Processing Foods By Convection Heat Transfer

Convection heat transfer is concerned with the mixing of fluids or the transfer of heat from a fluid to a surface. Therefore there is no solid-body thickness through which heat must be transferred, as in conduction. In normal food processing operations the concern is usually to transfer heat between a liquid and the surface of a solid or a fluid. Newton's law of cooling, which also applies to heating, defines the driving force as the temperature difference between the surface temperature (ts) and the bulk temperature (tf) of the fluid medium:

In this case the temperature of the surface is higher than the bulk temperature of the liquid and h is the convective heat transfer coefficient (commonly called the film coefficient), the experimentally determined proportionality constant that takes into consideration the flowing characteristics and liquid—solid interface effects. The units are W/m2 • K or Btu/(h)(ft2)(°F). Note that there is no dx or thickness factor so that the resistance R corresponding to conduction heat transfer is

Figure 2 depicts the type of temperature profile that occurs when a hot fluid is transferring heat to a solid (or liquid) surface. The equilibrium is between the bulk temperature of the liquid (tf) and the surface of the solid (£s). Note that the temperature drops in a nonlinear manner. This is caused by the very thin layer of fluid that is almost stagnant because of the friction of the moving fluid, often referred to as the edge effect. This layer is flowing in the streamlined region and, since there is essentially no mix

Hot fluid stream

Hot fluid stream

S Solid food

.^-Stagnant film

Figure 2. Temperature profile of hot flowing liquid near the surface of solid being heated.

S Solid food

.^-Stagnant film ing with the main liquid stream, the heat transfer through this layer is actually by conduction but without a thickness (dx) to consider. The magnitude of the edge or fictitious film effect varies widely depending on the solid surface and explains why this coefficient must be experimentally determined for any given condition. Some select convection film coefficients are given in Table 3. The wide range of film coefficient values is due to the surface and geometry over which the fluid is passing and the turbulence (Reynolds number) of the flowing fluid.

Over a period of time, scale deposits build up on the walls of vessels and pipes. The effects of these films or deposits are determined experimentally and expressed in units of the film coefficient h. Therefore films and scale-deposit film coefficients can be handled in the same manner as the thermal conductance of a flowing liquid.

Steady-state heat transfer processes occur in the operations that involve continuous heating or cooling of foods. These include continuous freezing tunnels, deep frying, and pasteurizing and sterilizing liquids. Many processing procedures in which a solid is being heated in a continuous moving system (eg, freezing or cooking a product on a moving belt) can be treated as a total steady-state system by using specific locations on the belt as base points for heat balances. In most of these cases steam, hot water or oil, or a refrigerant is the source for adding or removing heat, so that convection heat transfer is an important factor in these processes.

The only procedures that must be handled as batch, unsteady-state systems are those in which a product is being heated or cooled in place by a steady-state or non-steady-state source (eg, cooking in an oven, retorting canned foods, and freezing a product on a freezer plate). Normally the information required to calculate heat transfer in these situations is collected experimentally and then applied. For example, the thermal death time to ensure the sterilization of canned food is experimentally determined for each size of container and product and then applied to the commercial facilities. The actual retorting time is then increased by a sufficient time to insure that there is no error in obtaining sterilization. In the case of sterilizing products, the National Food Processors Association (in coordination with the Food and Drug Administration) provides

Table 3. Approximate Range of Convection Heat Transfer Coefficients

Conduction heat transfer film coefficient, h

Medium

Gases natural convection forced convection Viscous liquids, forced convection Water, forced convection Boiling water Condensing steam

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