What's an accurate evaporation rate for the hour long boil?

[tenbricks]

Percent doesn't work for me. Boiloff is usually a fixed amount no matter how much liquid is in the same kettle. I lose about 1.5 gallons in 70 minutes in my keggle regardless if I start with 13 or 8 gallons.

This.

Boiloff rate is determined by (heat in) = (heat out), the temperature remaining a constant 212. In practice I think heat out is almost entirely the boiling (vaporization) of the wort, much less so by surface area/size/shape. Heat in is determined by rate of propane burning and how efficiently it's heating the kettle, which will be a constant regardless of boil size. Thus, mostly it will be constant amount per time, not constant percentage.

The Blichmann Boilermaker 20 gallon kettle will boil off 2 gallons in 90 minutes. I'm attributing that to the larger diameter of the kettle compared to a keggle.

I have the same pot, and see a bit more than a gallon/hour for 5 or 10 gal batches. Must not have the propane as high up as EdWort does.

 

[wailingguitar]

I'm tweaking the Beersmith settings and the default evaporation rate is 11%. So I would collect 7 gallons of wort and boil down to 5.5 gallons in an hour. That sounds a bit high. Has anyone ever calculated this based on a turkey fryer setup? I have a 30 qt aluminum pot.

Or, what do you have your Beersmith evaporation rate set at? How close is it, typically?

That is about what I am getting on mine (33qt tamale pot on a Bayou Classic)... about a gallon and a half (7 to 5.5) boiled off in an hour as opposed to a gallon (6.5 to 5.5) when done on the electric stove top. I have never used Beersmith or anything like that, so I have no idea about it's setup or defaults.

 

[hausofstrauss]

I would agree with the arguments for a volumetric boil off rate related to the surface area of the water rather than a percentage of total volume.

I also would think that environmental conditions would have an impact on that number too.

A hot dry day would allow more water loss, versus a hot humid day would absorb less.

If you are always working indoors and have a relatively constant temperature and humidity, you are in the best case scenario.

 

[simchuck]

Listen to @blacklab and @bobbrews. The evaporation rate depends on the surface area of your brew kettle, the rate of boil (e.g., rolling boil), and to a lesser extent, the altitude. A percentage can be accurate if you always boil the same volume in the same kettle at the same rate, but as soon as you change one of these variables that percentage will be wrong.

If your kettle is tall and thin, it will generally have lower evaporation rate than a short and wide kettle. If you use a simmering boil, your evaporation rate will be much lower than a vigorous rolling boil. It doesn't matter how much you are boiling, because the boiling rate is the same (as judged from the surface - simmer, rolling, etc) and the surface area for the pot is the same.

If all we cared about is minimizing the evaporation rate (which would save water and energy), we might choose a tall, think kettle and simmer for an hour. But beer quality usually trumps these concerns, so the choice of brewpot and boiling rate is not so straightforward.

 

[qmax]

I concur on the physics. The evaporation rate depends on the heat balance.

The amount of heat that goes in the kettle is your BTU's which are almost directly proportional to the flow of gas to the burner (how high you set the flame) or the electrical power supplied to the heating element.

The amount of heat that is lost to the surroundings via convection and radiation is proportional to the surface area of your kettle and its temperature. Even if your kettle is half-full, its walls are going to have about the same temperature from the bottom to the top as if it were full, because of the high thermal conductivity of the metal.

The difference between the supplied heat and lost heat is spent on the latent heat of vaporization, and that's what determines your evaporation rate.

How much of the actual wort area is exposed is not so much of the factor as it would be in the case of evaporation at room temperature.

That being said, the amount of wort in the kettle does not have a significant effect on your evaporation rate. Hence it is a fixed value rather than a percentage.

 

[tskinsv]

I am fairly new to brewing and decided to do some 1 gallon batches to try out 4 different hops with the same wort. After I did the first couple of batches I was stunned at the water loss. I had used the percentage I used for 5 gal batches. But that percentage was not close in this case and I was left with ½ gal when I was expecting a gallon. That got me thinking that the amount of water that boils off is a constant for a specific brew pot.

After reading the many answers on this subject I was still left not really understanding what is going on with the amount of water that boils away when I brew. I was going to say evaporation but it turns out in science there is a difference in evaporation and boiling. We are not losing water through evaporation when in the boil. It also turns out that water temperate is the same for a simmer vs. a rolling boil. Closer you are to sea level the hotter your boil is going to be. Now the article talked about more heat the boil would be faster, like a rolling boil, but the article did not address if you lose more water at a rolling boil than a simmer.

I found a high school science write up on the subject which shed a lot of light. It also got me thinking how much altitude plays into boil time. Based on the fact that water boils at lower temperatures in altitude you might have to boil your hops longer to get the same IBU as someone who is down at sea level.

I am going to look to see if I can find a formula that provides an answer as I am sure one exists. If I find I will post. AT the bottom in quotes is the article write up I found.

The one thing that is clear is you should get the same amount of water loss using the same pot, and I am guessing the same heat resulting the same boil profile (rolling to simmer) no matter what the volume of wort you are boiling. Thus using a percentage is incorrect. Baselining your equipment boil off, keeping the heat the same, should provide a constant volume loss for planning purposes.

“As the temperature of the water increases, its vapor pressure increases. When the vapor pressure equals the atmospheric pressure on the liquid, the liquid will boil. At high altitudes, the boiling point of liquids is lower than at sea level. In Denver, Colorado, water will boil at about 94°C. Do not confuse boiling with cooking. Cooking pasta in Denver is a slower process because the water is at a lower temperature. Also, realize that water boiling rapidly is no hotter than water boiling slowly. The temperature of the water remains constant during the boiling process. And, the temperature of a boiling liquid never rises above its boiling point. No matter how much heat is applied, the liquid only boils faster, not hotter.

There are fundamental differences between evaporation and boiling. Evaporation occurs at temperatures lower than the boiling point of the liquid. Also, evaporation occurs at the surface of the liquid, whereas in boiling, bubbles of vapor arise inside the body of the liquid. For a bubble to form, the pressure of the atmosphere on the surface of the liquid must be overcome."


Added:

After researching some physics sites it is obvious there are a lot of variables in play. BTU output being applied, atmospheric pressure (including height from sea level), wind, ambient temperature, pot metallurgy and thickness, pot diameter.

To calculate it would change for each boil based on weather. I think the key is to figure out your heat source to get a consistent boil the way you want it. I prefer rolling boil. Then do a baseline on your pot with water. I chose to use a sick and mark the differences from start, boil start, completion of 1 hr. boil, cooled 80 degrees F. Then I used a volume equation to measure the actual boil off and shrinkage after boil off to 80F. I have a Blichmann Boilermaker 10 Gal. Using the measurements I calculated the following 0.72 gal of boil off then 0.24 for the shinkage from boiling to 80F. So basically 1 gal. So now I am going to test with a boil where I would like to have 1 gal of wort. Based on previous results the 1 gal of extra water sounds correct.

I live in Southern California near sea level (850 ft). Our temperatures are fairly consistant year round. If I wanted to get fancy I could baseline and measure barometric pressure and ambient temperature. Then as I brew I could see how those variables affect the outcome. Right now I am assuming they will not have a lot of affect and holding the BTUs being applied to get a consistent rolling boil is the variable I really want to control. IF I don’t get reasonable consistency I will look at other variables. But I know why many experienced brewers take wort sugar readings to determine if they need to continue boiling or have boiled too much and need to add water.

 

[mgortel]

I boil off 1.5 GPH on my setup and gas valve position.......determine your boil off rate by experiment and use that.

The key is to make sure you use the same position for your gas valve on burner every time so you are adding the same amount of heat on each brew.....if your flame is higher or lower from brew to brew your boil off rate will vary.

What oyu are looking for is a predictable boil rate so you can plan accordingly on your preboil volume to get the proper final volume

 

[AnOldUR]

The key is to make sure you use the same position for your gas valve on burner every time so you are adding the same amount of heat on each brew.....if your flame is higher or lower from brew to brew your boil off rate will vary.

In my opinion that sounds good in theory, but not in practice. There are too many other variables. Volume, ambient temperature and humidity the big ones. At a homebrew level it's more about the perceived intensity of boil; experience with your system. Not perfect, but the best we can do.

 

[mgortel]

In my opinion that sounds good in theory, but not in practice. There are too many other variables. Volume, ambient temperature and humidity the big ones. At a homebrew level it's more about the perceived intensity of boil; experience with your system. Not perfect, but the best we can do.

I agree that those other variables do effect the boil off rate slightly...but the most significant effect on the boil rate is heat addition......make that consistent and you will be pretty damn close. In practice it has worked for me. "Perceived intensity" is not a measurable quantity and therefore cannot be used for consistency....at least by this brewer

 

[lhommedieu]

I boil off 1.5 GPH on my setup and gas valve position.......determine your boil off rate by experiment and use that.

The key is to make sure you use the same position for your gas valve on burner every time so you are adding the same amount of heat on each brew.....if your flame is higher or lower from brew to brew your boil off rate will vary.

What oyu are looking for is a predictable boil rate so you can plan accordingly on your preboil volume to get the proper final volume

Agreed.

My pre-boil volume can vary as I use the SG and pH of the final runnings to determine when to end the sparge. I accept what I get. I use my gallon/hr rate as a rough predictor of when to measure OG - and then adjust accordingly.

 

[AnOldUR]

My pre-boil volume can vary as I use the SG and pH of the final runnings to determine when to end the sparge.

...but the most significant effect on the boil rate is heat addition......make that consistent and you will be pretty damn close.

The larger the volume the more heat intensity you need to maintain the same boil-off rate, so if the volume varies as lhommedieu says above, he would have to adjust his burner accordingly to get the same rate with each boil.

 

[lhommedieu]

The larger the volume the more heat intensity you need to maintain the same boil-off rate, so if the volume varies as lhommedieu says above, he would have to adjust his burner accordingly to get the same rate with each boil.

That's true - but there is another consideration at play: I'll always want to get enough wort to fill two 6 1/2 gallon carboys; if there is any wort left over, I'll pressure cook it in quart mason jars for use later as krausen, starter, etc.

I'll typically take an OG reading towards the end of the boil when my level starts to approach 15 gallons (I'm using leaf hops, so I have to account for what's going to be left in the boiler.) If the OG is too high, I may dilute with some water; if it's too low, I may boil for a while longer.

 

[tskinsv]

That's true - but there is another consideration at play: I'll always want to get enough wort to fill two 6 1/2 gallon carboys; if there is any wort left over, I'll pressure cook it in quart mason jars for use later as krausen, starter, etc.

I'll typically take an OG reading towards the end of the boil when my level starts to approach 15 gallons (I'm using leaf hops, so I have to account for what's going to be left in the boiler.) If the OG is too high, I may dilute with some water; if it's too low, I may boil for a while longer.

I have a refractometor, what can I use to sample wort during the boil that will not melt. They provided a plastic eye dropper with the refractometer but I would be afraid it would melt.

 

[lhommedieu]

I have a refractometor, what can I use to sample wort during the boil that will not melt. They provided a plastic eye dropper with the refractometer but I would be afraid it would melt.

Use a ladle and take a very small amount of wort out and pour it into a Pyrex measuring cup. The cup will absorb some of the heat of the wort and the sample will cool quickly due to the amount of surface area exposed to the air. In a couple of minutes, use the eye dropper to pick up a small amount of wort, and place your sample on the refractometer.

That said, I've never really had a problem with the plastic eye droppers that came with my refractometer; using the ladle just keeps me from burning my fingers with hot wort.