Heat Transfer

| Home | | Pharmaceutical Technology |

Chapter: Pharmaceutical Engineering: Heat Transfer

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.

Contact Us, Privacy Policy, Terms and Compliant, DMCA Policy and Compliant

TH 2019 - 2023 pharmacy180.com; Developed by Therithal info.