Lesson 12 - Heat Transfer End of Unit SAQ

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a)  Define the terms forced convection, natural convection, conduction, and radiation.

b)  Identify which of the above heat transfer modes are active in the following brewing processes:

(i)   Mash heat up in an agitated mash vessel

(ii)  Fermentation and cool down in a jacketed vessel

(iii) Beer chilling through a plate heat exchanger

Response:

a)  Convection is heat transfer that results from temperature differences within a fluid. As temperature increases in a fluid, the molecules have more energy and move around faster and further away from each other. The increased space between molecules results in a lower density.  

Forced convection is when convective heat transfer is enhanced by the mechanical action of the fluid, for example by pumping or agitation. Pumping or agitating fluid increases mixing of the fluid, ensuring that the bulk of the fluid is at a uniform temperature. This promotes faster and more uniform heat transfer throughout the fluid.

Natural convection is when convective heat transfer occurs due to differences in temperature between adjacent sectors of a fluid. The temperature difference results in a difference in density across the body of fluid. The higher density (lower temperature) fluid sinks downwards under gravity and displaces the lower density (higher temperature) fluid, which will move upwards due to buoyancy forces.

Conduction is when heat is transferred between two objects in contact with each other. More specifically, it is the transfer of kinetic energy from one atom or molecule to another, without bulk mixing. This kinetic energy is transferred as heat.

Radiation is energy in the form of electromagnetic waves, which travel through space and media such as air. When electromagnetic waves come into contact with a surface, they can be absorbed, emitted, or reflected. When the waves are absorbed, the energy they release causes the kinetic energy of the atoms or molecules of the substance to increase. This then manifests as an increase in the temperature of the substance.

b)

    (i) The heat transfer mechanism for heat up in an agitated mash vessel is a combination of forced convection and conduction.

Heat transfer from the surface of the vessel wall into the mash occurs through forced convection. The agitation forces the mash to move faster than it would under natural conditions. This increases the turbulence of the mash and improves the heat transfer rate. 

Heat transfer from one side of the vessel wall to the other occurs via conduction.

The utility fluid stream, typically steam, is also being forced through the jackets due to the pressure differential caused by condensing steam. Heat transfer from the steam to the vessel wall surface also occurs through forced convection.

   (ii) The heat transfer mechanism for fermentation in a jacketed vessel is a combination of natural convection and conduction. Furthermore, if the vessels are located outside, radiation may also play a part.

Fermenting wort inside a fermentation vessel will move and mix by natural convection. As the beer cools, the beer density increases, and it sinks to the bottom of the tank. The warm beer replaces the cool beer and the cycle continues as long as a temperature difference (gradient) is present. This natural convection drives the transfer of heat from the wort to the heat transfer surface, e.g. the vessel wall.

Heat is then transferred from one side of the fermentation vessel wall to the other via conduction.

The utility fluid stream, typically glycol, is pumped through the jackets and so heat transfer from the glycol to the vessel wall surface occurs through forced convection. 

The effect of radiation is most prevalent in external fermentation vessels. Depending on the standard of vessel insulation, radiation from the sun can cause the vessel temperature to rise significantly. This absorbed heat adds to the heat load that must be removed by the cooling fluid. 

    (iii) The heat transfer mechanism for beer chilling through a plate heat exchanger is a combination of forced convection and conduction. 

Forced convection is found in the chilling of beer in a plate heat exchanger because the warm beer is being pumped through the narrow gaps of the plates of the heat exchanger. This creates a turbulent flow and facilitates very good heat transfer between the warm beer and the cold fluid on the other side, which is also being forced through the narrow plate spacing of the heat exchanger.

Heat is transferred via conduction from one side of the heat exchanger plate to the other.