Understanding Heat Transfer in Chemical Engineering
Chemical Engineering Heat Transfer
The field of heat transfer explores the rate at which heat flows from a region of high temperature to one of low temperature. Heat flow occurs as molecules transfer their thermal energy, in the form of molecular motion, to nearby lower energy molecules or by fast moving molecules moving to another region of the system. The mechanisms of heat transfer can be categorized as occurring by convection, conduction, or radioactive processes. Convection occurs in a liquid or gas as high energy, fast moving molecules create an area of low density media that rises relative to the slower moving, denser regions. This molecular movement redistributes the energy in a system. This type of heat transfer is how heated air from a register moves around a room to reach a pleasant 22°C (72°F) during the winter. Conduction occurs as high energy molecules collide with lower energy molecules thereby transferring some of their kinetic energy to their collision partner. Conduction transfers the thermal energy around more evenly, allowing heat to travel from warmer to cooler regions. Conductive heating is used in electric stoves to heat pans for cooking. Radioactive heat transfer occurs when a warm object emits electromagnetic radiation. The radiation can be used to heat an object at a distance from the heat source.
The amount of heat transferred through by conduction is expressed by
Q/t = -kS (T1 – T2/ d)
Where Q is the amount of heat transferred, t is the time taken, k is the conductivity of the material, S is the surface area through which the heat is transferred, d is the distance between the two ends of the system, T1 is the temperature at the higher temperature end and T2 is the temperature at lower temperature end.
These mechanisms tell about how heat travels in systems, but rate of transfer is also important. The most common way to describe the heat transfer rate is through the use of thermal-conductivity coefficients, which define how quickly heat will travel per unit of length (or area for convection processes). Every material has a characteristic thermal conductivity coefficient. Metals have high thermal conductivity, while polymers generally exhibit low thermal conductivities. One interesting application of thermal conductivity is the utilization of calcium carbonate in blown film processing. Calcium carbonate is added to a polyethylene resin to increase the heat transfer rate from the melt to the air surrounding the bottle. Without calcium carbonate, the resin cools much more slowly and production rates are decreased.
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