Heat transfer unit operations
Heat transfer is defined as the movement of heat across the system's border due to a temperature difference between the system and its surroundings. In the food industry, this is a common occurrence. There are three types of heat transfer in food: conduction, convection, and radiation.
Evaporation, drying, freezing, refrigeration, heat sterilization and pasteurization all rest on the same physics: heat transfer. It is a dynamic process in which heat moves from one body to another that is cooler, and its rate is governed by the temperature difference between the two bodies — the greater the temperature difference, the greater the rate of heat transfer. Expressed simply, rate of heat transfer = temperature difference / heat flow resistance of the medium.
Three mechanisms operate in food plants. Conduction transfers vibrational energy from one molecule to adjacent molecules in a solid mass, with no physical movement of the body, as when heat passing from meat to cheese melts the cheese; it is described by Fourier's equation, where dQ/dT is the conduction rate of heat transfer, dT/dX is the temperature difference per unit length, and 1/kA is the resistance offered by the medium, with k the thermal conductivity of the body and A the cross-sectional area perpendicular to the direction of heat flow. Convection transfers heat through the movement of groups of molecules in a fluid, driven either by density changes or by forced fluid motion, as in boiling water; it follows Newton's Law of Cooling, Q conv = hA (T∞ - Ts), where h is the convective heat transfer coefficient, A the surface area being heated or cooled, T∞ the bulk temperature of the surrounding fluid and Ts the surface temperature of the object. Radiation transfers heat energy by electromagnetic waves, operates independently of the medium, and depends on relative temperatures, geometric arrangements and surface structures of the materials emitting or absorbing heat; it is governed by the Stefan-Boltzmann law, q = AσT⁴, where T is absolute temperature and sigma is the Stefan-Boltzmann constant (5.670374419 × 10⁻⁸ watt per square meter per kelvin). Baking in an oven, with radiation via resistors, is the everyday example.
In practice, how quickly heat reaches the food depends on more than the mechanism. Heat transfer characteristics of food are shaped by the type of product, the type, size and shape of the container, agitation of the container and the temperature of the retort. Heat penetrates faster through metal than through glass or plastics because of differences in thermal conductivity, and tall containers promote convection currents in convective heating foods. These are the variables a process designer works with when specifying thermal equipment and process schedules.