Excessive Kettle Evaporation Loss

[sshann001]

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. Has anyone else experienced losses this large? I notice that Beersmith has 5-15% of total volume as a guide.

 

[benno1973]

Percentages are fairly misleading when calculating evap loss. Evaporation occurs at a volume rate (i.e. 5L per hour) rather than at a percentage rate. So if your original boil volume is low, you'll notice that the evaporation percentage is quite high.

For an extreme example, think about boiling 1L or wort for an hour. Chances are, it'd boil dry. That's not 5-15%, that's 100% evaporation loss. If you're boiling only around 15L (which I'm guessing that you would be with a 19L pot), 30% evaporation would mean you'd be losing around 5L over the hour, which isn't excessive. I boil around 28L to end up with around 23L in the kettle, so I have a similar volume loss, but the percentages seem much more reasonable.

Obviously, evaporation rate is affected by numerous other factors, not least of which is how vigorous the boil is. You want a rolling boil, but it doesn't need to be violent. If you think you're boil is too high, just turn the gas down a little and you're evap loss will drop as well.

 

[thylacine]

I use same kettle. Start boil with 18 litres, after 90mins 15l remains. I measured every 30 min for the first few brews. 1 litre every 30min consistently. I used a somewhat gentle rolling boil as opposed to a vigorous one. Grain bills usually 33.2 to 3.5kg base, 300-400g spec. Fifteen litre wort in fermenter. Cheers...

 

[bkmad]

You will get a higher percentage when doing a smaller volume boil. In simple terms (not taking into account different surface area for different sized kettles), you might boil off 4L from a small 16L boil, giving a high 25% boil off. With a larger boil you may still only boil off 4L but for a boil size of 30L you only have 13% boil off.

 

[Nick JD]

I can easily boil 30% of say, 16L in a 19L pot in 60 minutes on the electric stovetop element. Frequently do.

The issue here is that a smaller volume has a much larger surface area proportion than a larger volume. More of the liquid comes in contact with the surface and more of the liquid at the bottom circulates to the top.

These two things are important when boiling small volumes: more rapid circulation, and bigger surface area/volume ratio.

The common two methods to fix the issue are: to boil a jug of water and top up your boil occasionally; or to reduce the heat significantly.

I do the second one - and while it appears that you are now no longer maintaining a "rolling boil" I've found that you still get acceptable evaporation losses due to the factors above taking lots of the DMS with it.

And you save money on power or gas.

 

[mika]

A 'rolling boil' is simply one that's turning the wort over in the kettle, there's no need for it to be jumping up and trying to reach for the roof. You'll find a steady rolling boil makes it a lot easier to deal with hop additions as well.

 

[Nick JD]

A 'rolling boil' is simply one that's turning the wort over in the kettle, there's no need for it to be jumping up and trying to reach for the roof. You'll find a steady rolling boil makes it a lot easier to deal with hop additions as well.

A rolling boil is a boil that when stirred it will still keep boiling.

In a 19L pot, this type of boiling will result in excessive evaporation losses.

 

[sshann001]

Thanks to all. Good to hear that it is not out of the ordinary. I will adjust the vigour of the rolling boil, and see how that helps.

 

[rude]

Good topic Shannbrewer as after hearing some answers I think I might be over doing it

I start with 34 L & finish with 25 L in the cube with maybe 1 to 2 L loss in the kettle never measured it

I use free gas though boil hard for the first 30 mins then depends on how I am going (check evap with dip stick) I'll slow it up a bit.

Sometimes Im getting a bit low I,ll put the lid half on

Is there any detriment to boiling too hard ?

 

[rude]

90 min boil there too

 

[Thirsty Boy]

A rolling boil is a boil that when stirred it will still keep boiling.

That's a terrible definition of a rolling boil... BUT, I think it might well be the simplest, easiest and most understandable way to describe how to TELL if you have a rolling boil that I have ever heard!! Nice.

Now to disagree with you -

In a boil, you need to get two things to happen, and you need to work out a balance that allows you to do it.

1] You need to get a certain amount of evaporation - NOT - an amount in litres, a percentage! in order to be sure that you have evaporated your DMS, aldehydes, bad tasting hop compounds and other unwanted volatile compounds, you want to evaporate a minimum of 8% of your starting volume per hour

2] You need to maintain a physical movement of the wort and the formation of bubbles which reach the surface. This is your rolling boil... the wort needs to physically roll and churn around the kettle, and bubbles need to form. The physical agitation and bubbles allow much of the chemistry in the boil to occur - a lot of protein chemistry happens on the surface of the bubbles, hop chemistry happens under physical force, volatiles come out of the wort and ride up to the surface inside the bubbles.

So - turning a boil down to the point where it no longer "rolls" cuts one of those aspects out. If you have a high surface area pot, you will still get the needed evaporation at a simmer, but you miss the agitation part.

Now you could just leave your flame up, boil and evaporate a large fraction of your beer - BUT - Heat load is bad for beer. The more heat you have to chuck at a wort in order to get it to meet conditions 1 & 2 - the less the final beer likes it. Too much heat load can effect flavour, colour, flavour stability, haze stability and propensity to staling. In general, less heat is better. Boiling "too hard" can do several of the same things and also mess up protein coagulation by breaking apart the bonds in the coagulated proteins and causing them to partially re-dissolve.

Now if your pot has a high surface area - it takes more energy shoved into the bottom to maintain a given level of rolling boil, heat is escaping through the surface (as increased evaporation) and you have to whack more in to get the same level of physical agitation in the wort.

So your high surface area ratio pot is hitting you with a double whammy - in order to maintain a rolling boil you are evaporating more liquid than is convenient or necessary, and to do it you are having to shove in more energy and higher heat load than is ideal (less being better) - you cant reduce your heat load or your evaporation by turning the heat down, because that will kill your boil vigour off. So what do you do??

Well ideally you get yourself a taller narrower pot, where you can natively get a good rolling boil and 8-15% boil off. Normally that's going to be a pot that will give you roughly a 1:1 ratio between the depth of your wort and the width of your wort. But that's a pretty loose target and will vary a fair bit.

The other way is to somehow reduce the surface area of your pot. The obvious way is to put a lid partially on - all sorts of people will tell you that a lid is a bad idea, but that's only true if you let too much of the liquid that condenses on it drip back into the pot (re-introducing the volatiles that have previously evaporated) A third on, Half on... its just not going to be an issue. If it still sounds like a bad idea - you could get tricky and use a tinfoil hat that allowed the opening to be partially covered.. but channel any condensation away to the outside of the pot. 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.

Back to disagreeing with Nick - Because I believe the above stuff to be true, and actually think its fairly important for beer quality, I think that to turn down your heat below the point of a rolling boil in order to cut evaporation losses (as Nick says he does) is basically a bad idea. I think a lid partially on, something floating on the surface of the boil, or ultimately a more appropriately shaped pot is preferable. That way any issues, small large or imagined are avoided, its not actually any harder or more complex; and you still solve your problem.

And his way of telling whether a boil is a rolling boil or not is still a cracker, and how I plan to describe it from now on.

Thirsty

 

[QldKev]

Good write up Thirsty. In my case I have always done what I would consider a strong rolling boil, but without boiling the sh!te out of it. I aim for 14% evap loss, one pot being a keggle shape kettle. Next time I may try the idea of floating something to reduce the surface area, and aim for a 10% boil off. I think lately I have had my beers slightly darker than to style, not much just a bit, and maybe this is from my caramelising some wort. Also the added benefit of better protein coagulation would also be excellent.

QldKev

 

[MeLoveBeer]

Good write up Thirsty.

+1, always educational Thirsty :icon_cheers:

My understanding was that a rolling boil was one where the rising bubbles actively shape the surface and a simmer was where the bubbles rise but the surface remains flat. I like the idea of floating something to reduce the surface area and evap rate... will give that a crack this weekend (currently getting high evap rates and having trouble supplying enough sparge water with my tiny HLT to compensate)

 

[MHB]

Again well put TB, and I to like Nicks description of a Rolling Boil.

In part where it's getting complicated is that all the research is done on big industrial kettles and we are talking about small domestic applications bound to be some confusion.

As a rule of thumb I would want to see 10% of the Knockout Volume (i.e. what you are putting in our fermenter) evaporated to ensure adequate stripping of volatiles. So for a 25 L batch 2.5 litres of evaporation.

When you are boiling a concentrated wort you would still need that same 2.5 L even tho it might be more than 10% of the kettle contents it's still 10% of the wort

MHB

 

[Nick JD]

That's a terrible definition of a rolling boil... BUT, I think it might well be the simplest, easiest and most understandable way to describe how to TELL if you have a rolling boil that I have ever heard!! Nice.

Now to disagree with you -

In a boil, you need to get two things to happen, and you need to work out a balance that allows you to do it.

1] You need to get a certain amount of evaporation - NOT - an amount in litres, a percentage! in order to be sure that you have evaporated your DMS, aldehydes, bad tasting hop compounds and other unwanted volatile compounds, you want to evaporate a minimum of 8% of your starting volume per hour

2] You need to maintain a physical movement of the wort and the formation of bubbles which reach the surface. This is your rolling boil... the wort needs to physically roll and churn around the kettle, and bubbles need to form. The physical agitation and bubbles allow much of the chemistry in the boil to occur - a lot of protein chemistry happens on the surface of the bubbles, hop chemistry happens under physical force, volatiles come out of the wort and ride up to the surface inside the bubbles.

So - turning a boil down to the point where it no longer "rolls" cuts one of those aspects out. If you have a high surface area pot, you will still get the needed evaporation at a simmer, but you miss the agitation part.

Now you could just leave your flame up, boil and evaporate a large fraction of your beer - BUT - Heat load is bad for beer. The more heat you have to chuck at a wort in order to get it to meet conditions 1 & 2 - the less the final beer likes it. Too much heat load can effect flavour, colour, flavour stability, haze stability and propensity to staling. In general, less heat is better. Boiling "too hard" can do several of the same things and also mess up protein coagulation by breaking apart the bonds in the coagulated proteins and causing them to partially re-dissolve.

Now if your pot has a high surface area - it takes more energy shoved into the bottom to maintain a given level of rolling boil, heat is escaping through the surface (as increased evaporation) and you have to whack more in to get the same level of physical agitation in the wort.

So your high surface area ratio pot is hitting you with a double whammy - in order to maintain a rolling boil you are evaporating more liquid than is convenient or necessary, and to do it you are having to shove in more energy and higher heat load than is ideal (less being better) - you cant reduce your heat load or your evaporation by turning the heat down, because that will kill your boil vigour off. So what do you do??

Well ideally you get yourself a taller narrower pot, where you can natively get a good rolling boil and 8-15% boil off. Normally that's going to be a pot that will give you roughly a 1:1 ratio between the depth of your wort and the width of your wort. But that's a pretty loose target and will vary a fair bit.

The other way is to somehow reduce the surface area of your pot. The obvious way is to put a lid partially on - all sorts of people will tell you that a lid is a bad idea, but that's only true if you let too much of the liquid that condenses on it drip back into the pot (re-introducing the volatiles that have previously evaporated) A third on, Half on... its just not going to be an issue. If it still sounds like a bad idea - you could get tricky and use a tinfoil hat that allowed the opening to be partially covered.. but channel any condensation away to the outside of the pot. 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.

Back to disagreeing with Nick - Because I believe the above stuff to be true, and actually think its fairly important for beer quality, I think that to turn down your heat below the point of a rolling boil in order to cut evaporation losses (as Nick says he does) is basically a bad idea. I think a lid partially on, something floating on the surface of the boil, or ultimately a more appropriately shaped pot is preferable. That way any issues, small large or imagined are avoided, its not actually any harder or more complex; and you still solve your problem.

And his way of telling whether a boil is a rolling boil or not is still a cracker, and how I plan to describe it from now on.

Thirsty

Or, you turn your stove's heat-o-meter to a heading of 275W maintaining what those in the Piss-Poor Brewing Monkey-Business call "A Wiggly Boil". I will take a video next time that will better describe this elusive creature.

 

[Thirsty Boy]

Again well put TB, and I to like Nicks description of a Rolling Boil.

In part where it's getting complicated is that all the research is done on big industrial kettles and we are talking about small domestic applications – bound to be some confusion.

As a rule of thumb I would want to see 10% of the Knockout Volume (i.e. what you are putting in our fermenter) evaporated to ensure adequate stripping of volatiles. So for a 25 L batch 2.5 litres of evaporation.

When you are boiling a concentrated wort you would still need that same 2.5 L even tho it might be more than 10% of the kettle contents it's still 10% of the wort

MHB

Actually that's one of the reasons why I think the old 8-15% rule is still a good one to follow... Lots of people will point you towards current research that talks about breweries that are only getting 3-4% boil off... But you are looking at modern kettles with internal calendria or even external boilers, wort strippers, low pressure dynamic boiling etc etc. And a home brewer just hasn't got any of that gear... We cant even close down the stack on our kettle to cut back evaporation a little.

The 8-15% thing however is an old rule of thumb that was developed when kettles were smaller, the heating method of choice was a flame under the bottom or at most a steam jacket and frequently even an open topped kettle. A hell of a lot closer to what you'll find in a home brewery.

Hell, brewing is a pretty forgiving process.. You're gonna have to try pretty hard to spoil your beer in the mash tun or kettle - but of all the ways I can think of, an inadequate boil is the one I think is most likely to come back and bite you.

If in doubt.... Too hard is way better than too soft.

TB

 

[Nick JD]

Too hard is way better than too soft.

Freud would have a field-day with ya, Thirsty. :lol:

 

[manticle]

you could get tricky and use a tinfoil hat

This doesn't work

http://people.csail.mit.edu/rahimi/helmet/

 

[Nick JD]

My guessing-o-meter says that reflux in wort boiling is bad. The steamy stuff needs to piss off.

Less steamy stuff = less piss off water = goodness beer. YEAH!

 

[PistolPatch]

Hell, brewing is a pretty forgiving process.. You're gonna have to try pretty hard to spoil your beer in the mash tun or kettle - but of all the ways I can think of, an inadequate boil is the one I think is most likely to come back and bite you.

If in doubt.... Too hard is way better than too soft.

TB

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 BIABrewer_Register.xls
(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.

The only practical advice that can be offered to another brewer until an impractical amount of research is done is to advise the good rolling boil. Anything less than this can quickly start to bring haze problems etc, etc..

Anyway, all I have found is the following and many people will disagree with it but I am a pretty good measurer when so inclined. I went through a stage where I went to great lengths to reduce my evaporation rate. All I now know is that...

1. Putting a lid over a kettle like mine (a straight-sided pot) did not do much to reduce the evaporation rate. Putting the lid even 3/4 over my kettle made little, if any difference.

2. Reducing to a simmer also made no or little difference.

I couldn't understand this at the time as so many people here told me it would. I then did some reading on the physics of evaporation and now understand well why 1 and 2 above were true.

Regarding 1 above, just think of your electric kitchen kettle. Most of the steam is out the spout before it even has a chance to even think of evaporating .

Regarding 2 above, this will take some reading up on the physics of liquids but basically there is little difference between a simmer and a gentle boil. In many "physical" ways they are the same. In other words, once you reach a critical point (the simmer) many things stay the same.

So, until us home brewers do a massive amount of research (which won't happen), a good rolling boil is the safest way to go. Concentrating on figures is certainly not the way to go.

Spot,
PP