Lesson 4 - Wort Boiling Technology and Process Control
This lesson explores the process of heat transfer during wort boiling, and how it can be controlled. It also considers the design principles of wort boiling systems and how energy use in the brewhouse can be minimised.
Dynamic Low Pressure Boiling
Whoever decided on the name for this system must have had a sense of humour. As you will soon see, at no point in the boiling process is the pressure ever lower than atmospheric.
Dynamic low pressure boiling is done with a kettle capable of withstanding pressures above atmospheric pressure with a sealing valve fitted on the stack to enable it to accumulate pressure. The system also features pressure measurement, a controller to regulate steam flow and the sealing valve to achieve the target boiling pressures. The system is usually heated with an internal calandria that has been modified to throw the wort back on the wort surface at two different levels. This is important because the wort’s potential to foam must be reduced before the pressure reductions begin or there is a risk of over foaming. The spreader is also equipped with vertical parts to improve the efficiency of the scrubbing of undesirable aroma components from the wort by increasing the surface area for evaporation.
The calandria is operated to ensure the kettle contents are circulated 20–30 times per hour, and there are no unheated zones.
In pre-boiling wort is heated at atmospheric pressure for three min at 100°C. The pressure is then periodically increased to 1.17 bar which allows the wort to boil at 104°C and then reduced to 1.05 bar for boiling at 101°C.
Figure: Temperature profile during dynamic low pressure boiling
Increasing the pressure and temperature of the boiling wort and then releasing it causes vapour bubbles to form throughout the kettle. This leads to extensive volatile stripping and the other changes in wort that result from vapour bubble formation. At the end of the process, there is a post-boiling phase of five min at atmospheric pressure and 100°C.
The system achieves the objectives of boiling with evaporation of around 4.5 % and shorter boil times that atmospheric pressure boiling. Dynamic low pressure boiling was developed by Huppmann who were bought out by GEA. The GEA Jetstar that we met earlier is designed to work with dynamic low pressure boiling and GEA have claimed that evaporation levels as low as 3% are possible while maintaining wort quality.