Cooling Towers. Principles and Practice

A tribute to my wife Joyce who, knowing what she faced willingly took on the task of typing another manuscript.
P.J.O.
Third edition
E. J. Pring, lEng, FInst SMM, MlPlant E, FIWSoc.
Peter D. Osborn, BScEng(Hons), CEng, FIEE
Butterworth-Heinemann London Boston Singapore Sydney Toronto Wellington
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First published by Carter Thermal Engineering Ltd, 1967 Second edition, 1970 Third edition published by Butterworth-Heinemann, 1990
© Butterworth-Heinemann Ltd, 1990
British Library Cataloguing in Publication Data Hill, G. B. (Gerald Bowen)
Cooling towers. —3rd ed. 1. Cooling towers I. Title II. Pring, E. J. III. Osborn, Peter D. (Peter Digby) IV. Stanford, W. (William) 1930-. Cooling towers
660.283 ISBN 0-7506-1005-0
Library of Congress Cataloging-in-Publication Data Hill, G. B. (Gerald Bowen)
Cooling towers/G. B. Hill,E. J. Pring, Peter D. Osborn.—3rd ed.
p. cm. Rev. ed. of: Cooling towers/W. Stanford, G. B. Hill. Includes bibliographical references and index. ISBN 0-7506-1005-0 1. Colling towers. I. Pring, E. J.
II. Osborne, Peter D. (Peter David) III. Stanford, W. (William). Cooling towers. IV. Title. TJ563.H55 1990 621.1'97—dc20
Photoset by Genesis Typesetting, Laser Quay, Rochester, Kent Printed and bound in Great Britain by Courier International Ltd, Tiptree, Essex.
Preface
The first and second editions of this book were published in 1967 and 1970 respectively; changes which have taken place since then have necessitated a major revision of the earlier work including the introduction of the SI system of units. In the light of progressive removal of European trade boundaries the change to SI was considered to be essential. In making the change, those older engineers, who, like myself, feel more comfortable with imperial units, have been provided for by comprehensive conversion tables.
The earlier editions were written against a background in which many small cooling requirements were catered for by 'once through' systems with warm water discharged to waste. Today's water costs and limited water resources render this approach quite untenable and it is now abundantly clear that water charges will go on rising at well beyond inflation rates in most industrialized countries.
Recirculation is, therefore, absolutely essential and the mecha nical draught evaporative cooling tower, in its many forms and sizes, is the cooling device with the lowest capital and running costs together with maximum operational flexibility. More effective technical press coverage combined with the development of compact factory assembled towers has, in the last two decades increased the system designers awareness of the advantages which these towers can offer; in particular, the recovery of the total capital investment, sometimes in a few months, from the savings in water charges.
The objective of this edition, as with the first two, is to provide the reader with a better understanding of the theory and practice, so that installations are correctly designed and operated. As with all branches of engineering, new technology calls for a level of technical knowledge which becomes progressively higher; this new edition seeks to ensure that the principles and practice of cooling towers are set against a background of up-to-date technology. The need for this thorough treatment of cooling tower engineering is
vi Preface
increased by the apparent neglect of the subject during higher technical education.
When the first edition was written, and in fact until the early 1980s, the cooling tower was perceived as an almost totally benign and beneficial piece of equipment. However, the outbreaks of legionnaires disease have understandably led to considerable public concern. The quality press and technical journals have kept the legionella hazard in perspective but the popular press and television have seized on the emotional aspects of the problem and exaggerated the dangers. All industrial activities, and indeed most human activities, carry with them some degree of risk, but safety consciousness both in industry and in everyday life can reduce those risks to an acceptable level. As far as is known, no properly designed and correctly maintained cooling tower anywhere in the world has been implicated in an outbreak of legionnaires disease. Tower manufacturers, water treatment specialists and filtration equipment suppliers have all been active and very thorough in providing the features and maintenance procedures essential to minimize the risk.
The best estimate is that there are about 100000 cooling towers operating in the United Kingdom. The efficiency of cooling towers is such that, if these were replaced by non-evaporative coolers, such as finned tube air blast coolers (even if practicable) the increased power requirement would be about 800 MW. As a standard for comparison the much debated Size well ' nuclear power station has a planned output of 1200 MW. The other crucial factor is that evaporative cooling can produce cooled water temperatures below the ambient air dry bulb temperature and these are commonly called for in industry and air conditioning systems. No other cooling method, apart from refrigeration, can achieve these temperature levels. The cost of refrigeration cooling systems can be up to eight times the cost of towers, they have much higher power consumptions and because they use CFCs they add to the problems of stratospheric ozone depletion and global warming.
Undoubtedly cooling towers are essential and will continue to be installed and operated; their safe operation will rest with manufacturers, installers, maintenance and water treatment staff, with premises management having the ultimate responsibility.
I must pay tribute to the contributions made to the earlier editions by my original co-author W. Stanford.
The preparation of this third edition has been heavily dependent on the dedication and expertise of Peter Osborn who is a very experienced technical author and has devised the new format
Preface vii
which we hope our readers will find convenient for reference purposes.
E. J. Pring, my other co-author, a well-known figure in the industry and a past Chairman of both the British Standards Committee on cooling towers and the Industrial Water Society, has been invaluable in helping to ensure that the latest technology has been incorporated.
R. S. Phull undertook the arduous but essential task of checking the calculations, graphs and nomograms.
Finally, my thanks to all those authorities and manufacturers who cooperated by supplying information and illustrations.
G. B. Hill
Acknowledgements
Thanks are due to the undermentioned organizations and manufacturers who assisted with the preparation of this book or who gave permission for information, photographs, drawings, diagrams or data to be reproduced.
British Standards Institution (see also bibliography): Charts at A.3.5.1, A.6.4.2 and A.6.4.3.
Chartered Institution of Building Services Engineers (see also bibliography): Psychrometric chart at B. 1.15.1. Maps at C.4.
The Industrial Water Society (see also bibliography) Maintenance schedule at A. 10.1.
Midland Research Laboratories, UK Ltd. 66 Hounslow Road, Twickenham, Middlesex TW2 7EX. Tel: 01 755 2661: Help in preparation of Section A.8 also photographs A. 12.3 and
A. 16.1 and diagrams A. 12.1 and A. 12.2. Bruel and Kjaer (UK) Ltd., Harrow Weald Lodge, 92 Uxbridge
Rd, Harrow Middlesex HA3 6BZ: Help in preparation of Section A.6.
Baltimore Aircoil Ltd., Corby, Northants: Illustrations A.2.2.2, A.2.3.4.
Carter Industrial Products Ltd., Birmingham: Illustrations A.2.3.5, A.2.4.5, A.2.4.6, A.2.4.7, A.2.5.2,
A.2.5.3, A.2.5.4, A.2.5.6, A.4.1.7, A.4.2.7, A.8.20.2, A.8.20.3, A.8.20.4, A.8.20.5, A.8.20.12, A.8.20.13, A.8.20.14, A.8.20.15, A.8.20.16, A.8.20.17, A.10.2.1.
Davenport Engineering Co. Ltd., Bradford: Illustrations A.l .2.3, A.2.1.2, A.2.3.6, A.4.1.6.
Film Cooling Towers Ltd, Richmond: Illustrations A.4.1.4, A.4.1.5.
Heenan-Marley Cooling Towers Ltd., Worcester: Illustrations A.2.3.3, A.2.5.5, A.2.6.2, A.4.2.6.
Plenty Ltd., Newbury: Illustrations A.8.20.8, A.8.20.9, A.8.20.10, A.8.20.11.
IX
A.4.2.5. Vokes Ltd., Guildford:
Cooling tower practice
A.l Fundamentals A. 1.1 Evaporative cooling When water changes its state from liquid to vapour or steam an input of heat energy must take place which is known as the latent heat of evaporation; this input energy must either be supplied from fuel as in a boiler or be extracted from the surroundings. Cooling towers take advantage of this change of state by creating conditions in which hot water evaporates in the presence of moving air; by this means heat is extracted from the water and transferred to the air and the process is known as evaporative cooling. The principle is very simple but the heat transfer processes are quite complex. Primitive cooling towers consist of no more than a four-sided wooden structure in which the hot water is introduced as a spray at the top of the tower, mixed with the cooling air and drawn off from a sump at the bottom; the water is thus cooled for return to the machine or process.
The principal criteria on which the design and manufacture of cooling towers is based are: • Achieving maximum contact between air and water in the tower
by the optimum design of tower packing and water distribution system as described below in A.4.1 and A.4.3.
• Assisting the flow of air by means of fans. • Minimizing the loss caused by water spray escaping from the
tower; control of spray loss is also of great importance in eliminating the risk of infectious diseases being transmitted to people by the warm moist air.
• Relating the design of the tower to the volume flow rate of the water to be cooled and to the three critical temperatures, i.e. ambient air wet bulb, warm water input and cooled water output.
• Ensuring that problems arising from the quality of the water such as corrosion, fouling and the growth of bacteria are properly understood and controlled.
1
2 Cooling tower practice
• Taking due account of space limitations at the tower's location and of the possibility that noise from the tower may be a source of nuisance to those living or working in the vicinity.
A. 1.2 Main components Figure A. 1.2.1 shows a schematic arrangement of a mechanical draught cooling tower and the cutaway section at A. 1.2.2 shows a typical layout of the main components which are defined below:
Casing or shell The structure enclosing the heat transfer process reinforced as necessary to carry the other main items. Air inlet and air outlet The positions at which cool air enters, and warmed air leaves the tower. In natural draught towers the inlet is normally protected by drip-proof louvres and the outlet by a suitable grill. Where an induced draught fan is used the outlet is the fan casing; with forced draught the fan casing provides the inlet.
Air outlet
1
s ^erflow _/T[ nn. T g i t le tX^
ah Louvres
_AIR INLET
Cold water basin
Figure A. 1.2.1 Schematic arangement of a typical mechanical draught cooling tower
3
Figure A.1.2.2 Main components of a mechanical draught cooling tower. 1, Fan housing; 2, axial flow fan; 3, mild steel outer panels; 4, air inlet louvres; 5, integral sump; 6, packing; 7, gravity flow distribution system; 8, drift eliminators
Figure A.1.2.3 Polypropylene spray nozzle for water distribution
Fan Correct selection of fan according to the tower duty is of major importance; volumetric air flow rate, fan pressure developed and noise from motor and fan impellor must all be considered according to the duty and location of the tower. Drift eliminators These are positioned in the outlet airstream so as to prevent water droplets from being carried away from the tower by the airstream. Warm water inlet The point at which warmed water from the process enters the tower. Water distribution system Water entering the tower must be spread as evenly as possible over the cross-section of the tower; some of the methods used are: spray nozzles (Figure A.1.2.3), trough and gutter (Figure A. 1.2.4 shows the Vee notches along
Figure A. 1.2.4 Typical trough and gutter distribution system
A.l Fundamentals 5
the edge of the gutters). Figure A.2.3.3 shows two large towers with open pan diffusion deck. The drawing at A. 1.2.2 shows the inverted cones or cups from which the water is splashed. Packing (also sometimes referred to as fill) Consists essentially of a system of baffles which slows the progress of the warm water through the tower and ensures maximum contact between water droplets and cooling air by maximizing surface area and minimizing water film thickness. There are many different types of packing and these are described in A.3. Cold water basin (also referred to as tank or sump) The point at which the cooled water is collected before return to the process. Cold water outlet The point at which the cooled water leaves the tower.
A. 1.3 Operating terms Operating terms are applied to air, water, temperature conditions and noise and these are further identified by Part 1 of BS 4485. Brief definitions of the main terms are given below and reference should be made to A.5 for details of terminology associated with noise:
Air flow Total quantity of air including the associated water vapour flowing through the tower. May be expressed in kilograms per second or in cubic metres per second and if the latter it must be related to temperature. Re-circulation That proportion of the outlet air which re- enters the tower. Fan power The power input to the fan in kilowatts. This excludes losses in driving motor, gearbox or power transmis sion, all of which, including mounting and support members are referred to as the fan drive assembly. Inlet water flow The quantity of hot water measured in cubic metres per second or per minute, flowing into the water distribution system. Drift loss Water loss caused by liquid drops carried away by the outlet air stream. Purge (also incorrectly referred to as blow-down) Water deliberately discharged from the system in order to reduce the concentration of salts and other impurities in the circulating water. Make-up Water added to the circulating water system to replace leakage, evaporation, drift loss and purge.
Concentration When water evaporates, dissolved solids and other impurities are left behind leading to an increase in concentration of these impurities. Concentration ratio Ratio of the total mass of impurities in the circulating water to the corresponding total mass in the make-up water. Water loading Flow of water related to the cross-sectional area of the packing normally expressed in kilograms per second (or per minute) per square metre of cross-section of packing. Heat load (or cooling load) Rate of heat removal from the water flowing through the tower expressed in kilowatts. Wet bulb temperature Temperature as measured by a wet bulb thermometer (see B.l.ll). Ambient air wet bulb temperature Wet bulb temperature measured on the windward side of the tower and free from the influence of the tower. Inlet air wet bulb temperature Average wet bulb temperature of the inlet air including any re-circulation effect. This is the wet bulb temperature used in the design of cooling towers, but it is difficult to measure with precision. See also B.4. Nominal inlet air wet bulb temperature An arithmetic average wet bulb temperature based on measurements taken within 1.5 m of the air inlets and between 1.5 m and 2.0 m above the basin kerb elevation on both sides of the cooling tower. Hot water temperature Temperature of water entering the distribution system. Re-cooled water temperature Average temperature of the water at the discharge point from the cold water basin excluding the effect of any make-up entering the basin. Cooling range Difference between the hot water temperature and the re-cooled water temperature. Approach Difference between re-cooled water temperature and the inlet air wet bulb temperature. It is clearly impracticable to cool the water to the inlet air level and the smaller the approach the more arduous tower design becomes. The minimum reasonable approach temperature is 2°C, however 4°C to 6°C is more usual.
A. 1.4 The physical mechanisms of cooling tower operation Theoretical aspects of psychrometry and heat transfer are developed in some detail in Section B and in particular B.2.2 covers the physical processes, basic formulae and calculations associated with cooling towers; at this stage the mechanisms by
A.l Fundamentals 7
which the water is cooled are best understood by reference to Figure A. 1.4.1 which illustrates a single droplet of water in the tower. The droplet is surrounded by a thin film of air which is saturated and remains almost undisturbed by the passing air stream. It is through this static film of saturated air that the transfer of heat takes place in three ways, i.e.:
Convected heat
Radiant heat
Layer of air at 100% humidity and same temperature as water
Bulk unsaturated air flowing past droplet
Figure A. 1.4. heat
Diagram showing the various ways in which a water droplet loses
By radiation from the surface of the droplet; this is a very small proportion of the total amount of heat flow and it is usually neglected. By conduction and convection between water and air; the amount of heat transferred will depend on the temperatures of air and water. It is a significant proportion of the whole, and may be as much as one-quarter to one-third. By evaporation; this accounts for the majority of heat transfer and is the reason why the whole process is termed 'evaporative cooling'.
Evaporation is the key to the successful operation of cooling towers and is covered more fully in B.2.2, B.2.3, and B.2.4; the main principles are summarized below:
The evaporation that occurs when air and water are in contact is caused by the difference in pressure of water vapour at the surface of the water and in the air. These vapour pressures are functions of the water temperature and the degree of saturation of the air, respectively.
In a cooling tower, the water and air streams are generally opposed so that cooled water leaving the bottom of the pack is in contact with the entering air. Similarly, hot water entering the pack will be in contact with warm air leaving the pack.
Evaporation will take place throughout the pack. It should be noted, that at the top of the pack, the fact that the air is nearly saturated, is compensated for by the high water temperature and consequently high vapour pressure. The amount of evaporation which takes place depends on a number of factors, including the total surface area the water presents to the air (which is why the pack design is so important) and the amount of air flowing. The greater the air flow the more cooling is achieved. This is because as the air rate increases, the effect of the water on its temperature and humidity will become less, and the partial pressure differences throughout the pack will be increased. The wet bulb temperature of the entering air has a very important effect. A lower wet bulb temperature produces a lower water-off temperature. The factors which influence the performance of a cooling tower may be summarized as follows:
1 The cooling range 2 The approach 3 The ambient air wet bulb temperature 4 The flow of water to be cooled (or circulation rate) 5 The rate at which air is passed over the water 6 The temperature level 7 The performance coefficients of the packing to be used 8 The volume of packing (i.e. height multiplied by horizontal
cross-sectional area) Item 6 is important because much greater cooling is possible at higher temperatures; this is apparent from Table C.6 which shows how the total heat or enthalpy of saturated air rises exponentially with temperature.
A.2 Types of cooling tower 9
Item 7 depends on how effectively the pack is designed and it follows that the volume of the pack (item 8) will directly affect tower performance.
A.2 Types of cooling tower There are four major components which go to make up a cooling tower, namely the packing, drift eliminators, the water distribu tion system, and (excepting natural draught towers) the fans. The relative disposition of these components is the main determinant of the different types of tower. They are all dependent on hot water entering at or near the top of the tower and descending under gravity through the packing to the basin. Early designs of tower were constructed almost entirely from timber but, although timber is still sometimes used for frames of large towers it has been largely superseded by materials such as glass fibre, PVC, polypropylene, and steel which may be galvanized, treated for corrosion resistance, or stainless.
A.2.1 Natural draught cooling towers Apart from the large hyperbolic concrete cooling towers which are a familiar sight adjacent to fossil fuel fired power stations, natural draught towers are rarely used today. Early designs of natural draught towers were constructed entirely from timber and were sited to take advantage of prevailing winds; this caused obvious limitations. Introduction of the hyperbolic shape enabled the chimney effect to be exploited and reduced the dependence on wind direction. The draught induced is a function of the difference in density between the ambient air entering the bottom of the tower and the air/water vapour mixture leaving the packing. Calculation of the operating air flow through the tower must take account of the draught induced and of the resistance to flow caused by packing and eliminators (reference should be made to BS4485: Part 2: 1988, Appendix E). The main features of a hyperbolic tower are shown at Diagram A.2.1.1 and the photograph at A.2.1.2 shows a group of towers for a power station.
A.2.2 Cross-flow forced draught designs (Figure A.2.2.1 and illustration A.2.2.2) Air is forced through the packing horizontally with drift eliminators on the outlet side; axial flow fans are normally used. A simple gravity hot water distribution system may be applied.
10
- ^ - r : , . . , :v . ' -
f' n?
-**£*·
Figure A.2.1.2 A group of hyperbolic cooling towers at a power station
11
Modular arrangements may be made to increase capacity by mounting two or more units side by side and such an arrangement facilitates control as fans can be switched on or off according to season and cooling demand.
A.2.3 Cross-flow induced draught designs (Figure A.2.3.1) Axial fans are normal for this arrangement; this tends to give more even distribution of air through the pack compared with the forced draught design, but makes control of drift rather more difficult.
Drift eliminator
Air flow
Hot water distribution pans Outlet air \
Figure A.2.3.2 Twin pack cross-flow induced draught cooling tower
Figure A.2.3.3 Two cross-tlow induced draught towers showing open pan gravity distribution system
Twin pack versions of this design are shown at Figure A.2.3.2 and illustrations A.2.3.3, A.2.3.4 and A.2.3.5; this arrangement enables vertical discharge of the outlet air to be effected. Figure A.2.3.6 shows a multi-cell double intake cross-flow tower with cast in situ reinforced concrete shell.
Fan power for a given performance is lower than with forced draught designs and a large area of drift eliminators can be accommodated. Fan motors are mounted in the warm moist air-stream and must be suitably protected, to IP55 weatherproof standards.
14
1*
' -mi
1
1 Figure A.2.3.4 Twin pack cross-flow steel frame induced draught cooling tower
Figure A.2.3.5 Twin pack cross-flow glass fibre towers
Figure A.2.3.6 Multi-cell double intake cross-flow tower with cast in situ reinforced concrete shell
A.2.4 Contra-flow forced draught designs (Figure A.2.4.1) Air is forced upwards through the pack by a fan mounted at low level. Axial or centrifugal fans may be used. Use of centrifugal fans enables the fan to be floor mounted with a resilient connection between fan casing and tower; such an arrangement reduces vibration and consequently noise, it also reduces the overall height of the tower where low silhouette is called for (Figure A.2.4.2, illustration A.2.4.5). With either fan type, re-circulation may be avoided where necessary by a canopy or directional louvres to concentrate the leaving air stream and increase its velocity. Modular designs with multiple fans may be used with fans switched in and out as needed; illustration A.2.4.6 shows an axial fan design. A two module centrifugal fan tower is shown at A.2.6.2 and A.2.4.7 shows a large timber frame axial fan tower.
The use of forced draught fans facilitates indoor siting of cooling towers as shown in Figures A.2.4.3 and A.2.4.4.
16
Outlet air
&«««««««««««««*
V Hot water distribution J —7 7 7 7 7 7 7—
Eliminator
Figure A.2.4.1 Contra-flow forced draught cooling tower with axial fan
Outlet air
t t t i I I I I I I I I I I I I I I I &«««««««««««««*+ Eliminator
Hot water distribution V_ J Floor mounted centrifugal fan
Figure A.2.4.2 Contra-flow forced draught cooling tower with centrifugal fan
17
t t t
'ΖΖΖΖΖΖΖΖΖΖΖΖΖΖΖΖΖΖΖΖΖΖΖΖ 7Δ^^ΖΖΖ7Ζ7ΖΛ Figure A.2.4.4 Plan of forced draught tower sited indoors
Figure A.2.4.5 Low height contra-flow forced draught cooling towers with centrifugal fans
A.2.5 Contra-flow induced draught designs (Figures A.2.5.1, illustrations A.2.5.2, A.2.5.3, A.2.5.4, A.2.5.5 and A.2.5.6) Axial flow fans are standard, and because the leaving air stream may be controlled in velocity and direction, re-circulation is minimized. Input air comes through louvred openings at the base of the tower and consequently performance can be affected by high winds; this can add to the airborne contaminants introduced into the cooling water. Multiple fan designs may be used enabling one or more fans to be switched off during periods of light load. Fan motors are exposed to the warm moist airstream and must therefore be suitably protected to IP55 weatherproof standards.
A.2.6 Indirect evaporative cooling towers (Figure A.2.6.1, illustration A.2.6.2) When applied to air conditioning systems this design incorporates a serpentine coil in the tower instead of packing. Hot water, from the refrigeration plant water cooled condenser, is circulated
Figure A.2.4.6 Forced draught contra-flow cooling tower
through the coil and cooled in the tower by the evaporative process (note that there are two independent water circuits). Although described as a closed circuit system water is still being evaporated in the tower and cooling efficiency is lower than with packed towers; a larger tower is needed with higher capital and running costs. Contamination of the closed cooling water circuit is avoided, but purging and treatment of the tower water is still required, and is likely to be more critical. Full evaporation cooling can be achieved by interposing a heat exchanger between the condenser cooling water circuit and a tower with standard packing.
A.2.7 Evaporative condensers (Figure A.2.7.1)
The principle is similar to that of indirect evaporative cooling towers, but in this case refrigerant is piped from the condenser to the cooling tower and cooled by the indirect evaporative method before return to the evaporator/compressor of the air conditioning system.
A.2.8 Some factors affecting the selection and performance of cooling towers Natural draught towers have limited specific application as described in A.2.1. For the majority of applications fans are essential and provide the only means of achieving a low approach temperature. For maximum cooling the contra-flow design with gravity water flow and vertical air stream is preferred. The cooled water meets dry air at the bottom of the tower and the hot water meets warm moist air at the top; this tends to ensure that evaporative cooling takes place throughout the pack.
O ut
le t
ai r
23
Figure A.2.5.5 Contra-flow induced draught cooling tower with glass reinforced casing showing spray nozzle distribution system
"<N Figure A.2.5.6 Two induced draught cooling towers designed specifically to meet British Airport Authority architectural requirements for Gatwick North Terminal (each tower cools 2500 gallons per minute of condenser water)
Hot water from **- process
Pump for circulation
^ . Hot water distribution
^Serpentine: :coil : 3>
Figure A.2.6.1 Indirect evaporative cooling tower
Centrifugal fans are larger, more expensive and occupy more floor space than axial flow fans, but they can generate higher pressures and have lower noise levels. Direct drive of axial fans up to 1.8 m diameter is possible; on larger fans where much lower speeds are called for belt or gear drive becomes necessary (see also A.4.5). Control of cooling may be effected by varying fan speed or by using multiple fans switched on or off as needed. Control of the flow rate of an individual fan may be by way of pole-change 2-speed motors, by variable pitch fan blades, or where economically justified by inverter control for motor speed variation. Electric motors for all outdoor cooling towers require a degree of protection IPW55 to BS4999 and this protection is satisfactory for
Figure A.2.6.2 Forced draught closed circuit cooling tower
motors driving induced draught fans which operate in the warm moist discharge air stream. For indoor forced draught towers standard TEFC motors to IP55 are satisfactory. The use of ventilated motors is unlikely to be justified except for indoor towers requiring large fan powers. Forced draught axial flow fans can be subject to icing up and because the clearance between fan impellor and fan casing is small this can have serious consequences. Fast tripping of the motor circuit under stall conditions can avoid damage but for critical
Vapour
Figure A.2.7.1 Use of cooling tower as evaporative condenser
installations a heater should be fitted or fans should be mounted on an inclined panel at 5-10° from vertical to ensure that water is drained back into the tower (see sketch at A.2.4.1).
A.3 Rating, duty and physical size of cooling towers An indication of the complexity of cooling tower selection and design is given in Sections A l , A2 and Section A5 gives some of the practical aspects of tower selection; calculation procedures are covered by Section B.
The notes which follow indicate some of the ways in which an impression may be gained of the space necessary to accommodate a tower for a given duty.
A.3.1 The problem of units SI units are used throughout this book; it is the only international system and once properly understood it makes calculation work much more straightforward. Table C.3. gives conversion factors
A.3 Rating, duty and physical size of cooling towers 27
for all the units likely to be encountered, but a reminder of those most relevant may be useful.
Consideration starts from the flow rate of water to be cooled and, particularly for small packaged towers it is convenient to use litres per second (1/s), but when carrying out calculations it is safer to use m3/s. (1000 gallons per minute = 0.0758m3/s or 75.81/s).
Because mass flow rather than volume flow should be used in calculations, note that, for the temperatures normally encoun tered, the density of water may be taken at 1000 kg/m3 (for more precise values refer to Table C.5). The specific heat capacity of water may be taken at 4.18 kilojoules per kg per °C (refer to Table C.5 for precise values).
The kilogram calorie (kg cal) may be enountered; this unit is defined in terms of water at 15°C and at that temperature the specific heat capacity of water is 4.187, thus one kg cal = 4.187 kJ.
For a given water flow rate the heat to be dissipated in the tower is:
Mass flow rate (kg/s) x specific heat capacity (kJ/kgK) x cooling range (K) (K is the kelvin measured on the absolute scale starting at - 273°C, an interval of one kelvin is the same as 1°C). The calculation will yield an answer in kJ/s which by definition is kW. (100000BTU/hr = 29.31 kW). One ton of refrigeration is the cooling load necessary to convert one ton of water to ice at freezing point. One ton = 907 kg so the rate is 37.79 kg per hour and this represents a cooling load of 3.52 kW (because heat rejected at the condenser is greater than the heat extracted at the evaporator it is usual to add 25% to obtain the cooling load on towers used in conjunction with refrigeration plant). For those engineers who prefer to visualize values in imperial units the main conversion factors are included in the Tables at C.9; also at C.9 will be found two-way conversion tables Fahrenheit/Celsius for easy reference.
A.3.2 The methods of specifying tower capacity • Most manufacturers will express the capacity of a given tower in
kW at a stated cooling range; this may be a nominal rating based on 5°C range or it may stipulate the three temperatures, i.e. 'hot water', 're-cooled water' and 'design air wet bulb'. Knowing the cooling load and cooling range enables a first approximation of tower size to be established.
• The volume rate of water flow is a main parameter of a given tower design and manufacturers will normally include this information in standard literature.
• The key to the capacity of a tower, and hence one of the main determinants of its physical size, is the cross-section area and the height of the pack. Because volume rate of air flow through the pack is critical to its performance manufacturers often give air flow rate for their standard ranges. The convenient unit is m3/s and the specific heat capacity of air may be taken as 1.0 kJ/kgK (for precise values see Table C.5). The density of air varies inversely with absolute temperature and at 15°C it is 1.226 kg/m3.
A.3.3 Design factors which affect tower size Figures A.2.2.1, A.2.3.1, A.2.3.2, A.2.4.1, A.2.4.2 and A.2.5.1 show in diagrammatic form the layout of standard towers and in considering space requirements for standard towers some general rules apply:
• The contra-flow forced draught arrangement shown in Figure A.2.4.1 occupies minimum floor space but tends to have a high profile because the full diameter of the axial fan must be accommodated in one side panel of the tower below the pack.
• The contra-flow induced draught tower shown in Figure A.2.5.1 requires less space below the packing but for satisfactory air flow the fan casing and air circuit necessitates a projection at the top.
• Minimum silhouette height is achieved by a forced draught floor-mounted centrifugal fan (Figure A.2.4.2), but this will increase floor space required. Floor space can be reduced somewhat by encroaching on the area needed for the cold water basin.
• Cross-flow towers are compact whether forced draught (Figure A.2.2.1), induced draught (Figure A.2.3.1) or twin pack (Figure A.2.3.2). The twin pack has the advantage over the other two because the air is discharged vertically.
The above comments refer only to tower height and floor space - there are many other factors which influence tower selection.
A.3.4 The use of selection charts For a given duty a tower may be selected from a standard range or it may be designed for that particular duty.
A.3 Rating, duty and physical size of cooling towers 29
Where selection is from a standard range a modular approach is used with two or more pack heights for a standard pack area. The modules are mounted side by side with interconnected pipework to and from the process.
The nomogram at A.3.4.1 enables the liquid loading for a typical contra-flow packing, as used in medium sized towers, to be determined from the three variables: Hot water temperature - Cooling range - Ambient air wet bulb temperature.
The pack plan area required is obtained by dividing the water flow rate in kg/s by the liquid loading in kg/sm2.
In the example shown on the nomogram 30°C hot water temperature, 6°C cooling range and 20°C ambient wet bulb temperature yields a liquid loading of 4.2 kg/sm2. The full calculation procedure is reviewed in B4 and B6. In practice the pack cross-section area and pack height will be selected from standard pack module sizes.
A.3.5 Effect on tower size of the variation of the main parameters The main determinant of the size of a mechanical draught cooling tower is the cross-section area and the height of the pack; relative overall height and floor space requirements will be modified with different types of tower as already described in A.3.3.
The following general statements may be made with regard to pack cross-section area: • It will increase steeply as approach temperature is reduced
towards the minimum practical level of 2°C. • Between the normal limits of wet bulb temperature (14 to 22°C
in temperate climates) the area required will be at a minimum at 14 and a maximum at 22°C.
• Area will be at a minimum when cooling range is at a minimum (around 6°C) increasing to a maximum when cooling range reaches 20°C.
These general statements are expressed in chart form at A.3.5 which is reproduced by permission from British Standard 4485: Part 3: 1988. In this chart the plan area factor is shown as unity for the average UK conditions, i.e. 12°C cooling range, 5°C Approach and 17°C Wet bulb temperature.
A typical waterloading value for these conditions is taken in BS4485 at 2.3 kg/sm2.
2. 0
3. 0
4. 0
5. 0
6. 0
L: L
iq ui
d lo
ad in
g (K
g/ s.
m 2 )
7. 0
CO CD CD C CO Q. o > CO CD cc
1.5
1.4
1.3
17
1.1
1.0
0.9
0.8
0.7
0.6
*νΛ %>
JL J I i L 14 15 16 17 18 19 20
Wet bulb temperature (°C)
J I L J I I J_ j
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Cooling range (K)
L _L J I 3 4 5 6 7 8
Approach (K)
Figure A.3.5.1 Mechanical draught counterflow tower: variation of tower size with cooling range, approach, and wet bulb temperature
Sample calculation. Water flow 0.75 m3/s Wet bulb 16°C Cooling
range from 35 to 22°C = 13°C Approach = 22° - 16° = 6° Relative plan factors (from the chart)
Cooling range (13°C) 1.04. Approach (6°C) 0.85 Wet bulb (16°C) 1.06 Overall plan area factor 1.04 x 0.85 x 1.06 = 0.95
Hence plan area = 0.75 (m3/,)x 1000 (kg^3)
2.3 (kg/m2s)
A.4 Cooling tower components and construction materials
The relative disposition of the four major components is determined according to the main categories of tower as reviewed in A.2. In addition to these four, i.e. packing, drift eliminators, water distribution system, and fans, the review which follows covers the cold water basin, the pumps, the structure or frame, and the cladding. The section ends with some general comments on materials used. The importance of maintenance and the influence which tower design can have on maintenance procedures are reviewed in Section A. 10.
A. 4.1 Packing
The traditional material used for cooling tower packing was timber, and, even though packs are now formed mainly from plastics, notably PVC (polyvinylchloride), timber remains the most straightforward material to consider in order to understand how the packing actually functions.
There are two fundamental approaches to packing design - the first is splash packing in which the hot water falling through the tower is encouraged to form droplets. The mechanism by means of which these droplets cause cooling has been reviewed in A. 1.4. The second approach to design uses the film principle in which the hot water is encouraged to spread out on a surface and form a thin film, thus providing the maximum surface area for evaporation and hence allowing cooling to take place.
33
* 4 ^J Water droplets
The splash mechanism is illustrated at Figure A.4.1.1 where staggered layers of timber laths are used to break the falling water into small droplets. There is a tendency for these droplets to agglomerate into larger drops at the edge of each lath, but as they fall to the next staggered layer they are broken down again. Because timber has a roughened surface thorough wetting is assured with consequent maximum evaporation as the water passes through the pack. Improved air flow is achieved by using timber laths of triangular cross section as shown in A.4.1.2. The photograph at A.4.1.3 shows a form of splash packing used in a large all-timber construction tower.
In early designs timber grids were used to maximize the film cooling effect; these grids consisted of 20 or more timber slats each
Figure A.4.1.3 Timber splash packing
A.4 Cooling tower components and construction materials 35
Figure A.4.1.4 Side elevation of PVC packing showing redistribution of water across the plate
35 to 50 mm deep by 10 to 15 mm wide with 20 or more grids mounted transversely one above the other to form the pack. This approach has been superseded by moulded packing which
provides the maximum surface area without the bulk of the timber slats; this has enabled the overall size of packing to be radically reduced.
Figure A.4.1.5 View from above of packing in Figure A.4.1.4
PVC is the most widely used material but other plastics have been used, including polystyrene, polypropylene (for high temperature applications) and polyethylene.
Vacuum formed PVC packings depend for their effectiveness on corrugations which ensure even distribution of the falling water whilst at the same time presenting the lowest resistance to air flow consistent with ensuring maximum evaporative cooling.
Figure A.4.1.4 illustrates a packing which has a primary waveform corrugation in one plane and a secondary smaller waveform at a skew angle to the primary. This double wave is well illustrated at A.4.1.5 which views the same packing from above.
The photograph at A.4.1.6 shows three types of plastic film packing with varying plate spacings and configurations for varying water qualities and thermal efficiencies. The fourth pack at the rear is of the plastic splash deck type.
A.4.1.7 shows packs in demountable plastic coated wire baskets. Plastic packings have many advantages, notably: • Lightness and consequent ease of removal and replacement • Inert in any water whether acid or alkaline • They do not break down to form a sludge as can happen with
timber or metal packing
Figure A.4.1.6 Three types of plastic film packing with a plastic splash deck type packing shown at the rear
Figure A.4.1.7 Plastic film packs in demountable plastic coated wire baskets
• Growth of scale is inhibited • They do not provide nutrients to support algae or bacteria • They are unaffected by electrolytic action • Easily formed to any shape required • Non-flammable (if PVC)
A.4.2 Drift eliminators The original purpose of drift eliminators was to control unnecessary loss of water and to reduce the nuisance caused to those close to a tower who might be subjected to a damp spray; drift loss of between 0.1 and 0.25% of the total water circulation rate was considered acceptable. The availability of PVC as a packing material and the complex shapes into which it could be formed enabled drift loss to be radically reduced; widespread concern about legionnaires disease (referred to in A.9) brought
into sharp focus the need for extremely tight control of drift. Losses below 0.005 and down to 0.001% can now be readily achieved and may be specified for cooling towers in sensitive locations.
Expressing drift loss as a percentage of total circulation can be misleading, as it implies that the loss in mg/m3 rises in proportion to the water flow rate, which it does not! In fact, within wide limits it is little affected by water flow rate. It is now believed that the
t t Air flow
Extruded plastic drift eliminators
spread of infection is related more to the number and size range of aerosols released than to the mass of water in mg/m3; the need is to devise a method of testing which relates eliminator efficiency directly to risk of infection.
The evolution of the modern drift eliminator is illustrated by the diagrams at A.4.2.1 and A.4.2.2. A.4.2.1 shows a metal lipped corrugated plate design which is heavy, prone to build-up scale, very difficult to clean and not very efficient. The extruded plastic eliminator shown at A.4.2.2 consists of aerofoil section plates of 150mm depth; performance is satisfactory at air velocities around
Air flow Figure A.4.2.3 Elevation of high efficiency plastic eliminator showing 1-, 2-, 3- and 4-pass arrangement
Figure A.4.2.4 Plan view of plastic eliminator shown at A.4.2.3
Figure A.4.2.5 High efficiency 2-pass drift eliminator module with cut-away section showing the pack formation
one metre per second; for velocities normally associated with mechanical draught towers (2.5 to 3.0m/s) the efficiency is poor. Figure A.4.2.3 shows, in elevation, the shape of a two, three, and four pass plastic eliminator and Figure A.4.2.4 shows the same eliminator in plan; the photograph at A.4.2.5 shows a two pass eliminator module mounted in a frame. Figure A.4.2.6 shows an alternative design with a panel removed to show the simplicity of installation. Eliminators of this type can restrict drift to 0.005 with single pass, to 0.001% with two pass and to nearly undetectable levels with 3 or 4 pass. Figure A.4.2.7 shows the complete eliminator fitted to a tower.
In contra-flow cooling towers the eliminators are fitted above the water distribution system and can be designed for single, two, or three/four pass as required by application. In cross-flow forced draught towers of the type illustrated in sketch A.2.3.1 the eliminator is fixed alongside the pack on the air discharge side and is sometimes integral with the pack.
Careful fitting and sealing of drift eliminators is essential to ensure that all discharge air passes through the eliminator passages.
42
Figure A.4.2.6 Alternative high efficiency eliminator without frame and with section removed to show simplicity of installation and facility for sealing
Figure A.4.2.7 Complete eliminator as fitted to a tower
A.4.3 Water distribution
All contra-flow towers, whether forced or induced draught have the hot water distribution system below the drift eliminators, whereas with cross-flow designs the reverse is the case (see Figures A.2.2.1, A.2.3.1, A.2.3.2).
There are four approaches to the design of water distribution systems: • The open pan or diffusion deck system is shown in the
photograph of an induced draught tower at A.2.3.3. This consists of a pan of the same area as the pack having a number of holes so as to give an even spread of hot water across the pack and should have a cover to reduce algae growth. Water may be delivered into the pan from an open pipe.
• The trough and gutter design with overspill as shown at Figures A. 1.2.2 and A. 1.2.4. The inlet water is delivered to a main trough which will normally be of suitably treated steel; there are a number of outlets in the base of the trough feeding the water into a series of gutters so as to cover the total area of the pack. Various designs are used to spill the water from the gutters on to the pack, examples being Vee notches or simple corrugations along the sides of the gutters.
• Spray distribution from nozzles as Figure A. 1.2.3. The nozzles are made from injection moulded PVC or polypropylene. The water is delivered to a main header pipe which has a series of branches running across the pack area with the distribution nozzles fitted into the branches. The nozzles need to be easily detachable for cleaning and any grommets used should be of material which does not provide nutrient for bacteria. The pipework can be either steel, acrylonitrile butadiene styrene (ABS), or UPVC (see A.4.6.2).
• A variation on the nozzle approach in large site constructed towers uses timber troughs into which are fitted a series of nozzles to direct the water into splash cups; this improves the distribution of water to the packing (refer Figure A. 1.2.2). All water distribution systems will require pump and suitable pipework to deliver the water to the top of the tower, but the size and mounting position of the pump will depend on volume flow rate and pump pressure requirements.
A.4.4 Cold water basin The cold water basin, also referred to as the sump, tank, or pond requires a number of connections:
• An inlet for make-up water from supply mains with float valve or other means of control to maintain the water level.
• Connections for filtration and water treatment (filtration and water treatment covered by Section A.8)
• Provision for thermostatically controlled electric immersion heater to prevent freezing of the pump suction outlet.
• Provision for purge which can be automatically controlled to limit the amount of dissolved solids in the water system.
• The basin must be provided with an overflow outlet, a connection for the cooled water return pipework and minimum 80 mm drains in floor of basin.
The design should be such that there are no internal up-turned flanges or pockets where sludge could accumulate, and preferably designed with sloping sides and base (see Figure A. 10.2.1).
A.4.5 Fans and fan drives Fan engineering is a complex and specialized subject and there are many inter-related factors which affect the selection of fans, notably: • The air flow rate varies directly as the fan speed. • The pressure exerted varies as the square of the fan speed. • The power absorbed varies as the cube of its speed. • In general the higher the fan speed the higher the noise level;
fan manufacturers publish characteristics of their fans which give details of noise levels under BS test conditions (refer also to Section A.6).
• The larger the diameter of a fan of a given type the greater the air flow rate it can handle.
• The available speeds of squirrel cage motors on 50 Hz supplies are 2920 - 1450 - 950 - 720 - 580 - 480r.p.m. and on 60 Hz supplies are 3520 - 1750 - 1150 - 840 - 690 - 560r.p.m. As the speed of the motor goes down the frame size and hence the cost of the motor rises and below around 500 r.p.m. it is more economical to use a standard 4-pole motor with belt or gear drive to the fan. Belt drive is invariably used for centrifugal fans.
Some of the main features of fan requirements for cooling towers are: • The volumetric flow is measured in m3/s and for standard
packaged cooling towers varies from around 0.5m3/s to 50m3/s. Large towers require very much higher volumes (of the order 500m3/s per cell).
• Standard practice is to express the pressure available from fans in kilopascal (kPa) and fan manufacturers will supply character istic curves for their various designs of fan showing the relationship between pressure generated and volume delivered. The pressure available falls away as volume increases until a point is reached where the pressure falls off very rapidly and the fan will no longer move the air. In any ventilation or air moving system the objective is to match the characteristics of the fan to the pressure and volume requirements of the system. With modern packing and drift eliminators the pressure drop in a cooling tower is low so that fans can be operated at close to their maximum volume.
A.4.6 Materials used in cooling tower manufacture The most important materials used in cooling tower manufacture are steel, timber, and plastics - the main characteristics and applications of these three groups of materials are reviewed below. Aluminium and ceramics are rarely used and asbestos cement, though once a popular material, is now ruled out because of health hazards from asbestos fibres.
A.4.6.1 Steel Due to its strength and ease of fabrication, mild steel has obvious advantages for the construction of cooling towers, but it is essential that it should be coated for protection against corrosion attack. Mill galvanized sheets will give good protection, but once sheets have been cut, drilled, welded and formed, areas are exposed for attack; even the smallest unprotected areas can lead to corrosion at a rate comparable to that of unprotected mild steel, (see A.8.12 and A.8.13). Protective paints can be applied to exposed areas but such protection must be inspected regularly and re-treated when necessary. Better protection can be obtained by the hot dipped galvanizing process after the tower sections have been cut, shaped, drilled and welded as necessary; thorough cleaning and smooth fettling of weld seams is essential before hot dipping. An alternative process is the electrolytic zinc coating of steel sheet, followed by an etch primer with finish based on epoxy, acrylic, or vinyl resins and final stoving.
Many other finishes may be used including bitumen, plastic coating, and synthetic rubber coating, but in every case thorough cleaning and degreasing is essential for the best results.
Even though relatively costly, stainless steel is used increasingly for cooling tower manufacture; with an \\Vi% chromium content
its corrosion rate is superior to that of untreated mild steel by a factor of up to 250 when used in severe marine industrial environments.
The use of steel pipework, in conjunction with plastic piping, should be avoided as far as possible, as differential expansion may cause problems.
A.4.6.2 Synthetic materials UPVC (unplasticized polyvinyl chloride) is a widely used material which can be vacuum formed into complex shapes and does not soften or distort in temperatures up to 60°C. It does not support combustion, but is subject to attack by some organic solvents. Used for packing, drift eliminators, and pipework; if correctly designed and suitably stiffened can be used for fabricated sections of towers.
GRP (glass reinforced plastic) covers a range of materials based on polyester which are widely used for the construction of small packaged towers (see Figure A.2.5.2). Suitable for service in temperatures up to 80-100°C. Should be treated with fire retardant.
ABS (Acrylonitrile-butadiene-styrene) has high impact strength and is suitable for temperatures up to 60-70°C. An alternative to GRP for tower construction.
Polypropylene, polystyrene and high density polyethylene can be used for packing and drift eliminators.
Polypropylene with glass reinforcement may be used for fan blades on axial fans. When used for packing polypropylene has the advantage of a higher softening temperature and can be used up to 80-90°C; it can be ignited but burns quite slowly.
Polystyrene can be used up to 60°C; however, high impact polystyrene, if ignited becomes a hazard as it burns very rapidly and gives off toxic fumes.
Polyethylene may be used up to 60°C; it ignites but is slow burning, and like polypropylene, will sustain combustion unless extinguished.
All plastic materials are subject to attack by organic solvents but UPVC is less vulnerable than the others.
A A. 63 Timber Correctly prepared and applied, timber remains a cost effective material for tower structures and very many towers have been in service with timber frames, cladding, and packing for 25-30 years. On large, site constructed, towers timber has also been used for
stairways, walkways, and fan casings. Timber should be treated after fabrication with suitable preservatives as specified in BS 4485: Part 4: 1988. The three timbers most widely used are:
Western red cedar which is the most durable and absorbs the least amount of preservative, but is costly.
Douglas fir is likewise resistant to the absorption of preserva tives.
Baltic redwood absorbs the largest amount of preservative, but when properly treated it has a useful life comparable to the other two.
Towers manufactured from timber structures can be expected to give good service for up to 30 years if well maintained.
The rotting of all types of timber takes many forms and is caused by various species of fungus; it is an essential part of the ecological process by which dead trees in their natural state are broken down and returned to the soil.
The materials from which trees are made up are cellulose and lignin; some fungi attack primarily the cellulose and others can attack both. The process of rot is associated with the life cycle of the fungus and starts with spores which are carried in the air to the surface of the timber; under suitable damp or wet conditions the spore germinates, releases enzymes which dissolve the cellulose or the lignin and enable the fungus to digest them as food. The next stage is for strands from the fungus to spread through the wood and extend the process of decay. The strands are known as hyphae and the mass of these hyphae is referred to as the mycelium or fungus plant; the mycelium eventually throws up a fruiting spur or sphorophore which, when ripe, breaks down into spores to start the cycle again. There are thousands of species of fungi, a number of which invade wood.
Massive decay of timber results from attack by fungi of the basidiomycetes group; this group includes the common mushroom and most of the fungi which produce noticeable sphorophores (fruiting bodies).
The different forms of wood rot are distinguished in ordinary language according to their appearance and main features, i.e. brown rot, white rot, wet rot, dry rot, soft rot, and stain (stain discolours the timber but does not destroy it).
True dry rot (serpula lacrymans) is not of great consequence as far as cooling towers are concerned and attacks mainly inside buildings where it has the ability to spread into bricks and concrete there to remain dormant and later spread into fresh timber.
Brown rot (which attacks only the cellulose) and white rot (which attacks both cellulose and lignin) both play their part in cooling tower timber, but the main culprits are wet rot and soft rot.
Wet rot is familiar in fence posts at close to ground level and causes decay in tower structural members which remain wet but are not immersed.
Soft rot occurs when the surface of the timber is softened and in the early stages the interior remains sound. Many fungi cause soft rot, the common ones being fusarium, gaphium, gliodadium and chaetomium globosum.
All timber used in towers can be affected by soft rot but timber packing is particularly susceptible because the fungi causing the condition can thrive even though the timber is continually wetted.
Treatment of timber to prevent fungal decay consists of causing fungicides to penetrate deep into the cellular structure of the timber; this is best achieved by first removing moisture from the timber in treatment plants by subjecting the timber to a vacuum to draw out the moisture. This is followed by flooding with fungicide liquid usually under a small pressure to ensure penetration. The timber is then drained and subjected to a further vacuum to remove surplus fungicide and to conserve material.
Preservatives which can be used with good effect are copper chrome arsenates and creosote, which give good results with thorough penetration.
A general disadvantage of the use of timber in cooling towers is that it provides a surface on which algae and bacteria can develop more readily; consequently the use of structural steel for the framework of large towers is increasing.
A.5 Practical aspects of tower selection A.5.1 Heat energy calculations The subject is treated fully in Section B.3.1, but a reminder of the ground rules is not out of place: • All energy including heat energy is measured in the practical
unit of kilojoules (kJ). • Rate of energy flow is measured in kilowatts (kW) and one
kilowatt equals one kilojoule per second. In every case the first objective is to establish the rate of energy flow which determines the cooling load on the tower. There are four sets of conditions:
A.5 Practical aspects of tower selection 49
(1) Where the cooling process is based on mains water running to waste and the requirement is to supply a cooling tower to reduce water charges. Data required are:
Mean temperature of mains water (outlet temperature required from tower) Normal temperature of water running to waste (inlet temperature to tower) Volume flow in litres per second
From B.3.1 The heat energy flow in kilojoules per second (kilowatts) is:
Lw x Cpw x (Tj —T2)
Lw = mass flow of water (kg/s). Cpw = specific heat capacity of water (kJ/kgK) Ti = inlet temperature (K) T2 = outlet temperature (K). K is the temperature in
kelvins (one kelvin = one degree Celsius)
Up to 45°C the mass of one litre of water is very close to 1 kg and over the same temperature range the specific heat capacity of water is very close to 4.2kJ/kgK (see Table C.5 for more precise values).
Thus for a flow rate of 50 litres per second and a temperature difference of 12°C the heat energy flow is:
50 x 4.2 x 12 = 2500 kW 2500 kW is the basis for tower design.
(2) Where heat is to be extracted from a process:
The rate of energy flow in kilowatts (Q) comes from the equation:
Q = m x Cp x At m = mass flow of material being cooled (kg/s) Cp = specific heat capacity of ditto (kJ/kgK) At = temperature fall of ditto (K)
The material being cooled can be solid or liquid.
(3) Where in addition to sensible heat extraction the process involves a change of state which will normally involve condensation of a gas to a liquid. The latent heat of evaporation of water at atmospheric pressure is 2250kJ/kg.
Thus the heat energy flow required to evaporate 2 litres of water per second is:
2 x 2250 = 4500 kW
The rate of heat extraction required to condense the equivalent amount of water vapour will also be 4500 kW
(4) Where the source of heat is power input either electrical or mechanical. Power output from an engine or an electric motor is expressed in kW and it is normal to assume that the whole of the power supplied to a process will be dissipated as heat in the process; thus for cooling tower design the total output is used. Remember that the input to an electric motor must take care of losses in the motor itself and these losses are dissipated by the cooling fan of the motor. This will warm the air but will not affect the process cooling requirement.
Having established the rate of energy flow the next objectives are to determine water flow rate, the re-cooled water temperature and the design wet bulb temperature; each of these is reviewed in the paragraphs which follow.
A.5.2 Determination of water flow rate
The majority of applications will involve a heat exchanger in the process itself and optimum design will therefore require a balance between the heat exchanger and the cooling tower. A low water flow through the exchanger associated with a high temperature rise will necessitate a large exchanger, but as the temperature range increases the cooling tower will become smaller and its flow rate will also be low.
The total pressure loss through the heat exchanger has an important bearing on the cooling water flow rate.
The detailed treatment of heat exchanger design is beyond the scope of this book; suffice to say that the flow rate through the tower can be arrived at only after a very thorough investigation of the process itself.
A.5.3 Determination of the re-cooled water temperature This is the desirable water temperature, in summer, to the inlet of the cooling process.
For a given air wet bulb temperature, the re-cooled water temperature has a considerable effect on tower size. To cool water to the wet bulb temperature of the air would require an infinitely
large cooling tower. More practically, as one tries to cool nearer to the air wet bulb temperature the tower size increases very rapidly. It is not usual to have an approach to the air wet bulb temperature of less than 3°C - a more usual figure is 4°C. However, reputable manufacturers will guarantee an approach to within about 2°C of the air wet bulb temperature if this is required.
When selecting a re-cooled water temperature, therefore, choose the highest possible temperature which will permit cooling water to do what is required of it. To do otherwise merely results in the selection of a larger (and therefore more expensive) tower than is necessary.
A.5.4 Choice of design air wet bulb temperature The choice of the design air wet bulb temperature is of vital importance; it is based on relevant meteorological information, but must be modified to take account of the consequences of the design figure being exceeded under operating conditions.
As the highest daytime air wet bulb temperatures recorded in meteorological data sheets refer to only short or peak periods of time, or to temperatures recorded at the same hour each day, it is normally possible to design for a figure less than the highest recorded, for two reasons: (1) Temporarily warmer re-cooled water from cooling tower
packing is quickly lowered in temperature by mixing with the cooling tower pond water.
(2) The thermal lag inherent in the cold water basin, and cooling system compensates for most of the peak time cooling tower performance, when re-cooled water leaving the tower packing may, for minutes only, be higher than desirable.
Hence, where a fairly liberal cold water basin capacity is specified, or where high summer conditions may correspond to times of reduced heat dissipation from process plant, or where a small temporary rise in cooling water temperature is acceptable, a design air wet bulb temperature of 2-5°C below peak tempera tures recorded is usually adopted.
Towers designed for air conditioning plant in the UK must be able to cope with a very few abnormally humid summer days and will therefore rarely be operating at full capacity; this is in contrast to tropical installations which operate close to full capacity for much of the summer period.
As a basis for assessing design values for both wet and dry bulb temperatures the Chartered Institution of Building Services
Engineers publishes in Section A2 of their Guide a series of maps of the UK showing isotherms averaged over the years 1960 to 1974 and covering the summer period June to September. Maps are available showing sets of isotherms based on percentage values 1 and 2x/2%; the percentage refers to the number of hours as a proportion of the total when the value shown on the isotherm is exceeded. The maps are reproduced at C.4.
If operating air wet bulb temperature rises above design level there will be a small rise in the temperature of the water leaving the tower, but this rise will be less than the excess over design level. The design temperature must be assessed on the merits of each application after careful consideration of the points referred to above.
To illustrate the effect of wet bulb temperature on tower performance the chart at Figure A.5.4.1 shows the results of wet bulb temperature variation on a specific tower. Here, for constant water flow rate, air flow rate, and temperature range, the variation in water-off temperatures with wet bulb temperature is given.
A.5.5 Effect of altitude Special consideration must be given when the site is at any appreciable altitude above sea level.
24 r-
23 Based on Water flow rate 2.4 kg/s per m2 of packing area Cooling range 6°C Air velocity 2.5 m/s Altitude up to 300 m
2 22 CD Ω. E 2 21
2 20
Ambient wet bulb temperature (°C)
Figure A.5.4.1 Relationship between re-cooled temperature and wet bulb temperature for typical contra-flow pack
The greatest effect of altitude is that the mass of air delivered by the fan is reduced. This is due to the reduction in density combined with the fact that a fan is essentially a constant volume machine (Table B.5.2 gives air density data). The tower is designed on mass flow of air per unit horizontal cross-sectional area of pack and care must be taken, therefore, when selecting the fan. In other words, a tower selected for a duty at sea level might be incapable of meeting that duty at a higher altitude.
The other factor is that as altitude increases, air at a given temperature is capable of holding a larger amount of water vapour. This tends to offset the density factor, but its effect is comparatively small particularly at low water temperature levels.
A.5.6 Choice of site The available site can often determine the type of tower to be used, or occasionally the type of tower necessary can determine the site.
In practice the engineer assesses the possible sites he might use, the types of tower available and then makes a judgement which depends upon various factors: • Facilities for mounting water circulating pumps and convenient
routes for pipework. A common problem arises when a tower is mounted below the level of a large capacity cooling system. When the pump is switched off the system empties into the tower basin and valuable water is lost down the overflow. The problem can be avoided by:
Mounting the tower on the same level as the system Making the basin large enough to accept the drain back Fitting non-return valves in the pipework
• The load-bearing capacity of the surface on which the tower is to be placed.
If a tower is to be mounted on a roof a cross-draught, low-silhouette tower (with large plan area) might be needed to limit the load on the roof. It should be remembered that the operating weight, i.e. including water in the distribution system, packing, and sump should be used when making roof stress calculations. • Restrictions to airflow to and from the tower.
This can be caused by: Air intake too close to a wall Re-circulation arising because discharge air is deflected back towards the air inlets; re-circulation will cause the wet bulb temperature to rise with consequent reduction in tower performance
If the obstruction is nearer than a distance corresponding to 150% of the air inlet height there are definite possibilities of excessive air inlet resistance, insufficient air supply and increased noise level. Good practice is to work to a minimum distance 200% of the air inlet height. The effect of obstructive walls on both incoming and outgoing air is more serious in a forced draught cooling tower.
Another cause of increased wet bulb temperature and reduction in performance is siting the tower near hot air or gas discharge points, such as ventilation outlets or boiler flues. • The need to site the tower indoors. If this is necessary due to site restrictions it is essential to duct the outlet from the tower through the roof or wall of the building. It is not always necessary to duct fresh air to the tower inlet as when conditions permit large louvres can be placed in an adjacent wall.
In these conditions, the layout of the forced draught tower makes it a very convenient choice. Figures A.2.4.1 and A.2.4.2 show the way in which this type of tower can be used to give a very compact arrangement. • The permissible operating noise level of the tower. Although noise level is normally of minor consequence in industrial cooling applications it can be of major importance if the tower is sited near residential or office buildings.
Limitation of noise level can be a major factor in deciding the size and type of tower and may necessitate changes in tower design or the use of sound attenuation methods. The subject of noise is dealt with in some detail at A.6.
A.5.7 Appearance Much progress has been made since the days of the unsightly timber tower with exposed framework, blemishes from water leakage and the depressing appearance of creosoted timber.
Modern advances in shell design and much improved cladding finished in suitable pastel colours have gone a long way towards
A.6 Noise and noise control 55
making the cooling tower more attractive in appearance. The illustrations at A.2.4.7, A.2.5.4 and A.2.5.6 are good examples of acceptable appearance.
A.5.8 Capital costs and operating costs As a general statement, for a given type of tower, it is possible to make an economic choice between a tower with high initial cost and low operating costs or with a low initial cost and higher operating costs. The former tower will have a larger pack and consequently require low fan power and the latter will be capable of handling the cooling load with a smaller pack but greater fan power. Running costs are also affected by the pump size which is itself determined by the head from the cold water basin to the warm water inlet pipe, together with pressure loss in the pipework.
In practice the type and size of tower to be used will be influenced by such factors as space available or indeed the value of the space which the tower will occupy.
With water charges already very high and set to rise more rapidly than other costs there is unlikely to be very much difficulty in justifying the cost of a cooling tower where cooling water was previously run to drain. A pay-back period of less than one year is likely.
A.5.9 Performance testing British Standard 4485: Part 2: 1988 deals comprehensively with testing of cooling towers with sections covering:
Conditions of validity of tests Instruments and methods of measurement Test checks and readings Test procedures Computation of results Evaluation of thermal performance
In addition the standard includes appendices, tables and figures relevant to cooling tower performance and testing.
A.6 Noise and noise control A.6.1 The subjective nature of sound Sound is caused by vibrations and can be transmitted through any fluid, but it is most familiar when carried by the atmosphere; the
human reaction to sound depends on the delicate and very complex mechanism of the ear. The limits of detection of sound by a young person with acute hearing are defined in 2 ways:
• Through the range of frequencies from the lowest tone which can be detected by the human ear to the highest note which can be recognized (some forms of life can detect sounds above the range of frequencies recognizable to humans).
• The power of sound, which is directly related to the pressure created in the atmosphere by the sound wave, spreads from a level which is just detectable to a level at which pain will be felt and damage to hearing may result.
The difference between sound and noise depends to a large extent on the reaction of the individual. Top ' music is enjoyable sound to some and intolerable noise to others; the dripping of a tap which is inaudible in the daytime can cause real distress in the silence of the night.
Subjective reaction and the level of background noise will have an important influence on whether noise from a cooling tower will be objectionable.
A.6.2 The basis of sound measurement Propagation of sound may be understood by considering the analogy of beating a drum; the vibration of the surface of the drum produces pressure variations in the air which move away from the drum as longitudinal vibrations.
Sound waves are said to be longitudinal because the pressure variations take place in the same plane as the direction of wave travel; this is in contrast to the wave produced by flicking the end of a flexible rope which produces transverse waves with disturbance at right angles to the direction of wave travel.
Sound measurement is based on the reaction of the human ear with a zero point at a pressure level corresponding to sound which is just detectable and extending to a maximum just beyond the threshold of pain.
The minimum pressure detectable is 20 micro-pascals (20 μΡζ) and the maximum tolerable is 20 million micro-pascals or 20 Pa. Because this range of pressures is so wide an arbitrary logarithmic scale of decibels is used with zero at 20 μPa and increasing by 20 decibels for each tenfold increase in pressure:
20 μΡ& corresponds to 0 decibels (dB) 200 μPa corresponds to 20 dB
2000 μPa corresponds to 40dB, and so on up to - 200 000 000 μPa corresponds to 140 dB
The system of measurement is applied to the range of frequencies recognizable, which, with optimum hearing extends from 20 hertz (20 vibrations per second) to 20000 hertz (20 kHz). The range of sound power levels is shown at Figure A.6.2.1.
As with any vibrations, frequency and wavelength are related by velocity and in the case of sound waves in air this is 344 metres per second at 20°C at sea level. This velocity is independent of frequency but increases with increasing temperature.
WAVELENGTH = SPEED OF SOUND FREQUENCY
200 000 000
2 000 000
Thus for a pure tone of frequency 20 Hz the wavelength will be 344/20 = 17.2 metres.
Pure tones of fixed frequency may be produced by tuning forks or by electronic means, but musical notes have quite complex waveforms because of the presence of overtones (defined scientifically as harmonics).
Noise in general consists of a mixed bag of frequencies which are not pleasing in the sense that musical tones can be. 'White' noise is an expression used in radio signal detection and consists of vibrations spread over the audible range to produce the 'rushing water' noise heard when a radio transmission is off tune.
The pressure of a sound wave, which in turn determines the sound power measured in decibels, is related to the amplitude of the sound wave as illustrated at Figure A.6.2.2.
Figure A.6.2.2 Sound wave - variation of pressure
On the logarithmic scale of decibels described above an increase of 6 dB represents a doubling of pressure, but for the sound to appear to be twice as loud an increase of 10 dB is needed (this is an example of how the decibel scale attempts to reflect the subjective nature of sound; a doubling of loudness is what most people will sense when the level is increased by 10dB).
The sensitivity of the human ear varies with frequency as shown in the family of equal loudness curves at Figure A.6.2.3. These curves show how some frequencies can be detected at lower
110 dB
2k 5k 10k 20k
Figure A.6.2.3 Curves showing frequency for equal loudness at sound power levels 10-130 dB
pressure levels than others, with the greatest sensitivity between 2 and 5 kHz. It is clear from the curves that in the 2 to 5 kHz band, high sound pressure levels will be least tolerable and therefore most likely to cause damage to the ear. Each of the curves is related to the sound pressure level at 1 kHz and by moving down the frequency scale of the 70 dB curve it can be seen that a sound pressure level of 85 dB will be necessary at 500 Hz to give the same loudness as at 1 kHz.
A.6.3 The measurement of sound Sound level meters consist of microphones connected to electronic circuitry which produce a digital display in decibels. Measurement is designed to take account of the equal loudness curves and thus to correct for the varying sensitivity of the human ear. An electronic network 'weights' the sound according to frequency and the weighting now universally accepted is the ' weighting which is based on equal loudness contours at low sound power levels (SPLs) and corresponds most closely with subjective tests. The ' and 'C' weightings were based on medium and high sound power
levels and are now rarely used. Thus sound power levels are now commonly expressed in dB(A). BS 4485: Part 2: 1988 Water Cooling Towers, Methods for Performance Testing stipulates that the ' weighting shall be used.
Because sound is a form of energy the damage to hearing which may result from exposure depends on the duration as well as the intensity of the sound. For this reason a value known as Leq has been devised; this enables a varying sound level to be analysed electronically and translated into an equivalent continuous sound having the same energy content. An alternative approach is the sound exposure level (SEL), which is defined as the constant level acting for one second which has the same amount of acoustic energy as the original source.
To a sensitive ear the minimum energy required to produce sound recognition is approximately 4 x 10~19 joules; hearing becomes painful and damage may ensue at 4 x 10~5 joules.
2 o Q.
Octave band level
_L J_ _L J_ _L _L 31.5 63 125 250 500 1 K 2 K 4 K 8 K
Frequency (Hz)
In order to make suitable allowance for the varying aural response to different frequencies the total audible spectrum of frequencies is divided up into Octave bands'. The definition of an octave band is the same as that for the diatonic musical scale where the frequency of the top note of the octave is double the frequency of the bottom note. Thus if the centre frequency is 1 kHz the span will be from 0.707 kHz to 1.414 kHz.
Instruments have been designed to measure the SPLs in each of the octave bands through the audible range and the result can be presented in a spectrogram showing the sound power levels in each band, (see Figure A.6.3.1). To assess the relative importance of the SPLs of the range of octave bands it is necessary to use a family of noise rating curves as shown in Figure A.6.3.2. Each curve has a noise rating related to the value of sound pressure level at 1 kHz (thus N70 intersects the vertical axis at 70dB at 1000Hz). Superimposing the spectrogram on the NR curves enables the frequencies causing the maximum noise problem to be identified (see Figure A.6.3.3).
NR
120
110
100
90
80
70
60
50
40
31.5 63 125 250 500 1 K 2 K 4 K 8 K Frequency (Hz)
Figure A.6.3.2 Noise rating (NR) curves to ISO R 1996
Octave band level
NR
" 120
110
"~ 100
"" 90
"" 80
- 70
" — 60
""" 50
—- 40
— 30
20
™ 31.5 63 125 250 500 1 K 2 K 4 K 8 K
Frequency (Hz)
Figure A.6.3.3 Identifying octave bands for maximum noise problem
A.6.4 The effect of distance on sound power levels Sound pressure levels are independent of distance but free field sound power levels decline with distance according to the chart at A.6.4.1 (free field sound power levels are those in which the effects of reflection and absorption of sound by buildings and other obstructions have been fully discounted). From the chart it can be seen that at a point 10 times the distance of the sound level meter from the sound source there will be a reduction in level of 20dB so that 80dB will sound like 60dB.
More precise information on attenuation for distance from source is given in BS 4485: Part 3: 1988 from which Figure A.6.4.2 is reproduced. This chart shows the effect of frequency on attenuation with separate curves shown for each of the octave bands from 63 Hz to 8 kHz. As can be seen, up to approximately 10 metres from the source attenuation is independent of
30 r
_J I I I I 5 10 15 20 25
Distance from source
Figure A.6.4.1 The effect of distance on sound power level
frequency, but beyond that point the low frequencies are more persistent than the high as distance increases.
The direction in which sound is projected from the tower also has an influence and is also frequency dependent. Figure A.6.4.3 from BS 4485: Part 3: 1988 shows how the maximum perception of noise from a tower is in positions close to vertically above the tower with a directivity index +5 dB for frequencies 500 Hz and above. Conversely at an angle of 20° or more below the horizontal level at the top of the tower the directivity index is approximately -18dB.
A.6.5 Subtraction and addition of noise levels The chart at A.6.5.1 may be used to assess the effect of background noise on a sound level measurement. Clearly, if the background noise is very close to the noise being measured the impact of the measured noise will be of no consequence. The measured noise must be more than 3 dB (A) above the background level before any adjustment can be assessed. As an example:
Total noise measured with cooling tower running 60 dB (A) Background noise alone 52 dB (A) Difference 8 dB (A) Correction (from chart) 0.8 dB (A) Noise from tower 60 - 0.8 = 59.2 dB (A)
Fo r
he m
is ph
er ic
al p
ro pa
ga tio
n a
n d
so ur
ce s
ou nd
p ow
er l
ev el
+10r-
+5
110°
round |
0 -45°
Octave band centre frequency (Hz) Figure A.6.4.3 Directivity correction
10 K
The chart at A.6.5.2 may be used when two sources of noise are to be added.
As an example:
Source No. 1. Source No. 2. Difference Correction (from chart) Total noise
78 dB (A) 84 dB (A) 6 dB (A)
1.0 dB (A) 84 + l = 85dB(A)
If the two sources are identical in value it can be seen from the chart that 3 dB (A) must be added to obtain the combined level.
A.6.6 Sources of noise from cooling towers A.6.6J Noise from fans The factors which contribute to the noise level of fans are:
• Noise increases with increased peripheral speed (the speed at which the tips of the fan blades move through the air)
• Noise increases as the power absorbed increases and as the static pressure developed incr

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Cooling Towers. Principles and Practice

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