Lesson #6: The thermodynamics of cold chain packaging
The following is some theoretical information on the heat transfer modes which can play a factor in cold chain packaging:
Conduction: conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole.
Essentially, when you have two or more solid materials at different temperatures which are touching, you can have heat transfer by conduction.
Convection: convection is heat transfer by means of a moving fluid. In this type of heat transfer, typically a solid and a fluid are in physical contact, which allows for the transfer of energy, when a temperature differential exists, from the warmer matter to the cooler matter.
When the fluid is air, convection above a hot surface occurs because hot air expands, becomes less dense, and rises. When the fluid is a liquid, such as hot water which is likewise less dense than cold water and rises, causing convection currents which transport energy.
Convection has two variants, natural and forced convection. Forced convection describes heat transfer by convection where the fluid is moved by an external element – an example of this would be a fan blowing on the coil of an air conditioning unit. Natural convection occurs because of the change in density described earlier – an example of this would be the convectors used in home heating applications, which do not use fans to move the air within a home’s room.
Radiation: Radiation heat transfer is concerned with the exchange of thermal radiation energy between two or more bodies. Thermal radiation is defined as electromagnetic radiation in the wavelength range of 0.1 to 100 microns (which encompasses the visible light regime), and arises as a result of a temperature difference between 2 bodies.
No medium need exist between the two bodies for heat transfer to take place (as is needed by conduction and convection). Rather, the intermediaries are photons which travel at the speed of light.
The heat transferred into or out of an object by thermal radiation is a function of several components. These include its surface reflectivity, emissivity, surface area, temperature, and geometric orientation with respect to other thermally participating objects. In turn, an object’s surface reflectivity and emissivity is a function of its surface conditions (roughness, finish, etc.) and composition.
The following image summarized all three types of heat transfer in one example:
The following are practical examples of how these modes of heat transfer actually affect a package’s internal temperature:
Conduction: conduction may exist between any two solid components of your thermal packaging system, and is considered to have the greatest relevance in cold chain packaging. Conduction takes place for the most part within the inside of the package because usually packaging components are touching each other.
Convection: convection typically is reduced to the exterior of the package, where the package is exposed to environmental air. Of course some of the sides of the package will be touching a surface of some type, and there conduction will be transferring heat into or out of the package.
Convection can also take place within the package, for example if air gaps exist between a frozen gel pack and the product. Such a scenario, for a refrigerated prodiuct, would be expected to reduce the heat transfer rate between the frozen gel pack and the product, therefore potentially reducing the possibility of freezing the product (vs. a frozen gel pack which is touching the product).
Radiation: radiation heat transfer typically only occurs when the thermal system is exposed to the sun, such as in the tarmac of an airport. Other rare scenarios would involve proximity to extremely hot objects such as an engine, or furnace. Because most packages are usually inside of a truck or other enclosure which does not allow exposure to the sun, radiation heat transfer is a relatively small factor.
Where radiation is a problem, materials which do absorb less radiation, such as aluminum foils or light colored paints can help. This is the reason, for example, that Envirotainer containers are painted white.