TABLE OF CONTENTS This chapter presents a basic overview of heat transfer fundamentals, particularly as they apply to HVAC. For a detailed, rigorous treatment, the reader should refer to a good college-level text on heat transfer or to the ASHRAE Handbook.1
Heat is transferred between any two bodies by one or more of three modes: conduction, convection, and radiation. Thermal conduction refers to the direct transfer of energy between particles at the atomic level. Thermal convection may include some conduction but refers primarily to energy transfer by eddy mixing and diffusion, i.e., by fluids in motion. Thermal radiation describes a complex phenomenon which includes changes in energy form: from internal energy at the source to electromagnetic energy for transmission, then back to internal energy at the receiver. Radiation transfer requires no intervening material, and in fact works best in a perfect vacuum. In accordance with the second law of thermodynamics, net heat transfer occurs in the direction of decreasing temperature. In this text, the Fahrenheit ( F) scale is used, or for absolute temperatures the Rankine ( R) scale: R = F + 460 .
For steady-state conduction in one direction through a homogeneous material, the Fourier equation applies:
| (18.1) |  |
where <i class="emphasis">q</i> = heat transfer rate, Btu/h<i class="emphasis"> k</i> = thermal conductivity, Btu/(h ft F)<i class="emphasis"> A</i> = area normal to flow, ft<sup2</sup><i class="emphasis"> dt</i>/<i class="emphasis">dx</i>
