Does boil off rate change as your wort level drops?

If you have a 15 gallon boil kettle, would your boil off rate change if you had 5 gallons in the kettle versus lets say 12? My thought is, is that it would not change but want to confirm this.

I tested my boil off rate recently using 5 gallons of water over 30 minutes and it boiled off 1.5 gallons, so in 60 minutes I'd be looking at a 3 gallon boil off.

No.

>>I tested my boil off rate recently using 5 gallons of water over 30 minutes and it boiled off 1.5 gallons, so in 60 minutes I'd be looking at a 3 gallon boil off.

Holy cow thats a lot of boil off. I think thats unnecessarily high. Try turning down the heat.

I know, I was surprised too. Thing is, the way my propane burner is it's pretty hard to turn it down and still be able to get a rolling boil. I just compensate for it by having a larger pre-boil volume.

ArcLight said:

>>I tested my boil off rate recently using 5 gallons of water over 30 minutes and it boiled off 1.5 gallons, so in 60 minutes I'd be looking at a 3 gallon boil off.

Holy cow thats a lot of boil off. I think thats unnecessarily high. Try turning down the heat.

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Physics says that changing the state of water from liquid to gas requires a certain amount of kilocalories per liter. If you add that much energy you boil off a certain amount of liquid. In more detail it also depends a little on the partial pressure of the gas but in our brewing case that won't matter much. So short answer is no.

RM-MN said:

Physics says that changing the state of water from liquid to gas requires a certain amount of kilocalories per liter. If you add that much energy you boil off a certain amount of liquid. In more detail it also depends a little on the partial pressure of the gas but in our brewing case that won't matter much. So short answer is no.

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RM-MN said:

Physics says that changing the state of water from liquid to gas requires a certain amount of kilocalories per liter. If you add that much energy you boil off a certain amount of liquid. In more detail it also depends a little on the partial pressure of the gas but in our brewing case that won't matter much. So short answer is no.

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The overall mass of the liquid is changed. If you're applying the very same amount of heat, the boiloff rate will increase.

However, most of us will adjust the heat down to keep a consistent rolling boil during the 60+ minutes, hence, the boil off rate stays roughly the same.

MC

Misplaced_Canuck said:

The overall mass of the liquid is changed. If you're applying the very same amount of heat, the boiloff rate will increase.

However, most of us will adjust the heat down to keep a consistent rolling boil during the 60+ minutes, hence, the boil off rate stays roughly the same.

MC

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Nope, for water to boil the entire mass of the water has to be ~ at the boiling point so you are not heating up more or less water, you are simply changing its state. Takes the same to boil off a gallon (ignoring the heat loss through the sides of the vessel) from a hundred gallons as it will for 5 gallons.

Thank you as well.

RM-MN said:

Nope, for water to boil the entire mass of the water has to be ~ at the boiling point so you are not heating up more or less water, you are simply changing its state. Takes the same to boil off a gallon (ignoring the heat loss through the sides of the vessel) from a hundred gallons as it will for 5 gallons.

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I dont understand why people are saying it doesnt change. Obviously its easier to boil a smaller amount of liquid than a larger amount. I am not sure it is necessary to worry about but your only question is does it change....yes. If i am boil a full pot it does so less vigourously then a small pot. There is a limit to how much you can boil with a given output so getting close to that limit will rapidly change the boil off. I think the difference most see in brewing is too small to worry about.

In absolute terms (either volume or mass) the answer, as long as you keep the heat input constant, the answer is no. If you express boil of in percentage terms, the instantaneous value will increase as the quantity in the kettle drops.

RM-MN said:

Takes the same to boil off a gallon (ignoring the heat loss through the sides of the vessel) from a hundred gallons as it will for 5 gallons.

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The thing is that the heat loss through the sides IS very significant.

Maintaining a boil with 5-gallons vs 10-gallons takes less energy (overall) due to less heat dissipation (smaller footprint). We're not talking about evaporation at this point.

A good point in case is that when I start my boil at 13-gallons, the amount of heat applied is X for a normal rolling boil, while at the end of the boil, I'm usually applying 0.9X. If I don't turn down the amount of heat, the kettle will boil over even after having been "quiet" for 45+ minutes at the same level of heat.

That being said, the evaporation level for the same kettle with a *proportionate* amount of heat (compensating for the lower volume) should generate roughly about the same evaporation rate.

MC