Page 141 - Industrial Ventilation Design Guidebook

4.3 HEAT AND MASS TRANSFER I 03

If we assume that the outlet air is saturated, the air state change process
is as presented in Fig. 4.22. The exact determination of the air humidity
at the end of the process would demand separate mass and heat transfer
examinations.

43 HEAT AND MASS TRANSFER

This section introduces the important subject of heat and mass transfer to
serve as a reference work to both the beginning engineer and the practicing in-
dustrial ventilation engineer.

4.3.1 Different Forms of Heat Transfer
For any method of heat transfer to take place, a temperature difference is nec-
essary between two faces of a solid body, or at the boundaries of a gas or va-
por. Heat transfer will take place only from a high-temperature source to a
lower-temperature sink and is an irreversible process unless acted upon by an-
other agency, as is the case with the refrigeration process.
Heat transfer may occur by one or more of three different modes:
• Conduction, which may be one-, two-, or three-dimensional or
internally generated
• Convection, which may be natural or forced
• Radiation, which may be symmetrical or nonsymmetrical
4.3.1.! Conduction
Conduction takes place at a solid, liquid, or vapor boundary through the col-
lisions of molecules, without mass transfer taking place. The process of heat con-
duction is analogous to that of electrical conduction, and similar concepts and
calculation methods apply. The thermal conductivity of matter is a physical prop-
erty and is its ability to conduct heat. Thermal conduction is a function of both
the temperature and the properties of the material. The system is often considered
as being homogeneous, and the thermal conductivity is considered constant.
Thermal conductivity, A, W irf °C~ , is defined using Fourier's law.

where

2
q = &/A = $" is the heat flow (W nT )
$> x is the heat flow in the x direction
dT/dx is the temperature gradient
The minus sign in the equation denotes that the heat flow is positive in the di-
rection of decreasing temperature.
Figure 4.23 represents a simple one-dimensional system with constant
heat flow <J> through the plate. The plate thickness is Ax (m) and the area of
2
the plate is A (m ).