Ayub, Z. H.

English

Purdue E-Pubs

Purdue UniversityPurdue e-PubsInternational Refrigeration and Air ConditioningConference School of Mechanical Engineering1988Design Techniques for R-717, R-22, and R-12Shell-and-Coil Subcoolers and IntercoolersZ. H. AyubE.L. Nickell Co.Follow this and additional works at: http://docs.lib.purdue.edu/iraccThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/Herrick/Events/orderlit.htmlAyub, Z. H., "Design Techniques for R-717, R-22, and R-12 Shell-and-Coil Subcoolers and Intercoolers" (1988). InternationalRefrigeration and Air Conditioning Conference. Paper 38.http://docs.lib.purdue.edu/iracc/38DESIGN TECfiNIQUES I'll< R-717, R-22, AND R-12 SHELL-AND-"COIL SUBCOOLEl',S ANI) INTERCOOLFRS Zahid H. Ayub E. L. NickeLl Co., Inc. Constantine, Michigan 1.,9042 ABSTRACT Subcoolers and 1nter<:oolers are becc:ming essential part of any refrigeration system. The most canmon type f:ound in industrial ·refrigeration is the Shell-and-Coil configLlration. This paper presents a design technique for s1.z..i.ng of coils for Ammonia, R-22, and R-12 systems. The e"aluation of inside .<;tnd outside heat transfer coefficients is based on well established correlations and canpL!ter programs. CorrelatJ.ons are also presented for pressure drop Bvaluation. ThB derived correlations s.re vali<i for t<emperature range 0 - 120 °F. TECHNIQUES DE CONCEPTlON DES SOUS-REF ROIDTSSEURS ET REf'RO!DlSSEURS lNTERMEDIAJRES MULTITUHULi\JRES POUR R717, R22 et Rl2. Resume ; Les soU>'"refroidisseurs et Ies refroidisseurs intermediaires sont devenus une partie essentielle de tout systeme frigorifique. Le type le plus courant dans le froid industricl est le type multitubuJaire. Cettc communication presente une technique de conception pour le dimens:onncment des serpcntins des >ysteme• a ammoniac, R22 et R12. L'evaluation des coefficients de transfert de chaleur interne et externe se fonde SUl' des correlations bien etablies et des programmes d'ordinateur. On presente aussi des correlations pour )'evaluation de Ia chute de pression. Les correlations etablies sont valables pour des temperatures de- 18 a so•c. 38 Dr:S i',;N '1iDi!H(!IiU:\ F'Jl'. 1<·-717, R··2 .: , N'U R-lL SliLI.L-ANP-CO LL SUilCWLffiS N~lJ lNTERCWLffiS Zahid H. Ayub E. L. Nickell Co., Inc. Constantine, Michigan 49042 Ai = inside surface s.rea, ft2 Am = log mean area, ft 2 NOMENCLAWRE Ao = outside surface area, ft2 Ch ~heat transfer factor, Bttt/hr." 2 -ft 2"'"- 'F COP = coefficient of performance Cp = pressure drop factor, lb-s 1"11 /ir!"' Di inside diameter., in. Do outside diameter, in, f Darcy's friction factor Fh constant for heat transfer, Btu/hr1 -ft•·• -"F Fp constant for pressure drop, lb/hr"' -ft'"' g gravitational acceleration, ft/s2 GTD = greatest tempera'~ure difference,' F h = enthalpy, Btullb hi inside heat transfer coefficient, Btuihr-f.t:"..,',< ho =outside heat transfer coefflCient, Btuihr-ft~~ 1< = refrigerant the~:-mal conductivity, Btu/hr-ft-'F K =coil thermal conductivity, Btulhr-ft-~ L = coil straight length, ft LMTD = log mean temperature difference, ~ LTD = least temperature difference, "F p = pressure d~:-op, psi Pr = Prandtl numbe~:-q in = heat input, Btu/hr R wall thermal ~:-esistance, hr-f~-"F/Btu Re Reynold's number Rffi inside fouling factor, hr-ft2 "FjBtu Rffo outside fouling factor, hr-ft- F/Btu Rw =net wall resistance, hr-ft-"F/Btu s entropy, Btu/lb-~ T = temperature, 'F Ts = saturation tempe-.;-ature, ~ T =temperature difference (range), "F Uo =overall heat transfer coefficient, Btulh>-ff-"F V = velocity, ft/s Wnet= mechanical work, Btu/hr P -refrigerant dens-~ty, lb/ft 3 ~ =bulk viscosity, lb/ft-hr Pavg =average viscosity for 0-l20°F range, lb/ft-h~:­~0 = viscosity at 0 ~, lb/ft-hr ~w viscosity at wall temperature, lb/ft-hr IN1RODUCTION In any refrigeration system there are various pressu~:-e vessels, heat exchangers, and other auxiliary components that are interlinked via complex network of piping. TI1e basic aim of this conglomeration is to transfer heat from one section of the system to another at a most economical cost. lt is important to note that the final system cost should be evaluated on the basis of the equipment cost, the installation and maintenance cost minus the energy savings, i.e., C- ES. The term, ES, will be zero for an ordinary design, but will b" greater than zero for an energy intensive modified system. Any modification will of course result in a rise in 'C', so that the actual indicator of net benefit is Z = CI(C- ES). There are nume~:-ous ways to increase this factor in a ~:-efrige~:-ation system. The most common method is to feed a subcooled refrigerant to the expansion valve. One way to attain subcooled refrigerant is to over design 39 the condenser so that besides condensing, the refrigerant is also subcooled. The problem with this method is that the condenser cannot achieve high level of subcooling. The tower water (in case of water cooled condensers) is usually available at 70'F to 80°F with a temperature range of about 10 degrees. The refrigerant condens~s at 95'F to 105°F. Hence, there is a limit on the approach temperature. The close approach temperature results in smaller LMTD which in turn results in larger surface area. Large surface area means larger shell and more tubes, which means extra material and labor cost. It is important to note that drilling holes in a tubesheet is a major component in determining the final cost of a heat exchanger. SHELL-AND-COIL SUBCOOLER The most common and widely used method is to install a subcooler before an expansion device as shown in figure la, A subcooler could either be a shell-and-tube type or a shell-and-coil configuration. The latter type is common in the industry because of its simplicity and lower cost. A typical configuration is shown in figure 2. The coil is submerged in a saturated pool of refrigerant at a saturation temperature corresponding to the low side pressure. To examine the net effect of a subcooler it is important to study the T-s chart as shown in figure lb. The refrigeration effect is defined as: ( 1 ) A system with no subcooling follows the broken line path between Point 2 and Point 4'. The refrigeration effect is: ( 2 ) On the other hand a system with a subcooler follows the solid line path between Point 2 and Point 4. The refrigeration effect for this case is: ( 3 ) Hence, the refrigeration effect is higher with the subcooler. The msgnitude of the enhancement depends upon the amount of subcooling achieved. DESIGNING A SHELL-AND-COIL SUBCOOUR Designing a correct coil could be cumbersome, involving single and two-phase flow analysis. The essence of this paper is to [ormulate a simple method that would enable a designer who ha~ the basic knowledge of heat transfer and fluid flow to size an appropriate coil. The equations and curves wm;·e developed on the basis of well known empirical correlation, a feedback from the previous jobs, and the computer program developed for flooded evaporators. The design technique for three most common and widely used refrigerants, R-717 (ammom.a), R-22, and R-12 is presented. The range of temperature common in practice is between 0- 120'F, therefore, the analysis is valid for this range. The transport properties for these refrigerants are shown in Table l. The overall resistance to heat transf~r based on the outside surface of the coil is given as, l/UoAo = l/hoAo + 1/h1A.1. + Rffo/ Ao + Rfei/ Ai + Rw (4) 1/Uo = l/ho + 1/hi (Ao/Ai) + Rffo + Rf.f~ (Ao/ki) + R (Ao/Am) l/Uo ~ 1/ho + l/hi (Do/Di) + Rffo + Rffi (Do/Di) + (Dof2L,K) ln(Do/Di) (5) where, Am= (Do-D~)/lnCDo/Di); R = (Do-D~l/24K. The foulin& resistan~es are a matter of choice. They depend upon the level of impurities in the system~ They could also be recommended by the specifier. A corrunon value for eac:h s~de i.s 0.0005 hr-ft2 .'Cf/Btu. To filcihtate calculations, the last term in equntion 40 "1• V,[l!_, C.\:ll\,l(lLI_t; fur dtf I r_ n.::nL l!lW c~li_"l;(,Jt1 ~~Lecl l-ljl(,' :-llZ~''J· l,lK' L~l.bUlt1ll: 1 1 valu~:., ,;.rc 3ivcn 1 n L'11Jlt~ /_. Ln order to evl'luate the second Le(m, l!hi, a Siedc:r-Tate ll l correlation Cor turbulent 1n-tub" flow is applied, i.e., hi= 0.027 k/lli Rg 9 P~3 ( l'/11wt"1"' (6) Substituting the transport properties from Table d>f€erent format results in, o·~ .1 o.J4 hi = 0.027 (V 101 ) Fh (1'/l'w) where, "' •• ., 1/3 ·2. ,_, Fh "' (k cp P IJ·'t ) , (Btulhr -ft -"F) and rewriting equation 6 ln (7) Table 1 indicat"s that Lhe variation in quantities of the term, Fh, 1s not large for the range 0 - l20°F, Hence, it is acceptable to substitute an average value for this term. The term, 1'/Mw, is replaced by, llavg/Mo, which is a safe assumption. The final equation for hi, is, ·S •2. 20 hi = Ch V /Di , (Btulhr-ft-Fl (8) where, Ch is : 388.7 for R-717 123.1 for R-22 98.6 for R-12 Equation 8 shows that in order to evaluate hi, the only parameters a designer has to know are the coil size and the refrigerant velocity. To evaluate the first term, ho, the outside heat cransfer coefficient, consult the curves in Figures 3 - 5 for the appropriate refrigerant. Ordinarily this would be a challenging part of the design as this coefficient depends on various physical and flow parameters [2]. Once the length of the coil is known, it is important to evaluate the pressure drop. In case the pressure drop is high, the designer has to either select a larger si7.e coil or select multiple circuit coil connected via a common header. The equation used for the pressure drop is, ·11 ll.p f(LIDi) (pV 2/2g) (fw/f) (1/144), (psi) (9) where, f = 0.2/Re• · 2 [3] or, ll.p -6 1·& 1•01 4.193*10 Fp (LV /Di ) (10) where, Fp ·10 ·II ·1'1- ·>. ._.t;. J1 P fW , (lb/hr -ft ) The factor Fp is evaluated using properties in Table 1 with l'w taken for the worst condition, i.e., at 0°F, l·· I·~ or LJ.P = Cp (LV /Dl l, where Cp is: 6. 762*10-5 1.220*10-4 1.349*10-4 (psi) for R-717 for R-22 for R-12 CONCLUSION (11) A slmplified design procedure lS presented for SlZlng subcooler and intercooler coils with R-717 (ammonia), R-22, and R-12 for a temperature rang~ of 0 - 120'F. TI1is procedure will prove to be a helpful tool for practicing engineers that do not want to be intensively involved in the complexities of two-phase flow and boiling heat transfer. REFERENCES [ ll Sieder, E. N., and Tate, G. E. , "Heat Transfer and Pressure Drop of Liquids in Tubes," Indust:r:ial Engineering Chemistry, Vol. 28, p. 1429 (1936). 41 [2] Kakac, S. , Bergles, A. E. , and Mayinger, F. , "Heat Exchangers: Thermal-Hydraulic Fundamentals and Design," Hemisphere Publishing Corporation, New York (1981). [3] Fraas, A. P., and Ozis:ik, M. N., "Heat Exchanger Design," John Wiley and Sons, New York (1965). [4) ASHRAE Handbook, "1981 Fundamentals," Atlanta (1982). Table 1. Fact.ors Fh, Fp. Properties taken from [4] T !/ k cp Fh Fp R-717 0 0.558 41.34 0.33'5"" 1.083 12.7714 17.4752 20 0.494 40.43 0.321 1.092 12.9430 17.0418 40 0.437 39.49 0.306 1.103 13.0709 16.6015 60 0.386 38.50 0.291 1.118 13.1837 16.1470 80 0.341 37.48 0.276 1.135 13.2645 15.6868 100 0.301 36.40 0.261 1.158 13.3213 15.2098 120 0.268 35.25 0.246 1.187 13.2853 14.7212 R-22 0 0.654 83.825 0.0630 0.271 4.3183 31.7552 20 0.599 81.597 0.0598 0.276 4.2787 30.9148 40 0.553 79.255 0.0566 0.283 4.2178 30.0584 60 0.513 76.773 0.0534 0.291 4.1346 29.1712 80 0.480 74.116 0.0502 0.300 4.0197 28.2478 100 0.449 71.236 0.0471 0.313 3.9052 27.2568 120 0.427 68.054 0.0439 0.332 3. 7506 26.1993 R-12 0 0. 767 90,659 0.0490 0.217 3.3520 34.9050 20 0.687 88.529 0.0467 0.220 3.3685 34.0218 40 0.620 86.296 0.0443 0.224 3.3628 33.1289 60 0.564 83.944 0.0420 0.229 3.3423 32.2211 80 0.517 81.450 0.0397 0.23L, 3.2962 31.2891 100 0.477 78.785 0.0373 0.240 3.2240 30.3206 120 0.441 75.906 0.0350 0.251 3.1581 29.2926 Table 2. Pipe size [3] and wall thermal resistance for carbon steel Nom. pipe Do Di Sch. II Po/Di Rw size (in) (in) (in) hr-fl-'t!Btu 3/4 1.050 0.824 40 1.2743 0.00038 0.742 80 1.4151 0.00055 1 1.315 1.049 40 1.2536 0.00045 0.957 80 1.3741 0.00063 1-1/4 1.660 1.380 40 1.2029 0.00046 1.278 80 1.2989 0.00065 l-1/2 1.900 1.610 40 1.1801 0.00047 1.500 80 1.2667 0.00067 2 2.375 2.067 L,O 1.1490 0.00049 1.939 80 1.2249 0.00072 2-1/2 2.875 2.469 40 1.1644 0.00066 2.323 80 1.2376 0.00092 3 3.500 3.068 40 1.1408 0.00069 2.900 80 1.2069 0.00098 3-1/2 4.000 3.548 40 1.12.74 0.00072 3.364 80 1.1891 0.00103 L, 4.500 4.026 40 Lll77 0.00075 3.826 80 1.1762 0.00109 42 B:Rece.iver C:Subcooler D:Exp. valve E: F.vaporator F:CQmpressor Fig. la VClpor compression loop with 6md without a subcooler increas~d refrigeration eff£!Ct: due to subcoo]i.ng Flg. lb T-s d.iagram for rcfrlgeral;:ion cycLe in Figure 1.:;~ 43 Fig. 2 Typical coil subcooler R-717 630 "'"' I ~ '"' ""' I 620 H -" -;-e 0 -" 610 600 20 40 60 80 100 120 Ts ("F) fig. 3 Outside heat transfer coefficient for Atnmonia 44 R-22 560 550 F I ~ "-< I ~ 540 .::::: 2 e 0 ""' 530 520 20 40 60 80 100 120 Ts ('F) l'fig. 4 Outside heat transfer coefficient for Freon 22 530 R-12 510 "" I ~ '"' I '" - 490 "-. 2 e 0 "' 470 450 20 100 120 Fig. 5 Outside heat transfer coefficient for Freon 12 45 

Design Techniques for R-717, R-22, and R-12 Shell-and-Coil Subcoolers and Intercoolers

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1037&amp;context=iracc

Design Techniques for R-717, R-22, and R-12 Shell-and-Coil Subcoolers and Intercoolers

Abstract

Similar works

Full text

Available Versions

Purdue E-Pubs