Heat transfer is a major unit operation in pharmacy. Heat energy can only be transferred from a region of higher temperature to a region of lower temperature.
Heat Transfer
INTRODUCTION
Heat
transfer is a major unit operation in pharmacy. Heat energy can only be
transferred from a region of higher temperature to a region of lower
temperature. Understanding heat transfer requires the study of the mechanism
and rate of this process. Heat is transferred by three mechanisms: conduction,
convec-tion, and radiation. It is unusual for the transfer to take place by one
mechanism only.
Conduction:
It is the most widely studied mechanism of heat transfer and the most
significant one in solids. The flow of heat depends on the transfer of
vibrational energy from one molecule to another and, in the case of metals, the
movement of free electrons with the occurrence of no appreciable displacement
of matter. Radiation is rare in solids, but examples are found among glasses
and plastics. Convection, by definition, is not possible in these conditions.
Con-duction in the bulk of fluids is normally overshadowed by convection, but
it assumes great importance at fluid boundaries.
Convection:
The motion of fluids transfers heat between them by convec-tion. In natural
convection, the movement is caused by buoyancy forces induced by variations in
the density of the fluid, caused by differences in temperature. In forced
convection, movement is created by an external energy source, such as a pump.
Radiation:
All bodies with a temperature above absolute zero radiate heat in the form of
electromagnetic waves. Radiation may be transmitted, reflected, or absorbed by
matter, the fraction absorbed being transformed into heat. Radiation is of
importance at extremes of temperature and in circumstances in which the other
modes of heat transmission are suppressed. Although heat losses can, in some
cases, equal the losses by natural convection, the mechanism is, from the
standpoint of pharmaceutical processing, least important and needs only brief
consideration.
Heat
transfer in many systems occurs as a steady-state process, and the temperature
at any point in the system will not vary with time. In other important
processes, temperatures in the system do vary with time. The latter, which is
common among the small-scale, batch-operated processes of the pharmaceutical
and fine chemicals industry, is known as unsteady heat transfer and, since
warming or cooling occurs, the thermal capacity, that is, the size and specific
heat, of the system becomes important. Unsteady heat transfer is a complex
phenomenon that is difficult to analyze from the first principles at a
fundamental level.
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