Boil off rates

[JerD]

Is boil off rate consistant in a given kettle with the same heat source, regardless of amount of wort or water in the kettle? For instance, if I start with five gallons and finish with four after an hour, would I finish with five gallons if I started with six gallons after an hour?

 

[samc]

No - firstly your math is wrong and secondly different volumes would effect the rate of boil off. To what degree I don't know but kettle size, ambient temperature, etc. would create different results.

If you lose 1 gallon from 5 that's 20%
If you lose 1 gallon from 6 that's 16.667%

 

[jds]

JerD: My experience is yes. For a given rig, the volume of wort boiled off over time is constant, and reasonably linear. (e.g. 1 gallon oer hour volume loss = 1.5 gallons in 90 minutes)

I don't follow, Samc.

I do know that my kettle on my burner boils off around 1.5 gallons per hour, regardless of whether I start with 7 gallons or 11.5 in the kettle.

Unfortunately, many brewing calculators (like beersmith) calculate boiloff in a percentage of batch volume lost per time, which is indeed dependent on batch size.

So, to sum up: Volume of boiloff per unit time = constant for my rig.
Percentage of volume lost per unit time = variable depending on batch size and boil time.

 

[JerD]

I'll just do a couple of experiments. Just hate to burn through the propane

 

[AnOldUR]

I'll just do a couple of experiments. Just hate to burn through the propane

+1 on what jds said. Don't go by percentage of your volume. The largest factor in evaporation is surface area and that doesn't change with volume. The only time that volume would effect it is if your burner is underpowered and won't give you the same rolling boil with the larger batch. Regardless, you will have to experiment with you rig to determine it's gallons/hour rate.

 

[Catt22]

Yes and as mentioned, use gallons per hour, not percentage. Typical boil off rates are 1-2 gallons per hour. I ususally shoot for 1-1/2 gallons per hour regardless of batch size. I monitor the volume throughout the boil and sometimes I will increase or decrease the heat for the last 1/2 of the boil to get to my target volume if need be. I've learned what to expect for a moderate roiling boil and very soon, you will too.

 

[WorryWort]

Upon reading this I agreed with Samc but after a little research it seems evaporation is function of many things - pressure, humidity, density, temperature, and surface area.

Volume however doesn't really appear to matter.

Practically, at least for the homebrew scale you'll get the biggest impact by controlling the boil intensity. If your boil is very intense you will boil of more because you are in fact increasing temperature and surface area when you boil more violently.

 

[The Pol]

Yup, boil off is pretty much a constant, it is not a percentage of the volume of wort. I know brewing software computes it this way many times, but it is wrong.

Run a test... boil it, and you will have your #s.

Yeah, pressure, humidity yadda yadda will "affect" it... but not to a measurable degree on your scale.

 

[weirdboy]

The largest factor in evaporation is surface area and that doesn't change with volume.

++++++++ a million times

A particularly vigorous boil will make a difference vs. a simmer, but the volume won't.

 

[Catt22]

Yup, boil off is pretty much a constant, it is not a percentage of the volume of wort. I know brewing software computes it this way many times, but it is wrong.

Run a test... boil it, and you will have your #s.

Yeah, pressure, humidity yadda yadda will "affect" it... but not to a measurable degree on your scale.

The Pol has it right!

 

[samc]

My logic is what it is although obviously many disagree with what I stated. Not sure why it is not obvious to some that when you have a kettle that has a larger amount of sidewall surface area above the liquid exposed you have a greater amount of steam collecting on the walls and falling back into the boil. When you have liquid nearing the top of a kettle that boil off has a greater and faster chance of being carried into the atmosphere. Similarly if you cover your kettle most of the steam hits the lid and falls back into the wort. It may not be a significant amount on any given system but I'd bet a beer that it happens all the time.

"Volumes and boil time: First, estimate the evaporation rate during the boil based on past experience. A typical evaporation rate is 1.15 gal/h (4.4 L/h) for 6.6-gal (25-L) batches in my converted half-barrel kettle. Evaporation rates of 10%/h are typical in commercial systems but may be larger (>15%/h) on amateur equipment. The evaporation rate may also vary with the batch size on a particular system."

BT - Recipe Formulation Calculations for Brewers

 

[WorryWort]

Samc - Covering a pot isn't the same as sidewalls because the sidewalls are very hot and will further evaporate this condensation. Ever have a boil over? It's a ***** because all the stuff burns to the sidewall, not to mention everything else, after it bubbles up (i.e. it loses liquid to evaporation).

This is way too much info for such a minor topic, but people are mixing a lot of terms:

Evaporation rate - is a ratio of the volume of water lost to initial volume used over a given period of time. It should be a percentage or ratio. NOT A VOLUME. It should also indicate how long your boil was. Otherwise it's useless, or it's not a rate.

Evaporation Loss - The total volume lost to evaporation. For relevance in brewing, we should note the boil time too.


The evaporation rate WILL change directly with different batch sizes. It will even change slightly with the same batch sizes - because several variables will change as we brew from batch to batch. We can't control air pressure for example, nor should we try, but the effects will be negligible on our scale.

If you make 5 gallons and lose 1 gallons that's 20%, but if you make 10 gallons and lose 1 gallon that's only 10%. Ergo - VOLUME CHANGES THE RATE OF EVAPORATION. But all else being equal it won't change the actual Evaporation LOSS.

So actual evaporation rate on a system IS primarily related to volume. IT WILL CHANGE WITH DIFFERENT BATCH SIZES. But the total evaporation loss will not change much.

For our scale

Of all the things on a homebrew scale, boil time, surface area (i.e. changing kettles) and boil intensity are going to matter most from batch to batch. Ambient temperature matters too - if you boil outside in Ohio you'll different amounts in the summer than you will in winter because there is such huge temp swings there. I can double my rate, which is usually 10% on 6g batch, by cranking up the boil.

If you don't believe it try a not-so scientific experiment:

1. Fill about half your kettle (or some other vessel) with water.
2. Measure the volume before you boil it.
3. Bring it just to a boil, so the top is just turn over but not 'rolling'
4. Boil for 15 minutes.
5. Measure.
6. Repeat the exact same steps but fill the vessel with twice the volume of water.
7. For each batch - subtract the volume final volume from the initial volume - that is your total loss - not your rate, yet
8. For each batch, divide your total loss by the initial volume multiply x 4. That is your rate of evaporation per hour, for each batch at different volumes. You will find they are roughly equal. They will certainly not be double. This is pretty much exactly what we do when we brew, only with bigger volumes.