Kettle Evaporation Rate

[blackbock]

All of this is very interesting but I wonder just how many of you do know exactly how much you are boiling off per hour? I still haven't worked out how to predict this accurately from batch to batch. I usually allow about 5L per hour, and sometimes this works out close to correct for me, other times I find that my batch has shrunk significantly and I scratch my head to work out why it varies so much.

A rolling boil with 25mm height sounds like a simple definition, but I'm sure as hell not about to try and measure the peaks and troughs of a 50L kettle of boiling wort. I figure as long as the wort is bubbling enough to move a hopsock around a fair bit it should be enough to do the job. It seems like a waste of energy to boil off nearly a quarter of a batch in an hour to me.

All of the advice I've ever heard suggests that keeping the lid off during the boil is the way to go, so that's what I do.

If fraser_john does work out a system to accurately predict the Kettle Evaporation Rate I want to hear all about it.

 

[devo]

All of this is very interesting but I wonder just how many of you do know exactly how much you are boiling off per hour? I still haven't worked out how to predict this accurately from batch to batch. I usually allow about 5L per hour, and sometimes this works out close to correct for me, other times I find that my batch has shrunk significantly and I scratch my head to work out why it varies so much.

A rolling boil with 25mm height sounds like a simple definition, but I'm sure as hell not about to try and measure the peaks and troughs of a 50L kettle of boiling wort. I figure as long as the wort is bubbling enough to move a hopsock around a fair bit it should be enough to do the job. It seems like a waste of energy to boil off nearly a quarter of a batch in an hour to me.

All of the advice I've ever heard suggests that keeping the lid off during the boil is the way to go, so that's what I do.

If fraser_john does work out a system to accurately predict the Kettle Evaporation Rate I want to hear all about it.

I use BrewAlchemy which has an evaporation calculator.

 

[blackbock]

Sounds worth a look Devo...

 

[devo]

Sounds worth a look Devo...

It's a pretty good piece of software that runs on an OSX platform. Updates come out quite regular as well.

 

[blackbock]

d'OH! Mac only...

 

[fraser_john]

If fraser_john does work out a system to accurately predict the Kettle Evaporation Rate I want to hear all about it.

Sorry, but I have the same quandry you do! Some days it appears I boil off about 10%, other days it gets as high as almost 20%!!! I use Promash boil off calculator using before/after volumes.

It seems dependant on so many factors, SG, weather, breezes, initial volume etc.

I am at the point of resigning my self to setting my Promash default setting to around 12%. If I get a little extra chilled wort at a lower SG, oh well, its a weaker beer and it goes further. If I get a little less chilled wort at a higher SG..... I go to bed earlier when drinking it.

 

[Julez]

In the past I have "estimated" my kettle evaporation rate, and it has been a dismal failure. Tomorrow night I am planning on a true experiment by measuring in 40 litres of water and doing a full hours boil and chilling through the CFC to get my rate figured out once and for all.

Have others done this or do they do their best estimation as well? If so, complete the polls so we get an idea.

Personally, I use a converted keg, I had an extra 6 inches of height welded in so I can do a full vigorous boil of a 38 litre batch without fear of a boil over.

This may be an old topic, but a very interesting one!! After my first AG with a HUGE loss due to evaporation that didn't tally much with any indicative evap figures I had previously read, I had to go in search of some answers. This thread was full of them, but the below cut + paste from Wikipedia sums up causes of evaporation quite nicely:

Factors influencing the rate of evaporation

• Concentration of the substance evaporating in the air: If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly. e.g. humidity in the tropics compared to dry areas
• Concentration of other substances in the air: If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating.
• Flow rate of air: This is in part related to the concentration points above. If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. This is result of the boundary layer at the evaporation surface decreasing with flow velocity, decreasing the diffusion distance in the stagnant layer. e.g boiling outside
• Concentration of other substances in the liquid(Impurities): If the liquid contains other substances, it will have a lower capacity for evaporation.
• Temperature of the substance: If the substance is hotter, then evaporation will be faster.
• Inter-molecular forces: The stronger the forces keeping the molecules together in the liquid state the more energy that must be input in order to evaporate them.
• Surface Area: A substance which has a larger surface area will evaporate faster as there are more surface molecules which are able to escape. e.g. brewing a small batch in a large pot - this was my problem. Surely this is also the reason most people seem to report similar evap amounts in litres, almost irrespective of batch size (assuming the same kettle is used). A number of posts reported, say 4L of evap, whether it was a 20 or 40L boil. If the kettle used is the same size for both batches, the surface area is also the same and presumably, the QUANTITY of loss would also be similar (but not the rate)!

It's clearly possible to have a "nice, rolling boil", but a very large loss due to evaporation, if your kettle size is mismatched to your batch size.

One question though - assuming you allow for a large loss due to evaporation, and match your hop additions and boil times accordingly, while also maintaining a gentle, rolling boil that everyone refers to, then is there a problem with say 40% loss in this scenario? This is what I had with a 15L batch in a 50L kettle. In my case, it caused me to stuff up the gravity and IBU calcs, but it was my first AG run and my losses were based on estimates only. If this loss rate is allowed for though, is it in itself a problem? I'd like to continue doing small batches in my largish kettle, rather than up the batch size unecessarily.

Cheers, Julez.

 

[domonsura]

Your evaporation rate is going to be fairly consistent( but with a small variance in it due to wind / atmospheric pressure / relative humidity), and is going to be more accurately given as a percentage of boiloff of your overall kettle volume, because the factors that affect it are boil vigour and surface are from which to evaporate from.

You will have the same surface area to evaporate from regardless of what volume you are actually boiling. The way you seem to be looking at it (apologies if I'm wrong and correct me if I am) you are expressing your evaporative loss as a percentage of the volume you are boiling which won't work.
In a 70 litre kettle, I used to get roughly 10 litres of evaporative loss over 60 minutes, which is just under 15% loss. As you are calculating it, if I was boiling 70 litres, I would lose that amount. However if I was boiling 35 litres, I still have the same surface area and same 10 litre loss, but 10 litres is nearly 30% loss of the boiled volume......(but still only 15% of the kettle volume.)
So when looking at evaporative loss, ignore the volume you are boiling (to a point) and calculate it solely on the volume of the kettle, as the actual amount of loss will not fluctuate all that much in my experience.

 

[Stuster]

Whatever you do, don't let Thirsty Boy see this thread.

As domonsura says, you're going to get roughly the same evap volume in your kettle every time, whatever the volume. That is unless you reduce either the flame or the kettle opening. The first is not going to necessarily a good idea as you might not get the right level of hop isomerisation, protein coagulation etc etc. So, you can follow the 'Darren method'. Keep the lid off for the first part of the boil, then partially cover the kettle. I do this to reduce the evap. rate for my kettle and it works for me.

 

[Guest Lurker]

Hi Julez

Yes, for the purposes of working out your water requirements, you should use a rate of evap in l per hour, as using a percentage will get you in trouble each time you change the batch size.

You will get different answers on the effect of evaporating too much, mine is it wont cause any problems, and you should use the same pot and sparge a bit more and top up the kettle prior to or during the boil. Remember in designing an AG brew you are aiming to hit a gravity rather than a volume, although once you get used to it you will accurately predict both.

 

[SpillsMostOfIt]

... Remember in designing an AG brew you are aiming to hit a gravity rather than a volume, although once you get used to it you will accurately predict both.

+1. Once you live this, life gets better, as does your beer.

 

[mika]

I must be working backwards then. On principle I refuse to water a beer down, so I aim for the 20L I want into the fermentor and whatever the gravity is, is whatever the gravity is. But then I'm also not fussed if I end up with a litre or two extra.

 

[Julez]

Your evaporation rate is going to be fairly consistent( but with a small variance in it due to wind / atmospheric pressure / relative humidity), and is going to be more accurately given as a percentage of boiloff of your overall kettle volume, because the factors that affect it are boil vigour and surface are from which to evaporate from.

You will have the same surface area to evaporate from regardless of what volume you are actually boiling. The way you seem to be looking at it (apologies if I'm wrong and correct me if I am) you are expressing your evaporative loss as a percentage of the volume you are boiling which won't work.
In a 70 litre kettle, I used to get roughly 10 litres of evaporative loss over 60 minutes, which is just under 15% loss. As you are calculating it, if I was boiling 70 litres, I would lose that amount. However if I was boiling 35 litres, I still have the same surface area and same 10 litre loss, but 10 litres is nearly 30% loss of the boiled volume......(but still only 15% of the kettle volume.)
So when looking at evaporative loss, ignore the volume you are boiling (to a point) and calculate it solely on the volume of the kettle, as the actual amount of loss will not fluctuate all that much in my experience.

Agreed! Percentage loss varies in relation to wort volume, but not by very much in relation to the kettle volume. THAT makes sense

 

[Julez]

You will get different answers on the effect of evaporating too much, mine is it wont cause any problems, and you should use the same pot and sparge a bit more and top up the kettle prior to or during the boil. Remember in designing an AG brew you are aiming to hit a gravity rather than a volume, although once you get used to it you will accurately predict both.

Thanks Guest Lurker, that's pretty much what I thought. It's obviously the overall balance that is important. Though volume does impact on that balance (if you get it wrong). But yep, agree with your comments, gravity (and IBU in relation to that gravity) is what you are aiming for, rather than volume. Cheers :icon_cheers:

 

[Maxt]

I now boil in a 50L keg shaped vessel, and lose about 6L in a 60min boil. My 80L kettle has such a massive surface area that I would lose over double that in a 60 min boil.

 

[Maxt]

I now boil in a 50L keg shaped vessel, and lose about 6L in a 60min boil. My 80L kettle has such a massive surface area that I would lose over double that in a 60 min boil.