Lesson 5 - Wort Boiling Technology and Process Control SAQ

*What are the three modes of wort boiling? Which mode is the best? Explain your reasoning.

Response:

The three modes of heat transfer are:

Film boiling occurs when there is a high temperature difference between the wort and the heating medium. It is also more likely when the heater surface is non-wettable.

In film boiling steam bubbles cling to the surface and join together to blanket it with a vapour film. This film reduces the ease with which heat can transfer from the surface to the wort. It acts as an insulator. With film boiling, the rate of fouling is increased as heat is not transferred effectively and the wort that does come in contact with the surface receives so much heat that it sticks.

Film boiling should be avoided as it results in poor beer flavour and poor flavour, colloidal and foam stability.

Forced convection occurs at low temperature differences, when wort velocity across the heating surface is high or when vapour bubble formation is suppressed by applying a high back pressure. The high wort velocities cause turbulent flow over the heating surface which aids heat transfer. While it is an effective way to heat wort, forced convection does not produce many vapour bubbles (some form as the wort leaves the calandria) and can also increase the level of shear on trub particles.

If shear can be minimised, and a pressure drop after heating can be achieved, forced convection can provide excellent boiling. Vapour bubble formation then takes place in the bulk of the wort in the kettle, providing effective trub formation, bitterness extraction and removal of undesirable volatiles. Also, the low ΔT means thermal load is low. However, the risk of shear means forced convection is not generally considered the best mode of boiling.

Nucleate boiling relies on steam bubble formation to create turbulence and improve heat transfer. The bubbles move away from the heating surface, taking heat with them. This allows unheated wort to come into contact with the heating surface and be heated. Vapour bubbles aid trub formation, volatile removal and hop acid isomerisation.

The steam bubbles are less dense than wort, so rise rapidly in the calandria tube. The mixture of steam bubbles and wort is called two-phase flow because it contains two phases, liquid (wort) and gas (vapour). This causes a thermosyphon which maintains the flow of wort through the calandria with no requirement for a pump.

As it achieves the objectives of wort boiling effectively, nucleate boiling is the best mode of wort boiling.