Air Conditioning Engineering

W.P. Jones, W Peter Jones

• Publisher: Routledge
• Year: 2007
• Print ISBN: 9780750650748
• eBook ISBN: 9781136430343
• Edition: 5
• Topic: Technology & Engineering
• Subtopic: Construction & Architectural Engineering
• Index: Technology & Engineering

Designed for students and professional engineers, the fifth edition of this classic text deals with fundamental science and design principles of air conditioning engineering systems. W P Jones is an acknowledged expert in the field, and he uses his experience as a lecturer to present the material in a logical and accessible manner, always introducing new techniques with the use of worked examples.

1

The Need for Air Conditioning

1.1   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 may 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 really correct to talk of air conditioning only when a cooling and dehumidification function is intended, in addition to other aims. 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. Full control over relative humidity is not always exercised, hence for this reason a good deal of partial air conditioning is carried out; it is still referred to as air conditioning because it does contain refrigeration plant and is therefore capable of cooling and dehumidifying.

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.

1.2   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 the arguments for air conditioning might be based solely on climatic considerations, but although these may be valid in tropical and subtropical areas, they are not for temperate climates with industrialised 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. In any case, the relief provided by natural airflow through open windows is only effective for a depth of about 6 metres inward from the glazing. It follows that the inner areas of deep buildings will not really benefit at all from opened windows. Coupled with the need for high intensity continuous electric lighting in these core areas, the lack of adequate ventilation means a good deal of discomfort for the occupants. Mechanical ventilation without refrigeration is only a partial solution. It is true that it provides a controlled and uniform means of air distribution, in place of the unsatisfactory results obtained with opened windows (the vagaries of wind and stack effect, again particularly with tall buildings, produce discontinuous natural ventilation), but tolerable internal temperatures will prevail only during winter months. For much of the spring and autumn, as well as the summer, the internal room temperature will be several degrees higher than that outside, and it will be necessary to open windows in order to augment the mechanical ventilation. See chapter 16.

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 per cent to about 20 per cent 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°C dry-bulb, with 50 per cent saturation, in the presence of an external summer state of 28°C dry-bulb, with 20°C wet-bulb, declining to an inside condition of 20°C dry-bulb, with an unspecified relative humidity, in the presence of an external state of -2°C saturated in winter.

The essential feature of comfort conditioning is that it aims to produce an environment which 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.

1.3   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.

Thus, a system might be specified to hold 21°C ± 0.5°C, with 50 per cent saturation ±2½ per cent, provided that the outside state lay between 29.5°C dry-bulb, with 21°C wet-bulb and -4°C saturated.

2

Fundamental Properties of Air and
Water Vapour Mixtures

2.1 The basis for rationalisation

Perhaps the most important thing for the student of psychrometry to appreciate from the outset is that the working fluid under study is a mixture of two different gaseous substances. One of these, dry air, is itself a mixture of gases, and the other, water vapour, is steam in the saturated or superheated condition. An understanding of this fact is important because in a simple analysis one applies the Ideal Gas Laws to each of these two substances separately, just as though one were not mixed with the other. The purpose of doing this is to establish equations which will express the physical properties of air and water vapour mixtures in a simple way. That is to say, the equations could be solved and the solutions used to compile tables of psychrometric data or to construct a psychrometric chart.

The justification for considering the air and the water vapour separately in this simplified treatment is provided by Dalton's laws of partial pressure and the starting point in the case of each physical property considered is its definition.

It must be acknowledged that the ideal gas laws are not strictly accurate, particularly at higher pressures. Although their use yields answers which have been adequately accurate in the past, they do not give a true picture of gas behaviour, since they ignore intermolecular forces. The most up-to-date psychrometric tables (CIBSE 1986) are based on a fuller treatment, discussed in section 2.19. However, the Ideal Gas Laws may still be used for establishing psychrometric data at non-standard barometric pressures, with sufficient accuracy for most practical purposes.

2.2 The composition of dry air

Dry air is a mixture of two main component gases together with traces of a number of other gases. It is reasonable to consider all these as one homogeneous substance but to deal separately with the water vapour present because the latter is condensable at everyday pressures and temperatures whereas the associated dry gases are not.

One method of distinguishing between gases and vapours is to regard vapours as capable of liquefaction by the application of pressure alone but to consider gases as incapable of being liquefied unless their temperatures are reduced to below certain critical values. Each gas has its own unique critical temperature, and it so happens that the critical temperatures of nitrogen and oxygen, the major constituents of dry air, are very much below the temperatures dealt with in air conditioning. On the other hand, the critical temperature of steam (374.2°C) is very much higher than these values and, consequently, the water vapour mixed with the dry air in the atmosphere may change its phase from gas to liquid if its pressure is increased, without any reduction in temperature. While this is occurring, the phase of the dry air will, of course, remain gaseous.

Figures 2.1(a) and 2.1 (b) illustrate this. Pressure-volume diagrams are shown for dry air and for steam, separately. Point A in Figure 2.1(a) represents a state of dry air at 21°C. It can be seen that no amount of increase of pressure will cause the air to pass through the liquid phase, but if its temperature is reduced to -145°C, say, a value less than that of the critical isotherm, t_c (-140.2°C), then the air may be compelled ...

Table of contents

• Front Cover
• Half Title
• Title Page
• Copyright
• Preface to the Fifth Edition
• Preface to the First Edition
• Acknowledgement
• Contents
• 1. The Need for Air Conditioning
• 2. Fundamental Properties of Air and Water Vapour Mixtures
• 3. The Psychrometry of Air Conditioning Processes
• 4. Comfort and Inside Design Conditions
• 5. Climate and Outside Design Conditions
• 6. The Choice of Supply Design Conditions
• 7. Heat Gains from Solar and Other Sources
• 8. Cooling Load
• 9. The Fundamentals of Vapour Compression Refrigeration
• 10. Air Cooler Coils
• 11. The Rejection of Heat from Condensers and Cooling Towers
• 12. Refrigeration Plant
• 13. Automatic Controls
• 14. Vapour Absorption Refrigeration
• 15. Airflow in Ducts and Fan Performance
• 16. Ventilation and a Decay Equation
• 17. Filtration
• Index