Chapter 3 - Heat and Mass Transfer, Basic Principles

Abstract

This chapter deals with heat transfer and mass transfer principles that are based on essentially similar physical principles. In principle, they all obey a universal law, similar to the familiar Ohm's law, which can be expressed, in general terms as: the rate of transport (i.e., the quantity transported per unit time) is proportional to the driving force and inversely proportional to the resistance of the medium to the transport. The basic principles of heat and mass transfer are described together while their applications are treated separately. All transport phenomena (fluid flow, heat and mass transfer, electric current etc.) are the result of lack of equilibrium between parts of the system. Heat transfer occurs via three fundamental mechanisms: conduction, convection, and radiation. Conduction refers to the transfer of heat through a stationary medium. Convection occurs when heat travels along with a moving fluid. Radiation is the transfer of heat in the form of electromagnetic radiation. Unlike the former two mechanisms, radiative heat transfer does not require the presence of a material medium between the two points. Further, heat exchangers are devices for the exchange of heat between two fluids separated by a heat conducting partition. Heat exchangers are extensively used in the food industry for heating (e.g., pasteurizers), cooling (chilled water generators), and heat induced phase change (freezing, evaporation). Each one of the two fluids may be confined or unconfined (free), stagnant or flowing. The partition is a heat conducting solid wall, usually made of metal. The design of a heat exchanger usually involves two main domains, namely thermal analysis and hydraulic calculations.