PART 1: INTRODUCTION
I. The meaning of air conditioning
Full air conditioning implies the automatic control of an atmospheric environment either for the
comfort of human beings or animals or for the proper performance of some industrial or scientific
process.
The adjective 'full' demands that the purity, movement, temperature and relative humidity of the
air be controlled, within the limits imposed by the design specification. (It is possible that, for
certain applications, the pressure of the air in the environment will also have to be controlled.)
Air conditioning is often misused as a term and is loosely and wrongly adopted to describe a system
of simple ventilation. It is correct to talk of air conditioning only when a cooling and
dehumidification function. This means that air conditioning is always associated with refrigeration
and it accounts for the comparatively high cost of air conditioning.
Refrigeration plant is precision-built machinery and is the major item of cost in an air conditioning
installation, thus the expense of air conditioning a building is some four times greater than that of
only heating it.
The ability to counter sensible and latent heat gains is, then, the essential feature of an air
conditioning system and, by common usage, the term 'air conditioning' means that refrigeration is
involved.
II. Comfort conditioning
Human beings are born into a hostile environment, but the degree of hostility varies with the season
of the year and with the geographical locality. This suggests that air conditioning can be based
solely on climatic considerations, but although these may be valid in tropical and subtropical areas,
they are not for temperate climates with industrialized social structures and rising standards of
living.
Briefly, air conditioning is necessary for the following reasons. Heat gains from sunlight, electric
lighting and business machines, in particular, may cause unpleasantly high temperatures in rooms,
unless windows are opened. If windows are opened, then even moderate wind speeds cause
excessive draughts, becoming worse on the upper floors of tall buildings. Further, if windows are
opened, noise and dirt enter and are objectionable, becoming worse on the lower floors of buildings,
particularly in urban districts and industrial areas. Mechanical ventilation without refrigeration is
only a partial solution.
The design specification for a comfort conditioning system is intended to be the framework for
providing a comfortable environment for human beings throughout the year, in the presence of
sensible heat gains in summer and sensible heat losses in winter. Dehumidification would be
achieved in summer but the relative humidity in the conditioned space would be allowed to
diminish as winter approached. There are two reasons why this is acceptable: first, human beings
are comfortable within a fairly large range of humidities, from about 65 % to about 20 % and,
secondly, if single glazing is used it will cause the inner surfaces of windows to stream with
condensed moisture if it is attempted to maintain too high a humidity in winter.
The major market for air conditioning is to deal with office blocks in urban areas. Increasing land
prices have led to the construction of deep-plan, high-rise buildings that had to be air conditioned
and developers found that these could command an increase in rent that would more than pay for
the capital depreciation and running cost of the air conditioning systems installed.
Thus, a system might be specified as capable of maintaining an internal condition of 22~ dry-bulb,
with 50 % saturation, in the presence of an external summer state of 28~ dry-bulb, with 20~ wet-
bulb, declining to an inside condition of 20~ dry-bulb, with an unspecified relative humidity, in
the presence of an external state of-2~ saturated in winter.
The essential feature of comfort conditioning is that it aims to produce an environment that is
comfortable to the majority of the occupants. The ultimate in comfort can never be achieved, but
the use of individual automatic control for individual rooms helps considerably in satisfying most
people and is essential.
III. Industrial Conditioning
Here the picture is quite different. An industrial or scientific process may, perhaps, be performed
properly only if it is carried out in an environment that has values of temperature and humidity
lying within well-defined limits. A departure from these limits may spoil the work being done. It
follows that a choice of the inside design condition is not based on a statistical survey of the
feelings of human beings but on a clearly defined statement of what is wanted.
Questions:
1. Define the following: dry bulb temperature, wet bulb temperature and humidity.
2. Differentiate refrigeration to air-conditioning.
3. What is the difference between dry air and atmospheric air?
4. Can the water vapor in air be treated as an ideal gas? Explain.
5. What is vapor pressure?
