Document 1Qz7KpYx1M2w2g6yXYDwL9KpX

64 CHAPTER 4 1965 Guide And Data Book Table 8 .;;. Heat Transfer Coefficient* for film Type Condensation ' 1/ (quid filot temporctefu tmA -- 0J5 Ai 1: Vertical Surfaces, height L, - Laminar Condensate Flow, Nki = 4r/v < 1800 '- Turbulent Flow, Nr* -- 4r/*i/ > 1800 2. 'Outside Horizontal Tubes, N rows in a vertical plane, Length L, T-ominar Flow Finned Tubes `. 3. Simplified Equations for Steam; - f ; Outside Vertical Tubes, Nr* -- 4r/#/ < 2100' Outside Horizontal Tubes, Nr, 4r/p/ < 1800 8ingle Tube Multiple Tubes ' 4. Inside Vertical Tubes-. ` ! ' - - d<?i 5. Inside Horisontal Tubes,----- < 5,000 mi ' d<7. / i\1/1 1,000 <---( --) <20,000 pi V p/ ;. .^ ' ' . dG.//>t\w 20,000 <--M --) < 100,000 Ml V P./ , dGi - dG, / pi\ri ' - For--- > 5,000-- ( --) > 20,000 f : Ml Ml V P./ i .'.*l . . ^ > |A - 1.13 Fi(A/,/LAI)U - ,IA - 1.11 Pt(b/v>i)U* ... fA = 0.003 (F,)*(AiL/wlA/,)u* ' U - 0.0077 Fi{Nr,)**(1/m/)w ' -* ' ' /CD (2) (3) (4) : i (A - 0.79 FtQiiJNdAiyiy ... ` U - ll05 'A - 0.689 F,(A/#/niZ)n)w ' ' - Where Dm,is determined from . 1 . "" (5) (6) (7) 1 -130 '- -.4*.' *r ..i;-:, - - \(7a) (Dn)' - AniL^yi* - d,,(d):: - i- A = 4ooo/(Ljv4(ni)' , " .. i ... A = 3100/(d')U4(dl)W A - S800/(Nd*)`/*(AI)1'* - A = 0.065 (~^rt^UlGm '' ; Where' ' c ^ -|- GdG. + GL*y* (8) .- ;[ - (9) . '(9a) (10) -` (10a) (ID `r,- (rr(^nd:-(:)T (12) ..-(13) Where G. G, (13a) -r -fig. T 3 1 Film-Type' Condensation- ' Heat Transfers T ye 9 Values of Condensing Coefficient-Factors for ? -'Different Refrigerants* F9n Toop faht. ' Refifgwu*7 I, =* w - 0JS (Al) -f ; . ft Refrigerant - - -75 100" 125 Refrigerant r" 75 100 125XX ; Refrigerant ; , 75 , 22 - ' : 100 tr-V-' 125 -' Sulfur Dioxide 75 110205 154 ' 822 153 ; , 815 , 151 803 : 133 672 '' ' 122 60S 112 538 " 7 153 822' 144 755 132 676-' 290 299 - 318 1920 * 2000 2170' 75 409 3040 . 100 408 3035 fr. ' ' ........1 125 - ' `408 - / - 3030 ' Propane .i. 75 100 125 159 850 .. 157 : - 845 ,... , ; , 154. ^ , 836 .. 75'** . 100 125 - Property Valaca from Chapter 19.' Ft - 156 840 .. 156 - : , 843; . . , 157 845 . . / (Btu)(tb.) yiy' A (hr)`(ft)?(^)*/ 7 (BUi)Hib.);y: \ (hr)4(ft)?(F)7 / . Impurities and Noncondensable Gases ' The vapor catering the. condenser often contains a small percentage of impurities such'as oil. The oil forms a film on the Poremeter: T/pc, tM T Eqwl flow rote, pounds per (hoar) (fool); P " liquid duaaty, pounds'per cubfc foot; and i "* surface tendon of liquid relative te water, the values of pa* velocity ufJCced in cafafafmg f and Ne ore cafculoted as though no liquid was present. -- - fig. 14.... Friction Fartors for Gas Row Inside Pipes with Wetted1 Walls*7- *'< 65 condensing surfaces which offers additional resistance to-the heat i transfer. Some allowance. should be made for this, especially when no oil separator is used' and when the dis charge lines from the compressor to the condenser are short. Experiments show that the presence of noncondensable gases in the condenser adversely affects the heat transfer. The de crease in the heat transfer coefficient is reported by Chapman? to be approximately linear with the" weight fraction of the noncondensable gas present. Pressure Drop for a general discussion of pressure drop with two-phase flow, refer, to Ghapter 6, Fluid Flow. Pressure drop data for two-phase flow in'a horizontal tube with condensing -Re frigerant 22 are reported by Altman, Staub and Norris.71 EXTENDED SURFACE Heat transfer from a prime surface can be increased by attaching fins or extended surfaces, which increase the, area available for heat transfer.1 In' general; the advantage of fins is that they provide a more compact heat exchanger with lower material costs for a given performance. In order to achieve an optimum design, finsarc generally provided on the side of the heat exchanger -where tthe heat-transfer coefficients are low (such as the air side of an air-tOrwater coil): Examples of equipment employing extended surface are natural and forced convection coils, and shell-ond-tube evaporators and condensers. Fins arc also used inside tubes in condensers and dry expansion evaporators. Rn Efficiency . As heat flows from the.root of a fin to its tip,.there is a tem perature drop due to the thermal resistance of the fin material: Therefore,.the temperature difference between the fin and the surrounding fluid .varies, being greater.at the .root than atthe tip, and causes a corresponding variation in the heat flux. For this reason, increases in the length of a fin .result in propor tionately less additional heat transfer. To .account for , this effect, a factor is introduced called the fin efficiency. The fin efficiency is defined: by Equation 18 as the ratio of.the actual heat transferred from the fin to the heat which would be transferred if the entire fin were at its root or base tempera ture. J Hi-l.)dA *------------------------------ (18) - J h(t. - QdA where is the fin efficiency' 1,- is the temperature of the sur rounding environment, and t, is the temperature at the fin root. The fin efficiency will be low for.long fins'*, thin fins, or fins of low thermal conductivity material; Furthermore, the X efficiency will decrease as the Heat transfer coefficient in creases because of the increased heat flow through the fin. For natural convection in air-cooled condensers and evapora tors, where A for the air side is low, fins can be made fairly large and of low conductivity materials, such as steel, instead of copper or aluminum. On the other hand, for condensing and boiling, where large heat transfer coefficients are involved, fins must be very short to achieve optimum usage of material. 'The heat.transfer-from a finned surface, such as a tube, which includes both finnari or secondary area, A* and unfinned-or prime area, dp, is given by Equation 19. / ; q - (MVv+-*Mi)Ur - O (19)