What determines boil off rate

[oasisbliss]

What determines boil off rate? Furthermore, is it your altitude or ??
my question is my boil off rate on my gas oven is about 1.25 per hour.
I am going to get a propane turkey cooker and do the boils in my garage now can I assume the same 1.25 boil off or should i retest it ?

thanks

 

[Gixxer]

I would test it. If you are living in like Denver CO, then altitude affects boil compared to sea level. Pot dimensions and how hard you stick the spurs to it with the propane burner all affect boil off.

 

[milesvdustin]

Test for sure mine tends to vary sometimes. I have crappy burner though. I boiled off three gallons last time I made a ten gallon batch! It was cold and really windy outside too.

 

[mgortel]

Boil off rate is determined by HEAT ADDITION primarily...........

Liquid surface area of pot, and size of pot, and ambient temperature to a lesser extent.

Like the others said.....do experiment with your setup....and determine boil off rate for your specific pot.

NOTE the location of your oven....when you do ur experiment.....make sure you write it down....i.e. 50% open on gas knob......do your test...start with a specific volume of water.....let it boil for 60 mins....measure final volume and voila....you now know your boil off rate.

Just make sure when you brew....to put knob in same exact location...i.e. so you have same exact heat addition.

 

[oasisbliss]

thanks to all this really helps !

 

[kanzimonson]

You'll probably have more control with the turkey fryer. I aim to boil 15% of the wort off and end up with 6gal at the end of the boil. So that means I need to start with 7.06gal.

But in reality what I do is fill the kettle to 7gal, check the gravity, use math to figure out what the gravity will be at 6 gal, and then boil until I reach it. I check at the 30min mark to make sure I'm around halfway there gravity-wise. If I'm under, I turn up the flame. Over and I turn it down. Eventually you'll just know what your boil looks like to achieve 15% boil off. It's way less vigorous than you expect.

 

[acidrain]

The boil off is not a percent.
It is a function of surface area of the pot.
The same amount will boil off whether it's 10 gallons or 20 gallons in the same pot.
The variables are surface area, ambient temp, ambient humidity, barometric pressure, and heat applied.

 

[Yooper]

It's not just about the heat applied- it's also about humidity. If you live in the desert, you'll boil off far more than someone who lives in Miami.

I boil off more in the winter, even on my own indoor setup, due to the big changes in humidity.

 

[kingwood-kid]

The boil off is not a percent.
It is a function of surface area of the pot.
The same amount will boil off whether it's 10 gallons or 20 gallons in the same pot.
The variables are surface area, ambient temp, ambient humidity, barometric pressure, and heat applied.

What he said, except that applying a given amount of heat to 10 gallons will yield a different result than applying same amount to 20 gallons, namely a higher boil-off.

 

[Yooper]

What he said, except that applying a given amount of heat to 10 gallons will yield a different result than applying same amount to 20 gallons, namely a higher boil-off.

How? I don't get that.

In the same pot, boiling 7 gallons or boiling 12 gallons will still boil off, say, 1.5 gallons. It's not possible that the boil off changes IF the pan is the same, the temperature is the same, and the climate is the same. I boil off 2 gallons in the winter with my system, whether I start with 3 gallons or 13. The amount in the kettle doesn't matter.

 

[kingwood-kid]

Don't you have to turn the heat up more to achieve that? If not, I stand corrected.

 

[Pivovar_Koucky]

Absolutely retest:
rate heat is applied, geometry of pot, ambient temperature, pressure and humidity all affect boil rate.

Acidrain is right, you don't boil off a % of your total volume in a given time. You boil off a certain volume over a certain time depending on the factors mentioned.

 

[HopSong]

Yah.. time is relative here.. above factors plus ambient temperature and wind. With my old burner, at times I'd have a difficult time getting the pot to a boil when it was windy outside.. I could have had the fire on for hours and still not had the boiloff that I'd have on a warm, calm day.

 

[kanzimonson]

All I'm saying is that if you're a good brewer you're aiming somewhere around 15% boiloff rate per hour.

 

[Gameface]

All I'm saying is that if you're a good brewer you're aiming somewhere around 15% boiloff rate per hour.

Why does boiling off 15% make you a good brewer, or not doing so make you a bad brewer? I'm really confused.

I live in a high altitude city with usually very low humidity. Does that make me a good brewer or a bad brewer?

 

[Yooper]

Why does boiling off 15% make you a good brewer, or not doing so make you a bad brewer? I'm really confused.

I live in a high altitude city with usually very low humidity. Does that make me a good brewer or a bad brewer?

Oh, no! I must suck. When I make a 5 gallon batch, I start with 7 gallons. When I make a 10 gallon batch, I start with 12 gallons (I boil off 1.5 gallons per hour). I must suck at 5 gallon batches, I guess.

 

[kanzimonson]

Haha, I knew that would be a nice trap. Sorry for trolling.

But for real, over 15% and you have to wonder if you're generating too much melanoidins (and wasting fuel for that matter). If your boil is too low then the question is if you're driving off stuff like DMS.

 

[Yooper]

Haha, I knew that would be a nice trap. Sorry for trolling.

But for real, over 15% and you have to wonder if you're generating too much melanoidins (and wasting fuel for that matter). If your boil is too low then the question is if you're driving off stuff like DMS.

I agree- but targeting a % is not the way to go! It's because, as you can see, I boil off the same volume whether it's a 10 gallon batch or a 5 gallon batch, so of course a smaller batch will have a higher percentage of boil off, but it's not an issue at all.

I think most people would agree that in a "normal" situation, it's pretty routine to boil off 1- 1.5 gallons per hour. It'll be more in a very dry climate, or with a super wide pot, and less in a tall narrow pot and in a humid climate.

A good boil is a rolling boil. Not a simmer, but you also don't have to have the pot shaking and boil so vigorously that you lose half your wort!

A nice rolling boil will have the bubbles breaking at the top, will have a nice hot break, and will boil off about 1-1.5 gallons per hour.

 

[kanzimonson]

Here's a question for Yooper. So since it sounds like you're using the same pot for 5 and 10gal batches, and you expect the same boiloff volume for every batch regardless of size, I assume with the 10gal batches you have to sparge with less water so that your preboil volume is lower than normal.

Or put another way, your preboil 10gal volume is not double your preboil 5gal volume?

If this is correct, do you have to adjust your efficiency for different batch sizes?

 

[Yooper]

Here's a question for Yooper. So since it sounds like you're using the same pot for 5 and 10gal batches, and you expect the same boiloff volume for every batch regardless of size, I assume with the 10gal batches you have to sparge with less water so that your preboil volume is lower than normal.

Or put another way, your preboil 10gal volume is not double your preboil 5gal volume?

If this is correct, do you have to adjust your efficiency for different batch sizes?

Let's see- Yes. No. No. No.

Unless I have a huge grainbill and so undersparge as to stop when I"m at my preboil volume, my efficiency is relatively constant over all size grainbills and all size batches.

I boil off a tad over 1.5 gallons per hour, so start with 7 gallons for 5.25 gallons in the fermenter and 12 gallons for 10.5 gallons in the fermenter.