Document KRJ2JBMbMnm3aZXJwEGbadbrX

708 CHAPTER 43 1965 Guide And Data Book Boiling'film coefficients in evaporators when 'the re frigerant fkrwe inside plain or finned'tubes are-influenced by: . 1. Tube size' (coefficient increases with a decrease in' tube' di ameter). j 2.; Method of expansion (direct or recirculated). 3.- Refrigerant distribution. 4. Presence of'oil:: 5. liquid subcooling prior to expansion. '* 6. Superheat of leaving refrigerant.: 7. Evaporator temperature. 1 - . 8. Refrigerant pressure drop.. - Attempts to correlate the film coefficient with the mass ve locity of the refrigerant have been generally unsatisfactory. Whether tire refrigerant is evaporated on the inside or outside of tubes, the film coefficients increase as the loading (or beat flux) is increased. An approximation of this effect is:: ! h, -C, + C.(^-) (4) taftcre Ci and Ct are constants.,. ' This above straight line relationship appears to be reasonably accurate forhalocarbon refrigerants within the range 1000 <?/A <5000, in Btuh/(sq ft). The nature of the tube surface has a pronounced effect upon the temperature difference required to promote boiling or, conversely, the amount of heat that can be transferred per degree of temperature difference across the refrigerant film. A roughened surface provides more points on which nucleate, boiling can form and promotes the .formation of tial1PF gas bubbles which are readily swept away to be replaced by liquid refrigerant. (Refrigerant velocities within the tubes of'dry expansion coolers must be kept above certain minimum values' to obtain good refrigerant distribution and usually to obtain satisfactory oil return. A-minimum pressure drop for good performance is the result. It is common practice to rate the performance of an evapo rator in terms of the saturated refrigerant temperature based upon the exit refrigerant pressure. Pressure drop within an evaporator results in an average refrigerant temperature' higher than that of the rating hyna. Within the pressure1 drop ranges given previously, a decrease in' the triievalueof* the ATm due to an increase in pressure drop is usually offset' by an improved film coefficient or improved refrigerant dis^' tribution. ` - :~ The overall boiling refrigerant film coefficient K varies in an: inverse manner with superheat. A change,in-superheat from-; 8 F deg to 14 F deg in a dry expansion cooler can decrease the valueofAbyasinuchas 12percent. Dimensionless correlations have been developed*-11 whereby the boiling film coefficients can be mathematically,determined when the physical properties' of the fluid and the surface characteristics are known. Despite this work, experimental- determination of the boiling film coefficients for each refriger ant in each style of evaporator and with each method `of 're frigerant flow control is the safest method. Chilled Liquid Film Coefficient The Reynolds number and other HimffngtnnlpBH groups in volving physical constants of the liquid and the surface characteristics can often be successfully applied in predicting liquid film coefficienta.4'"*"-M For turbulent flow of liquids or gases inside of stnught dean round tubes. Equations 3 or- 5 of Tablefi in Chapter! 4, Heat Transfer are generally used as the basic equation for the liquid film coefficient. Fig. 7 .... Typical Wilson Plot The term 0i/p)*J4 in Equation 5 of Table 6 in Chapter'4 lyn be considered as unity for most applications nring water or brine. . For liquid water'at temperature up to 220 F,'Equation 14 of Table 6 in Chapter 4 is used. -' - For turbulent' flow of liquids or gases across rows of tubes, Equations 25 and 26.of .Table 6, Chapter 4,-are generally used to determine'the film coefficient. ' ' '.The method formerly used to predict values of the fluid film coefficient h* when the fluid is on the outside of the tubes was to predict values for cross-flow and apply a factor to ac count for bypass and leakage around'the baffles and tube bundle.1* - InvestigationsKt'*,4,w have produced a method for accu rately determining the fluid film coefficient ho when the fluid is on the outside of the tube surface. * -v ' Low-tonnage' evaporators frequently employ''the multn tube, counter-flow principle. The fluid being cooled flows on the outside of the tube and parallel to the axis of the tube or tubes. The flow is considered as similar to. flow through tubes It is customary to consider d to be four times the hydraulic radius based'upon the total wetted perimeter when pressure drop calculations are concerned, and d to be four' times the hy draulic radius based upon the portion of the wetted perimeter through which heat flows for calculations involving the value of h*. Film coefficients for flow of fluids either inside or outside tubes are greater with turbulent flow, tow viscosity, high' thermal conductivity/ small or closely packed tubes,' and minimum bypass and leakage in transverse-baffle heat ex changers. 1 Separation of Coefficients:--Analysis of Test Results One of the most useful analytical plots at the disposal of the engineer is the Wilson Plot. An outline of a Wilkin Plot is shown in Fig. 7. The reciprocal of. (/.is plotted against the reciprocal of the velocity or of the fluid flow rate of the fluid being cooled. The value of the exponent n depends upon whether,tKe fluid being cooled is inside tubes, flowing parallel to the axis of the tube, or is flowing acres tubes by means of transverse baffles. For flow through tubes and parallel to the axis of tubes n = 0.8. Various investigators have reported val ues of n from 0.55 to 0.65 for flow of fluids across tubes in transverse baffled heat exchangers. A reasonably safe value is n = 0.6. lines A and B represent constantly held conditions of operation other than the velocity of the fluid being cooled. For example line B might represent the operation , of the evaporator at.half of the load represented'by. line.A. line B might also represent the performance of a brine cooler whose brine viscosity is greater than that of the brine'represented Evaporators for Liquid Cooling Kb XaV tlj 709, Table * .... Recommended Fouling Factors' for. Use' tn .Calculating Heat.Tfansfer-RdtesV ' ' Application Neo-farrow Tabes Tabes:, Brine Coolers (Brine Side) inhibited salt brines Non-inhibited salt brines Solvent brines (methylene. chlo-. ride, fluorinated hydrocarbon' refrigerants 1 . 0.0005 0.001 None 0.001 0.002 . None ' -Water Coolers (water side) Recirculated water, closed systems Recirculated water, open .systems - .0.0005 0.001 ; b'.ooi: . . 0.002 . Non-reciiculated water ; . . Use of.that allowed for ' condenser,! service but in no care less than 0.0005. FMQnf fecton an in (7 dec ) ftr) f*q ft) per Btn. by line A and which is at a tower evaporator temperature than that used to determine line A. The scale 1/p* must start at zero to make the plot of value. When 1/p* equals zero or infinite velocity the resistance on the fluid side theoretically equals zero. The intercepts of lines A and B on the 1// scale, therefore, represent the sum of all other resistances. The fluid ride coefficients can be determined from the slope of fines A and B. Fouling Factors Resistance due to fouling depends upon the material of which the tube is made, nature of the fluid, and fluid velocity. Copper tubes usually become fouled at a slower rate and to a. lesser degree than steel tubes. Fluid velocities less than approximately 3 fps promote additional fouling by permitting static accumulation of deposits. Recommended fluid velocities for. copper and steel tubes are from 5 to 10 fps providing pressure drop limitations are not exceeded. Table 4 lists recommended fouling resistances. The values are expressed as l/K*. Water, and brine coolers'operating atnormalconditions do nbtreqairethe addition of a fouling resistance to the refriger-1 ant side. .Caution should be exercised when designing eyapo^ ratora. for service below.-- 20 F because the oil ,that,'is circu-, toting through the refrigerant circuit may.congeal on the tube surface and add;an appreciable'resistance to heat1flow. This is particularly true of the:dry expansion type oooler.: ....Comprehensive' .tables of folding/rkistont^iafeigiven^in Reference.!; ? t o. -i.r-iW;/';' r.; Overall Heat Transfer Rates. r Due to the many;Variables- that enter into,the determina tion of overall heat .transfer rates, Table 5 gives only approxi mate minimumandmaximuiQ values'of'(/;v` 1 ., The.higher values of-'(/generally apply to;Ioadings,(<2/A)V corresponding to a (ATj) of 12-F degor more, at the higher velocities.' The tower-values of (/`apply to -loading!} corre- spondingtb (AT.) of8 F deg and less, and at tow fluid'yelocity. Water and Brine Properties' "'Ji .'"."V. - The physical properties of a fluid required in order to cal culate associated heat transfer film coefficients h and pressure drop are: (1) density or specific gravity, (2) specific heat, (3) thermal conductivity, and (4) viscosity. The properties of water are given in Reference 12. The properties of calcium and sodium chloride brine are given in Chapter 21. Some properties of ethylene and propylene brine are given in Chapter 21. Additional information may be ob tained from Reference 7 of Chapter 21. Properties of the alcohol brines are given in Reference 17. REFERENCES 1 Standard* of Tabular Exchanger Manufacturer! (Tubular Exchanger Manufacturer* Aaociation, New York, 1952, 3rd ed.). 1 Joseph Fabregaa: Design heat exchangers more exactly (ChemicalEngineering, April 1956, p. 181). * Townsend Tinker: Shell side characteristics of *h*ll tube heat exchangers (ASME Paper No. 56-A-123, 1956). * O. P. Bergelin, K. J. Bell, and M. D. Leighton: Heat Trans fer and Fluid Friction During Flow Across Ranks of Tubes (University of Delaware Engineering Experiment Station Bulletin No. 4, 1952, Revised April 1958). Table 5 .... Overall Heat Transfer Coefficients for liquid Coolers Type ot frapofgfor Flooded sheU-and-plain-tube (water to Refrigerants 12, 22, and 717) Flooded sbeD-and-finned-tube (water to Refrigerant 12 or 22) Flooded sheU-and-plain-tube (brine to Refrigerant 717) Flooded ahell-and-ptoin-tube (brine to Refrigerant 12 or 22) Dry expansion, sheU-and-plain tube (water to Refrigerant 12, 22, or 717) (Refrigerant in tubes) Dry expansion, sheU-aad-mternai-finned-Uibee (water to Refrigerant 12 or 22) (Refrigerant in tubes) Dry expansion, sheU-and-plain-tube (brine to Refrigerant 12, 22, or 717) (Refrigerant in tubes) Dry expansion, sheU-and-mternal-finned-tubes (non-salt brines to Refrigerant 12, or 22) Shelt-and-plain-tube coil (water in aheU) (Refrigerant 12, 22, or 717 in coil) iJaudelot cooler, flooded (Refrigerant 12 or 22 to water) oauaelot cooler, dry expansion (Refrigerant 717 to water) Jiaudetot cooler, dry expansion (Refrigerant 12 or 22 to water) Double-pipe cooler (Refrigerant 717 to water) Double-pipe cooler (Refrigerant 717 to brine) Py type shell-and-tube water coolers (Refrigerants 11, 12, 22, 113, or 717) Tank-and-agitator, coil type water cooler (flooded. Refrigerant 717) ianfrand-agitator, coil type water cooler (flooded, Refrigerant 12 or 22) T'u* ""poo** (Refrigerant 717) to brine cooling, coils between can in ice tank rank, high velocity raceway type (Refrigerant 717 to brine) Maxidata 130 190 90 140 45 100 30 90 80 160 160 250 60 140 100 170 10 25 100 200 60 150 60 120 50 150 50 125 150 250 80 125 60 100 15 40 80 110