Beer Forum

I am trying to figure out how to calculate the power required to maintain a rolling boil in a brew kettle (I may create a web site calculator for this).

I know how to calculate energy loss to the surrounding environment and how I may use the "enthalpy of vaporization" to calculate the energy required to boil the wort. Question is - when do you get a rolling boil? My gut feeling tells me this mainly depends on the top surface area of the wort and the evaporization rate, but I am hoping that someone knows this better than I do. Is there a scientific definition of a rolling boil or even a good rule-of-thumb?

Good luck. There's a lot of variables involved. A rolling boil means different things to different people but in my 15 years working in commercial kitchens I've mainly seen a boil divided into 3 classifications: smiling, rolling & hard/vigorous. When the liquid 'smiles' at you is when there's minimal but noticeable surface activity. A vigorous/hard boil is sustained at boiling temp for that altitude. Rolling is the few degrees between.

This is why we calculate by evaporation rate. Different altitudes are going to require slight adjustments in the rate, but it's a calculable, repeatable number. Part of the brewer's skill is to be able to 'read' the surface at a known elevation & know the evaporation rate.

The amount of energy needed for a rolling boil also changes with wort gravity.

It may be that I do not actually need to solve the problem of "what is a rolling boil"" - let me rephrase that - "can you calculate the evaporation rate that gives you a good boil?". I still expect that the proper evaporation rate needs to be calculated from the surface areas involved (wide kettles probably needing more power than tall ones).

And, yes, I am going to end up with an approximation, but approximations (if good enough) can put people on the moon. I would be happy if the formula I end up with could give better guidance to people buying heating elements for electrical breweries.

It's a matter of personal preference. What you're proposing is very similar to 'how long do you cook a pizza?' - it depends. There's already an excepted range (for both), do you like thin or thick crust, do you like a deep gold or darker tan bottom ( ), do you like ...

I've long since forgotten what the full range of an acceptable boil is since long ago I set mine at 13% and left in there. Why 13? 12 wasn't enough & 14 was a touch too much. I've heard others say they like 14% or 15%. It depends. You can calculate it by volume reduction over time. If you boil 100L for 1 hour & you have 87L at the end, you have a 13% evaporation rate. Your job as the brewer is to relate that number to what the boil looks like on a particular system. As stated before, the energy requirement will change per recipe, but a 13% looks like a 13% on a session blonde as it does on a 1.100 barleywine.

Lastly, there's really not that many choices when it comes to elements, certainly not enough to warrant a calculator. The most important thing is having the lowest density element possible. After that, I'm of the opinion to get the most powerful one possible while not sacrificing the density. Let the PID do the work; it's what it's there for.

Ozwald wrote:Lastly, there's really not that many choices when it comes to elements, certainly not enough to warrant a calculator. The most important thing is having the lowest density element possible. After that, I'm of the opinion to get the most powerful one possible while not sacrificing the density. Let the PID do the work; it's what it's there for.

Thank you for your input! I actually find that a calculator like this would have been very helpful for me when I was planning my brew setup. Here (in Sweden) the normal electrical outlets will give you 10A at 230V, while some outlets give you 16A at 230V. Above that you have the expensive three-phase power systems. So beginning brewers are faced with a number of options - and very little guidance. I ended up buying a setup that could not give me a proper rolling boil, despite having read on forums that "yes - I use X kW and it works for me, it will be fine". I ended up doing an expensive upgrade to my system.

I agree that once you invest in three-phase power and a PID, there is little use for a calculator. However, there are plenty of electrical brewers who do not have those options for one reason or another (I live in an apartment for instance - I cannot alter the wiring).

If those are the only 2 options that's real easy. It doesn't matter how big of an element you put on those circuits. The 10A circuit will max out at 2300 watts & the 16A at 3680 watts.

(Sorry I forgot about this thread for a little bit there)

Ozwald wrote:If those are the only 2 options that's real easy. It doesn't matter how big of an element you put on those circuits. The 10A circuit will max out at 2300 watts & the 16A at 3680 watts.

But the question I am trying to answer is really - what size (and possibly shape) of brew kettle could you get if you are limited to N watts? Trial and error is not a good process for planning a brewery. I realize that the question I am asking is a difficult one (googling it really yields nothing), but I do think there is a good answer to it. I cannot do experiments since I have no way of limiting the power (dimmers for the kW range are not cheap), otherwise I would have (volume, shape and gravity as variables).

I am not really looking for the answer for my own brewing at the moment (I have done my upgrades), but it would have been nice to give other new brewers some help. Also... I kind of enjoy solving problems...