Transferencia de calor

[PROCESSING]
by J. Peter Clark

O

Getting to the Heart of Heat Transfer
f the significant transport processes—heat, mass, and momentum—heat transfer is often of greatest interest to food processors. Thermal processing—heating foods to sterilize them—speaks for itself. Other examples include dehydration, freezing, cooking, evaporation, frying, freeze-drying, and cooling. In each of these,transfer of energy into or out of food determines rate, costs, and quality. Most people have an intuitive understanding of the law of thermodynamics that says that a cup of coffee or bowl of soup seems to cool it faster. We all know it is cooler in the shade on a sunny day. Each of these commonplace observations reveals a fundamental fact about heat transfer. Engineers quantify these phenomena tomake predictions and calculations useful in design of equipment and processes. Non-engineers can benefit from some understanding of the mechanisms of heat transfer as they assist in troubleshooting and interact with designers and operators.

Conduction Heat Transfer
Conduction refers to the movement of energy within a substance that is at rest, that is, not moving. While the bulk of the materialis at rest, the molecules and atoms of which it is composed are restless. They are constantly moving and colliding with each other and with their surroundings. Their average velocity may be zero, but the absolute value of their velocity is a function of the temperature. The average is zero because motion in one direction cancels the motion in the opposite direction. Collision of one atom ormolecule with another at a different energy level results in transfer of energy from one to the other. Thus, if a bar of copper is heated at one end by a flame, for instance, it is not very long before the other end is too warm to hold in a bare hand. Energy from the flame has moved along the bar as atoms of copper collide with one another. Copper is a good heat conductor. If the bar had been wood, abare hand could have held it until the flame reached the holder, because wood is a poor conductor of heat. Poor conductors of heat are insulators, and have their valuable uses. The intrinsic property of materials that describes how good or poor a heat conductor they are is the thermal conductivity, symbolized as k, with units in the SI (metric) system as watts/meter degree K (W/m K). Copper has avalue at room temperature of 386 while wood has a value of about 0.1 in these units, according to a table in Introduction to Food Engineering (Singh and Heldman, 2001). Other intrinsic properties that affect conduc-

Tetra Pak’s Tetra Plex plate heat exchanger in this Belgian Tropicana plant is employed in an aseptic drink processing application.
Photo courtesy of Tetra Pak

energy moves froma region of high temperature to one of lower temperature. They may also grasp that materials differ from one another in some property that affects how they conduct heat and therefore how they feel to the touch. Metal, glass, and stone feel cool while wood, plastic, and paper feel warm by comparison, all supposedly at the same temperature. Why is that? Also intuitive is the observation thatstirring

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tion heat transfer include density and heat capacity. Density in SI units is expressed as kilograms/ cubic meter and heat capacity is expressed as kilojoules/kilogram degree K. One connection among these units is the definition that a joule is equal to one watt-second. Without belaboring the mathematics, it is important to know that the rate of heat flowthrough a material by conduction is proportional to the temperature difference, known as the driving force, the cross

greater. Transfer from the surface onward is usually by conduction if the surface is of a solid. Transfer may be by conduction or convection if the surface is of a liquid. The rate of heat transfer is also proportional to the surface area and to a coefficient, symbolized by h,...