HEAT TRANSFER PERFORMANCE OF A DRY AND WET/DRY ADVANCED COOLING TOWER CONDENSER

Hans D. Fricke, David J. Webster, Kenneth McIlroy Union carbide Corporation - Linde Division, Tonawanda, New York

John A. Bartz Electric Power Research Institute, Palo Alto, california

ABSTRACT

This paper describes an EPRI-funded experimental evaluation of advanced air-cooled ammonia condensers for a phase. change dry/wet cooling system for power plants. 'Two condenser surfaces with dIfferent air-side augmentation were tested in an ammonia phase change pilot plant (0.6 MWth) located at UCC/Unde. The first unit consisted of an integral shaved-fin-extruded aluminum tubing designed fordry operation. Heat transfer and air-side pressure loss characteristics were measured under varying air face velocities (1 to 5 m/s) and initial temperature differences, lTD (11 to 33K). Measured overall heat transfer coefficients, U, ranged between 40 and 49 J /m2 s.K (based on air-side surface). The second configuration constituted an aluminum plate-fin/tube as sembly, which was tested in both dry and wet (water deluge) modes at 1 t04 m/s air face velocities and lTD's of 5 to 33K. Deluge rates varied from 1 to 6 m3/s per meter of core width. In the dry mode, U ranged from 42 to 63 J/m2 .s.K. Water deluge enhanced the heat rej ection up to 4.5 times over dry operation.

INTRODUCTION

For over a decade, considerable interest has existed in dry (nonevaporative) cooling to reject waste heat from steam-electric power stations. Dry cooling offers the elimination of thermal pollution of lakes and streams caused by once-through cooling, as well as a methodio avoid water makeup, blowdown, and fogging problems associated with wet (evaporativel cooling. In addition, dry cooling increases siting flexibility, particularly for locations in arid Western coal fields.

However, dry cooling requires considerable capital investment for the cooling towers. Hence, the development of effitient (low cost) heat transfer surfaces in conjunction with neW cooling processes is very important for this approach to be economica lly feasible. Extensive studies have indicated that although heat exchangers contribute about 35% of the cost, the entire heat transport system, not merely the heat exchangers, need to be evaluated to achieve an attractive alternative, Ref. (1). ThiS must include acceptable working fluids to transfer the heat from the steam condenser to the ambient

414

air, effective methods to transfer heat and methods of achieving augmented cooling during hot dry weather.

Beginning in 1975 the Electric Power Research Institute (EPR!) has sponsored a program of dry cooling research proposed by Union Carbide Corporation/ Linde Division (UCC/Unde) and Westinghouse Electric Corporation. The primary object of this effort was to study, select and demonstrate a heat-rejection cycle of substantially increased thermal performance to reduce the large penalties commonly associated with conventional dry cooling where circulating water (the coolant) is heated in the steam condenser and, subsequently cooled in the cooling tower. The final recommended concept consists of an isothermal ammonia-phase-change heat-rejection cycle with heat transfer enhancements on the heat exchanger surfaces in the loop. The waste heat from the power plant is rejected to liquid ammonia in a steam condenser/ammonia reboiler and the ammonia vapor thus generated is condensed in a cooling tower by heat rejection to the atmosphere.

In order to demonstrate the technical feasibility of ammonia phase change for dry cooling, a test pilot plant (0.6 MWth) was designed, built and operated by UCC/Unde under EPRI contract. Primary obj ectives were to demonstrate the integrated performance of this cooling concept using UCC/Unde I s heat transfer enhancements in the steam condenser/ ammonia reboiler and to test the performance of advanced air-cooled ammonia condensers in the pilot plant's cooling tower. This paper will address only the experimental studies relating to the ammonia condenser. Other efforts of this program are described in Ref. (2). Specifically, the heat transfer and pressure loss characteristics of two ammonia condenser configurations 1 with differing air-side augmentation are discussed: the Curtiss-Wright integral shaved fin-extruded aluminum tubing, designed for dry operation, and the Hoterv aluminum platefin-tube condenser which was tested in the dry and wet (water deluged) mode. Water deluge entails wetting the air-side surface of the condenser to improve the heat transfer performance of dry cooling

1 of approximately equal projected cost for a 500 MWe power station cooling system.

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towers during periods of high ambient air temperature. Augmentation of the conve ctive and conductive tube-to-air heat transfer is achieved by the evaporative cooling of the water as it flows over the external surface. This technique is expected to be an economical peak shaving approach with minimal water consumption.

TEST APPARATUS

An ammonia phase change dry/wet cooling pilot plant was built and tested for demonstrating the concept and for measuring the heat transfer performance of augmented heat exchangers. A simpli~ fied schematic is shown in Figure 1.

enters the phase separator. The ammonia vapor from the separator is then condensed in the air-: cooled condenser from which liquid is pumped back to the surge tank. Steam is condensed in the condenser/reboiler on the outside of the tubes which also incorporate a UCC/Linde condensing enhartcement. The condensate is collected in the hotw~ll and returned to the steam generator to be recyded.

The system was designed for a nominal heat load of 0.6 MWth when condensing steam at 1.7 x 104 Pa absolute pressure (Tsat = 330 K) with an: ambient air temperature up to 307 K. At design :condition, this would translate to condensing steam at the rate of 908 kg/hr, boiling and condensing 2043 kg/hr of ammonia at 2.1 x 106 pa (Tsat = 327 K) with an air flow rate of 127,440 m3/hr through the ammonia condenser.

STEAM (FROM STEAM

GENERATOR)

__.... TO ] HOTERV CONDENSER TEST SECTION

___ FROM (SEE FIG.3)

AMMONIA VAPOR

L10UID AMMONIA

AMMONIA CONDENSATE

AMMONIA AMMONIA RECIRCULATION PUMP PUMP

FIG. I - Advanced dry cooling pilot plont flow schematic

There are two primary flow loops: the anhydrous ammonia isothermal phase change circuit which transports the heat from the steam condenser to the ammonia air-cooled condenser in the cooling tower and the steam/condensate loop which provides the source of low pressure steam to the condenser from a commercia I steam generator.

Basically, the liquid ammonia is pumped from the surge and phase separator tanks into the steam condenser/ammonia reboiler where it boils inside High Flux2 tubes, extracting heat of condensation from the shell-side condensing steam. Twophase ammonia exits the condenser/reboiler and

2 Dee/Linde patented porous boiling surface which greatly enhances heat transfer by typically 5-10 times that of a plain surface.

Two separate ammonia vapor loops were u;tilized for testing the ammonia condensers. One: loop (indicated in Figure 1) contained a cooling tower, housing two Curtiss-Wright tubed condensers which were tested strictly in the dry mode. The configuration of this particular condenser is not sUitabl for a wet operation since water-deluge cannot easily be supplied to each horizontal, shaved-fin tube. 3 : The heat exchangers consisted of aluminum shaved-fin tubing (developed by Curtiss-Wright) which was manifolded horizontally between two headers (a vapor and liquid channell. The frontal area was 5.95 m2 with a core depth of 0.12 m. The air-tp

3 Changing the design/orientation of the shavedfin tube assembly, however, could conceivably accommodate water-deluge.

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condensing side area ratio was 7.2. Two such units were installed at opposite sides of the cooling tower cabinet. A photograph of this extended surface tubing and geometry is shown in Figure 2. Noteworthy are the individual rectangular ammonia flow channels, which assure strength and promote turbulence for improved heat transfer. As noted in Figure 2, air flow is horizontally acros s the fins. A variable-pitch fan mounted on top of the cabinet controls the induced ambient air flow across the heat exchanger cores. The condenser was tested under air face velocities varying from 1 to 5 mls and ITO's from 11 to 33 K.

FIG. 2 Curtiss-Wright shoved-- fin lublnQ

The second loop consisted of a separate vapor line, also originating from the top of the phase separator, an air duct with a variable speed blower, and a water deluge system for testing the Hoterv condenser in the wet mode. Figure 3 presents a more detailed schematic of this branch circuit. Saturated vapor flows from the pilot-plant phase separator to the Hoterv test unit where it is condensed. The liquid is then returned to the pilot-plant surge tank. The ammonia pressure in the loop (and in the pilot plant) can be varied from 1.03 x 106 to 2.06 x 106

pa in order to control the ITO. 4 An ammonia pump was not required since the condenser is elevated above the surge tank, providing sufficient hydrostatic head.

A rectangular duct with the blower was utilized to supply room temperature air to the test unit. Screens and baffles were installed inside the duct, upstream of the heat exchanger, to assure uniform air flow across the face of the unit. The centifugal fan provided air flow velocities between 0.6 and 4 mls at the inlet of the heat exchanger. A traversing pitot-static probe connected to a manometer and a turbine meter monitored the air approa ch ve locity

A water deluge system was installed above the Hoterv condenser for testing in the wet mode and was designed to uniformly wet the heat exchanger up to 6 x 10-4 m3Is per meter of heat exchanger width. A water softener and heater (not shown in Figure 3) were employed in the loop upstream of the heat exchanger to condition the augmentation water. The water softener was used to reduce variations in the water chemistry and to minimize any corrosion or deposition tendencies on the external heat exchanger surface during the tests. Mineral deposition or scaling could cause an increase in the air-side frictional pressure loss and in the thermal resistance. The Hoterv condenser was inclined at a 16 angle with respect to the vertical to assure uniform water coverage. This was based upon water-deluge experiments which Battelle Pacific Northwest Laboratories, Ref. (3) had conducted on a similar heat exchanger. As noted in the figure thermocouples (in the ammonia, water, and air stream), pressure transducers (absolute and differential) , and turbine flow meters were provided to measure the heat transfer and air-side pressure-loss characteristics of the test condenser in the dry and wet mode.

The Hoterv condenser is an aluminum platefinltube configuration which was developed and manufactured by Hoterv Institute, a Hungarian organization. Babcock & Wilcox is now the licensee and U S. distributor. This heat exchanger (0.46 m2 frontal area) was nominally rated for a heat duty of 0.06 MWth and condenses ammonia at a rate of 202 kglhr with an air velocity of 1.8 mls in the wet mode. As noted in Figure 4, the assembly basica lly consists of two rectangular headers, manifolding the staggered horizontal tubing which pass through a closely spaced stack of plate-fins. A Hoterv heat exchanger is typically fabricated using aluminum tubes and fins. However I because of the high pressure used in the tests, carbonsteel liners (1.46 cm I .0. x 1.70 cm 0.0.) were employed inside the aluminum tubes (l.8 cm 0.0.) to accommodate the 2.4 x 106 Pa operating pressures. The air-to-condensing side area ratio was 17.3.

4 Initial Temperature Difference = Ammonia Saturation Temperature - Inlet Air Temperature.

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FI

l-d'\I'V,\PU''"1"TEST

AIR ~ ~ IN~ ~ .-v'\.I'\r"".I'-I=: VELOCITY

TURBINE METER

r---

I I I I I I I I I I I

~ AIR VELOCITY INDICATOR

CV

P 5 I E L P o A T R

AP T L o A R N T

DELUGE

DELUGE WATER FILTER SUPPLY

DELUGE WATER DISTRIBUTION HEADER

AIR ....

~~

CONDENSER

AMMONIA CONDENSATE

TE

WATER DRAIN

cv PILOT PLANT SURGE TANK

FIG.3 Pilot plant auxiliary

NH3 VAPOR I I i I I ~LET WATER DELUGE DIRECTION

! ~. ~ ~- -~-- 76 cm -------...1

1 I

I I

RECTANGULAR HEADER

(15cm)( 20cm; I 2cm WALL THICKNESS) :

I

LpLATE FINS (3.5 FINS/em)

+ NH3 CONDENSATE

OUTLET FIG.4 Hoterv ammonia condenser outline

As illustrated in Figure 5 the plate fins of the condenser feature patterns of slots I and raised turbulators to enhance the air-side heat transfer.

Condenser performance was measured for air face velocities varying from 1 to 4 m/s, lTD's between 5 and 33 K and water deluge changing from 1 to 6.2 m3/s per linear meter of condenser width.

loop for wet / dry testing of ammonia condenser

6 TUBE ROWS IN DIRECTION OF AIR FLOW ~

0.05 cm TURBULATOR

---15.0cm--~-2.5 cm

~~~~~]ocm -1.8 cm 0.0.

PLATE FIN SLOT

FIG. 5 Hoterv condenser plate fin configuratIon

RESULTS AND DISCUSSION

As previously indicated, the shaved-fin Surface is used for an all dry ammonia condensing tower 1 5 while the plate-fin/tube core is designed

5 Possibly augmented with a separate wet tower

for peak-shaving (during high ambient air temperatures)

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such that the surface can be operated in the dry mode, or the heat transfer performance can be enhanced by applying water deluge when high ambient air temperatures occur. A parameter of key interest to the designer is the power consumed by the tower fans employed to draw the air through the heat exchanger core. Hence, in Figure 6, the measured heat rejection capability of the two surfaces is compared as a function of fan power.

90 DELUGE RATE HOTERV (3.1 X 104 m1s, m)

80 HOTERV (2.1 X 10-4 m)s m)

70

r

ri. 0 t-U (t z Q t !:J U D: \L CURTISS-WRIGHT CORE C) z Z .3

0~ z ~

o 10 20 30 40 50 60 70 80 90 100

AIR RELATIVE HUMIDITY

FIG. 10 Deluge enhancement vs. air relative tl.lmidity. 0WET/OORY at equal fan power. Air face velocity ...... 2.03 m/s

Curtiss-Wright surface as a function of air Reynolds N number.

3 ....

E Water deluging the Hoterv ammonia condenser improves its heat transfer performance manyfold.

... 200 - 1 r- -r I f DELUGERATE ::0, ITDRANGE~56-112'C (m3/s ml I The greatest enhancement is achieved at low relaE R H RANGE: 40 -60 or. - 3,.0 It 10-4 tive humidities. The improvement increases with ~ - : . +. higher deluge rates to a point where a further infZ crease in deluge offers little or no improvement.w Q Fortunately the augmentation offered by deluging is lJ

lJ greatest at low lTD's when it is needed most--duringW8 10.0 high ambient air temperatures. 0:: However, it must be emphasized that even W lJ

ACKNOWLEDGEMENTS NOMENCLATURE

The program described in this paper was sponsored by the Electric Power Research Institute (EPR!); Contract No. RP 422-2.

Special recognition is given to Messrs. C. F. Gottzmann, F. Notaro and J. B. Wulf (UCC/Linde) , Mr. G. J. Silvestri (Westinghouse), and Dr. R. W. Zeren and Dr. J. Maulbetsch (EPRI) for their efforts in originating and developing the study of the ammonia phase'-change dry-cooling concept.

The authors gratefully acknowledge the assistance of their many colleagues in various facets of this particular effort; in particular Messrs. M. H. Gallisdorfer, T. D. Craig, R. A. McClellan, and Ms. C. E. McHale. The authors also wish to acknowledge their colleagues at Battelle (PNL) for helpful information exchange during the test program.

REFERENCES

1 McHale, C. E., etal "New Developments In Dry Cooling of Power Plants," paper presented at the 41st American Power Conference, Chicago, Ill, April, 1979.

2 Fricke, H. D., etal "Power Plant Waste Heat Rejection Using Dry Cooling Towers", ucci Linde Interim Report For RP 422-2 EPRI contract, Performance Period: December 1976 through December 1979 - dated December, 1979.

3 Parry, H. L., et al "Augmented Dry Cooling Surface Test Program: Analysis and Experimental Results",Battelle (PNLl Report No. PNL - 2746, September, 1979.

ratio of total air-side heat transfer surfa~e to frontal area of the condenser, dimendonless

Cp specific heat of air, JIkg K f Fanning friction factor, dimensionless

I. D. inside tube diameter, cm

ITD initial temperature difference, K or 0 C

Colburn air-side heat transfer coefficient, dimensionles s

LMTD logarithmic mean temperature difference, K or 0 C

m air mass flowrate per m2 of frontal area, 2kgls m

O. D. outside tube diameter, cm

P fan power-based on frontal area, W/m 2

O/A heat flux, J/m 2 s q heat rejection rate per m2 of fronta 1 areq,

2J/s mOdry heat rejection rate-dry mode, JIs 0wet heat rejection rate-with water deluge, J{s R.H. air relative humidity, %

Tsat saturation temperature, K or 0 C

U overall heat transfer coefficient, J1m2 s K U* overall heat transfer coefficient (wet mope)

J/m2 s K boHin enthalpy di fference of saturated air at t~e

ammonia temperature and the enthalpy of the air at the inlet temperature and humidity:', Jlkg

bop air-side frictional pressure loss throughi condenser, Pa

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1981 Proceedings Volume II.pdf