Kettle Evaporation Rate

[Ross]

Never had a problem with the burner Ross but I haven't brewed on a really windy day, just noticed that when there's a bit of a breeze I get a bigger boiloff - strictly qualitative observation I should add.

Do you have a shield around your burner?

No shield, as I'm in a fairly protected spot under the verandah, but with the burner turned down low once the boil starts, any breeze makes a big difference.
Not doubting your observations, but intrigued how a breeze gives you a significant increase in boiloff.

Cheers Ross

 

[Cortez The Killer]

I've wondered if the presence of a jumbo hop bag affects evaporation

I have a converted keg kettle and with the hop sock in there there is not a terrible amount of room (I'm thinking about increasing the size of the hole).

Would the sock interfer with the evaporation and make it condense again and fall back into the kettle?

Here's a pic of the kettle - is the hole big enough to achieve sufficient boil off?

Cheers

 

[braufrau]

No shield, as I'm in a fairly protected spot under the verandah, but with the burner turned down low once the boil starts, any breeze makes a big difference.
Not doubting your observations, but intrigued how a breeze gives you a significant increase in boiloff.

Cheers Ross

This is bit OT but I was pondering the logistics of AG the other day and thinking what do the outdoors guy do if, say, an autumn leaf lands in the kettle! :unsure:

 

[tangent]

don't boil under a tree
nothing on the leaf will last longer than a few seconds

 

[RobW]

No shield, as I'm in a fairly protected spot under the verandah, but with the burner turned down low once the boil starts, any breeze makes a big difference.
Not doubting your observations, but intrigued how a breeze gives you a significant increase in boiloff.

Cheers Ross

I assume from increased evaporation - the same reason washing dries faster on a windy day.

 

[BrotherNutz]

I worked out the evap based on 4.8KW over 1 hour. Came out to 8 liters. As it happens, that is just what I get!

Heat source is two 2.4KW immersion heaters. Boiler is an old 50L keg.

 

[Guest Lurker]

Not doubting your observations, but intrigued how a breeze gives you a significant increase in boiloff.

Cheers Ross

As the water evaporates, it starts to saturate the air sitting above the kettle to the point where it cant take any more water vapour. The faster you blow away the "wet" air above the kettle, and replace it with "less wet" air, the more evaporation can occur, assuming everything else stays the same. Haven't tried it, but supposedly for a barley wine to get extra evap you can point a fan at the top of the kettle.

 

[BrotherNutz]

As the water evaporates, it starts to saturate the air sitting above the kettle to the point where it cant take any more water vapour. The faster you blow away the "wet" air above the kettle, and replace it with "less wet" air, the more evaporation can occur, assuming everything else stays the same. Haven't tried it, but supposedly for a barley wine to get extra evap you can point a fan at the top of the kettle.

That doesn't sound like solid science there....

If I am applying X amount of power to the boil, it will seek to go no higher than 100degC (ignoring altittude adjustments) regardless of any extra power I add to it. It will simply boil faster, and evap more. The energy applied needs to go somewhere. If it can't dissipate, the boiler will build up pressure and THEN the temp of the boil can go past 100.

If evap were constrained in some way by the 'air' above the boil, either the pressure or the temperature of the boil would have to rise. And I think you would agree that that is highly unlikely.

 

[RobW]

That doesn't sound like solid science there....

If I am applying X amount of power to the boil, it will seek to go no higher than 100degC (ignoring altittude adjustments) regardless of any extra power I add to it. It will simply boil faster, and evap more. The energy applied needs to go somewhere. If it can't dissipate, the boiler will build up pressure and THEN the temp of the boil can go past 100.

If evap were constrained in some way by the 'air' above the boil, either the pressure or the temperature of the boil would have to rise. And I think you would agree that that is highly unlikely.

The issue isn't about how much power goes into the boil. If you leave a keg of water for long enough at any temperature you will get evaporation. It's about removing moisture laden air from the liquid/air interface and replacing it with "dry" air that can take up more water. The more "dry" air you blow over the liquid/air interface the quicker the liquid evaporates. The evap rate in still air is the baseline and by increasing the air movement you increase the evap rate (all other factors being equal).

 

[andreic]

Hi,

I have a 60L aluminium pot and a HP wok burner. I've only done 9 brews but I now generally aim for a 90min boil, starting at 34-35L and ending at 24-26L. Recently I've had a few odd results. On a few occasions I have ended up with much lower evaporation rates. One time I ended up with around 28L, the other time I boiled real hard for the last 45 mins, extended my boil time and just made it down to 26L. I am a night brewer outdoors in Melbourne... I think the nights in question were rather cold, and one was a bit wet, so I may boil longer or start with a lower volume for my next few brews.

I am now trying to take more detailed notes to see if I can get a bit more consistency in future.

Andrei

 

[PistolPatch]

I lose 7+ litres per 35 litre rolling boil in a 70 litre stockpot (450mm dia) over 90 minutes using a NASA.

:beer:

I've got to the stage where any threads involving measurements scare me :unsure:

A few months ago we had an intersesting discussion on evaporation rates in the BIAB thread. Thirsty gave some good reasons to not boil too vigorously.

After that I attempted to get my evaporation rate down. No matter what I did, I found it impossible to get down to 15%. I tried lid mostly on, reduced flame etc but I'm still up around matti's results. I lose around 10 litres in a 60 minute boil. It's got me buggered as TidalPete and I have exactly the same pot :blink:

I'm starting to shy away from doing too much measuring now - too depressing!
Pat

 

[sinkas]

Just to change the perspective, I always thought that one of the reasons for open boiling was to boil off, off flavours, impurities, etc, so I wonder why I am hearing about peoples "Lids half on" boils, is it not defeating much of the reason for boiling?

 

[Thirsty Boy]

I boil off 12-15% of my starting volume. Yes, I use the % figure and I actually change my setup (flame level, lid, pot size etc etc) to make sure that I fall in that range.

People constantly misunderstand what the % figure of boil off is for... its not really about how much you boil off in an hour, the figure is a tool for expressing the level of vigor in your boil.

If you are boiling off somewhere in the vicinity of 12-15% per hour, your boil is vigorous enough to do everything that its supposed to do, but its not so vigorous that you might be in danger of suffering some of the possible negative effects of boiling too hard. This is independent of the volume of your boil.

if you are always doing roughly the same batch size... then it just doesn't matter if you express your boil off as a percentage or a Litres/hour figure, but if you significantly vary your batch size then it will.

eg: I start with 22.5litres looking for 15% per hour (19into fermentor) and I boil off 3.4litres. Great, every time I do a batch I can just use 3.4ltres per hour... right?? Same pot, same burner settings.

Except when I do an 10 litre batch, then even if the rate was the same (it isn't) then I lose nearly 40% of my volume. The boil goes apeshit and is obviously far to "hot"

Conversely, if I do a 45lire boil volume, the same level of heat hardly makes the boil come to the surface, it barely ticks over! I'm probably still getting about 3.4litres per hour, but the boil is patently not "hot" enough.

Its obvious really, same energy in, same kettle geometry... changing the volume is going to change the vigor/hardness/anger/heat/level whatever you want to call it, of the boil.

But you dont want the vigour of the boil to be random, or variable depending on your batch size, you want it to be what you planned it to be... which is where the % figure comes in.

15% of 10 litres is 1.5lires/hour - a nice gentle rolling boil..
15% of 22 litrs is 3.4lires/hour - a nice gentle rolling boil..
15% of 45 litres is 6.75litres/hour - a nice gentle rolling boil..

Thats what the percentage figure is for. People who say that it is pointless or a waste of time, are usually applying the figure in a way that isn't really its purpose.

And Jeezus H Christ do people really boil off at 25% per hour?? bloody hell !! what for?? hell guys, the pro's/micros are going with something around 8% and the megas are doing closer to 4%... That high a rate just isn't needed; and it might be doing your beer a bit of no good.

Save yourself some money and turn the gas down a bit... sheesh

 

[AndrewQLD]

The purpose of a boil off rate is to aid in calculating final volumes in the boiler, using lt/hr rate I don't have to adjust my boil rate/vigour to come up with the same figure, I know I get 6lt evaporation everytime. Very simple and straight forward. I think the % rate is really for people that brew the same volume constantly and just makes the calculations more complex for those that brew varying sized batches.

But you dont want the vigour of the boil to be random, or variable depending on your batch size, you want it to be what you planned it to be... which is where the % figure comes in.

15% of 10 litres is 1.5lires/hour - a nice gentle rolling boil..
15% of 22 litrs is 3.4lires/hour - a nice gentle rolling boil..
15% of 45 litres is 6.75litres/hour - a nice gentle rolling boil..

I may have misunderstood the above comments but it seems you have contradicted yourself here. It looks to me like you Will have to vary the boil vigour based on your figures above in order to achieve the same evaporation amounts.

Cheers
Andrew

 

[warrenlw63]

As the water evaporates, it starts to saturate the air sitting above the kettle to the point where it cant take any more water vapour. The faster you blow away the "wet" air above the kettle, and replace it with "less wet" air, the more evaporation can occur, assuming everything else stays the same. Haven't tried it, but supposedly for a barley wine to get extra evap you can point a fan at the top of the kettle.

The issue isn't about how much power goes into the boil. If you leave a keg of water for long enough at any temperature you will get evaporation. It's about removing moisture laden air from the liquid/air interface and replacing it with "dry" air that can take up more water. The more "dry" air you blow over the liquid/air interface the quicker the liquid evaporates. The evap rate in still air is the baseline and by increasing the air movement you increase the evap rate (all other factors being equal).

So in layman's terms all that's occuring is a scaled down version of what creates rain I suppose? :blink:

Bulk buy for barometers anybody?

Warren -

 

[BrotherNutz]

The issue isn't about how much power goes into the boil. If you leave a keg of water for long enough at any temperature you will get evaporation. It's about removing moisture laden air from the liquid/air interface and replacing it with "dry" air that can take up more water. The more "dry" air you blow over the liquid/air interface the quicker the liquid evaporates. The evap rate in still air is the baseline and by increasing the air movement you increase the evap rate (all other factors being equal).

Yep, I agree with that contention, but you are now talking about a natural evaporation rate versus a forced phase change to a gas under huge amounts of applied energy.

"The issue isn't about how much power goes into the boil"

Yes, this does matter. I can boil with one element, or I can boil with two. My evap rate using maths alone is halved. In practice this is not the case due to energy lost through radiated heat etc.

"It's about removing moisture laden air from the liquid/air interface and replacing it with "dry" air that can take up more water"

The air is not moisture laden. Steam is a gas, not a liquid. Once the gas condenses, then it is a liquid/visible moisture.

Yes, I agree that if one blows air over a body of water for a couple of weeks, one will get more evaporation than the same body of water in still air. Air temp and humidity all being equal. However, we are not talking about boiling for weeks! We are talking about 60 to 90 min boils here.

Basically it comes down to this, know what your evap rate is and use these figures in ProMash and ye will be a happy beer brewer! :chug: :beer:

 

[fraser_john]

Last night I conducted an experiment, I added 32 litres of water to my kettle and boiled it, fairly vigorously, but not too hard. It was 4 degrees outside and I had a fan extracting the moist air outside very rapidly.

After 30 minutes of boiling, I chilled it and measured what I had left over. 27.5 litres (including what was left in the kettle).

Giving a loss rate of 9 litres/hour or 28% !!!!! Cripes! But, given this is only over a 32 litre starting volume. Assume that the loss rate is the same and I started with a 48 litre wort it would still result in an 18.75% boil off rate. Given some of the discussion in this thread, that is still very high!

Now, does pure water evaporate out of a boil faster than when a wort is boiled? I cannot imagine that it would be that much different? What about the head space left in a kettle whilst boiling, does that affect the rate?

Maybe it was my rapid movement of air across the top of the kettle and the low temperature that helped leach away the vapor?

I dunno, but I am still going to be struggling with how to get consistent end wort sizes into my ferment unless I get my kettle figured out :angry:

 

[Tony]

When we start out AGing*, every measurement we hear about seems really important. From what I have brewed or the beers I have tasted that others have brewed, none of these measurements are critical. Some people make them sound critical but they are not.


If you have a good thermometer and a good recipe (a good grain and hop bill) you should feel really confident in those and not worry too much about meaurements especially if you start low-tech. You might end up with a little more wort than you can fit in your fermenter. So what? Chuck it out. It's only a dollar or two of ingredients. You might end up with less than will go in your fermenter. Top it up with tap water.



Seriously though, if brewing was like housing, assuming you have the right basic equipment, I would yell not, 'Location. Location. Location.' I'd yell, 'Recipe. Recipe. Recipe.'

Pat Pat......... that was poetry.

And SOOOOOOOOO true :super:

All new AG'ers read these three phrases and memorise them!

I will add 1 more very critical item to good beer. Fermentation Temperature!

Now back on track....... All i did with my new kettle when i first started was what Pat just said. I didnt wory about the amounts too much. I actually worked it to undershoot the target volume cause you can just top it up with water to what it would have been if i got it right so who cares. You top a kit up from the tap dont you? I always did.

I worked on a starting number of 3 liters/hr from memory.

I measured the volume in the kettle with pre-marked stick in liters and then measured it when i shut off the burner after 60 minuites.

I measured 4 or 5 liters gone from memmory so my brew came out at 21 liters instead of 23. I fired up the hose and what do you know, back to what it should be.

These days i tend to deliberatly under shoot. 50 liter batches are heavy and a 45 liter batch is easier to lift. So with my system, i just fill the MT to the top with sparge water ( i dont measure it, i just pump it in till its full) stir it, recirc till claer and pump it in.

Sometimes it comes in close, sometines im 5 liters short but who cares. When i run it into the keg, i top it up with a few liters and its back to what it should be.

cheers

 

[SpillsMostOfIt]

So with my system, i just fill the MT to the top with sparge water ( i dont measure it, i just pump it in till its full) stir it, recirc till claer and pump it in.

In my 75litre boiler, my measurements tend to be along the lines of '22cm into boiler'. I don't know what the actual volume of that is - the shape is a bit wonky, so trying would be pointless. I sparge with '15cm into big saucepan'. I end up with enough to fill a 20 (24litre) jerry, which becomes 23litres in fermenter.

Disgustingly unscientific, but it works!

I'm running in a very small batch kit, which I fire on the stove and am trying to do the same thing. The height measurements don't add up and I figure I might have to throw more science at it... *sigh*

 

[PistolPatch]

Pat Pat......... that was poetry.

LOL Tony! While you were writing your post I was doing a very heavy edit on mine. I just deleted all the stuff you quoted!!! Good to see some of it made sense :beerbang:

Spot from Pat Pat