1、BRITISH STANDARD BS4485-3: 1988 Water cooling towers Part 3: Code of practice for thermal and functional design UDC66.045.53/.54BS4485-3:1988 This British Standard, having been prepared under the directionof the Civil Engineeringand Building Structures Standards Committee,was published underthe auth
2、ority of the BoardofBSI and comes into effect on 30 September 1988 BSI 06-1999 First published April 1977 First revision September 1988 The following BSI references relate to the work on this standard: Committee reference CSB/23 Draft for comment85/15356DC ISBN 0 580 16169 2 Committees responsible f
3、or this British Standard The preparation of this British Standard was entrusted by the Civil Engineering and Building Structures Standards Committee (CSB/-) to Technical Committee CSB/23, upon which the following bodies were represented: Association of Consulting Engineers British Effluent and Water
4、 Association British Gear Association Chartered Institution of Building Services Engineers Concrete Society Electricity Supply Industry in England and Wales Engineering Employers Federation Engineering Equipment and Materials Users Association Health and Safety Executive Hevac Association Industrial
5、 Water Society Institution of Chemical Engineers Institution of Structural Engineers Process Plant Association Amendments issued since publication Amd. No. Date of issue CommentsBS4485-3:1988 BSI 06-1999 i Contents Page Committees responsible Inside front cover Foreword iii 1 Scope 1 2 Symbols and u
6、nits 1 3 Thermal design principles 2 3.1 Cooling process 2 3.2 Heat transfer diagram for counterflow conditions 3 3.3 Crossflow tower calculation 8 3.4 Natural draught tower calculations 8 4 Types of cooling tower and their relative merits 8 4.1 General 8 4.2 Natural draught towers 10 4.3 Mechanical
7、 draught towers 11 5 Siting, spacing and environmental considerations 12 5.1 General 12 5.2 Siting 13 5.3 Spacing 13 5.4 Environmental considerations 13 6 Guidance on specified operational requirements 17 6.1 Selection of design conditions (including probability curves) 17 6.2 Guidance on the effect
8、 of variations of parameters 21 7 General thermal design 22 7.1 Packings 22 7.2 Drift eliminators 24 7.3 Recirculation 26 7.4 Effect of altitude 27 8 Hydraulic requirements of cooling towers 27 8.1 Water distribution 27 8.2 Water losses 28 8.3 Cold water basin design 32 8.4 Water quality 35 8.5 Desi
9、gn for winter operation 36 8.6 Use of saline water 37 8.7 Effect of salinity on heat transfer 39 9 Mechanical equipment 39 9.1 Fans 39 9.2 Gearboxes 40 9.3 Drive shafts and couplings 40 9.4 Fan drivers 40 9.5 Supports for mechanical equipment 41 9.6 Vibration cut-out switches 42 9.7 Keys and keyways
10、 42 10 Safety precautions 42 10.1 General 42 10.2 Mechanical plant 42 10.3 Electrical plant 42 10.4 Access 42 10.5 Fire precautions 42BS4485-3:1988 ii BSI 06-1999 Page 11 Maintenance 42 Appendix A Enquiry and suggested tender information for cooling towers 43 Appendix B Evaluation of noise in coolin
11、g towers 45 Figure 1 Air-water flow paths in crossflow, counterflow and mixed flow towers 4 Figure 2 Conditions of air and water in a cooling tower 5 Figure 3 Heat transfer diagram 5 Figure 4 Demand and characteristic curves 6 Figure 5 Performance curves for mechanical draught tower 7 Figure 6 Perfo
12、rmance curves for natural draught tower 7 Figure 7 Illustrations of basic types of cooling tower 9 Figure 8 Spacing 14 Figure 9 Screen dry bulb and wet bulb isotherms for the UK for temperatures exceeded for the stated percentage of hours during June to September: period 1964-70 18 Figure 10 Mechani
13、cal draught counterflow tower: variation of tower size with cooling range, approach and wet bulb temperature 20 Figure 11 Natural draught tower: variation of duty factor with cooling range, approach and wet bulb temperature (at constant70%r.h.) 21 Figure 12 Natural draught tower: variation of base d
14、iameter with circulating water flow corrected for duty factor 23 Figure 13 Drift eliminators 24 Figure 14 Basic recirculation allowance 25 Figure 15 Correction for cooling range and approach at17C wet bulb temperature 26 Figure 16 Effect of altitude on heat transfer diagram 27 Figure 17 Natural drau
15、ght tower: water distribution 29 Figure 18 Mechanical draught tower: water distribution 30 Figure 19 Variation in evaporation losses with ambient conditions 31 Figure 20 Purge, make-up and evaporation quantity balance 32 Figure 21 Relation between purge ratio and concentration factor 32 Figure 22 Po
16、nd characteristic 34 Figure 23 Anti-icing arrangements 38 Figure 24 Effect of salinity on heat transfer diagram 39 Figure 25 Typical arrangement for induced draught fan drive assembly 41 Figure 26 Attenuation for distance from source 47 Figure 27 Directivity correction 48 Table 1 Symbols and units 1
17、 Table 2 Standard reference sheet for physical dimensions 10 Table 3 Addition of decibels 46 Table 4 Typical corrections to obtain octave band spectra for various fan types 47 Table 5 Tower noise calculation 48 Publications referred to Inside back coverBS4485-3:1988 BSI 06-1999 iii Foreword This Par
18、t of BS4485, which has been prepared under the direction of the Civil Engineering and Building Standards Committee, deals with the thermal and functional design of natural draught, mechanical draught and factory prefabricated cooling towers. This Part of BS4485 is a revision of BS4485-3:1977, togeth
19、er with its Addendum No.1(1978) both of which are withdrawn. In this revision the following principal changes have been made. a) The guidelines on water treatment have been expanded as these were not considered sufficient for good operating practice. b) Reference has been made to the potential healt
20、h hazard arising from the bacterial population of cooling towers, as it relates to the design of the towers. However, operational techniques for control in this area are outside the scope of this Part of BS4485. c) Factory prefabricated cooling towers previously dealt with in Addendum No.1 to BS4485
21、-3:1977 have been covered by suitable modification of the main text of this Part of BS4485. d) Information to be supplied by the purchaser and the manufacturer, which was included in Addendum No.1 to BS4485-3:1977, now appears as Appendix A in this Part of BS4485 as it is considered relevant to all
22、types of cooling towers. e) Materials of construction have been omitted as these are dealt with in BS4485-4 1) . f) A new clause on maintenance has been added which was in Addendum No.1 to BS4485-3:1977. This Part of BS4485 provides information on design principles, siting and spacing. Guidance is g
23、iven on specific thermal and hydraulic requirements, on mechanical equipment and on environmental aspects such as discharge into rivers and cooling tower noise. The other Parts of BS4485 are as follows. Part 1: Glossary of terms; Part 2: Methods for performance testing; Part 4: Structural design of
24、cooling towers. Where necessary, definitions have been included in the revisions of BS4485-2, BS4485-3 and BS4485-4 so that when they have all been published BS4485-1 can be withdrawn. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standar
25、ds are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i to iv, pages 1 to 48, an inside back cover and a back cover. This sta
26、ndard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. 1) Under revision.iv blankBS4485-3:1988 BSI 06-1999 1 1 Scope This Part of BS4485 gives recommendations for the thermal and functional design
27、 of industrial and natural draught water cooling towers and factory prefabricated cooling towers. NOTEThe titles of the publications referred to in this standard are listed on the inside back cover. 2 Symbols and units For the purposes of this Part of BS4485, the symbols and units given in Table 1 a
28、pply. Table 1 Symbols and units Symbol Quantity Unit a Area of effective transfer surface per unit of tower packing volume m 2 /m 3 A s Area of sound propagation m 2 A p Total packing area normal to air flow m 2 B Width or length dimensions perpendicular to tower axis m c Specific heat capacity of w
29、ater kJ/(kgK) C Concentration factor at equilibrium C T Concentration factor at time T C 1 Original make-up concentration of impurities % C 2 Stable state concentration of impurities in circulating system under continuous purge % D Diameter m g Acceleration due to gravity G Mass flow of dry air per
30、unit plan area of packing kg/(m 2 s) h Enthalpy aof air-water vapour mixture kJ/kg h m Mean driving force kJ/kg h G Enthalpy aof air-water vapour mixture passing through the packing kJ/kg h L Enthalpy aof saturated air film in contact with and at the temperature of the water passing through the pack
31、ing kJ/kg H Height (vertical distance above or below basin kerb level) m H e Effective height of shell, normally taken as height from middle of packing to top of shell m K Coefficient of mass transfer defined in terms of difference in absolute humidity kg/m 2 s(kg/kg) L Mass water flow per unit plan
32、 area of packing kg/(m 2 s) m 1 Mass of solute kg m 2 Mass of solvent kg M 1 Relative molecular mass of solute M 2 Relative molecular mass of solvent n Mole fraction of solvent N Number of velocity heads representing the system resistance p Total pressure Pa p 2 Vapour pressure of pure solvent Pa p
33、3 Vapour pressure of solution Pa p 5 Sound pressure N/m 2 p 0 Sound pressure reference datum N/m 2 Q 1 Circulating water flow m 3 /s R Surface radius from sound source m a All enthalpies relate to1kg dry air and associated water vapour.BS4485-3:1988 2 BSI 06-1999Table 1 Symbols and units 3 Thermal d
34、esign principles 3.1 Cooling process A water cooling tower is a heat exchanger in which warm water falls gravitationally through a cooler current of air. Heat is transferred from the water to the air in two ways: a) by evaporation as latent heat of water vapour; b) by sensible heat in warming the ai
35、r current in its passage through the tower. As a general measure, about80% of the cooling occurs by evaporation and about20 % by sensible heat transfer. The transfer of heat is effected from the water through the boundary film of saturated air in contact with the water surface. This air is saturated
36、 at the water temperature. From this saturated air film, heat transfer occurs to the general mass of air flowing through the tower. Symbol Quantity Unit S w Sound power level reading at a point source dB S p Sound pressure level reading some specified distance away from the source dB T Time h t b Te
37、mperature of water with which boundary vapour is associated C t m Mean water temperature C t DB Dry bulb temperature C t E Temperature of mixture of recooled water and make-up leaving cold water basin C t WB Wet bulb temperature C t 1 Hot water temperature at inlet C t 2 Recooled water temperature C
38、 V Effective packing volume per unit area of packing m 3 /m 2 V b Volume in cold water basin m 3 v e Evaporation rate m 3 /h v p Purge rate m 3 /h V s Volume in system excluding pond m 3 w 1 Atmospheric moisture content of ambient air condition kg/kg w 2 Atmospheric moisture content at mean water te
39、mperature saturated conditions kg/kg W d Fan driver power kW W o Sound power threshold W W s Sound power W x Pond surface area m 2 X Spacing m Density of air kg/m 3 %h Change in air enthalpy kJ/kg %t Cooling range K % Change in air density kg/m 3 Approach K Power W KaV/L Tower characteristic L/G Wat
40、er/air ratioBS4485-3:1988 BSI 06-1999 3 In the interests of efficiency, it is essential that both the area of water surface in contact with the air and the time of contact be as great as possible. This may be achieved either by forming a large number of water droplets as repetitive splash effects in
41、 one basic kind of tower packing, or by leading the water in a thin film over lengthy surfaces. Air flow is achieved either by reliance on wind effects, by thermal draught or by mechanical means. The direction of air travel may be opposed to the direction of water flow giving counterflow conditions,
42、 or may be at right angles to the flow of water giving crossflow conditions. Although the methods of analysis may be different for counterflow and crossflow conditions, the fundamental heat transfer process is the same in both cases. In some designs mixed flow conditions exist. These patterns of flo
43、w are illustrated in Figure 1. Present cooling tower technology relies on the fact that, with acceptable error, the effects of evaporative and sensible heat transfer can be combined into one dependent on enthalpy difference. The difference concerned is that between the enthalpy of the film of air su
44、rrounding the water surface (taken to be at water temperature) and the enthalpy of the general mass of air flowing through the tower. This enthalpy difference varies according to the point of measurement in the tower, but at all points it provides the enthalpy potential or driving force for the heat
45、 transfer. The so-called combined transfer theory depends upon certain approximations, which are reasonable at normal cooling water temperatures and particularly when the characteristics of the packing have been determined in accordance with the theory. However, the approximations become progressive
46、ly less valid with increasing water temperature and a more exact analysis should be adopted in applications where the mean water temperature exceeds35C. The air and air-water film conditions, in passage through the tower, may be illustrated on a psychrometric chart as shown in Figure 2. The cooling
47、range of the tower corresponds to the difference in temperature of the air-water film between entry to and exit from the tower. Air enters the tower having wet and dry bulb characteristics dependent on the ambient conditions. It is generally in an unsaturated state and achieves near-saturation in pa
48、ssing through the tower. It may be considered saturated at exit in all but very dry climates. The enthalpy of the entering air is considered, with acceptable error, to be equivalent to the enthalpy of air saturated at the wet bulb temperature. For the purposes of enthalpy differences in heat transfe
49、r, only the wet bulb temperature of the ambient air is therefore of significance. The dry bulb temperature has, however, to be considered for draught assessment purposes in those towers whose air flow relies on thermally created draught. The thermal draught is defined by the change of density of the air between entry to and exit from the cooling tower multiplied by the effective shell height, and the dry bulb temperature as well as the wet bulb temperature is of significance in determ
