Chapter hvac engineering Fundamentals: Part 1 Introduction

Part 3
Heat Transfer
18.1
Introduction
This chapter presents a basic overview of heat transfer fundamentals,
particularly as they apply to HVAC. For a detailed, rigorous treat-
ment, the reader should refer to a good college-level text on heat trans-
fer or to the ASHRAE Handbook.
1
18.2
Heat Transfer Modes
Heat is transferred between any two bodies by one or more of three
modes: conduction, convection, and radiation. Thermal
conduction
re-
fers to the direct transfer of energy between particles at the atomic
level. Thermal
convection
may include some conduction but refers pri-
marily 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 en-
ergy at the receiver. Radiation transfer requires no intervening ma-
terial, 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
⬚
.
Source: HVAC Systems Design Handbook
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