Excessive Kettle Evaporation Loss

[Thirsty Boy]

This whole area is not researched very well as it all "boils" down to kettle shape and volume.

Working on percentages doesn't work very well practically. For example, in my 70L pot, if I do a single batch my evaporation rate is around 19%. At the same boil vigour, if doing a double batch, my evaporation rate is about 9.5%. Both brews taste the same. You can see some evaporation figures in this file..
View attachment 39750
(My figures are the PP ones.)

Unless you know my kettle shape and boil volume telling me to make sure I achieve an evaporation rate of x percent is silly. Telling me I need to boil off x litres per hour is silly.

Using percentages to "work out" your boil off is silly, agreed. But, (and I am starting to run out of ideas about how to differently explain this to people who seem persistently unable to get it) THATS NOT WHAT YOU ARE SUPPOSED TO USE THEM FOR.

You measure you boil off... In litres per hour. That's how you are going to work out you start and finish volumes. Easy. Now that you have that.... You, convert it into a % figure. If that % figure is above 8% you are good... Stop worrying and go back to thinking in litres per hour. Done.

If your rate is low, In a given pot you can, absolutely, done it many times, physics says I should be able to do it and I have - change the boil off rate by increasing the amount of heat put in and thus the vigor of the boil. Turn up the gas and boil harder - more liquid will boil off. Reducing it isn't so easy, but reducing it isn't the goal.

In your double batch... You get 9.5% - great, that's all you want to know, nothing to change. If you do a single though, you are getting 19% - and that to me would indicate that you are putting in more heat than ideal. You have a high surface area to depth ratio, and in order to maintain an acceptable rolling boil, the heat flux through your pot is higher than it need be, lower heat flux is better.

So you can turn down the heat... Which kills your rolling boil (bad) and you still get mostly the same boil off anyway, or put on a lid, which also only minorly effects the boil off until it's almost all the way on. So those two options are no use are they? BUT, what is less obvious, is that you get a much more active boil when lids are even part on..... So you drop the heat, which lowers the boil, but then you pop a lid partially on, which makes the boil pick back upmto where it was. And the combination of the lid and the lower heat flux, will drop your evaporation too. Remember, the goal isn't particularly to have a low evaporation..... The high evaporation is simply an indicator that something else is not as it should be - and you need to take action to fix it.

In your case the high evaporation rate is telling you that your 70L pot is too big to do single batches in. If you want to keep your heat flux low and not have to worry about topping up anyway. Lid mostly on, heat down lower....back a few percent to under 15% ... And that's all that's needed. Or you buy a smaller slimmer pot for single batches, where everything happens as it should without interference. But we aren't all made of money so a solution that doesn'tmcost a whole new pot would be nice - As I said, my preferred option is to float something on the surface of the boil.... Insant reduction in surface area to depth ratio, heat gets turned lower to maintain the boil level, evaporation rate drops to let me know its working and thus instant resolution of problem.

It's not about the "figures" and obsessively trying to get them right. It's about understanding what the figures are telling you about what's happening in the boil.... The figures you hear bandied about aren't just there for fun, because that's what the writer of the text book got and therefore so should you. They are the figures that "indicate" that all is well, or not!

If you boil off 8-15% of your kettles starting volume per hour.
AND
If you have a nice rolling boil the whole time you are boiling
THEN
All is well

If you have a lower or higher boil off, no matter what the size or shape of your pot, then there are things about your boil that could be considered less than optimal and you might like to consider changing something.

That's all those figures are.. Little hints about where you can look for improvement, or where you might find problems hiding. You use them as you will.

TB

 

[Screwtop]

Using percentages to "work out" your boil off is silly, agreed. But, (and I am starting to run out of ideas about how to differently explain this to people who seem persistently unable to get it) THATS NOT WHAT YOU ARE SUPPOSED TO USE THEM FOR.

You measure you boil off... In litres per hour. That's how you are going to work out you start and finish volumes. Easy. Now that you have that.... You, convert it into a % figure. If that % figure is above 8% you are good... Stop worrying and go back to thinking in litres per hour. Done.

Spot on TB, been trying to get this across for a long time. Boiloff percentage is important as above 8% all of the processes required of a boil are achieved. But for calculations of total brewing liquor or pre boil volume a % is pointless, for this a known boil off volume in litres is required. Beersmith uses the boil off % when calculating total brewing liquor, the result can be confusing to the brewer. Any time batch volume is altered from the default, open Beersmith's equipment details on the recipe page and change the boil off % to reflect your known boiloff volume in litres. This provides correct total brewing liquor and all targets should be close, depending upon variables such as ambient atmospheric conditions where you operate your kettle.

Screwy

 

[thylacine]

"...Or, you could do what I do, which is float something on the surface of the liquid. That way anything that evaporates stays evaporated, but the surface area of the liquid is reduced anyway. In a smallish pot, you could just float a takeaway container or a heatproof bowl in there. Experiment with different sized things till you find one that allows you to keep your rolling boil, turn down your heat and maintain a lower % evaporation rate..."

Floating an object in my 19L kettle right now. It made an instant boil out of a simmer. (18L boil on gas stove-top). When I floated my hop bag I could turn the heat down even more.

Thanks TB

 

[PistolPatch]

What a great idea thylacine! Love it. A totally simple way of reducing the wort surface area :icon_cheers:.

There's hope for me yet Thirsty getting my evap rates down on the single batches .

Thanks and :beer:,
Pat

 

[King Brown]

After reading TB's post I understand why people do 2 hour boils for certain styles. Having a short and fat brewpot I think I'm gonna have to try the method of floating something on top of the wort for my next batch. Lot's of great info in this thread.

 

[PistolPatch]

My apologies and queries...

Thirsty, I was just re-reading your post above and I see that I have given credit to thylacine for floating something in the kettle (here and on BIABrewer.info) whereas it was your post above that first mentioned it. (I'll correct the BIABrewer.info post* I made when I finish here but thanks to you also thylacine. Thirsty's posts are as long as mine so without yours I would have missed the floatie bit entireley ).

My only query for you Thirsty is the bit where you say that having a lid part on lowers the evaporation rate by giving you a boil with a higher vigour. The increased pressure of a partly covered kettle is minimal and the wort surface area does not change. The change in evaporation rate is miniscule.

This is what I was trying to say about the physics of evaporation. My personal experience and physics tells me that, with a flat lid covering 3/4s of your kettle, very little difference will be noticed in evaporation rate. There is little difference in evap rate once you reach a simmer.

It's all fun though and I might even do some experiments this weekend as a matter of interest with my 70L and 20L pots just with water as I find this sort of thing quite interesting.

More simmering/boiling/measuring and less talk I reckon ,
Pat

*Corrections made here.

 

[Nick JD]

If you float something on your beer does it lose carbon dioxide slower?

The evaporation is happening in the bottom of the kettle in those little bubbles of gas called water vapour. Unless you're simmering.

 

[beerbog]

As someone stated before, you can control your loss by varying the boil intensity, ensuring you have a rolling boil to start with.

Your kettle opening diameter is important too.

I can vary mine anywhere from 4 - 5 litres per hour for a 40cm diameter kettle just by changing the aggressiveness of the boil. :beerbang:

 

[Thirsty Boy]

My apologies and queries...

Thirsty, I was just re-reading your post above and I see that I have given credit to thylacine for floating something in the kettle (here and on BIABrewer.info) whereas it was your post above that first mentioned it. (I'll correct the BIABrewer.info post* I made when I finish here but thanks to you also thylacine. Thirsty's posts are as long as mine so without yours I would have missed the floatie bit entireley ).

My only query for you Thirsty is the bit where you say that having a lid part on lowers the evaporation rate by giving you a boil with a higher vigour. The increased pressure of a partly covered kettle is minimal and the wort surface area does not change. The change in evaporation rate is miniscule.

This is what I was trying to say about the physics of evaporation. My personal experience and physics tells me that, with a flat lid covering 3/4s of your kettle, very little difference will be noticed in evaporation rate. There is little difference in evap rate once you reach a simmer.

It's all fun though and I might even do some experiments this weekend as a matter of interest with my 70L and 20L pots just with water as I find this sort of thing quite interesting.

More simmering/boiling/measuring and less talk I reckon ,
Pat

*Corrections made here.

Your lid may perhaps function differently to mine - mine, when put partially over the kettle, cuts the evaporation rate down by a small but noticeable amount.... your reading of the physics of the matter is different to mine, but I gave physics up after second year uni and so might be wrong.

Besides... as I have tried a couple of times to point out... it isn't the actual evaporation rate that matters -- the evaporation rate is indicative of other things. If you put a lid partially on your pot, as anyone who has ever even boiled an egg can tell you, the pot boils harder... so in order to get the pot boiling at the same vigour, you can turn the heat down - and that's what you want to do - add less heat for the same boil vigour, which will as a consequence, reduce your evaporation rate. A lid is a way to do this, something floating on the boil is a way to do this, a pot with a smaller surface area is a way to do this

You need to evaporate 8% of your starting volume per hour - that is a given for volatile removal. Above that point, your evaporation rate is a tool for you to use, to help you understand how your boil is performing.

TB

 

[manticle]

If you float something on your beer does it lose carbon dioxide slower?

The evaporation is happening in the bottom of the kettle in those little bubbles of gas called water vapour. Unless you're simmering.

Not quite sure if I've misinterpreted what you're saying but it seems akin to suggesting that if CO2 can escape a vessel that isn't sealed so can water vapour. If that were exactly the case, placing a lid on something wouldn't effect evaporation loss which we both know is not correct. Placing something like a foil takeaway container sounds like it would act like a small lid - if water vapour does hit it it will condense and reform into a liquid state.

CO2 vapour and water vapour have different properties

Sorry if I've misunderstood.

 

[bum]

float something on the surface of the liquid. That way anything that evaporates stays evaporated, but the surface area of the liquid is reduced anyway.

Genius. Never would have thought of this.

Manticle, that link may only be used to mock the paranoid. Your attempt to use it for borderline relevant reasons in an affront to decency.

 

[Nick JD]

Not quite sure if I've misinterpreted what you're saying but it seems akin to suggesting that if CO2 can escape a vessel that isn't sealed so can water vapour. If that were exactly the case, placing a lid on something wouldn't effect evaporation loss which we both know is not correct. Placing something like a foil takeaway container sounds like it would act like a small lid - if water vapour does hit it it will condense and reform into a liquid state.

CO2 vapour and water vapour have different properties

Sorry if I've misunderstood.

The bubbles of water vapour that rise in the rolling boil when they hit the object floating on the surface will move around it and into the atmosphere just the same as a CO2 bubble will go around an ice cube. The foil container will be very close to 100C - not a great condenser, most likely doing zero condensing.

The only reason a lid causes less less evaporation is because condensed water is refluxing back into the pot.

Let's take this to the extreme and imagine a kettle at rolling boil entirely covered with a floating object. What will happen then? Will all the rising water vapor bubbles strike it and recondense? Or will it find one area that will be jetting out vapour?

Convection currents will force more of the rising bubbles away from the floating object and the rolling boil will be more intense on the other side of the object.

And again - evaporation is happening at the bottom of the kettle in a rolling boil.

 

[Thirsty Boy]

The bubbles of water vapour that rise in the rolling boil when they hit the object floating on the surface will move around it and into the atmosphere just the same as a CO2 bubble will go around an ice cube. The foil container will be very close to 100C - not a great condenser, most likely doing zero condensing.

The only reason a lid causes less less evaporation is because condensed water is refluxing back into the pot.

Let's take this to the extreme and imagine a kettle at rolling boil entirely covered with a floating object. What will happen then? Will all the rising water vapor bubbles strike it and recondense? Or will it find one area that will be jetting out vapour?

Convection currents will force more of the rising bubbles away from the floating object and the rolling boil will be more intense on the other side of the object.

Except for the inconvenient fact that I float something in my boil every single time I brew... and it both increases the vigour of my boil and decreases the evaporation... markedly.

Therefore your theory disagrees with empirical observation and is thus incorrect.

 

[Nick JD]

Except for the inconvenient fact that I float something in my boil every single time I brew... and it both increases the vigour of my boil and decreases the evaporation... markedly.

And you have your heat setting the same?

Breweries use Batch Rectification techniques when all they need to do is get a whole stack of Chinese Takeaway containers? :lol:

Bubbles of water vapour will get into the air regardless of obstacles - read up on vapour pressure. Emperical observation favours the observer's bias.

 

[Thirsty Boy]

And you have your heat setting the same?

. . . . . Emperical observation favours the observer's bias.

ahhhh - no. What with having specifically stated that I turn the heat down and that the object of the exercise was in fact to be able to turn the heat down.... I kind of thought that was implicit.

Empirical observation is how science works... observer bias is simply something to be accounted for in experimental design.

 

[Nick JD]

....be able to turn the heat down...

So you are seeing increased vigour in the boil - but obviously only in the area that's not covered - right?

How much of the surface is covered by the floating object?

Can you refer me to some science on how this under-takeaway-container-condensation works?

I say you'll get the same result from just boiling less vigorously and leaving the floatie off.

 

[Thirsty Boy]

So you are seeing increased vigour in the boil - but obviously only in the area that's not covered - right?

How much of the surface is covered by the floating object?

Can you refer me to some science on how this under-takeaway-container-condensation works?

I say you'll get the same result from just boiling less vigorously and leaving the floatie off.

Why yes you would, approximately - for the evaporation rate part... which as I have been at pains to point out is the indicator, not the object. The boil vigour increases, as it does if you put a lid on a pot, allowing you to turn the heat down - which is the point. I don't need to refer you to science texts... Instead I refer you to the kitchen where anyone can try it and see. Give it a wee go... if it doesn't work, then pop back and let us know how right you were.

as for the vigour of the boil only being increased in the areas "not" under the lid -- perhaps thats true... I see a general increase in wort movement, a larger number of bubbles breaking the surface and generally a lot more activity in the kettle. It may be my imagination, or it may be your observer bias in action... but I have been homebrewing from grain for 5 years and I work in a professional brewery, I have observed literally thousands of wort boils over the years, and in my opinion, the boil vigour with the float is higher than without - if it weren't, I wouldn't do it.

 

[manticle]

I'm a little bit confused Nick. Empirical observation seems to be good enough for your brewery regardless of science and you seem to expect others to take you at face value (or at least try it before canning it) . Why is Thirsty's experience less valuable than yours?

Or is that the joke?

 

[beerbog]

Just boil the *******! :beerbang:

 

[Nick JD]

It may be my imagination, or it may be your observer bias in action...

It may be. It may not be.

That is the question. This is the question:

"I have just tested my kettle before I do my first BIAB brew, and I have found that I lose around 30% of my total kettle volume for a 1 hour boil. My kettle is a Big W 19L SS pot."

I'm just not that sure that "float a takeaway container on the surface" is a great answer.