How do you calculate the evaporation rate?

[Finlandbrews]

On beer Smith the evaporation rate is 12% by default. Is that a percentage that relates to boiling tap water at sea level, in a 1.2 height to 1 width kettle with outside temp of 20 degrees celsius?

Does anybody know how the evaporation rate is calculated?

Thanks!

 

[GrogNerd]

was something I tested on my own equipment. 6 gallons of water in my 30 qt pot, timed how long it took to get to boil, boiled for an hour, figured out how much I had leftover

not sure it can be calculated. estimated or measured, but not calculated

 

[Homercidal]

I think of boil-off as more of a constant, like 1 gallon per hour. The actual amount will vary depending on how hard you boil and the surface area of your kettle, plus maybe the temp/humidity at the time.

I think you should estimate the first time and take a careful measurement of the volume of wort just before the boil starts, and then just after your 60 minute mark. This will give you a pretty close idea of your BO rate.

 

[ajdelange]

It's going to depend on a lot of factors but you can bound it from above by dividing the heat output of your burner by the enthalpy of vaporization of water (40.657 kJ/mol @ 100 C). That gives you the number of moles of water that would be vaporized per unit time if 100% of the energy of the burner went into the water. If you have an electric heater delivering 7.5 kw (7.5 kJ/sec) most does get coupled into the water and so your theoretical maximum would be 7.5 (kJ/sec)/40.657(kJ/mol) = 0.18 mol/sec = 3.2 grams/sec. Of course even with an electric kettle you will lose heat through the sides and bottom so all 7.5 kW don't go to transforming water to vapor. How much goes to the surrounding air depends, of course, on how hot that air is, whether it is circulating and so on. In effect you have to measure the rate for your equipment and practice under typical conditions.

As liquid boils away there is less area of kettle in contact with liquid to lose heat but more area in contact with the vapor on which it may recondense. These effects should cancel to a certain extent and the rate, in mass per unit time should, thus, be approximately constant.

 

[Finlandbrews]

So with 3.2 kw I am about at 4 grams per second maximum evaporation rate, right? That would be 1 litre per 4 minutes, so that is an impossible result in natural environment! Now you mean if I don't have an insulating material such as reflectix and I would brew in very cold conditions my evaporation would be lower than with reflectix and high temperature in the air? I know there are other factors but if weather is warmer and I have reflectix there is higher evaporation then?

 

[JonW]

Just run an experiment with some boiling water and you'll answer the question!

 

[LLBeanJ]

On beer Smith the evaporation rate is 12% by default. Is that a percentage that relates to boiling tap water at sea level, in a 1.2 height to 1 width kettle with outside temp of 20 degrees celsius?

Does anybody know how the evaporation rate is calculated?

Thanks!

From the BeerSmith 2 Help pdf (p 55)...

"((post_boil_vol / pre_boil_vol) - 1.0) times 100 (which gives you a percentage)"

 

[NoIguanaForZ]

Can someone explain to me why Beersmith (and a lot of brewing advice, for that matter) uses a percentage when that's, physically, not how evaporation works?

 

[LLBeanJ]

Can someone explain to me why Beersmith (and a lot of brewing advice, for that matter) uses a percentage when that's, physically, not how evaporation works?

BS doesn't really "use" it. You set the boil-off rate in gallons/hour and it will calculate and display the boil-off percentage, which may be useful for someone.

 

[PGEduardo]

So with 3.2 kw I am about at 4 grams per second maximum evaporation rate, right? That would be 1 litre per 4 minutes, so that is an impossible result in natural environment! Now you mean if I don't have an insulating material such as reflectix and I would brew in very cold conditions my evaporation would be lower than with reflectix and high temperature in the air? I know there are other factors but if weather is warmer and I have reflectix there is higher evaporation then?

I think it's just easier to measure it, unless you have access to a complete weather station at your home and calibrated heating equipment, etc.

Personally I would estimate low, so that if more boiled off than expected you could simply top off to dilute without having to extend the boil (and thus bitterness from hops).

Estimate something like 0.9 gallons/hour, keeping track of your pre/post boil volumes to calculate your true rate, for that particular setup.

 

[NoIguanaForZ]

BS doesn't really "use" it. You set the boil-off rate in gallons/hour and it will calculate and display the boil-off percentage, which may be useful for someone.

That makes a lot more sense.

 

[kh54s10]

Not sure what how it is calculated, either by BS or other. It depends on so many things, pot geometry, altitude, humidity, heat intensity etc. I learned how much my system was boiling off while doing kits, before using BS to make my recipes. So, I knew that I boil off about 2 gallons in an hour and input that in the profile in BS. I then made small adjustments in the equipment profile and now I end up with very close to the proper volume.

 

[ajdelange]

So with 3.2 kw I am about at 4 grams per second maximum evaporation rate, right?

No, actually you would be at about 1.6 grams/sec. When throwing numbers around in my last post I reached into that cluttered hodgepdoge of my brain for grams per mole of water (18) and pulled out moles pf water per liter (55). Sorry about that.

That would be 1 litre per 4 minutes,

More like 1 litre in 10 minutes. You still won't get that because of efficiency.