1.2 System of Units
Before looking at the three distinct modes of transfer, it is appropriate to introduce some terms
and units that apply to all of them. It is worth mentioning that we will be using the SI units
throughout this book:
The rate of heat flow will be denoted by the symbol Q . It is measured in Watts (W) and
multiples such as (kW) and (MW).
It is often convenient to specify the flow of energy as the heat flow per unit area which is
also known as heat flux. This is denoted by q . Note that, q Q / A where A is the area
through which the heat flows, and that the units of heat flux are (W/m2).
Naturally, temperatures play a major part in the study of heat transfer. The symbol T will be
used for temperature. In SI units, temperature is measured in Kelvin or Celsius: (K) and (qC).
Sometimes the symbol t is used for temperature, but this is not appropriate in the context of
transient heat transfer, where it is convenient to use that symbol for time. Temperature
difference is denoted in Kelvin (K).
The following three subsections describe the above mentioned three modes of heat flow in more
detail. Further details of conduction, convection and radiation will be presented in Chapters 2, 3
and 4 respectively. Chapter 5 gives a brief overview of Heat Exchangers theory and application
which draws on the work from the previous Chapters.
The conductive transfer is of immediate interest through solid materials. However, conduction
within fluids is also important as it is one of the mechanisms by which heat reaches and leaves
the surface of a solid. Moreover, the tiny voids within some solid materials contain gases that
conduct heat, albeit not very effectively unless they are replaced by liquids, an event which is not
uncommon. Provided that a fluid is still or very slowly moving, the following analysis for solids
is also applicable to conductive heat flow through a fluid.