Request: boil evaporation calculator

[golson3]

I've been dealing with wild seasonal swings in humidity (MN is a desert in the winter, and GA jr in the summer) that I believe have resulted in huge swings in boil evaporation. Through the winter, this wasn't that big of a deal, as I was extract brewing. It wasn't ideal, but I could top off with pre boiled water when too much wort boiled off. Last batch, I overestimated the evaporation rate and had to deal with adding some pasteurized DME to set things right. I'm about to switch to AG, though, and would rather not mess with topping up or adding extract anymore. I did a quick search on google and here and came up with nothing. There has got to be a way to estimate evaporation volume by plugging in temp, boil time, dew point/relative humidity, and kettle diameter. Does such an animal exist?

Edit: hell, maybe even wind could factor into it?

 

[doug293cz]

I've been dealing with wild seasonal swings in humidity (MN is a desert in the winter, and GA jr in the summer) that I believe have resulted in huge swings in boil evaporation. Through the winter, this wasn't that big of a deal, as I was extract brewing. It wasn't ideal, but I could top off with pre boiled water when too much wort boiled off. Last batch, I overestimated the evaporation rate and had to deal with adding some pasteurized DME to set things right. I'm about to switch to AG, though, and would rather not mess with topping up or adding extract anymore. I did a quick search on google and here and came up with nothing. There has got to be a way to estimate evaporation volume by plugging in temp, boil time, dew point/relative humidity, and kettle diameter. Does such an animal exist?

Edit: hell, maybe even wind could factor into it?

Wind is a big factor, but humidity probably not much if at all. Only two primary factors affect boil off rate:

1. Heat input to the boil kettle.
2. Heat loss from the sides of the BK.

It takes a fixed amount of heat to boil off a fixed amount of water. It is a well known physical constant. Heat loss from the sides of the BK is primarily affected by temperature, wind velocity, and what the sides are radiating towards. Heat input from a burner is variable depending on gas setting, and can also be affected by wind. With electric heating you can control the heat input much more precisely.

The boil off rate for well controlled conditions can be modeled, but real world brewing set ups are seldom controlled well enough for modeling to be accurate.

Best approach is to measure the boil off with your system with water, under conditions like those you brew in.

Brew on

 

[golson3]

I guess I can start charting boil off volumes with time of the year or ambient air temps to get an idea of what I'll be dealing with. Wind isn't usually a huge factor for me, since I'm protected on 3 sides in my garage. The swing in evaporation between winter and summer has been drastic (along with a 95F temp swing between the coldes and hottest brew days), with far more being evaporated in the winter. I wouldn't think that colder air would increase evaporation, which is why I thought it would have something to do with the amount of water in the air (and then the air suddenly being heated up, and being able to hold much more water). Dunno, I'll chart it out and see what happens, but the worst during the winter resulted in 2.5 gallons being boiled off from 7 gallons I'd started with, while I only lost a gallon on my last batch. The winter batch had a lot of grains being steeped, but water loss in the grains is already factored in. Maybe I'll ask around on the MNHBA FB page, since there are some experienced home brewers there that are used to dealing with the local climate. I'd really like to get advice from anybody else in the upper midwest or nearby parts of Canada.

 

[MikeInMKE]

The extract doesn't get boiled off, so you should never add DME to make up for water lost through evaporation.

At flameout, if you discover you boiled off too much water (taking into consideration there'll be shrinkage from chilling it down), add some more water to get you to your desired post-boil pre-chilled volume. As long as the result is still above 180 degrees, the water you add will be pasteurized.