Boil-off Rate

[Vale71]

I haven't confused anything. The evaporation rate depends on the boil, time of year, humidity, etc.

I think you misread/misunderstood. Boil off rate aka evaporation rate. I wasn't rating about the boil setting.

Once again, unless you don't actually boil but wait for water to evaporate spontaneously neither humidity nor time of year play any role at all. Okay, if you brew outside then extreme temperature swings between summer and winter might make a small but noticeable difference, not so much if you brew in a temperature-controlled room indoors.

The only thing determining boil off rate is the power output of the heating system, its yield and any thermal losses your system has during boil which would further decrease its yield.

 

[hawaiibrewer]

I've found that the surface area of the boil, ie. 16 in. diameter kettle opening versus 24 in. does indeed impact the boil off rate. As a reference, my 15 gal keggle loses 1/2 gal per 30 min with a rolling boil. Bigger surface area, more loss.

 

[Vale71]

I've found that the surface area of the boil, ie. 16 in. diameter kettle opening versus 24 in. does indeed impact the boil off rate. As a reference, my 15 gal keggle loses 1/2 gal per 30 min with a rolling boil. Bigger surface area, more loss.

Do you adjust the power to get the same "appearance" of rolling boil? If that's the case then the increased power output is the reason for the increased boil-off rate, not the surface area.

Energy can neither be destroyed nor created, if you inject the same amount of energy into a system you will get the same result unless the yield (i.e. the losses) change. Some of the results can be counter-intuitive. For example, larger surface area in an open kettle means increased losses due to irradiation from the surface which in turn means lower boil-off rate. That is, unless you adjust the power setting thus changing a major variable making a simple side-by-side comparison meaningless.

 

[hawaiibrewer]

I brew with propane....I use my eyes to determine a rolling boil. When I had a large diameter 20 gal kettle, I lost more in boil off then I do with my smaller diameter 15 gal keggle. Use what ever science you want to explain it.

 

[Vale71]

To sum it up: you increased the burner's power output and observed a higher boil-off rate. You "chose" to attribute that to the kettle's larger surface area, presumably because that is the theory that on the surface (pun intended) is the most appealing to you, and ignore the obvious fact that you've been putting more energy into the system and that energy has to go somewhere (can you guess where?).

 

[hawaiibrewer]

That science is great, but the amount of energy/gas I put into the kettle is actually irrelevant to me since I'm concerned with the amount of wort I end up with to transfer into my fermenter. When I used a larger diameter kettle, I had fewer gallons to transfer. I'm not concerned with btu's needed, kWh used, etc. but rather gallons available to transfer.

 

[Vale71]

What's (ir)relevant in your case might not be (ir)relevant to some else. Your claim that "larger surface = larger boil off rate" is in any case wrong and could be misleading to others.

 

[Gozie Boy]

Cliff Notes are what we used in High School to cheat in English class.

My 10th grade English teacher was onto that, and asked us questions about items not contained in the Cliffs Notes. Doh!

 

[JohnDBrewer]

I use propane and keggles. As soon as I get about 4 inches of wort in the bottom of the BK I fire up the propane full blast. I fly sparge so it takes 25 or 50 minutes to fill depending on a 5g or 10g batch. I'm usually boiling when I hit my desired volume. I keep it wide open the whole boil except for short periods to control boil overs. I boil off close to 1.5 gallons per hour. I do this with all my beers to keep the process consistent so I'll be able to replicate the beer on future brews.

 

[Tobor_8thMan]

Once again, unless you don't actually boil but wait for water to evaporate spontaneously neither humidity nor time of year play any role at all. Okay, if you brew outside then extreme temperature swings between summer and winter might make a small but noticeable difference, not so much if you brew in a temperature-controlled room indoors.

The only thing determining boil off rate is the power output of the heating system, its yield and any thermal losses your system has during boil which would further decrease its yield.

Nope! Boil rate, humidity, time of year all factor into the boil off amount. Even as something as simple as a coffee pot sitting on the warming plate loses more volume when there is less humidity in the air than when the air is humid.

 

[Vale71]

Nope! Boil rate, humidity, time of year all factor into the boil off amount. Even as something as simple as a coffee pot sitting on the warming plate loses more volume when there is less humidity in the air than when the air is humid.

You can repeat it as often as you want but that won't make it right. Again, you're confusing spontaneous evaporation such as you'd have at the surface of a lake or any large water body with actually boiling water on a stove. Albeit related they're completely different phenomena.

 

[VikeMan]

You can repeat it as often as you want but that won't make it right. Again, you're confusing spontaneous evaporation such as you'd have at the surface of a lake or any large water body with actually boiling water on a stove. Albeit related they're completely different phenomena.

By my reckoning, even if it could be argued to add a full (spontaneous) evaporation rate (which is affected by humidity) to the boil off, and thus kludge together a case for the total loss being slightly different with varying humidity, we'd be talking about something on the order of thousandths of a percent of loss over the course of an hour or two boil. IOW, unmeasurable.

 

[stickyfinger]

So, the boil off rate as a percentage or gallons/hr should be the same whether you are doing a 5, 10 or 15 gallon batch in a 20 gallon kettle? That seems to mean that if I am getting 8% boil off at X% PID power for 15 gallons that I could use X/3% power for 5 gallons or X*2/3 for 10 gallons to be injecting the same amount of energy into the kettle? Does this work in practice?

As a side note, to get into the "optimal" boil off rate range what are people using on their Kal-clone setups with the ULWD curvy 5500W elements to get into that range for various volumes?

I want to try doing some beers in the optimal range and see if I can tell any difference.

 

[stickyfinger]

Found this at BYO:

https://byo.com/article/boiling-advanced-brewing/
"The amount of evaporation varies with the size and geometry of the kettle, the surface area exposed to the air, the intensity of the heat source and to a lesser extent other variables such as the ambient temperature, humidity, air pressure and any movement of air surrounding the kettle."

This seems to be true in practice, which means you can't just give an exact percentage and say that it is optimal for everyone. Right?

 

[Vale71]

A lot of what you'll find on that website is nonsense, I'm afraid.

 

[stickyfinger]

Finally read the Brungard article in Zymurgy about wort boiling. I'm interested to try doing some lower power boils. He of course has a lot of focus on making sure you're not getting DMS in your wort with the low power method. That makes sense. There is a trade-off between damaging the wort with excessive boiling vigor and not getting enough exchange of the wort at the surface with a very weak boil, which could result in off-flavors, such as DMS.

An interesting method he mentions is doing a 30 minute boil at very low power with the lid completely on to convert SMM to DMS and then do a more vigorous 30 minutes to convert more SMM and drive off all of the DMS. Seems like this would be an excellent way to get the benefits of having the wort at hot temps for a longer time without having to provide so much heat to the wort while still getting turbulence in the kettle. Anyway, I think I'll try the 30-30 method for my next beer, probably a NEIPA.

He also talks about using temperatures below boiling with pumping to cause the turbulence and wort exchange at the surface. Does anyone just do a wort recirculation at like 207F for 60 minutes? I could try that as well, using my two HERMS pumps back to back and recirculating through my ports in my kettle.

 

[Vale71]

I wouldn't try the recirculation of boiling wort with our cheap pumps. I fear that prolonged exposure to such high temperatures will certainly shorten their life expectancy.

The thing of boiling half the time with the lid on makes absolutely no sense. With the lid on you're reducing the radiative losses from the surface, with the lid literally acting as a mirror and reflecting the radiative heat back into the wort. This makes the wort boil more vigorously because there is more energy available for the vaporizatin of water, so basically you are boiling with higher available power just like you would do with a higher power setting.

 

[Franktalk]

A kettle with a steam condenser is the perfect solution to what Martin suggests. The lid being always on allows you to boil at very low power, and the condenser helps the DMS escape. As a matter of fact, when boiling with a condenser, you can smell the DMS in the effluent of the condenser. With my condenser my boil off rate is just over 5% and no DMS perceived in the finished beer.

 

[stickyfinger]

Yeah, I think I'll skip trying the recirculation approach.

I understand your point on the 30-30 approach. Of course you would turn down the heat input to a minimum to maintain the wort temp while the lid is on. Is using the trapped heat to maintain the wort temperature for 30 minutes better than just using a higher heat input with an open lid for 60 minutes better? I don't konw.

I wouldn't try the recirculation of boiling wort with our cheap pumps. I fear that prolonged exposure to such high temperatures will certainly shorten their life expectancy.

The thing of boiling half the time with the lid on makes absolutely no sense. With the lid on you're reducing the radiative losses from the surface, with the lid literally acting as a mirror and reflecting the radiative heat back into the wort. This makes the wort boil more vigorously because there is more energy available for the vaporizatin of water, so basically you are boiling with higher available power just like you would do with a higher power setting.

 

[stickyfinger]

I finally got around to trying a lower PID % for my boil. Used my 20G Concord pot with 5500W element. Set the PID at 59% (I would normally have used maybe 75-80%.) I would normally expect 1.5-1.7 Gal/hr. Today I got 0.85 Gal/hr with 14.7 Gal pre-boil, to give an evaportion rate of around 5.8%. I had the lid off the entire time and the PID stayed at 212F. I think I could go down more on the PID and still maintain 212F. I guess I'll fine-tune it with time. I hope to scale the PID% with the volume (since I do all different sizes of batches) and see if I can get repeatability.

 

[Tobor_8thMan]

Depends. We always want a nice rolling boil and not some crazy, volcanic activity. Once satisfied, adjust the equipment profile in your favorite brewing software appropriately.

For me, in my situation, 75% boil, works. I realize everyone's setup is different, but 75% works for me.

 

[chipmunk]

The thing of boiling half the time with the lid on makes absolutely no sense. With the lid on you're reducing the radiative losses from the surface, with the lid literally acting as a mirror and reflecting the radiative heat back into the wort. This makes the wort boil more vigorously because there is more energy available for the vaporizatin of water, so basically you are boiling with higher available power just like you would do with a higher power setting.

I believe the net energy available for vaporization will be similar between lid on and lid off. The radiative heat losses “seen” from above the pot will be similar - it’s just the difference between radiative heat loss from water/steam at 212F and radiative heat loss from a pot lid at 212F. In other words, the mass loss of the water (i.e., net boiling rate leaving the pot) will be very similar with lid on or off. Although it is true you will get a more vigorous boil due to “reflective effect” within the pot, much of this steam condenses on the lid (...and maintains the lid at 212F). I’d expect the net boil off rates to be very similar. I’m sure there will be differences in heat losses due to emissivity differences (water vs lid) and/or convection and pot surface temps, but these should be minor.
So to me it does make sense to boil with the lid on to allow a higher degree of agitation. Although you will get a higher boiling rate in the kettle, the excess steam will condense back into the wort.