar slants, the liquid was so close to the boil when I took it out of the microwave, that by putting in a spoon to stir it the whole lot bubbled up the spoon, over the top of the jug and onto my fingers.

Getting a bit OT here, but that may actually be superheating Wolfy, best explained here, see superheating section down a page or two. The teaspoon provided the nucleation points perhaps. Anyway, I've experienced this myself, it is novel but really no fun when you're on the receiving end. Agar does need to be boiled however...

 

[Nick JD]

...it increases that actual rate at which the liquid boils. I have in fact tested it quite thoroughly.

What's the "rate" at which a liquid boils?

I thought boiling was the transition of a liquid to a gas. Liquid nitrogen boils at a rate controled by the energy put into it, not its container.

Am I missing something?

 

[Acasta]

Studying Fluid dynamics.
Basic things that may help.
Closing the lid will = more pressure = more heat, therefore i'd imagine more evaporation as its hotter throughout.

 

[Scruffy]

Studying Fluid dynamics.
Basic things that may help.
Closing the lid will = more pressure = more heat, therefore i'd imagine more evaporation as its hotter throughout.

If the lid's still on, look up condensation

 

[bum]

You have no right to correct him, Scruffy. He has every right to talk whatever nonsense he likes, however.

 

[Thirsty Boy]

Is all more complex than poor laymen types like me can understand by just trying to extrapolate what "should" happen. So thats why I'm running a few little tests... I am interested in what actually happens - why it happens is a minor (but interesting) part of it. I'll think about theory and see what explanations I can come up with after I observe the real behavior.

First (moving straight into a contradiction of what I just said I was going to do) - the Pressure Argument. The lid I am using is a round plastic take-away container lid. It weighs 6g and has a radius of 5.5cms. convert that into pressure needed to lift it and you have about - 0.0009psi. Now using a chart I found on engineeringtoolbox.com, you need an extra 0.31psi to raise the boiling point of water from 100C to 101C. So the pressure this lid could apply at best is 3/100ths of that needed to raise the boiling point by 1 degree C. AND - in the combination of this lid and the beakers being used, you can see on the photos there is a gap left at the spout of about 1cm square. So its not even going to be that amount of pressure. This increase might be enough to visibly change the behavior of the boil, but I don't think so. And I also don't think pressure would significantly change the nature of the boil even if it were higher, it would just change the temperature at which things happen.

Second - in these experiments I took into account what Pat had mentioned about the amount of headspace and the surface to volume ratios. So I looked at my brew kettle, which is a decent example of the situations I am trying to address. ie: its a little big for my batch size and I either get too low a boil or too high an evaporation rate using it as it is.
So i worked out how much liquid in the beakers, would give the same wort depth to width (diameter) ratio as I get in my kettle. This was about 400-500 ml in the slightly more than 1L beakers. This results in wort that is about twice as wide as it is deep. I used 400-500ml as the starting volume for all the following tests. This left slightly more than 50% headspace in the beakers.

SOME STUFF ABOUT LIDS

(almost all the way on - means lid slightly ajar, perhaps 5 or 6 mm from the outside edge of the lid to the edge of the beaker)

Lid "almost" all the way on - vs - Lid 2/3rds on... adjust boil vigor

Beakers filled to 500ml, brought to hard boil and heat off. adjusted back to 500ml with boiling water from kettle. Lids on, flame back on - boil vigor adjusted visually to the same level. boil till one of the beakers hits 200ml.

Result - small but definite difference in boil vigor. Final difference in amount boiled off was a small but noticeable decrease for the beaker with the lid mostly on. On the order of 40ml

Lid 'almost' all the way on - vs - lid 2/3rds on. Equal amounts of heat applied

Beakers filled to 500ml, brought to hard boil and heat off. Adjusted back to 500ml with boiling water from the kettle. Lids on, flame back on. Beakers swapped from burner to burner every 5 minutes for a total boil time of 30 mins.

Result - Medium increase in boil vigor for beaker with lid on. Virtually no difference in amount boiled off.

Lid 'almost' all the way on - vs - No Lid at all. Equal amounts of heat applied.

Beakers filled to 500ml, brought to hard boil and heat off. Adjusted back to 500ml with boiling water from the kettle. Lid on one beaker most of the way, flame back on. Beakers swapped from burner to burner every 5 minutes for a total boil time of 30 mins.

Result - distinct increase in boil vigor in beaker with lid. Virtually no difference in amount boiled off.

Lid "fully" on - vs - No lid at all. Equal amounts of heat.

Beakers filled to 500ml, brought to hard boil and heat off. Adjusted back to 500ml with boiling water from the kettle. Lid fully on one beaker, flame back on. Beakers swapped from burner to burner every 5 minutes for a total boil time of 30 mins.

Result - A small but noticeable difference in the amount of liquid boiled off. On the order of about 25-35ml

Result summary - In all the cases but one, where the heat applied was the same, the amount of liquid lost was near enough to the same. The exception was the case where the lid was completely on, where a small decrease in boil off was observed.

There was an observable increase in boil intensity in all cases where a lid was on to any extent at all, with a larger increase where the lid was closer to being fully on.

In the case where the boil intensity was "matched", the lid being more "closed" resulted in the amount of liquid lost being reduced by an observable, but with the precision of the scale used, not very measurable amount.

Conclusions - (or speculations really) Any decrease in evaporation that might be brought about by placing a lid on a boiling vessel, will be (roughly) balanced by the increase in vaporization brought about by an increase in boil intensity when the lid is placed on the vessel. So unless an effort is made to balance the vigor of the boil in the vessel, putting a lid on will not decrease the amount of liquid lost during the boil. Although a reduction might be seen if the lid completely closes the pot, this may present other brewing problems.

If however the boil intensity in a vessel with a lid (even less than fully on) is reduced to match in intensity the boil before the lid was put on... then a reduction in the amount of liquid lost will be observed.

 

[Thirsty Boy]

SOME MORE STUFF ABOUT LIDS

The conclusion from my last experiments depends on the observation that boil intensity increases noticeably when you put on a lid, even partially. So.... is it true or is it just my imagination? It being true supports my pre-conceptions, and perhaps the decreases in liquid losses are just due to me turning down a boil that had in fact not increased in intensity at all. So to be certain, I better test it to make sure I am observing something real rather than something I just want to see.

Boil Intensity in a vessel with a lid - vs - Boil Intensity in a vessel without a lid given the same amount of applied heat.

1 beaker, filled to the 400ml mark and brought to hard boil, heat reduced and boil adjusted to a very low boil/simmer. Maintain boil without adjusting flame for several minutes to let it "settle". Count the number of bubbles produced in one minute. (bubbles were considered to be countable when they reached the top of the liquid and caused an observable ripple as they broke the surface.)

Without adjusting flame, put a lid fully on the beaker. Allow to settle for several minutes, count bubbles produced in one minute.

Repeat the counts with lid off and lid on several times to lessen the impact of changes in gas pressure, ambient temp, breeze etc.

Repeat the series of tests again, but instead of putting the lid all the way on the vessel, only put it "halfway on".

Results -

Lid fully on tests

1. Off = 56 bubbles. On = far more than I could count. Hundreds.
2. Off = 40 bubbles. On = far more than I could count. Hundreds
3. Off = 68 bubbles. On = far more than I could count. Hundreds.

Average with Lid Off = 54.7
Average with Lid On = More than countable, hundreds.

Lid Half on Tests

1. Off = 60 bubbles. On = 107 bubbles
2. Off = 50 bubbles. On = 95 bubbles
3. Off = 43 bubbles. On = 77 bubbles

Average with Lid Off = 51
Average with Lid Half On = 93

Conclusions

I wasn't imagining it. Putting a lid even partly on a boiling vessel increases the intensity of the boil quite significantly. This is in little beakers though and I imagine that it would be less noticeable in larger vessels with a higher volume to surface area ratio.

 

[Thirsty Boy]

STUFF ABOUT EVAPORATION VS VAPORIZATION

Evaporation is where molecules of a liquid, in this case water, just break off from the surface of a body of liquid and wander off into the atmosphere. it happens at room temperature, but happens a lot faster when the liquid is hot. A bunch of other things affect evaporation too.

When a liquid is heated beyond its boiling point, liquid changes state to gas and bubbles away. The liquid stays the same temperature once it reaches its boiling point and any extra energy added goes into changing liquid to gas. I think the term is vaporization, but am not sure. For my purposes here, thats what I'm going to call it at any rate, sorry if I am getting it wrong.

Surely in a boiling liquid, the vast majority of liquid lost will be from vaporization rather than evaporation? I think thats its less clear than you might suspect. Let test it.

Volume lost when you Boil the Liquid

Beaker filled to 500ml, brought to hard boil and heat off. Adjusted back to 500ml with boiling water from the kettle. Heat back on and heat turned to lowest pre-set. This gives a boil that in a brew kettle I would call medium to medium-high. Boil for 30 mins.

Result -

Start Volume = 500ml
End Volume = 225ml

Total boiled away in 30min = 275ml = 550ml/hr

Volume lost to evaporation in nearly boiling but still water

Beaker filled to 500ml, brought to hard boil and heat off. Adjusted back to 500ml with boiling water from the kettle. Heat back on and turned down until all signs of boiling stopped. Thermometer placed at about half depth, attempt to maintain heat at as close to 100 as possible without letting any actual boil happen. Maintain for 1hr.

Results -

Able to keep the temp between 93 & 97 for the vast majority of the hour. It slipped to 89 for a minute or two, and once or twice it broke into actual bubbles getting to the surface for a few seconds before I managed to adjust the heat down.

Start Volume = 500ml
End Volume = 325ml

Total volume Evaporated = 175ml/hr

Volume lost to evaporation in a vessel that is heated to boiling, then removed from the heat totally

500ml into beaker and brought to a hard boil. Adjusted back to 500ml with boiling water from the kettle. No more heat applied to vessel. Leave for an hour.

Result -

Start Volume = 500ml
End volume = 450ml

Total Volume Evaporated = 50ml
(see, that proves it wasn't just me getting the fire wrong.. Happens significantly even with no extra heat)

Conclusion/Speculation

In a boiling vessel, liquid is lost to the atmosphere in two different way. By the obvious bubbling away of steam through vaporization & by the less obvious process of evaporation which occurs even if there is no vaporization happening at all.

Although the results above would almost certainly not be directly transferable to brewing sized vessels, in the above test it seems that given a total loss of 550ml/hr in a boiled vessel, at least 175ml/hr or 32% of the water lost could be ascribed to evaporation rather than vaporization. And considering that in a boiling vessel the temperatures would be even higher than in the test, and the surface area would be vastly increased by the fact that it was being stretched, agitated and wobbled etc by the boiling action... It seems to me that the percentage would actually have been even higher. Now vaporization increases with boil intensity, so the ratio of evaporation to vaporization would drop as boil intensity increased the amount of total vaporization, with the evaporation staying relatively constant. Increasing the volume to surface area ratio would have a similar effect of increasing the importance of vaporization vs evaporation and the visa versa is also true.

While the total effect will be variable from brewery to brewery - measures taken to reduce/increase evaporation in a brew kettle will reduce/increase the total amount of liquid lost during the boiling period. Not as dramatically as measures taken to reduce or increase boil vigor (assuming you always maintain a reasonable amount of actual active boiling) but certainly by amounts that could be significant.

 

[Wolfy]

Conclusions - (or speculations really) Any decrease in evaporation that might be brought about by placing a lid on a boiling vessel, will be (roughly) balanced by the increase in vaporization brought about by an increase in boil intensity when the lid is placed on the vessel. So unless an effort is made to balance the vigor of the boil in the vessel, putting a lid on will not decrease the amount of liquid lost during the boil. Although a reduction might be seen if the lid completely closes the pot, this may present other brewing problems.

I do not doubt the conclusions based on what you observed in the conditions of your experiment.
However, I'd suggest that, the conclusion that the lid fully on 'might' reduce evaporation, is simply the result of the artificial test conditions - small plastic lid, beaker with a large spout etc - and that those conditions do not match real life cooking or brewing conditions enough to validate extending it to those situations. Most saucepans, pots and brew kettles have heavy close fitting lids, so I'd like to see the same experiment repeated with two larger sized pots, one with lid on and one with lid off (or just use the same pot and the beaker to measure before/after volume with the same flame/boil intensity for the same period of time).

...
1 beaker, filled to the 400ml mark and brought to hard boil, heat reduced and boil adjusted to a very low boil/simmer. Maintain boil without adjusting flame for several minutes to let it "settle". Count the number of bubbles produced in one minute. (bubbles were considered to be countable when they reached the top of the liquid and caused an observable ripple as they broke the surface.)
...
Conclusions
I wasn't imagining it. Putting a lid even partly on a boiling vessel increases the intensity of the boil quite significantly. This is in little beakers though and I imagine that it would be less noticeable in larger vessels with a higher volume to surface area ratio.

Again I am not questioning the results you observed, however this time I'd like to question your interpretation of 'boil vigor', is counting the number of bubbles released at the surface an adequate or even satisfactory measure of 'boil vigor'. By putting a lid partly over the container you are changing the conditions at the liquid/gas interface, which as you observed changes the number of bubbles released. However, is this extended all the way through the boiling container, or is it only a surface effect? What would you observe if you used some food coloring or semi-floating balls in the liquid, would putting a lid on change the behavior of all the liquid in the container or just that at the surface?

My logic for this question is as follows: Given that a 'rolling boil' is usually recommended for brewing so that there is a good exchange of liquid from the bottom of the kettle to the top, if someone followed your conclusion and used a lid to cover 1/2 of their kettle so they could turn down the gas but still have what appears to be the same 'boil vigor', would they actually still be getting a complete 'mixing' due to convection currents and other process within the boiling wort as a whole, or does it only change what is observed at the surface?

 

[Nick JD]

Key words: Vapour Pressure and Latent Heat of Vapourisation/Evaporation.

Check it out Thirsty - it'll explain a lot for you. It's the same phenomenon that controls the rate at which your clothes dry on the line depending on the humidity of the air. Same thing that makes a hot day okay because it's a "dry" heat.

Filling the layer above the liquid with saturated vapour by putting a lid (even partially) on means not nearly as much of the molecules on the surface of the liquid (and on the "surface" of the rising bubbles) can "change" into their gas state as easily. This has a massive impact on the system and it's something you have completely disregarded. A++ for effort though.

 

[drsmurto]

Key words: Vapour Pressure.

Check it out Thirsty - it'll explain a lot for you.

It's not quite that simple though Nick, once you open up that can you need to take into account atmospheric pressure, relative humidity, temperature, wind speed above the kettle and let's not even start discussing how the density of a fluid changes the relative vapour pressure.

We could start a discussion about the laws of thermodynamics and fluid dynamics......I am already salivating at the thought and my jeans are suddenly a tad more restricting...... :huh: :lol:

Sorry for the off topic, I'll go back to my mechanistic study of ketone oxidation :icon_drool2:

 

[Nick JD]

It's not quite that simple though Nick, once you open up that can you need to take into account atmospheric pressure, relative humidity, temperature, wind speed above the kettle and let's not even start discussing how the density of a fluid changes the relative vapour pressure.

We could start a discussion about the laws of thermodynamics and fluid dynamics......I am already salivating at the thought and my jeans are suddenly a tad more restricting...... :huh: :lol:

Sorry for the off topic, I'll go back to my mechanistic study of ketone oxidation :icon_drool2:

It's the "lid on" stuff I'm refering to, Doc.

EDIT: although it probably does relate somewhat to the winter/summer boil losses discussed earlier.

 

[drsmurto]

It's the "lid on" stuff I'm refering to, Doc.

EDIT: although it probably does relate somewhat to the winter/summer boil losses discussed earlier.

Ah, missed that. Carry on.

 

[Thirsty Boy]

Remember... I don't really care "why" I care what happens. If evaporation, vaporization or vigor changes with a lid on... Great, if it doesn't... Great. Don't care, just want to know if. Why, I am happy to think about after I know what.

Wolfy... Measuring boil intensity by bubbles. I agree, but measuring the velocity of the wort was a bit beyond me. Anyway, most of the movement and velocity pretty obviously comes from the bubbles... They form at the bottom and rise to the top stirring the wort as they go... Sure not all of them make it to the top because they don't have enough vapor pressure, which is why I was only counting the ones that did make it to the top... In a brew kettle we can't see through the wort, so we need to judge boil vigor by what we see on the surface, and that means mostly how much it is disturbed and the amount of bubbles . It's probably far from a perfect model to say that twice as many bubbles equals twice as vigorous, (which is why I didn't suggest anything like that) but I can't see that a boil with more bubbles in total forming and breaking the surface isn't going to also be more vigorous than one with less. Or am I missing something obvious? The boils were obviously more vigorous to look at.... The bubbles were just a way to try and stick a number to it in an attempt to remove my observer bias as much as I could.

The Lid fully on thing... Absolutely, that lid isn't the same sort of thing as a proper brewing lid. It almost certainly wouldn't emulate a proper close fitting heavier lid all the way on a brewing pot. Actually, that was part of the situation I wanted to test... I don't think that absolute pressure inside the pot would terribly effect the outcomes of these tests... So using a lid that wasn't capable of causing any significant increase in gross pressure was a way of mostly taking that out as a variable. If something was happening, it wasn't happening because of an increase in total pressure. Any lid at all would produce the same increase in local vapor pressure and temperature, so in the lid partly on tests the light plastic lid was as good as any other.

That was my thought process anyway... Anymore holes you can pick? (don't be too thorough though or I'll have to do all the damn tests again)

Thanks for taking the time to think about it.

TB