Processing Foods By Conduction Heat Transfer

The relationship between the factors involving heat transfer by conduction can be intuitively realized by considering the flow of heat energy. One would reason that heat flow (q) perpendicular to the direction of flow would be proportional to the area (A) of contact or flow and the temperature difference (dt) along the path of flow. Likewise, the thickness of the food or path through which the heat is flowing (dx) would be inversely proportional to this flow. Although modern mathematics allow one to derive such relationships, the basic conductive heat transfer relationship, known as Fourier's law, was derived empirically and confirmed by experimentation.

In addition to the factors, discussed above, the physical and chemical properties of a material have a significant effect on heat flow and can be represented by an experimentally determined proportionality constant called the thermal conductivity (k). The thermal conductivity is related to the number of free electrons in a material, varying from being high in metals to low in gases. It can be considered the flow of heat per unit time (watts or Btu per hour) through a given area (square meters or square feet) per unit thickness (meters or feet). The mathematical relationships involving conduction heat transfer can be represented by Fourier's law:

The resistance in the heat transfer equation (2) equals the thickness (Ax), which is directly proportional to resistance to heat transfer divided by the area and thermal conductivity, both of which decrease the resistance,

The reciprocal of the resistance is the conductance (C), where

Since the total heat being transferred through each of a series of materials in contact is the same for each material,

Table 1. Selected Coefficients of Thermal Conductivity


SI units

English units

q is the heat flow

watts (W)

Btu heat flow Btu/h

h is the thermal

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