Lesson 2 - Wort Boiling Theory

Another means of estimating evaporation is measuring the mass of steam supplied to the kettle. We say estimate because it is not a direct measurement.

Steam is supplied to the calandria or heating jackets as a gas and as it gives up its latent heat it condenses into a liquid. This heat is transferred to the wort. This means that the mass of steam supplied is proportional to the heat supplied to the wort, which is proportional to the amount of water evaporated from the wort. Using this means of measurement assumes that all the steam supplied is used to heat the wort. It also assumes that there are no heat losses due to leaks or faulty steam traps. Steam traps are devices fitted in the steam line after the calandria/jacket that allow only liquid to flow back to the boiler feed tank. If they are faulty, steam can flow back to the boiler feed tank without being used to heat the wort.

The relationship between the mass of steam supplied to the mass of water evaporated can also be different depending on the design of the kettle. Some designs are able to achieve more evaporation per unit of energy supplied by providing a larger surface area of wort exposed to the atmosphere over the course of the boil.

The mass flowrate of steam can be measured to estimate the rate of evaporation and used to control boiling. This is usually only done in large breweries and even in those it is checked against changes in gravity to maintain accuracy.

The total mass of steam supplied is used in some breweries to determine when the boil is complete. The total steam mass and the flowrate required can be calculated using these equations:

1.017 is a factor added to account for the change in the volume of wort between 75°C to 100°C, and 65 is the factor to account for radiant heat losses. The equation is based on 3 bar steam. Let’s say we have 100 hL and we want to evaporate 5% in the boil. How much steam do we need to supply to the kettle?

As a rule of thumb, just over 1 kg of steam is required to evaporate 1 kg of water. This makes sense as the energy to evaporate the liquid water comes from the steam as it condenses.

Using a target mass of steam to determine when the boil is complete means that as the heating surface in the boiling system becomes fouled, the boil will take longer. This is because the heat is transferred into the wort at a reduced rate (so steam is used at a slower rate), and the steam system can only supply steam to replace what is used. The time of the boil can also be monitored and when it reaches an upper limit, the kettle must be cleaned to remove the fouling. In small breweries using electrical elements metering the kW of electricity supplied to the kettle can be used in the same way, although it seldom is.

Point of reflection: Your specification states that the wort should have a start of boil gravity of 10.2°P, and an end of boil gravity of 10.8°P. However, on today’s brew, the end of boil gravity is 10.6°P. What could have caused this? If the start of boil gravity was 10.0°P, what could this indicate?

Give yourself 5-10 mins to think about this and then click ‘Next’ to continue with this lesson.