Lesson 6 - Radiation and Combined
This lesson deals with the principles and application of a form of heat transfer called radiation. This lesson will also combine the heat transfer methods you learned previously to provide an overall picture of heat transfer processes in a brewery.
Radiation Enhancement and Mitigation
Radiation is a function of area, emissivity, and surface temperature. Energy that is absorbed or emitted is directly proportional to the surface area. As surface area increases, so does the absorbing or emitting potential.
As discussed earlier, emissivity is a characteristic of a surface material. To minimise solar energy absorption, buildings and vessels may be made of or coated with a material that has a low emissivity. In contrast, when radiation energy is desired, utilisation of a material with a high emissivity will absorb the most heat energy.
Recall that the Stefan-Boltzmann law for radiation heat transfer involves the temperature to the fourth power. It therefore follows that the surface temperature plays a large role in the rate of heat transfer. Therefore, a small reduction in surface temperature will make a big difference in the radiant heat transfer.
Insulation systems to reduce heat loss
Insulating hot surfaces on process equipment is key to reducing radiant heat loss.
Insulation should be made of a low thermal conductivity material with low water vapour permeability.
A layer of insulation adds a layer of resistance to heat transfer. Thermal insulation is essential to preventing both heat loss from hot surfaces and piping, and heat gain in refrigeration systems. In a brewery, pipes that transport steam, hot water, chilled water, and glycol are commonly insulated to reduce all forms of heat loss, including radiant heat loss.
To minimise heat loss or gain in an insulation system, it is important to avoid ingress of water into the insulation. The reason for this is the thermal conductivity of water significantly reduces the effectiveness of the insulation by significantly raising the overall thermal conductivity of the insulation layer. In addition, in the case of cold insulation, freezing of entrapped water will cause expansion of the insulation and often rupture of the outside cladding over the insulating material.
Now that we have explored the third and final form of heat transfer – radiation – we can now look at some scenarios when multiple forms of heat transfer occur simultaneously.