How to use a Jacketed Boiler

First published on Distillique's website 2015 GM Bosman

When Lost Creek Distillery first started up their brand new 250lt and 500lt jacketed stills from Distillique and for a "stress test" they filled the boilers with water to test for water and vapor leaks. However, they just not could get their boilers to boil - not even after a few hours of trying to boil it - although the water in the jacket boiled vigorously and created a massive amount of steam escaping from the filling hole. 

The water jacket heating water was filled to the top and inside the boiler, there was water simulating their mash.

On the 250lt boiler they had three, 4.5kW heating elements. When they switched on the three elements it took a while and then the jacket water started boiling. However, the temperature of the mash water only very slowly started rising and even after 5 hours they could not get it to boil. ... what now? what went wrong? They can't wait that long!

Let's look at the problem with a bit of an "engineering eye":

• The heating elements transferred their heat/energy to the jacket water.
• The jacket water transfers its heat/energy to the stainless steel jacket and;
• The stainless steel jacket transferred its heat/energy to the mash water (and off-course the outside air on the outside of the jacket)

 Looking at this problem brings a very important issue to the front: "Heat transfer coefficient" (we'll just call it the HTC from here). This parameter tells us how much heat can be transferred from one environment to the other. In this particular case there are quite a few  HTCs  at play:

• The HTC between the jacket water and the stainless steel,
• The HTC between the stainless steel and the mash water.
• The HTC between the stainless steel (top of boiler) and outside air
• The HTC between the stainless steel (sides of jacket) and the outside air.
• The HTC between the heating elements an dthe jacket water

For the purpose on finding a solution, we combined some individual HTCs into a single one to describe the heat transfer:

The HTC(WSM) between the jacket Water, through Stainless steel into the Mash water.

The HTC(WSM) in this case is about 50 Watt per square meter per degree Celcius. HTC(WSM) = 50W/(m^2.C)

In other words, for each degree temperature difference between the jacket water and the mash water, there was only 50 watts of energy transferred for every square meter of jacket (Square meter of stainless steel between the jacket water and the mash water)

The heating elements transferred so much energy to the jacket water that the jacket water boiled vigorously but the heat transfer between the jacket and the wash water was too slow. ...and to worsen the problem: As the mash water's temperature rose, the less heat was transferred because the temperature difference got smaller and smaller.

The solution is quite simple (but definitely not intuitive):

Don't fill the jackets of jacketed boilers more than what is required to protect the heating elements

This counter intuitive solution can be rationalized as follows:

As the heating elements boils the jacket water, steam rises in the jacket. The heat transfer coeficient between Steam-stainless steel-mash water is about 300. i.e. HTC(SSM)=300 W/(m^2.C).

The steam transfers MUCH more heat to the mash through the jacket than boiling water (although both are at the same temperature) and;

As the steam rises and transfers heat to the mash, it loses energy in the process and condenses against the jacket sides (mostly the outer jacket).  As it condenses it runs back towards the bottom to get boiled again ... and very little steam escapes form the jacket while also not building up any pressure in the jacket.

Moral of the story: Don't fill the water in jacketed boilers more than just above the heating elements (and a little above for safety purposes). It will then heat the the mash much quicker, is more energy efficient and doesn't require regular water top-ups for the jacket!

 ...and off-course, it will work even better when using real mash as the mash's boiling temperature is lower than that of water.

The above is the solution to get your mash much quicker to the boiling point.

The temperature difference between the mash and the jacket water will be at its smallest when the mash is boiling.

With this small temperature difference, much less energy will be able to pass from the jacket water to the mash. It is important now to REDUCE the heating to the jacket water to prevent it from  boiling too fast and having steam escaping from the jacket. Once the mash starts boiling - reduce the energy input until no more steam escapes from the jacket. This will be the maximum heat that can be transferred to the mash (as the heat transfer is limited by the heat transfer coefficient and more heat will NOT boil the mash faster).