Lesson 2 - Principles of Heat Transfer and Conduction

It is important for the brewer to understand that the demand or rate of heat transfer for a heating or cooling process can change throughout the process. The driving force for heat transfer is the temperature difference between the heat source and the heat destination. Therefore, when the temperature difference changes, so does the rate of heat transfer.

Steady state heat transfer occurs when the temperature difference remains constant, while unsteady state heat transfer occurs when the temperature difference changes throughout a heating or cooling process.

Furthermore, steady state heat transfer is not dependent on time while unsteady state transfer is dependent on time. Most of the examples in this unit will focus on steady state heat transfer however many brewing processes are unsteady and this will be explained further in the lesson on Jacketed Vessels.

To illustrate the difference, consider a process such as heating and eventual boiling of wort. In the beginning the temperature difference between the heat source and the wort is large compared to when the wort is boiling. The larger temperature difference results in a larger heat transfer rate in the beginning than the end. We could say this process is unsteady state heat transfer. Therefore, the heating up process is unsteady while the boiling process is steady.

Now that you have been introduced to the general principles of heat transfer we will discuss the three main heat transfer mechanisms; conduction convection and radiation (and combined) heat transfer, in more depth. Convection and radiation (combined) will be covered in the lessons that follow. For now, let us turn our attention back to conduction.