fouling of heat exchangers, elsevier (1995

Fouling of Heat Exchangers by T. R. Bott • ISBN: 0444821864 • Pub. Date: April 1995 • Publisher: Elsevier Science & Technology Books PREFACE There are many textbooks devoted to heat transfer and the design of heat exchangers ranging from the extreme theoretical to the very practical. The purpose of these publications is to provide improved understanding of the science and to give guidance on the design and operation of process heat exchangers. In many of these texts the problem of the accumulation of deposits on heat transfer surfaces is ignored or at best, dealt with through the traditional fouling resistance. It is common knowledge that this approach is severely limited and inaccurate and may lead to gross errors in design. Furthermore the very arbitrary choice of fouling resistance more than offsets the accuracy of correlations and sophisticated methods, for the application of fundamental heat transfer knowledge. Little attention was paid to the heat exchanger fouling and the associated inefficiencies of heat exchanger operation till the so-called "oil crisis" of the 1970s, when it became vital to make efficient use of available energy. Heat exchanger fouling of course reduces the opportunity for heat recovery with its attendant effect on primary energy demands. Since the oil crisis there has been a modest interest in obtaining knowledge regarding all aspects of heat exchanger fouling, but the investment is nowhere near as large as in the field of heat transfer as a whole. Although books have appeared from time to time since the 1970s, addressing the question of heat exchanger fouling, they are largely based on conferences and meetings so that there is a general lack of continuity. The purpose of this book therefore, is to present a comprehensive appraisal of current knowledge in all aspects of heat exchanger fouling including fundamental science, mathematical models such as they are, and aspects of the practical approach to deal with the problem of fouling through design and operation of heat exchangers. The techniques of on and off-line cleaning of heat exchangers to restore efficiency are also described in some detail. The philosophy of the book is to provide a wide range of data in support of the basic concepts associated with heat exchanger fouling, but written in such a way that the non-mathematical novice as well as the expert, may find the text of interest. T.R. Bott December 1994 oo vii ACKNOWLEDGEMENTS The author wishes to record his sincere gratitude for the skill, dedication and persistence of Jayne Olden, without which this book would never have been completed. All the diagrams and figures in this book were drawn by Pauline Hill and her considerable effort is acknowledged. ix NOMENCLATURE Note: In the use of equations it will be necessary to use consistent units unless otherwise stated Area or area for heat transfer A~ Constant A n Hamaker constant a+, ao aD, ar Vector switches associated with the dimensionless deposition parameters N I, N o N o and N r respectively B Correction term Equation 12.28 C Circulation rate C Cunningham coefficient or a constant c Concentration cb C m Concentration in bulk blowdown water Concentration in make up water cp Specific heat % c~ Specific heat of solid Concentration of cells in suspension D Diffusion coefficient or dimensionless grouping as described by Equation 10.50 D c Collector diameter d Diffusion coefficient for particles Diameter E Activation energy or dimensionless grouping as described by Equation 10.49 Fouling of Heat Exchangers Eo F Eddy diffusivity Shear force F~ Adhesion parameter F~ Repulsion force FS Slagging index F. Van der Waals force f Friction factor f~ Ball frequency (Balls/h) Lifshitz - van der Waals constant K Transfer coefficient or Constant x~ Mass transfer coefficient of species A x~ Deposition coefficient x. Mass transfer coefficient allowing for sticking probability Mass transfer coefficient Mass transfer coefficient of macro-molecules xo Constant in Equation 10.31 X,o Solubility product x, Transport coefficient Kt * Dimensionless transport coefficient k~ Rate constants Equation 12.10 Rate constant Length t. Characteristic length M Mass flow rate M* Asymptotic deposit mass Nomenclature xi m~ m Mass of fouling deposit Mass N Dimensionless deposition parameter (Equation 7.40) N~ Mass flux of cells Dimensionless interception deposition parameter Dimensionless diffusion deposition parameter or mass flux away from reaction zone N, Dimensionless impaction deposition parameter N~ N~ N~ Mass flux of macro-molecules Mass flux of reactants or precursors Dimensionless thermophoresis deposition parameter Particle number density Integer on concentration factor P Sticking probability e, Po P~ Probability of scale formation Sticking probability for impacting mechanisms Sticking probability for non-impacting mechanisms Overall sticking probability P Pressure Ap O Pressure drop Rate of heat transfer Heat flux R Universal gas constant or parameter defined in Equation 9.14 Fouling resistance or Fouling potential (see Chapter 16) Overall fouling resistance Fouling resistance at time t o~ Xll Fouling of Heat Exchangers a, R T R| Slagging propensity Total resistance to heat transfer Asymptotic fouling resistance t" Radius Rate of oxygen supply r~ Rate of corrosion Rate of oxygen supply Stopping distance or parameter defined by Equation 9.13 SR Silica ratio Temperature L Cloud point temperature Tcv Temperature of critical viscosity rl r, t Freezing temperature Pour point temperature Time Induction time U Average ball circulation time Electrophoretic mobility of charged particles Overall heat transfer coefficient for clean conditions Overall heat transfer coefficient for fouled conditions Velocity U o Initial velocity or velocity in the absence of thermophoresis //r U T Radial velocity Stokes terminal velocity /At Velocity due to thermophoresis Nomenclature xiii U* f, Friction velocity Mean particle volume V Volumetric flow v, Energy associated with double layers Total energy of adsorption Energy associated with van der Waals forces Electrophoretic mobility X Number of cells per unit area Number of cells to cover completely unit area Thickness or distance Subscripts Av Average Bulk B~o Biomass C Cold or clean Critical Crystal face f Foulant, or freezing g Gas or growth H Hot Impact Induction, or initiation, or interference or inside /n Inhibitory irr Irreversible xiv Fouling of Heat Exchangers m Mean, or metal max Maximum P Particle or sticking probability p Pressure rev Reversible Scale, surface or solid, saturation t Time w Wall or surface x~ Adsorbed cells Asymptotic or infinite Dimensionless numbers Re = dvp Reynolds number r/ Pr = cp ~ Prandtl number St =~ Stanton number 17vc p ad Nu = Nusselt number Sc = r/ Schmidt number pO Sh = KI. Sherwood number D Bi = a/~ Biot number 2 Nomenclature xv Greek a P r/o ~tot P 2"* .(2 Heat transfer coefficient Time constant Distance over which diffusion takes place Induced EMF Viscosity Particle collection efficiency Combined collection efficiency for non-impacting mechanisms Overall particle collection efficiency Fraction of surface Thermal conductivity Thermal conductivity of foulant deposit Thermal conductivity of scale Kinematic viscosity Dimensionless group described by Equation 10.48 Density Foulant density Shear stress or particle relaxation time Dimensionless particle relaxation time Rate of deposition Particle flux Particle volume Particle volume function (see Equation 7.45) Rate of removal Scale strength factor Water quality factor [...]... means of estimating the required heat transfer area, i.e A= Q (2.16) The choice of the individual fouling resistances for the calculation of Uo can have a marked influence on the size of the heat exchanger and hence the capital cost For a heat exchanger transferring heat from one liquid to another with the individual liquid heat transfer coefficients of 2150 and 2940 W/mZK and fouling Fouling of Heat. .. effects of fouling may be made on this basis [Bott and Walker 1971 ] Change in = Change due to heat transfer thermal coefficient resistance of foulant + Change due to roughness of foulant + Change due to change in Re caused by the presence of the foulant (4.6) The purpose of any fouling model is to assist the designer or indeed the operator of heat exchangers, to make an assessment of the impact of fouling. .. the cost of fouling were mentioned It is the purpose of this chapter to give more detail in respect of these costs Attempts have been made to make estimates of the overall costs of fouling in terms of particular processes or in particular countries In a very extensive study of refinery fouling costs published in 1981 [van Nostrand et al 1981] a typical figure was given as being of the order of $107... this will constitute a loss of output that, if the remainder of the equipment is running to capacity still represents a loss of profit and a reduced contribution to the overall costs of the particular site The consequences of enforced shutdown due to the effects of fouling are of course much more expensive in terms of output Much depends on a recognition of the potential fouling at the design stage... COST OF REMEDIAL ACTION The use of additives to eliminate or reduce the effects of fouling has already been mentioned An example of the effectiveness of an antifoulant on the preheat stream of a crude oil distillation unit has been described [van Nostrand et al 1981 ] These data show that considerable mitigation of the fouling can be achieved by this method Fig 3.1 demonstrates the fall off in heat. .. CONCLUDING COMMENTS ON THE COST OF FOULING A number of contributions to the cost of fouling have been identified, however some of the costs will remain hidden Although the cost of cleaning and loss of production may be recognised and properly assessed, some of the associated costs may not be attributed directly to the fouling problem For instance the cost of additional maintenance of ancillary equipment such... eds Fouling Mechanisms - Theoretical and Practical Aspects Editions Europ6ennes Thermique et Industrie, Paris 22 Fouling of Heat Exchangers Cudett, P.L and Impagliazzo, A.M., 1981, The impact of condenser tube fouling on power plant design and enconomics, in: Chenoweth, J.M and Impagliazzo, A.M eds Fouling in Heat Exchange Equipment HTD, Vol 17, ASME Garrett-Price, B.A et al, 1985, Fouling of Heat Exchangers,. .. and Operation of 13 269 Heat Exchangers to Minimise Fouling 14 The Use of Additives to Mitigate Fouling 287 15 Heat Exchanger Cleaning 357 Fouling Assessment and Mitigation in Some Common 16 409 Industrial Processes 17 Obtaining Data 479 Index 517 CHAPTER 1 Introduction The accumulation of unwanted deposits on the surfaces of heat exchangers is usually referred to as fouling The presence of these deposits... 1988, Fouling of heat transfer surfaces: an historical review 25th Nat Heat Trans Conf ASME Houston CHAPTER 2 Basic Principles The accumulation of deposits on the surfaces of a heat exchanger increases the overall resistance to heat flow Fig 2.1 illustrates how the temperature distribution is affected by the presence of the individual fouling layers FIGURE 2.1 Temperature distribution across fouled heat. .. the other hand relatively little consideration has been given to the problem of surface fouling in heat exchangers A review [Somerscales 1988] that traces the history of heat exchanger fouling suggests four epochs in the development of an understanding of the problem of fouling The chronology follows in general, the development of science and measurement techniques over the same timescale In the first . practical approach to deal with the problem of fouling through design and operation of heat exchangers. The techniques of on and off-line cleaning of heat exchangers to restore efficiency are. problem of surface fouling in heat exchangers. A review [Somerscales 1988] that traces the history of heat exchanger fouling suggests four epochs in the development of an understanding of the. o~ Xll Fouling of Heat Exchangers a, R T R| Slagging propensity Total resistance to heat transfer Asymptotic fouling resistance t" Radius Rate of oxygen supply r~ Rate of corrosion