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Deluge evaporative cooling performance of wavy fin and tube-inclined heat exchangers.

INTRODUCTION

Compact round tube-and-fin heat exchangers (RTHX) are widely used as condensers or coolers in power generation, air conditioning, refrigeration and process cooling applications. Typically, air flows over the tubes and fins on air side of the heat exchanger (HX) and cools the process fluid circulating through the tube side. Since heat transfer in such HXs is constrained on the air side due to high thermal resistance, enhanced fin surfaces are typically utilized to increase fin-tube HX performance (Wang et al. 1999a). Extended surfaces are often incorporated with a series of flow interruptions such as louvers, off-strips and wavy fins that induce turbulent mixing of airflow and prevent growth of thermal boundary layer from leading edge (Chang and Wang 1997; Webb and Jung 1992).

Due to low manufacturing cost, louver or wavy fins are more commonly used to enhance heat transfer in fin-tube HXs. However, at high ambient air temperature, even enhanced fin HX capacities reduce considerably, and higher condenser temperature reduces thermodynamic cycle efficiency by up to 1% for every degree increase in condensing temperature (Leidenfrost and Korenic 1979). In addition, further increase in fin density provides a marginal increase in HX capacity, often at the cost of added air-side pressure drop ([DELTA][P.sub.a]) or fan power.

One way to enhance heat transfer in compact HXs is to utilize evaporative cooling where a thin water film is applied over HX fins. As a result, air-side heat transfer is enhanced through both forced convection on liquid film and latent heat of evaporation at the interface of flowing air and thin water film. Compared to air-cooled condensers or fluid coolers, evaporative cooling could help reduce electric energy consumption of heating, ventilation and air-conditioning (HVAC) systems by 20%-40%, while providing same cooling capacity obtained using water-cooled units (Knebel 1997). Evaporative cooling may be employed either using deluge flow distributors or spray nozzles. Although spray cooling involves lower spray rates and provides a higher area-to-volume ratio for evaporation, deluge cooling is a relatively simple technology. Unlike spray cooling, deluge distributor does not clog due to dirt in wetting water, may require less maintenance, and provides higher enhancement for HXs when narrow fin spacing may not allow spray droplet penetration in HX depth. This forms the objective of work presented in this paper, which experimentally evaluated the performance of compact wavy-fin RTHX used as hybrid cooler. Hybrid coolers/condensers utilize air cooling for a major portion of the year, and during high ambient air temperature conditions, evaporative cooling is employed to enhance capacity.

A number of experimental studies have published performance data of air-cooled RTHX with different fin configurations. Air-side heat transfer and pressure drop correlations have also been presented either in terms of Colburn j and f factors as a function of Reynolds number and HX geometry (Wang et al. 1998; Wang et al. 1999b; Wang et al. 1999c; Wang et al. 1999d). In comparison to condenser performance data of RTHX in dry conditions, published experimental studies in wet conditions are limited in the open literature. Table 1 presents an overview of experimental studies on evaporative cooling of HXs. Although limited in number, published experimental studies have established that evaporative cooling could significantly enhance heat transfer capacity of fin-tube HXs and could enhance power plant efficiency, refrigeration cycle coefficient of performance (COP). and offer substantial energy savings. Currently, lack of availability of wetting water, corrosion issues, and restriction on its use for industrial cooling, limits the widespread application of evaporative cooling technology; however, corrosion issues can be mitigated to a large extent using reverse osmosis (RO) or treated water and selective wetting of the coils.

The magnitude of capacity enhancement obtained using evaporative cooling is directly related to uniform wetting of fin surfaces and evaporation rate. Some of the authors have therefore attributed lower capacity enhancement factors (Hauser et al. 1982; Hauser and Kreid 1982; Kreid et al. 1979a) to complex fin geometries, narrow fin spacing, and HXs with longer depth in the direction of airflow. Ideally, a thin layer of wetting water should be maintained over the cooling surface, but such a layer is often not formed on the entire fin area, i.e., uniformly in depth of HX. This is one of the factors limiting overall heat transfer augmentation when a significant portion of coil remains dry.

In a recent study, Lee et al. (2005) experimentally tested evaporative cooling enhancement on untreated and hydrophilic porous layer coated inclined surfaces, and found that latent heat transfer could be enhanced by approximately 80% for hydrophilic coated surfaces. Ma et al. (2007) tested effect of hydrophilic coating on air-side heat transfer and friction factors on wavy fin and tube HXs under dehumidifying conditions (HX used as evaporator) and found that hydrophilic coatings could reduce air-side pressure drop by up to 44% and improve heat transfer by up to 35%. The authors attributed improved performance to thin film of condensate water on hydrophilic coated fins compared to droplets of condensate on uncoated fins. Kim and Kang (2003) observed similar heat transfer enhancement for hydrophilic coated tubes in the evaporator of absorption chiller due to higher heat transfer area of thin films compared to sessile drops on untreated tubes. Thus, hydrophilic coatings could offer a potential solution for nonuniform wetting of HX fins, but to the best of authors' knowledge, no study on deluge evaporative cooling performance of hydrophilic coated wavy-fins was found in the published literature. Therefore, the objectives of this study are to 1) provide reliable and accurate experimental data for cooler performance of RTHX with herringbone wavy fins in both dry and wet conditions using deluge evaporative cooling, and 2) investigate the effect of wetting-water mass flow rates, airflow rates, fin spacing and hydrophilic coating on HX capacity enhancement factor (CEF [ {[[??].sub.wet]/[[??].sub.dry]} ]) and air-side pressure drop penalty ratio ([PR.sub.[DELTA]Pa]).

EXPERIMENTAL FACILITY OVERVIEW

The experimental setup mainly consists of air side, process-fluid, and wetting water-side loop described in this section. All experiments were conducted within an environmental chamber that simulates and controls desired ambient temperature and relative humidity (RH) conditions at HX inlet. Temperature can be controlled within an uncertainty of [+ or -] 0.5[degrees]C ([+ or -] 0.1[degrees]F) and RH within [+ or -] 2%. A vapor compression system provided cooling or heating while RH was controlled using a proportional-integral-derivative (PID) controlled steam humidifier and silica solid desiccant wheel.

Air-Side Loop

The schematic of an air-side loop in an experimental facility, which is a typical calorimetric wind tunnel consisting of an axial fan, nozzles, air mixer, guide vanes, visualization section, HX, and wetting-water drain, is presented in Figure 1. The variable frequency drive (VFD) controlled the speed of the axial fan, which drives the airflow out of the wind tunnel at the desired test case velocity. Each RTHX was installed in the wind tunnel with a provision of varying angle of inclination of HX with vertical from approximately 0[degrees] to 60[degrees] as shown in Figure 1. RH and temperature of air were recorded at the inlet and outlet of HX using a capacitive RH sensor and a 3 x 3 type T thermocouple grid, respectively. Air mixer, guide vane, and settling means were installed to ensure uniform flow and accurate measurement of RH and temperature measurement at the outlet. The differential pressures across the HX and nozzles were measured using differential pressure transducers in accordance with ASHRAE Standard 41.2 (1987). The differential pressure across nozzles was then used to calculate air velocity and flow rate. Gaps between the HX frame and the walls of the test section were sealed to prevent bypass of airflow.

Process Fluid Loop

A schematic of heat transfer fluid-side loop in an experimental facility is presented in Figure 2. Hot water used as process fluid enters the test HX at 43[degrees]C (109.4[degrees]F) and is cooled by air flowing through wavy fins and tubes in dry case tests. When wetting water was used, air and wetting-water flowed in a cross-flow pattern to cool the process fluid flowing through cooler tubes. The temperature of hot water was measured at the inlet and outlet of test HX using high-precision resistance temperature detectors (RTD). Valve V2 provided water used for priming the centrifugal pump prior to start.

A VFD controlled centrifugal pump flow through heat transfer fluid loop, which consisted of a water filter placed before turbine flow meter. The desired temperature at the test exchanger inlet was maintained using heat supplied by a typical R-22 heat pump cycle using a fixed speed scroll compressor, and auxiliary heater. Therefore, the designed test facility could test HX capacities up to 18 kW (61,418 Btu/h). Heat pump cycle was incorporated with a hot-gas bypass valve between the compressor outlet and evaporator outlet, which enabled reducing the capacity of the test HX without changing the speed of the compressor by allowing certain amount of refrigerant to bypass from the compressor discharge line back to the suction line. A water chiller was utilized to cool overheated water and serve as additional temperature control along with bypass and temperature control valves. Thus, process fluid temperature at the HX inlet could be maintained within 43[degrees]C [+ or -] 0.05[degrees]C (109.4[degrees]F [+ or -] 0.1[degrees]F).

Wetting Water Loop

A schematic of heat transfer fluid and wetting water loop in an experimental facility is presented in Figure 2. Deluge water which is required for testing cooler performance in wet conditions was distributed evenly over the leading edge of RTHX using an overflow-type distributor attached on top of HX as shown in Figure 3.

Tap water supply was treated using a RO system and water quality parameters measured using a photometer before and after treatment are listed in Table 2. Treated RO water was used as both heat transfer fluid within the tubes and as evaporative cooling fluid on the air side. Reduced salt content reduces potential scaling issues and facilitates consistent test conditions. Temperature of wetting water was measured at the inlet and outlet of the test exchanger using high precision RTD. A VFD-controlled gear pump drove flow through wetting water loop, which also consists of a coriolis flow meter. Wetting water can be cooled or kept at a constant inlet temperature using a vapor compression water chiller.

TEST CONDITIONS AND MEASURING INSTRUMENTATION

Geometric specifications of three herringbone wavy-fin HXs tested in this study are presented in Table 3.

Coils and 1 and 2 were geometrically similar, i.e., fin spacing (Fp = 2.4 mm [0.09 in.]) but coil 1 had untreated aluminum fins and coil 2 has a hydrophilic plasma coating on aluminum wavy fins. Coil 3 had a slightly larger Fp = 3 mm (0.118 in.). Therefore, effects of fin spacing and hydrophilic coating on HX capacity and [DELTA][P.sub.a] were studied in both dry and deluge cooled conditions. RTHX set at 20[degrees] angle from vertical placed in test section is shown in Figure 4.

An experimental test matrix and air, process fluid, and wetting water parameters for testing herringbone wavy-fin HXs are summarized in Tables 4 and 5, respectively.

Dry case tests were performed at air velocities varying from 1.5-3.0 m/s (4.9-9.8 ft/s), with process fluid inlet temperature at 43[degrees]C (109.4[degrees]F), air inlet temperature at 28[degrees]C (82.4[degrees]F), and air inlet RH approximately 45%. Wet case tests were conducted under similar conditions with the addition of deluge wetting water flow rates. Deluged cooling water is kept at constant inlet temperature of 28[degrees]C (82.4[degrees]F).

DATA REDUCTION

Experimentally measured air, process-fluid, and wetting water parameters such as flow rate, temperature, and RH were used to calculate HX capacity, energy balances (E), and enhancement factors in both dry and wet conditions.

Dry case air-side capacity, [[??].sub.a, dry], is calculated using Equation 1.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

Both dry and wet case heat transfer fluid-side capacity, [[??].sub.pf], is calculated using Equation 2.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

Equation 3 is used to calculate dry case energy balance error in %, [E.sub.eb, dry],

[E.sub.eb, dry] = 100 x (1 - [Q.sub.pf]/[Q.sub.a, dry]) (3)

Wet case air-side capacity, [[??].sub.a, wet], is calculated using Equation 4.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

Wetting-water capacity, [[??].sub.ww], is calculated using Equation 5.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII](5)

Equation (6) is used to calculate wet case energy balance error in %, [E.sub.eb,we].

[E.sub.eb, wet] = 100 x (1 - [[Q.sub.pf]/[Q.sub.a,wet] + [Q.sub.WW])) (6)

The total uncertainty of measured variables, such as airside pressure drop, was calculated using the sum of systematic error of each measuring instrument, as summarized in Table 6, and random error, which is the standard deviation of measured variable in respective test case.

A data acquisition system was used to record experimental data at a frequency of 1 s. For each test case, steady state was obtained and maintained for at least 15 min., upon which data was recorded for 20 min. If energy balance was found to be more than 5%, data was discarded and the test case repeated.

RESULTS AND DISCUSSION

In this section, experimental heat transfer rate and air-side pressure drop for three wavy-fin HXs working as hybrid evaporative coolers i.e. dry and wet conditions is presented.

Dry Case Results

Dry case results were first obtained for setting a baseline performance. Figure 5 presents the capacity and [DELTA][P.sub.a] for herringbone wavy fin HXs set at 20[degrees] from vertical as a function of air velocity.

Both capacity and [DELTA][P.sub.a] were observed to increase linearly with increase in air velocity. Also, it was observed that compared to uncoated HX, both capacity and [DELTA][P.sub.a] were reduced by up to 8% for HX with hydrophilic coating. Since the difference is almost within measurement uncertainty range for most cases it is difficult to conclude if hydrophilic coating is affecting capacity and [DELTA][P.sub.a], which is in line with previous dry case studies published in literature (Hong and Webb 1999; Liu 2011). On the other hand, increasing fin spacing from Fp = 2.4 mm (0.09 in.) to Fp = 3 mm (0.118 in.) reduces dry case HX capacity by up to 21%. However, this also corresponds to approximately 21.7% fin area reduction and approximately 22.8% fin material reduction.

Wet Case Results

Deluge evaporative cooling tests were conducted with wetting water and air flowing in cross-flow configuration through the HX. CEF and [PR.sub.[DELTA][P.sub.a]] of wavy-fin HXs using deluge evaporative cooling are presented in Figure 6. As a result of evaporative cooling on HX fin surface, capacities were significantly enhanced for all HXs. However, the highest enhancements were obtained at higher deluge flow rates, which cause water bridging between fin-adjacent surfaces and results in higher values of [PR.sub.[DELTA][P.sub.a]]. Although [PR.sub.[DELTA][P.sub.a]] was close to 1 for low-deluge flow rates (0.015 kg/s [0.03 lb/s]), it also corresponds to lowest CEF values.

Effect of Hydrophilic Coating. Hydrophilic coated coil achieves CEF and [PR.sub.[DELTA][P.sub.a]] of 1.32 to 2.78 and 1.07 to 2.28, respectively. and untreated coil with same fin spacing achieves lower CEF and PR^ values from 1.13 to 2.28 and 1.0 to 2.0, respectively. Therefore, compared to untreated coil, hydrophilic coated coil CEF is approximately 21.7% higher when comparing to the dry-case baseline performances of respective HXs, which may be due to improved surface wetting. This is also evident from Figure 7, which shows the contribution of deluge water evaporation to overall HX capacity. The trends for ratio of [Q.sub.evap]/[Q.sub.total] were in line with the CEFs in Figure 6, and so heat transfer enhancement is attributed to enhanced evaporation rates.

To compare the effect of hydrophilic coating, percentage enhancements in HX capacity and [DELTA][P.sub.a] relative to each dry and wet case of uncoated HX with same Fp = 2.4 mm (0.09 in.) are plotted in Figure 8.

It is interesting to note that compared to the uncoated coil with same Fp, heat transfer was enhanced by 5%-30% when deluge cooling was applied to hydrophilic coated coils but [DELTA][P.sub.a] was not enhanced in most cases.

Although the highest CEFs were obtained for coated coil at [m.sub.ww] = 0.166 kg/s (0.36 lb/s) (Figure 6), it is also observed that, relative to uncoated HX, highest capacity enhancement was obtained at [m.sub.ww] = 0.08 kg/s (0.17 lb/s) as shown in Figure 8a. This reduction in enhancement may be due to film blowout, which was observed for coil 2 at [V.sub.a] [greater than or equal to] 2 m/s (6.5 ft/s).

Figure 9 shows HX capacities as a function of [DELTA][P.sub.a] for wavy-fin HXs under dry and deluge cooling. It can be noted that at constant [DELTA][P.sub.a] values, hydrophilic coated HX achieved higher capacity compared to uncoated coil at approximately half the deluge flow rates for the same fin spacing, thereby offering substantial potential for wetting water savings.

Effect of Fin Spacing. CEF and [PR.sub.[DELTA][p.sub.a]] of 1.22 to 2.0 and 1.06 to 1.59, respectively, were obtained for HX with Fp = 3.0 mm (0.118 in.) as shown in Figure 6. A HX with higher Fp achieved the lowest contribution of deluge water evaporation to overall heat transfer rates compared to other test coils, as observed from Figure 7. To compare the effect of fin spacing, percentage enhancements in HX capacity and [DELTA][P.sub.a] relative to each dry and wet case of uncoated HX with same Fp = 2.4 mm (0.09 in.) are plotted in Figure 10.

It was found that compared to coil 3, heat transfer was enhanced by 2%-30% and [DELTA][P.sub.a] increased by 33%-58% when deluge cooling was applied to coil 1. The highest enhancement in both capacity and [DELTA][P.sub.a] was obtained at the highest deluge flow rate of [m.sub.ww] = 0.166 kg/s (0.36 lb/s). At lower flow rates [m.sub.ww] = 0.015 kg/s (0.03 lb/s) and 0.08 kg/s (0.17 lb/s) no significant difference in heat transfer rate was obtained for coil 1 and 3. However [DELTA][P.sub.a] increased considerably for deluge flow rates higher than 0.015 kg/s (0.03 lb/s) due to deluge water film blocking the fin space. At the highest deluge flow rate tested, deluge water was observed to completely cover the frontal face of HX which lead to a higher wet case [DELTA][P.sub.a] when compared to the dry case measurements at same air velocity. Although [DELTA][P.sub.a] values were higher for coil 1, in both dry and deluge cooling, it may be recommended to use smaller fin spacings due to the fact that in many cases such HXs may be utilized as hybrid fluid coolers or condensers. Therefore, a compact fin spacing helps obtain a higher heat transfer rate for a given HX volume for the major portion for the year in both dry operation and deluge evaporative cooling mode.

On the other hand if [DELTA][P.sub.a] or fan energy consumption is a critical parameter, a higher fin spacing would be beneficial, which would save 22.8% fin material at the cost of up to 13% lower heat transfer rate for moderate deluge rates up to 0.08 kg/s (0.17 lb/s). Furthermore, deluge cooling may even be utilized throughout the year, which would further reduce the HX volume required and lead to fin material savings. In such conditions a higher fin spacing may be desirable if higher [DELTA][P.sub.a] or fan power consumption is undesirable. So a trade off exists between fan power consumption, deluge flow rate, and HX capacity, which is typical for hybrid evaporatively cooled HXs.

CONCLUSIONS

An experimental study was conducted to evaluate the performance of three cross-flow herringbone wavy-fin HXs working as hybrid evaporative coolers set at 20[degrees] from vertical in both dry and wet conditions using deluge cooling. The effect of fin spacing and hydrophilic coating, on deluge cooling HX performance under varying air velocities and wetting water flow rates is presented in terms of capacity enhancement factor and air-side pressure drop penalty ratios. It was found that in dry operation, hydrophilic coating of coils reduces dry case HX capacity by 4%-8% and increasing fin spacing reduces capacity by up to 21%.

Capacity enhancements due to deluge cooling were accompanied by significant increase in air-side pressure drops with maximum capacity enhancement factor (CEF) of 2.78 obtained for hydrophilic coated HX at [PR.sub.[DELTA][p.sub.a]] of up to 2.28. Furthermore, at a given [DELTA][P.sub.a] values, hydrophilic coated HX achieved higher capacity compared to uncoated coil at approximately half the deluge flow rates for the same fin spacing, thereby offering substantial potential for wetting water savings. Also, it was found that compared to the coil with Fp 3.0 mm (0.118 in.), heat transfer was enhanced by 2%-30% when deluge cooling was applied to HX with Fp 2.4 mm (0.09 in.) and [DELTA][P.sub.a] increased by 33%-58%. Future work is underway to develop evaporative cooling techniques that could offer similar or higher CEF obtain using deluge cooling but with [PR.sub.[DELTA][P.sub.a]] =1.

ACKNOWLEDGEMENTS

We gratefully acknowledge the support of this effort from the sponsors of the Alternative Cooling Technologies and Applications Consortium and the Center for Environmental Energy Engineering (CEEE) at the University of Maryland, and Guntner AG.

NOMENCLATURE

Parameters Cp = specific heat, kJ/kg.[degrees]C (Btu/lb-[degrees]F) FPI = fins per inch, -- Fp = fin spacing (mm [in.]) h = enthalpy (kJ/kg [BTU/lb]) [??] = mass flow rate, kg/s (lb/s) [PR.sub.[DELTA][P.sub.a]] = air-side pressure drop penalty ratio ([P.sub.wet]/[P.sub.dry]) [??] = heat transfer rate [rho] = density, kg/[m.sup.3] (lb/gal) T = temperature [degrees]C ([degrees]F) [??] = volume flow rate, [m.sup.3]/s (gal/s) v = velocity (m/s [ft/s]) [omega] = humidity ratio [kg.sub.w]/[kg.sub.a] ([lb.sub.w]/[lb.sub.a])

Subscripts a = air eb = energy balance dry = dry case experiment evap = evaporation in = inlet stream out = outlet stream P = pressure pf = process fluid wet = wet/deluge case experiments ww = wetting water

REFERENCES

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Hasan, A., and K. Siren. 2003. Performance investigation of plain and finned tube evaporatively cooled heat exchangers. Applied Thermal Engineering 23:325-40.

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Hauser, S.G. and D.K. Kreid. 1982. Dry-wet performance of a plate-fin air-cooled heat exchanger with continuous corrugated fins. Richland, WA: US Environmental Protection Agency Report.

Heyns, J.A., and D.G. Kroger. 2010. Experimental investigation into the thermal-flow performance characteristics of an evaporative cooler. Applied Thermal Engineering 30:492-98.

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Lee, D.Y., J.W. Lee, and B.H. Kang. 2005. Experimental study on the hydrophilic porous film coating for evaporative cooling enhancement. Journal of Air-Conditioning and Refrigeration 13:99-106.

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Liu, L. 2011. Effect of air-side surface wettability on the performance of dehumidifying heat exchangers. PhD Thesis, University of Illinois at Urbana-Champaign, IL.

Ma, X., G. Ding, Y. Zhang, and K. Wang. 2007. Effects of hydrophilic coating on air side heat transfer and friction characteristics of wavy fin and tube heat exchangers under dehumidifying conditions. Energy Conversion and Management 48:2525-32.

Popli, S., Y. Hwang, and R. Radermacher. 2012a. Enhancement of Round Tube Heat Exchanger Performance Using Deluge Water Cooling- Paper #2331. 14th International Refrigeration and Air Conditioning Conference, West Lafayette, IN, USA, July 16-19.

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Sahil Popli

Student Member ASHRAE

Yunho Hwang, PhD

Member ASHRAE

Reinhard Radermacher, PhD

Fellow ASHRAE

Sahil Popli is a graduate research assistant at the Center for Environmental Energy Engineering (CEEE) and a PhD candidate in the Department of Mechanical Engineering, University of Maryland, College Park, MD. Yunho Hwang is an associate director at the CEEE, and a research professor in the Department of Mechanical Engineering, University of Maryland. Reinhard Radermacher is director at the CEEE and a professor in the Department of Mechanical Engineering, University of Maryland.

Table 1. Overview of Experimental Studies on
Evaporative Cooling of HXs

Author                        Fin Geometry          Deluge Rate1,
                                                    g/s/[m.sup.3]
                                                 (lb/h/[in..sup.3])


Yang and Clark (1975)         Plain-finned,            151-328
                                louvered,            (33.9-73.7)
                             and perforated

Kreid et al.                    Slit fin
(1979a)                     (9 fins per inch
                                 [FPI])

Kreid et al. (1979b)            Slit fin              1771-2360
                                                     (397.8-530)

Hauser et al.                 Spiral, wavy,           993-2,482
(1982)                        and slit fin           (223-557.5)

Hauser and Kreid (1982)        3 row wavy             1657-2210
                                                    (372.2-496.5)

Hasan and Siren (2003)     Bare and plate fin        8741-19,424

                                                    (1963.6-4363)

Hosoz and                                                --
Kilicarslan (2004)

Vrachopoulos et                   RTHX
al. (2007)

Heyns and                     Bare tube HX           4500-11,750

Kroger (2010)                                       (1011-2639.5)

Bell et al. (2011)             Round tube
                               louver fin

Popli et al. (2012a)        Slit-fin (16 FPI)       1600 (359.4)

Popli et al. (2012b)        Flat-tube louver         5039-13,600
                             fin HX (22 FPI)         (1132-3055)

Author                     Major Findings

Yang and Clark (1975)      * Spray cooling did not affect pressure
                           drop and heat transfer coefficient (HTC)
                           was enhanced by approximately 40%-45% for
                           air [Re.sub.a] of 1000 and 500,
                           respectively

                           * Maximum overall HTC enhancement of 1.5

                           * Lower enhancement in condenser capacity
                           at higher [Re.sub.a] = 1000 to break up of
                           liquid film on fin surface

                           * Spraying water and ethylene glycol
                           provided similar improvement

Kreid et al.               * CEF of 9.8 at inlet temperature
(1979a)                    difference (ITD) = 5[degrees]C
                           (9[degrees]F); lowest CEF of 1.4 at ITD =
                           14[degrees]C (25.2[degrees]F)

                           * Heat and mass transfer analogy based
                           model to predict deluged finned HX capacity

Kreid et al. (1979b)       * CEF between 2 and 7, at inlet air RH
                           between 10%-70%

                           * Film blown out at [V.sub.a] > 1.83 m/s (6
                           ft/s), and > 620 g/s x m (139.2 lb/h/
                           [in..sup.3]) of HX

                           * No change in capacity when changing HX
                           angle from 0[degrees]-16[degrees]

                           * Addition of surfactants increased heat
                           transfer by 20%-30% and [DELTA] [P.sub.a]
                           by 100% due to bubble formation

                           * Deluge cooling provides rivulet flow not
                           thin film on fins

                           * HX performance increases at higher deluge
                           flow rate, and lower ITD between inlet air
                           and process fluid

Hauser et al.              * Spiral wound HX capacity 40% higher and
(1982)                     [DELTA] [P.sub.a] higher by 50% and 100%
                           compared to split and wavy coils

                           * Wavy fin best overall hybrid performance

                           * Spiral fin HX capacity reduced by 20%
                           when HX angle increased from 15[degrees]-
                           20[degrees]

                           * Film blown out at [V.sub.a] > 1.37 m/s
                           (4.5 ft/s)

Hauser and Kreid (1982)    * CEF of 2 at ITD 27.7[degrees]C
                           (50[degrees]F), 75% rh; CEF of 5 at ITD
                           11.1[degrees]C (20[degrees]F), 25% rh

                           * Film blown out at [V.sub.a] > 1.8-2.4 m/s
                           (5.9-7.8 ft/s)

Hasan and Siren (2003)     * Increase in finned HX capacity 92%-140%
                           higher than bare tube HX

                           * No water bridging observed

Hosoz and                  * Capacity and COP of evaporatively cooled
Kilicarslan (2004)         unit

                           * 31% and 14.3% higher than air-cooled unit

                           * 2.9%-14.4% and 1.5%-10.2% lower than
                           water cooled unit

Vrachopoulos et            * COP enhancement and energy savings of
al. (2007)                 211% and 58% respectively for spray-cooled
                           refrigeration system

Heyns and                  * Film HTC is a function of air mass
                           velocity, deluge water flow rate, and
                           deluge water temperature

Kroger (2010)              * Air mass transfer coefficient (MTC) and
                           [DELTA] [P.sub.a] is a function of air
                           velocity and deluge water flow rate

Bell et al. (2011)         * 0.6 kg (1.32 lb) particulate fouling
                           increases [DELTA] [P.sub.a] by 50%, thermal
                           performance not affected in dry cooling

                           * Wet conditions fouling has no impact on
                           HX capacity or [DELTA] [P.sub.a]

Popli et al. (2012a)       * CEF from 2.6-2.8 and PR[DELTA] [P.sub.a]
                           from 2.3-2.4, respectively

                           * Increasing inclination angle of HX from
                           0[degrees] to 20[degrees] increases
                           capacity by 8.6%, and [DELTA] [P.sub.a] is
                           unaffected

                           * Film blown out at [V.sub.a] > 2.5 m/s
                           (8.2 ft/s)

Popli et al. (2012b)       * CEF from 2.7 to 3.5 and [PR.sub.[DELTA]
                           [P.sub.a]] from 7-14

                           * Increasing inclination angle of HX from
                           0[degrees] to 20[degrees] increases
                           capacity from 23%-47%, and [DELTA]
                           [P.sub.a] by 100%

Note: (1) Deluge flow rate in g/s/[m.sup.3]
(lb/h/[in..sup.3]) of HX volume.

Table 2. Water Quality Parameters before and after RO Treatment

Parameter                    RO Treated      Tap Water
                                Water          Supply

Alkalinity (CaC[O.sub.3]    20 (1.67e-4)    40 (3.3e-4)
  mg/l [lb/gal])
Hardness (CaC[O.sub.3]        25 (2e-4)      60 (5e-4)
  mg/l [lb/gal]))

Table 3. Geometric Specifications of Herringbone Wavy-Fin HX

Parameter                                 Unit           Coil 1

Hydrophilic coating                        --              No
Number of tube banks                       --               6
Number of tubes per bank                   --              10
Tube outer diameter                     mm (in.)       12.7 (0.5)
Horizontal spacing                      mm (in.)       24.9 (0.98)
Vertical spacing                        mm (in.)       50.0 (1.96)
(Fp) Fin pitch (mm)                     mm (in.)       2.4 (0.09)
([DELTA]f) Fin thickness (mm)           mm (in.)      0.14 (0.005)
HX length                               mm (in.)       492 (19.37)
HX depth                                mm (in.)       150 (5.90)
HX height                               mm (in.)       500 (19.68)
Fin wave length                         mm (in.)        3 (0.118)
Fin wave height                         mm (in.)       1 (0.0393)
Total air-side heat transfer area       [m.sup.2]     29.9 (332.2)
                                      ([ft.sup.2])

Parameter                                Coil 2          Coil 3

Hydrophilic coating                        Yes             No
Number of tube banks                        6               6
Number of tubes per bank                   10              10
Tube outer diameter                    12.7 (0.5)      12.7 (0.5)
Horizontal spacing                     24.9 (0.98)     24.9 (0.98)
Vertical spacing                       50.0 (1.96)     50.0 (1.96)
(Fp) Fin pitch (mm)                    2.4 (0.09)      3.0 (0.09)
([DELTA]f) Fin thickness (mm)         0.14 (0.005)    0.14 (0.005)
HX length                              492 (19.37)     492 (19.37)
HX depth                               150 (5.90)      150 (5.90)
HX height                              500 (19.68)     500 (19.68)
Fin wave length                         3 (0.118)       3 (0.118)
Fin wave height                        1 (0.0393)      1 (0.0393)
Total air-side heat transfer area     29.9 (332.2)    23.4 (251.7)

Table 4. Experimental Test Matrix for Testing RTHX

Case          HX Frontal Air          Wetting Water
           Velocity, m/s (ft/s)        Flow Rates
                                       kg/s (lb/s)

Dry         1.5, 2.0, 2.5, 3.0
           (4.9, 6.5, 8.2, 9.8)

Deluge      1.5, 2.0, 2.5, 3.0     0.015, 0.08, 0.166
cooling    (4.9, 6.5, 8.2, 9.8)    (0.03, 0.17, 0.36)

Case                 Test HX               Number
                                          of Cases

Dry          Fp = 2.4, 3.0 (uncoated)        12
           Fp = 2.4 hydrophilic coated

Deluge       Fp = 2.4, 3.0 (uncoated)        34 (1)
cooling    Fp = 2.4 hydrophilic coated

(1) No experimental data could be recorded at [V.sub.a] = 3.0 m/s
(9.8 ft/s) in deluge conditions due to excessive wetting water
blow out from test HX.

Table 5. Air, Refrigerant, and Wetting Water Parameters
for Testing of Herringbone Wavy-Fin RTHX

Parameter               Average Value           Unit

Air Parameters

Inlet temperature        28.0 (82.4)         [degrees]C
                                            ([degrees]F)

Inlet RH                     45.0                 %

Refrigerant Parameters

Inlet temperature        43.0 (109.4)        [degrees]C
                                            ([degrees]F)

Mass flow rate           0.35 (0.77)         kg/s (lb/s)

Wetting Water Parameters

Inlet temperature        28.0 (82.4)         [degrees]C
                                            ([degrees]F)

Mass flow rate       0.015, 0.080, 0.166     kg/s (lb/s)
(deluge cooling)      (0.03, 0.17, 0.36)

Table 6. Systematic Errors of Measuring Instruments

Instrument          Type       Range               Systematic
                                                   Uncertainty

                                  Air-Side

Thermocouple       Type T      -250[degrees]C-     [+ or -]
                               350[degrees]C       0.3[degrees]C
                               (-418[degrees]F-    ([+ or -]
                               662[degrees]F)      0.54[degrees]F)

Relative         Capacitive    0% rh-90% rh        [+ or -] 2% rh
humidity
sensor

[DELTA]            Strain      0 Pa-1245.8 Pa      [+ or -] 1.0% full
pressure                       (0 in. water-       scale
transducer                     5 in. water)
(nozzle)

[DELTA]            Strain      0 Pa-249.1 Pa       [+ or -] 1.0% full
pressure                       (0 in. water-       scale
transducer                     1 in. water)
(HX)

                       Hot-Water and Wetting Water Side

RTD                PT 100      -100[degrees]C-     [+ or -]
                               400[degrees]C       0.06[degrees]C
                               (-148[degrees]F-    ([+ or -]
                               752[degrees]F)      0.11[degrees]F)

Wetting water     Coriolis     0 kg/s-0.2 kg/s     [+ or -] 0.1%
flow meter                     (0 lb/s-            reading
                               0.44 lb/s)

Hot-water          Turbine     0.157 kg/s-         [+ or -] 0.02%
flow meter                     1.84 kg/s           full scale
                               (0.34 lb/
                               s-4 lb/s)
COPYRIGHT 2014 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. (ASHRAE)
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2014 Gale, Cengage Learning. All rights reserved.

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Author:Popli, Sahil; Hwang, Yunho; Radermacher, Reinhard
Publication:ASHRAE Transactions
Article Type:Report
Date:Jul 1, 2014
Words:6124
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