Factors Effecting Boil-Off of Liquid

[Gremlyn1]

Hi all,
I came across your forum looking for some help to settle a debate. A few fellow home brewers and I are debating what factors influence the rate of loss during a boil. The original question came from a brewer who switched to a wider pot and discovered that he lost more liquid over the course of his hour long boil than he previously did with his old pot.

The side opposing me on the debate is stating that the ONLY thing that matters is how much energy is put into the liquid and the wider pot simply manages to use the heat more efficiently.

I say that while the energy put into the system is certainly an important factor, the extra surface area of the wider pot has to come into play because evaporation occurs at all temperatures, and of course occurs faster a higher temperatures.

Sooooooo, who is right and why?

Thanks for your time!

 

[Danger]

Welcome to PF, Gremlyn.
The liquid itself can't rise above its boiling temperature, regardless of how much heat you put in, unless it is pressurized. A wider pot might reach that temperature sooner, but it's moot after that. I therefore agree that the surface area determines loss rate.
Someone better educated might have a different opinion, though, so wait to hear from them before running off in triumphant joy.

 

[russ_watters]

Yes, the only thing that matters when you are boiling in a container open to the atmosphere is the [net] rate of heat absorption. However, the shape of the pot can affect the rate of heat absorption: a wider base may effectively trap more of the heat from the heat source, such as a burner. Ie, if the flames are licking-up the sides of the pot instead of being fully underneath it, a very substantial fraction of the heat will be lost to radiation and convection around the pot.

 

[Gremlyn1]

So you're saying two pots with different widths will lose the exact same amount of liquid in a give time given equal energy inputs. Why is it that evaporation plays no part? And just to be clear, we're not talking about some perfect system in controlled conditions. There are many factors like wind, humidity, etc also at play.

 

[russ_watters]

Why is it that evaporation plays no part? And just to be clear, we're not talking about some perfect system in controlled conditions. There are many factors like wind, humidity, etc also at play.

Because "boiling" and "evaporation" are not the same thing. When water evaporates, it is below its boiling point and water vapor comes off the surface due to interaction with the air above it. Wind and humidity have an enormous impact on this interaction.

But when water boils, water is converted from liquid to gas by the heat input at the bottom of the pot. How fast that water vapor is generated is a function only of the heat input. Once created, the bubbles of water vapor rise through the pot and are released into the air, regardless of what is going on above the pot. Since the temperature of the bubbles is above the boiling point, not even 100% humidity above the pot can keep those bubbles of water vapor from being relesed into the air.