AGITATOR SELECTION AND DESIGN

94 Pages·1963·3.991 MB·English

by  ENGINEERING EQUIPMENT USERS ASSOCIATION

LONDON
PUBLISHED BY
Constable and Company Ltd.
J0.J2 Orange Street W.C.2
© 1963 Engineering Equipment Users Association
PRINTED IN GREAT BRITAIN BY J'. W. ARROWSMITH LTD., BRISTOL
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THE ENGINEERING EQUIPMENT USERS ASSOCIATION
The Engineering Equipment Users Association is an association of
industrial users of engineering equipment, materials and stores. It was
founded in 1949 by a number of large companies who realised the
important role that would be played in the future by standardisation
and simplification of engineering products, and by the free exchange of
technical information on engineering matters.
The principal objects of E.E.U.A. are to assist its members to pro
mote a common policy for the standardisation of engineering materials,
equipment and stores, through the British Standards Institution or
otherwise; to foster the preparation and adoption of national standards
for such products, and to collate and summarise information in order
to give guidance on their nature and use. Membership is restricted to
companies and other bodies who are predominantly users rather than
manufacturers of engineering products.
The work of the Association is entirely complementary to that of
the B.S.I. with whom close liaison and collaboration are always main
tained; it does not duplicate the activity of that Institution in any
way. Everything possible is done to enlist the early support of manu
facturing interests in standardisation work which E.E.U.A. considers
should be promoted in the national interest.
This particular handbook was prepared by a Panel drawn from
member organisations interested in the problems of agitator selection
and design for various duties, and was first issued to members in 1957.
The Council of E.E.U.A. decided that this was a case where the results
of the Panel's work ought to be made known outside the Association;
consequently the handbook has been reviewed, amended where neces
sary, and published for general sale. The Council hopes that it will
prove a useful guide to all responsible for the selection and use of
agitator equipment, and will welcome any information on the results
obtained by following the recommendations given.
This Handbook was prepared by E.E.U.A. Panel M/16, Agitating
and Mixing Equipment, which was constituted from representatives of
the ~ollowing E.E.U.A. organisations:
Courtaulds Limited.
Distillers Company Limited.
Dunlop Rubber Company Limited.
Imperial Chemical Industries Limited.
Monsanto Chemicals Limited.
Unilever Limited.
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AGITATOR SELECTION AND DESIGN
(E.E.U.A. Handbook No. 9. Revised 1962)
CONTENTS
Page
FOREWORD I
SECTION ONE-SCOPE AND DEFINITIONS 3
SECTION Two-RECOMMENDATIONS ON PLANT 5
l
(a) Types and General Features 5
(i) Mixing or Agitation Vessels 5
(ii) Impeller Diameters and Clearances 6
(iii) Vessel Fittings (including Baffles) 8
(b) Sizes, Proportions and Speeds 9
(i) Mixing Vessels 9
l' (ii) Impellers (Paddles, Turbines and Anchors) 9
(iii) Baffles 10
SECTION THREE-GUIDE TO THE CHOICE OF IMPELLER TYPES
AND SPEEDS 12
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(a) Factors Influencing Agitation 12
(b) Selection of Impeller Type and Speed 13
(i) General Considerations 13
(ii) Basis of Impeller Selection 15
(iii) Impeller Type and Speed 16
(c) "Scaling-up" from Given Data 19
\./"" SECTION. FOUR-CHOICE OF DRIVE 22
i (a) General 22
(b) Type of Gearbox 22
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(c) Mounting the Drive 22
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Page
SECTION FIVE-POWER ASSESSMENT 23
(a) Factors Affecting Power Consumption 23
(b) Methods of Computing Power Consumption 23
(i) Power Absorbed by the Impeller 24
(ii) Power Absorbed by Baffles and Vessel Fittings 25
(iii) Transmission and Gland Losses 26
(iv) HP of Driving Motor 27
(c) Tables Giving Horse-Power Requirements of Impellers
(Tables 4 to 23) 28
SECTION Six-MECHANICAL DESIGN OF AGITATORS 45
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(a) Materials of Construction 45
(i) The Impeller 45
(ii) The Vessel 46
(b) Type of Service 47
(i) Continuity of Operation 47
(ii) Possibility of Modification 47
(iii) Stepwise Processes 47
(iv) Corrosive Service 48
(c) The Design ·of Shafts and Bearings (see also Appendix D) 48
(i) Assumptions Made 48
(ii) Application of the Assumptions Made 49
(d) Glands, Bushes, and Bearings Inside Vessels 50
(e) Drive and Bearing Arrangements (see also Section Four) 52
APPENDIX A-RECOMMENDED DIMENSIONS FOR VERTICAL
CYLINDRICAL VESSELS, BAFFLES, AND IMPELLERS 54
(a) Proposed Standard Diameters for Vertical Cylindrical
Vessels 54
(b) Recommended Baffle Dimensions 55
(c ) Recommended Dimensions for:
(i) Paddle Impellers 56
(ii) Turbine Impellers 57
(iii) Anchor Impellers 58
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Page
APPENDIX B-ASSESSMENT OF THE POWER LOST IN STUFFING
59
BOXES
APPENDIX C-FORMULAE FOR HEAT TRANSFER IN AGITATION
VESSELS 61
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APPENDIX D-THEORY AND CALCULATIONS FOR THE DESIGN
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OF IMPELLERS, SHAFTS, AND BEARINGS 64
(a) General Requirements Relating to Design of Impeller
Shaft 64
(b) Calculations for Shaft Diameter 64
(c ) Gear Selection when an Impeller is Supported by the
Gear Output Shaft (or Bearing Load Calculations) 67
(d) Calculation of Critical Speeds 69
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l (e ) Calculation of Deflections 71
(/) Design of I111peller Blades 72
(g) Stresses in Shafts and Bearings where a Subsidiary
Bearing is Fitted 73
(i) Bearing Loads 73
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• (ii) Bending Moments 74
(iii) Critical Speeds 74
(h) Suggested Working Stresses for Shaft Materials 74
(i) Worked Examples 75
APPENDIX E-A NOTE ON SOURCES OF INFORMATION' WITH
SOME LITERATURE REFERENCES 81
(a) Brief Guide to the References Cited 81
(b) Derivation of Individual Sections of this Handbook 83
(c) Some Literature References 86
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FOREWORD
This E.E.U.A. Handbook has been prepared to give some guidance to
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those responsible for the selection or design of agitation or mixing
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equipment for various duties.
At present many types of mixers with widely differing character--
istics are used industrially, even for similar or identical applications;
performances and costs vary correspondingly. Examination of pub
lished data and the experience of member firms strongly suggests that
such variety is unnecessary, except perhaps for certain highly specialised
processes.
Many design methods have been evolved for predicting power
requirements and mixing characteristics. Some methods seem reason
ably satisfactory in that the resulting designs prove to be effective and
economical over a limited range of conditions and for a few specific
types of mixers. Because of these limitations, however, the results
obtained cannot readily be woven into a single reliable theory, and
there are many agitation problems which cannot be solved without
further investigation or even specific research.
Nevertheless, it is believed that the majority of agitation problems
can be tackled by methods or recommendations given in this Hand
book, though it is not claimed that they will always prove the most
suitable or give the best reusults.
Much of the information given in this Handbook is derived from
experience within the E.E.U.A. Some, however, has of necessity been
taken from the published technical literature briefly discussed in
Appendix E.
It is considered that the proper use of the Handbook will help to
solve or clarify some outstanding problems and will also contribute to
a fuller understanding or a later review of the subject.
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SECTION ONE-SCOPE AND DEFINITIONS
(a) SCOPE
This E.E.U.A. Handbook gives guidance on the selection and design
of agitation equipment for liquid media in vertical cylindrical vessels
or tanks with impellers on vertical shafts.
While the use of other impellers is briefly discussed, the recommenda
tions given are based on the use of paddle, turbine and anchor-type
impellers in vessels having "standardised" diameters ranging from
2 ft 3 in. to 15 ft. Within these limitations the Handbook will help to
solve agitation design problems involving liquids with liquids, solids or
gases over a mixture viscosity range of l ·Oto 1 million cp., and specific
gravity range of 0·6 to 1.4.
The information given is believed to be the best available at the time
of publication; much of the data referred to, however, has not yet
been checked against results obtained in practice, and special care
should be taken when using the Handbook for the design of agitators
where the agitation requirements are or may be critical.
Recommendations regarding the use of the marine type propeller
have been omitted, not because this type of impeller is considered
unsuitable or inefficient, but because of the difficulty of specifying its
shape, on which its performance so largely depends.
(b) DEFINITIONS
For the purposes of this Handbook the following definitions are
employed:
Agitator: The total assembly of impeller, impeller shaft and
drive; including any gland, bearing, etc., used in
conjunction with these.
Impeller: The actual element which imparts movement to the
liquid.
Impeller Types (see also Table 1 in Section Three-page 14)
Anchor: An impeller which is profiled to sweep the wall of
the containing vessel with a small clearance.
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Paddle: An impeller having four or fewer blades, which is
not a propeller.
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Plate: A paddle impeller having a blade depth greater
than the blade radius.
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4 .A.GIT.ATOR SELECTION .AN°~~GN
Propeller: An impeller imparting essentially an axial thrust to
the liquid, and whose pitch does not vary with
radius.
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Turbine: An impeller with more than four blades, all on the
same boss.
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Baffie: An element fixed inside the vessel to impede swirl-
1ng.
Draught Tube: A tubular fitting which is arranged to direct the
liquid flow produced by the impelfur:-
Gland, stuffing box, mechanical seal: Devices to prevent the passage
. of liquid or gas through the :point of entry of a
shaft into a vessel.
Filling Ratio: (Vertical cylindrical tanks only):
. Liquid Depth I•
The rat10 of: V D' t r
esse1 1ame er
(See Section Two (b) (i), page 9).
The continuous rotation of liquid about a fixed
axis.
Vortex: A depression in the surface of a liquid produced by
swirling.
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SECTION TWO-RECOMMENDATIONS ON PLANT
(a) TYPES AND GENERAL FEATURES
(i) Mixin~ or A~itation Vessels
Vertical cylindrical vessels or tanks with dished bottoms are
usually chosen for fluid mixing operations (4),* and in this section
of the Handbook the recommendations made relate to vertical
cylindrical vessels with flat, dished or shallow coned bottoms. Some
generalisations are also made, however, concerning vessels of other
shapes, such as horizontal cylindrical tanks, and rectangular tanks
of various proportions.
The following factors are significant when considering vertical
cylindrical vessels:
1. Ratio of Liquid Depth to Tank Diameter (filling ratio)
The value of this ratio, termed the filling ratio, is normally
between 0·5 an~ l ·5 (16), and a value approximately equal to l ·O
is recommended for most purposes. When dispersing gas in a
liquid, however, a filling ratio of about 2·0 is recommended in
order to maintain a sufficiently long period of contact between the
gas and the liquid. In this connection it should be noted that for
the same agjtating effect, the power consumption per unit volume
increases as the filling ratio departs from unity.
2. Shape of the Bottom of the Vessel
The importance of this factor increases as the filling ratio is
reduced. Other things being equal, vessels with dished, bottoms ,
tend to be most economical in power consumption.
Flat-bottomed and coned-bottomed vessels have the disad
vantage of low agitation efficiency in the corners formed between
the walls and the bottom, as well as in the apex of the cone in the
case of coned bottoms. This is particularly significant wheri mixing
involves the suspension of heavy solids in a liquid. In such cases,
fillets should be inserted in the corners between the bottoms and
t~walls of flat-bottomed or coned-bottomed tanks (4).
3. Roughness of Vessel Walls
A rough-walled vessel consumes more power in agitation than a
smooth-walled vessel because of increase in local turbulence at
• Throughout this Handbook the numbers given in parentheses refer to the corre
sponding literature references listed in Appendix E-A Note on Sources of Information
with Some Literature References.
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