Thermal Performance of Submerged Coil Heat ...

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onsi:::dng of a submerged coil in a storage tank. This paper ... Thermal capacity rate of the cold stream (product ... tank water temperature at several locations.
Thermal Performance of Submerged Coil Heat Exchangers Used in Solar Energ}" Storage Tanks (2)

p. E. Klett, 1 D. Y. Goswami, 1 and M. T. Saad 1

Introduction Tne use of a heat excr.anger in the energy collection loop of a solar energy system imposes a penalty on the system performance, the magnitude of which depends on the thermal perfo:mance characteristics of the heat exchanger. Typkall:y. 3 single-\\all heat exchanger reduce~ the overall system performance by 2-5 percent relative to a system with no heat exchanger. Howev-er, double-wall heat exchangers can reduce th:! s)stem preformance by as much as 15 percent [l ~3]. Many solar domestic hot water systems use internal heat exchangers ;;onsi:::dng of a submerged coil in a storage tank. This paper discusses thermal performance testing of submerged coil heat exchangeL'>. Feiereisen, et al., [4] also recently reported on test'> of submerged coil heat exchangers similar in design to the neat exchangers tested in the present work.

The preceding equation assumes negligible stratification in the tank which is- indeed what was. observed ~xperimentally during the tests. ldeaUy, T T should be an average temperature in the vicinity of the coiL However, it was observed experimentally that due to convection currents, the temperatures in the vicinity of the coi! change rapidly a11d erracticaUy. It was also observed experimentally that average temperature in the vkinity of the coil was slightly higher b>.H close to the average tank centerline temperature. Therefore, equation (2) would give reliable and conservative values of effectlveness. Effectiveness is thus determined by measuring instantaneous values of coil inlet and outlet temperature and tank water temperature at several locations. The tank heating process is sufficiently slow that quasi~steady state is assumed for each set of temperature readings that determine an efw fecti\·eness value.

Typical Test Results Thermal Performance Parameters There are three parameters commonly used to express the ;herma! performance of heat exchangers: The overall heat uar.sfer coefficient, U. the number of heat transfer units, ,V, 11 , and the heat exchanger effectiveness, c [5]. Of these, heat exchanger effectiveness is the most easily utilizable parameter with the currently popular solar system design methods. FCHART [6) and SOLCOST [7). Heat exchanger effectiveness is defined as: Actual Heat Transfer Rate t= Maximum Transfer Rate For a

shell~and-tube

type of heat exchanger,

t

can be written

as: T";~)

(l)

r:,tr.)

Thermal capacity rate of the cold stream (product of mass flow rate and specific heat) Thermal capacity rate of the hot stream The lesser of C, or C n lnlet temperature of cold stream Outlet temperature of cold stream Inlet temperature of hot stream Outlet temperature of hot stream Given the operating conditions of the heat exchanger Th.ir., T,_.,_n, Ch, and C 0 the effectiveness completely defines the heat transfer rate of the heat exchanger. 1n the case of submerged coil heat exchangers, equation 0) is not appropriate since Cm, is zero when no water is being drawn from the storage tank. A different equation is thus req'Jired to evaluate effectiveness for submerged coil heat exchangers. Feiereisen, et aL, [4] defines the effectiveness for immersed coBs in terms of an average tank temperature, T r, computed as the average of several temperatures taken along the tank centerline. The maximum heat transfer rare from the coil win occur if the fluid flowing through the coil leaves the coil at temperature T T· The effectiveness is then given by

A schematic of the test equipment ls shown in Fig. 1. Two glass-lined water heaters manufactured by Bock Corporation with capacities of 300 1 (BOgal), and 450 1 (120 gal) and fitted with copper-finnedwtube heat exchangers were tested for thermal performance. Each tank was tested with a single~wall coil and a double-waH coil. The coils were identical in other respects. Table J gives the specifications of the finned-copper tube used in these heat exchangers. Initially the storage tank was filled with city supply water at about 25"C. Tests were started v.lith the coil supply water at the htghest temperature that couid be achieved in the existing facility (about 72"C). At a constant flow rate through the coil. temperatures were noted at intervals of S min, untH the temperature differential between Tc,;:.ir: and T 7 was about 5"C. This procedure was repeated for flow rates ofO.l26 l/s (2 GPM), 0.189 lis (3 GPM), and 0.252 lis (4 GPM). These flow rates are typical for solar domestJc hot water systems with 300~450-liter stOrage tanks. The heat exchanger ef~ Table 1

Specifications of the copper finned tube*

·····--···--·----

m Finned sec·i-ion Pipe o.d. Finned section pipe i.d. Fin height Fins per in. Outside area

0.019 0.011 0.003

*Spiral coil consuuction. Double-walled coil is made by press-fiaing: a finned tube over a smooth tube,

~---.--·---

' De:,x:.rtment of :Vtechankal Engineer:ng, North Carolina A&T State l___'r;l,crsity,

Gn:e;-~:At Of Sf>tAR E~•ERDY E'(ill-.HR!l'G. ;...tanus.:ript recei\ed b} the Solar Encrg;. D:\i;ion. ~ay 14. 1983.

Journal of Solar Energy Engineering

Flg. 1 Schematic of test facility

AUGUST 1984. Vol.1061373

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value F' ;F" ranges only from 0.%~0.98. For the doublewall heatexchanger at 0.125 1/s the effectiveness ranges from 0.48-0.57 and Ff;./F,r; ranges from 0.95~0.97. Since doublewall heat exchangers display less variation of

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Effectiveness of double-wall coil in 300 1 tank Fig. 5

(ectiveness was determined from the temperature data with equation (2) for the different flow rates and temperature differentials. The res.uits presented here were obtained with no flow through the tank. In actual practice~ heat transfer to the storage tank can occur both when there is no flow through the rank and when water is being drawn from the tank. Thermal performance tests conducted with no flow through the tank give conservative values of effectiveness since the convection coefficient on the external coil surface is not augmented by forced convection. Figure 2 presents effectiveness values versus temperature differential ( Tt:c:l.m - T r) for the single-wall coH in the 300 1 tank for three different flow rates. Figure 3 gives the results for the double-wall coiJ in the 300 1 tank. Figures 4 and 5 give results obtained for the 450 l tank for the single-wall coil and double~waH coil, respectively. The curves are second-order least square fits to the data. Three representative data points from reference (4j are included on Fig. 5 for comparison. In all cases, the effectiveness is a strong function of coil flow rate. Effectiveness also appears to be a function of tern3741 VoL 106, AUGUST 1984

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Fig. 7 Haat exchanger penally versus effectiveness for a typical system

Effectiveness of double-wall coil in 450 1 tank

Collector area = 6m 1 , Pp UL = 3.75 W /m 2 "C, collector loop fl.ow 0.125 lis, collector fluid specific_ hear 4180 J/kgQC. For the single~wall coil and 0.125 1/s ~ow rate~ the effectiveness range~ from 0.55~0.72. Using either extreme

perature difference between coil inlet temperature and tank: temperature. The de_pendenc-e on temperature difference is less for the double-wall coil than for the single-wall coil because of the greater wall thermal resistance of the doublewaH configuration~ due to imperfect contact between the innerand the outer tube. For a given flow rate, the ratio of effectiveness for the double-wall coil to effectiveness of the single-wall coit ranges: from 0. 7 to 0.8 as a function of tern~ perature differential with the greatest penalty imposed by the double wan at higher temperature differentials. Figure 6 compares the values off at 0.25 1/s for the two tank sizes for both single and double-wall coils. Tank size bas no significant effect on f for the double-wall coil but does effect e of the smgle-wall coil at lower temperature differentiab. The greater waH thermal resistance of the double-waH coil appears to mask other effects that influence the effectiveness of the single~ wall coiL Figure 7 is a ptot of FfJFR versus t: for a typical solar domestic hot water system with the following parameters:

Transactions of the ASME

-'-.

Journal of Solar Energy Engineering

1 de Winter. F., "Heat E:.:c~a.nger Penalties in Double-Loop So:ar Water HeMing Systems," SoforEnergy. VoL 17,197$, p. 335, , . _. . 2 de \\'inter, F,, and Hore-L J. Q_, "Heat Exchanger Pena_t:csm Smglc Loop Proceeding.s of the !978 Annual !Antifreeze) Solar Water Heatmg ~1eeung of AS ofiSES. VoL 2. l, p. 3 Gosv.ami, D. y .. and Kktt, D. E .. "0:1 Thermal Pen'orm\im:e Teslirtg ~~ Compact Heat Exchangers for Low Temperanre Solar Ener~y s~ste:il.'-, Proctvdings of the ASME Solar Energy Dmsion Fourth Ar.nu:;;l Conference. Alb·.tquerque, K~1ex., April 26-29. 1982. , " . 4 Feiereisen. T. J., Klein, S. A., Du!fie, J, A., and Be_::lo.man, \\·A., Hea, Transfer :rom Immersed Coils," AS\1£ Pa;:>