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Effect of salt spray corrosion on air-side performance of finned-tube heat exchanger with hydrophilic coating under dehumidifying conditions.

INTRODUCTION

Finned-tube heat exchangers are widely applied as evaporators of air conditioners. The aluminum fins are usually coated with hydrophilic materials in order to promote the hydrophilicity of fins and the air-side performance of the finned-tube evaporators under dehumidifying conditions (Hong 1996; Wang and Chang 1997; Ma et al. 2009). The employment of hydrophilic coating can effectively reduce the contact angle of the condensate water and improve the condensate drainage so that the higher heat transfer coefficients and the lower pressure drops can be achieved. However, the hydrophilic coating on fins may be destroyed by salt spray corrosion (SSC) (Yang 2003; Hao et al. 2007; Bao et al. 2008), resulting in the change of the heat transfer and pressure drop performance. Salt spray corrosion is a corrosion caused by the deposition of a certain amount of [Cl.sup.-]on fin surfaces (Ahn and Lee 2005), and it often happens in high salt concentration districts, e.g., coastal areas. Therefore, it is necessary to pay attention to the effects of SSC on the air-side performance of finned-tube heat exchangers with hydrophilic coating, including the effects on hydrophilicity, air-side heat transfer, and pressure drop performance.

A lot of hydrophilicity-related research focuses on the long-term hydrophilicity of hydrophilic-coated fins (Shin and Ha 2002; Min et al. 2000; Min and Webb 2002; Kim et al. 2002). It is found that the hydrophilicity of hydrophilic-coated fins generally changes with service time. Both the advancing and the receding dynamic contact angles obviously increase with the increase of wet/dry cycles, indicating that the hydrophilicity of fins is degraded with the increase of wet/dry cycles (Min et al. 2000; Min and Webb 2002). The reason for the degradation of the hydrophilicity may be that the hydrophilic coating is partially dissolved by the condensate water. However, the hydrophilicity of plasma-hydrophilic-coated fins does not change with service time, obviously. Kim et al. (2002) did experiments on the long-term hydrophilicity for the finned-tube heat exchangers with plasma-hydrophilic coating, and the experimental results showed that the air-side pressure drops did not change with the increase of wet/dry cycles.

The impact of hydrophilic coating on the air-side heat transfer of finned-tube heat exchangers has been researched, and it is found that the impact at dry conditions is different from that at wet conditions (Wang et al. 2002; Hong and Webb 1999, 2000). In dry conditions, only sensible heat transfer occurs and the sensible heat transfer coefficient is hardly affected by hydrophilic coating, so the effect of hydrophilic coating on heat transfer is negligible (Wang et al. 2002). In wet conditions, latent heat transfer and sensible heat transfer occurs simultaneously. The latent heat transfer coefficient could be obviously enhanced under wet conditions by hydrophilic coating (Wang et al. 2002; Hong and Webb 1999; Hong and Webb 2000), while existing research on the effect of the hydrophilic coating on the sensible heat transfer coefficients under wet conditions could not reach a consistent conclusion. Wang et al. (2002) found that the sensible heat transfer coefficients degrade as the effect of hydrophilic coating, and the degradation of the sensible heat transfer coefficients may be up to 20%. However, the experiments conducted by Hong and Webb (1999, 2000) indicated that the hydrophilic coating has no influence on the sensible heat transfer coefficients.

The impact of hydrophilic coating on the air-side pressure drop of finned-tube heat exchangers has been researched, and it is found that the impact is related to the working conditions (Wang et al. 2002; Hong and Webb 1999, 2000). In dry conditions, the effect of hydrophilic coating on the air-side pressure drop can be negligible (Wang et al. 2002). In wet conditions, the effect is obvious and related to the inlet air humidity. The larger the inlet humidity, the greater the impact on the pressure drop. Compared with the finned-tube heat exchanger without hydrophilic coating, the air-side pressure drop of those with hydrophilic coating degrades by 15%-40% under wet conditions (Wang et al. 2002; Ma et al. 2007).

About the effects of SSC on the air-side performance of heat exchangers, the existing research mainly focuses on the anticorrosion of aluminum alloy fins (Birol et al. 2002), the anticorrosion of the anti-corrosive layer of aluminum fins (Lifka and Vandenburgh 1995), and the evaluation method of corrosion degree of vacuum brazed aluminum heat exchangers (Scott et al. 1991). However, there is no publication about the effect of SSC on the hydrophilicity, the air-side heat transfer, and pressure drop performance of finned-tube heat exchangers with hydrophilic coating.

The purpose of this study is to investigate the effect of SSC on the hydrophilicity, the air-side heat transfer, and pressure drop performance of finned-tube heat exchangers with hydrophilic coating. For this purpose, experiments are done on heat exchangers with different corrosion degrees as well as on those without corrosion, and the results are compared.

EXPERIMENTAL PROCESS

Heat Exchanger Geometry

Four finned-tube heat exchangers made of aluminum herringbone wavy fin and copper tube were used in the experiments, and the fins were coated with hydrophilic coatings. The employment of hydrophilic coating can effectively reduce the contact angle of the condensate water and improve the condensate drainage so that the higher heat transfer coefficients and the lower pressure drops can be achieved. In the present study, each fin surface is coated with an anticorrosive layer and a hydrophilic layer, as shown in Figure 1. The hydrophilic-coating material is a kind of organic resin. The coating process includes degreasing, acid washing, and drying and coating, as done by Ma et al. (2007). After the coating process, the water contact angle of the fin is 18[degrees]. Three heat exchangers were corroded with salt spray, and the remaining one remained uncorroded. The geometric details of the tested heat exchangers are shown in Figure 2, and the geometric dimensions of the tested heat exchangers are shown in Table 1.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]
Table 1. Geometric Dimensions of the Tested Finned-Tube Heat
Exchangers

N  [D.sub.o],  [D.sub.c],  [P.sub.t],  [P.sub.l],  [F.sub.P],
    mm (in.)    mm (in.)    mm (in.)    mm (in.)    mm (in.)

2     9.52        9.74        25.4        22.0         1.8
    (0.375)      (0.383)     (1.0)      (0.866)      (0.071)

N  [[delta].sub.f],  [theta],
       mm (in.)      mm (in).

2       0.115         23.0
       (0.005)       (0.906)


Salt Spray Corrosion Test

An artificial accelerated method of SSC (Howard et al. 1999; Hao et al. 2007; Bao et al. 2008) on the finned-tube heat exchangers with hydrophilic coating was used for simulating the corrosion process of the actual corroded heat exchangers. The SSC experiments were performed based on ISO 9227 (2006). The schematic of the salt-sprayed cabinet is illustrated in Figure 3. Three finned-tube heat exchangers with hydrophilic coating were salt sprayed by a neutral 5 wt.% NaCl solution at 35[degrees]C (95[degrees]F) for 100, 200, and 400 hours, respectively, and the solution was in the pH range of 6.5 to 7.2. The deposition rate of sprayed solution maintained 0.1~0.15 L*[m.sup.-2]*[h.sup.-1] (0.009~0.014 L*[ft.sup.-2]*[h.sup.-1]), and the inclined angle between specimens and horizontal plane was 70[+ or -]1[degrees]. After the exposure, the specimens were washed with clean running water to remove any salt deposits from their surfaces, and were then dried.

[FIGURE 3 OMITTED]

In the salt-sprayed process, an extra thin salt-containing moisture film forms on the surface of the fins due to salt deposition, and it adsorbs and dissolves [O.sub.2], [H.sub.2]O, and [CO.sub.2] in the air (Yang 2003). These dissolved atoms and molecules are active and can diffuse easily to the surface of the fins and react with aluminum. [Al.sub.2][O.sub.3]*[3H.sub.2]O is produced and causes the damage of the fins after the salt corrosion. The actual corrosion hours can generally be obtained with the actual corrosion rate, the experimental corrosion hours, and the experimental corrosion rate (Kolotyrkin 1985), as shown in Equation 1.

[t.sub.act] = [t.sub.exp] * [[u.sub.exp]/[u.sub.act]] (1)

where [t.sub.act] is the actual corrosion hours, [t.sub.exp] is the experimental corrosion hours, [u.sub.act] is the actual corrosion rate, and [u.sub.exp] is the experimental corrosion rate.

The corrosion rate of the aluminum-fin samples in the salty environment shows the corrosion depth per year. The mean corrosion rate of the three fin samples evaluated with linear polarization method (Rocchini 1993) is 9.25 [micro]m/year. The actual corrosion rate is related to the locations. If the actual corrosion rate of a city is known, the actual corrosion time in the city corresponding to the accelerated corrosion time in the laboratory can be obtained by using Equation 1. For example, the actual corrosion rates of aluminum are 0.30 [micro]m/year (12 x [10.sup.-6] in./year) in Qingdao, China, and 0.23 [micro]m/year (9 x [10.sup.-6] in./year) in Guangzhou, China, so 400 corrosion hours in the present experiments represent almost 2640 hours and 3443 hours of the aluminum fin with hydrophilic coating being corroded in Qingdao and Guangzhou, respectively.

In order to clearly show the corrosion degree of the fins of the corroded heat exchangers, three aluminum-fin samples with 30 x 50 mm (1.18 x 1.97 in.) area were corroded for 100, 200, and 400 hours, respectively, in the same environment where the finned-tube heat exchangers with hydrophilic coating were corroded. The photos of uncorroded and corroded fin samples are shown in Figure 4. The surface appearances of the uncorroded fin and the corroded fins were observed with naked eyes. Numerous corrosion pits were observed on the corroded fin surfaces. With the increase of corrosion hours, the amount of the pits increases gradually.

[FIGURE 4 OMITTED]

Measurement of Contact Angle

The static contact angle (SCA), the advancing dynamic contact angle (DCA), and the receding dynamic contact angle (DCA) of fins were measured using sessile drop technique with a contact angle goniometer (Min et al. 2000). For measuring the SCA, distilled water droplets were dripped on the fin surface, and the SCA was measured within 1 min after the droplet stabilized on the fin, as shown in Figure 5a. For measuring the DCA, distilled water droplets were dripped on the fin surface, the water drop volume enlarged gradually with injection by a micro-injector, and the advancing DCA was measured until the contact line of liquid/gas interface moved, as shown in Figure 5b. Likewise, the water drop volume reduced gradually with extraction by a micro-injector, and the receding DCA was measured until the contact line of liquid/gas interface moved, as shown in Figure 5c. Five locations selected at random on the surface of each fin sample were used to measure the SCA, the advancing DCA, and the receding DCA. The average of five measured values of a fin sample was used as the representative contact angle of the fin sample.

[FIGURE 5 OMITTED]

Measurement of Heat Transfer and Pressure Drop Performance

In order to investigate the effect of SSC on the air-side performance of finned-tube heat exchangers with hydrophilic coating, the air-side heat transfer and pressure drop of the corroded and the uncorroded heat exchangers were measured.

The experimental apparatus is schematically illustrated in Figure 6, which includes a closed-loop wind tunnel, a water flow loop, a data acquisition system, and the tested heat exchanger. Air and cold water were used as working fluids in the closed-loop wind tunnel and the water flow loop, respectively.

[FIGURE 6 OMITTED]

The closed-loop wind tunnel was used to conduct the airflow through the tested heat exchanger. The wind tunnel was made of galvanized steel sheet and had a 210 x 210 mm (8.27 x 8.27 in.) cross section in the test section. A variable-speed centrifugal fan (0.75 kW [2559.1 Btu/h]) was used to circulate the air, which passed through the nozzle chamber, the air-conditioner box, the mixing device, the straightener, and the tested heat exchanger in order. The airflow rate was detected by multiple nozzles based on ASHRAE Standard 41.2 (ASHRAE 1987). A differential pressure transducer with [+ or -]5.0 Pa (0.73 x [10.sup.-3] psi) precision was used to measure the pressure difference across the nozzles. A pressure transducer with [+ or -]1.0 kPa (0.145 psi) precision and a dry-bulb and wet-bulb temperature transducer with [+ or -]0.3[degrees]C ([+ or -]0.54[degrees]F) precision were used to measure the inlet air conditions of the nozzles. The air-conditioner box was used to control the temperature and humidity of inlet air, which allowed [+ or -]0.2[degrees]C ([+ or -]0.36[degrees]F) and [+ or -]3% fluctuation range. The pressure difference of air across the tested heat exchangers was measured by a pressure difference transducer with a [+ or -]0.2 Pa ([+ or -]2.9 x [10.sup.-5] psi) precision. The dry-bulb temperature and relative humidity of the inlet air were measured by two temperature-and-humidity transducers with [+ or -]0.1[degrees]C ([+ or -]0.18[degrees]F) and [+ or -]1.4% precision, respectively. Eight temperature-and-humidity transducers with [+ or -]0.1[degrees]C ([+ or -]0.18[degrees]F) and [+ or -]1.4% precision were distributed evenly at the downstream of the test section for measuring the dry-bulb temperature and relative humidity of the outlet air, respectively. Condensation phenomena on the fin surface were recorded by a CCD camera, which was located at the outlet of airflow. The existence of the camera changed the pressure loss of the airflow through the heat exchanger by less than 1.1%, so the effects of the camera can be omitted (Ma et al. 2007).

The water flow loop consisted of a thermostat, a centrifugal pump, and a magnetic flowmeter with [+ or -]0.15 L/min precision. The purpose of this loop was to provide the cooling capacity for the tested heat exchangers. After the water reached the required temperature, it was pumped out of the thermostat, delivered to the tested heat exchanger, and then returned to the thermostat. The water temperature difference between the inlet and outlet of tested heat exchangers was measured by two K type thermocouples with a calibrated accuracy of [+ or -]0.1[degrees]C ([+ or -]0.18[degrees]F).

The inlet and outlet relative humidity of air, the inlet and outlet temperatures of air and water, and flow rates of air and water were recorded when the experimental system was stable.

Experimental uncertainties were tabulated in Table 2 according to the analysis method proposed by Moffat (1988). Experiments for testing the air-side heat transfer and pressure drop performance on three corroded heat exchangers with hydrophilic coating and one uncorroded heat exchanger with hydrophilic coating were performed under dehumidifying conditions. The experimental conditions were as follows:
Table 2. Summary of Estimated Uncertainties

                         Uncertainty
   Parameter
                    Min. (%)      Max. (%)

   [m.sub.a]      [+ or -]0.9   [+ or -]1.7
   [m.sub.w]      [+ or -]1.7   [+ or -]2.3
   [Q.sub.a]      [+ or -]1.6   [+ or -]3.0

                         Uncertainty
   Parameter
                    Min. (%)      Max. (%)

   [Q.sub.w]      [+ or -]2.5   [+ or -]4.2
[[DELTA].sub.p]   [+ or -]0.4   [+ or -]3.3
   [h.sub.o]      [+ or -]5.2   [+ or -]6.7


1. Air inlet dry-bulb temperature: 27[degrees]C [+ or -] 0.2[degrees]C (80.6[degrees]F [+ or -]0.36[degrees]F)

2. Air inlet relative humidity: 50% [+ or -] 0.2%

3. Air velocity: 0.5 (5905.5), 1.0 (11811.0), 1.5 (17716.5), 2.0 m/s (23622.0 ft/h)

4. Water inlet temperature: 5[degrees]C [+ or -] 0.5[degrees]C (41[degrees]F [+ or -] 0.9[degrees]F)

5. Water Reynolds number inside the tube: 1.7 x [10.sup.4]~2.3 x [10.sup.4]

During the test, the water temperature inside the tube changed no less than 2.0[degrees]C (3.6[degrees]F) in order to measure the water temperature change accurately (Wang et al. 2000).

DATA REDUCTION

The data measured by the pressure difference transducer can be directly used to show the effect of hydrophilic coating on the air-side pressure drop. However, experimental data should be transformed in order to show the effect of hydrophilic coating on the air-side heat transfer.

The air-side sensible heat transfer coefficient ([h.sub.o]) is typically obtained from the overall heat transfer equation with the Wang and Chang (1997) method, and it can be determined by using Equation 2.

[h.sub.o]=[[C.sub.p,a]/[[b.sub.[w,m].sup.'][(1/[U.sub.o]-[[[b.sub.r.sup.'][A.sub.o]]/[[h.sub.i][A.sub.t,i]]]-[[b.sub.t.sup.'][x.sub.t][A.sub.o]]/[[k.sub.t][A.sub.t,m]])*([A.sub.t,o]/[[b.sub.[w,t].sup.'][A.sub.o]]+[[A.sub.f][n.sub.f,wet]]/[[b.sub.w,m][A.sub.o]])-[y.sub.w]/[k.sub.w]]]] (2)

where

[U.sub.o] = the overall heat transfer coefficient

[h.sub.i] = the tube-side heat transfer coefficient

[k.sub.t] = the thermal conductivity of tubes

[[eta].sub.f,wet] = the wet fin efficiency

[A.sub.o] = the total surface area

[A.sub.f] = the fin surface area

[A.sub.t,i] = the inside surface area of tubes

[A.sub.t,o] = the outer surface area of tubes

[A.sub.t,m] = the mean heat transfer area of tubes

[C.sub.p,a] = the moist air specific heat at constant pressure

[b'.sub.t] and [b'.sub.r] = the enthalpy-temperature ratios

[b'.sub.w,t] and [b'.sub.w,m] = the slope of saturated enthalpy curve evaluated at the outer mean water film temperature at the base surface and at the fin surface, respectively

1. The overall heat transfer coefficient ([U.sub.o])

The overall heat transfer coefficient ([U.sub.o]) can be determined by using Equation 3.

[U.sub.o] = [Q/[[DELTA][i.sub.m] * F * [A.sub.o]]] (3)

where

Q = the heat transfer rate

[[DELTA]i.sub.m] = the log mean enthalpy difference

F = the correction factor

The heat transfer rate (Q) used in the Equation 3 is the mathematical average of the air-side heat transfer rate and the water-side heat transfer rate, namely,

Q = [Q.sub.a] + [Q.sub.w]/2 (4)

[Q.sub.a] = [m.sub.a]([i.sub.a,in] - [i.sub.a,out]), and (5)

[Q.w.sub.] = [C.sub.p,w][m.sub.w](T.sub.w,out] - [T.sub.w,in] (6)

where m is the mass flow rate, i is the enthalpy, T is the temperature, and [C.sub.p,w] is the specific heat at constant pressure of water. Subscripts a, w, in, and out represent air, water, inlet, and outlet, respectively. Only the data that meet the ASHRAE 33-78 requirements are considered in the final analysis. Namely, the energy balance criterion function |[Q.sub.a]-Q|/Q is less than 0.05.

The log mean enthalpy difference ([[DELTA]i.sub.m]) used in the Equation 3 is determined from

[DELTA][i.sub.m] = [i.sub.a,in] + [[([i.sub.a,in] - [i.sub.a,out])]/[ln([[i.sub.a,in] - [i.sub.a,out]]/[[i.sub.a,out] - [i.sub.r,in]])]] - [[([i.sub.a,in] - [i.sub.a,out])([i.sub.a,in] - [i.sub.r,out])]/[([i.sub.a,in] - [i.sub.r,out]) - ([i.sub.a,out] - [i.sub.r,in])]]. (7)

The correction factor (F) is described by Equation 8, as presented by Spalding and Taborek (1983).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where R, P, and S are dimensionless parameters as defined by Equations 9 through 11.

R = [T.sub.w,in] - [T.sub.w,out]/[T.sub.a,out] - [T.sub.a,in] (9)

P = [T.sub.a,out] - [T.sub.a,in]/[T.sub.w,in] - [T.sub.a,in] (10)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

As Equation 8 is an implicit equation, solving Equations 8 through 11 for determining F is done by trial and error.

2. The tube-side heat transfer coefficient ([h.sub.i])

For a single-phase fluid, the tube-side heat transfer coefficient ([h.sub.i]) is evaluated with the Gnielinski correlation (Gnielinski 1976), and can be determined from Equations 12 through 14.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where

[h.sub.i] = the tube-side heat transfer coefficient

[D.sub.i] = the inside tube diameter

[Re.sub.i] = the tube-side Reynolds number

Pr = the Prandtl number

[f.sub.i] = the friction factor

[rho] = the density

V = the velocity

[micro] = the dynamic viscosity

3. The wet fin efficiency ([[eta].sub.f,wet])

For the wet fin efficiency, Wang et al. (1997) derived the corresponding formula by the equivalent circular area method.

[[eta].sub.f,wet] = [[2[r.sub.i]]/[[M.sub.T]([r.sub.o.sup.2] - [r.sub.i.sup.2])]]x[[[K.sub.1]([M.sub.T][r.sub.i])[I.sub.1]([M.sub.T][r.sub.o]) - [I.sub.1]([M.sub.T][r.sub.i])[K.sub.1]([M.sub.T][r.sub.o])]/[[K.sub.0]([M.sub.T][r.sub.i])[I.sub.1]([M.sub.T][r.sub.o]) - [I.sub.0]([M.sub.T][r.sub.i])[K.sub.1]([M.sub.T][r.sub.o])]] (15)

where

[r.sub.i] = the distance from the center of the tube to the fin base

[r.sub.o] = the distance from the center of the tube to the fin tip

[M.sub.T] is determined from

[M.sub.T] = [square root of [[2[h.sub.o][b.sub.w.sup.']]/[[k.sub.f][[delta].sub.f][C.sub.p,a]]]]. (16)

RESULTS AND DISCUSSION

Effects of SSC Hours on Hydrophilicity Performance

Figure 7 illustrates the effects of SSC hours on the SCA, the advancing DCA, and the receding DCA of aluminum fins with hydrophilic coating. The contact angles were recorded at a temperature of 23[degrees]C [+ or -] 0.5[degrees]C (73.4[degrees]F [+ or -] 0.9[degrees]F), and a relative humidity of 50% [+ or -] 1%. It can be seen from the figure that the SCA of the fin samples increases with the increase of SSC hours. The SCA of the uncorroded fin (corroded for 0 hour) is 18[degrees], while the SCA of the fin after 100 h corrosion is 35[degrees], and then the SCA further increases to 41[degrees] and 45[degrees] for the fins after 200 h and 400 h corrosion, respectively. Figure 7 also shows that both the advancing DCA and the receding DCA increase with the increase of SSC hours. The advancing DCA is 23[degrees] for uncorroded fin and increases to 72[degrees] for the fin after 400 h corrosion, while the receding DCA is 0[degrees] for uncorroded fin and increases to 23[degrees] for the fin after 400 h corrosion. The increase of contact angles indicates that the hydrophilicity of hydrophilic-coated fin degrades with the increase of SSC hours. The deterioration of hydrophilicity may increase the possibility of water bridges forming on the adjacent fins, while water bridges may cause the degradation of heat transfer performance and the increase of air-side pressure drop.

[FIGURE 7 OMITTED]

Effects of SSC Hours on Heat Transfer and Pressure Drop Performance

Figure 8 depicts the effects of SSC hours on the air-side heat transfer coefficients of finned-tube heat exchangers with hydrophilic coating. As shown in the figure, the air-side heat transfer coefficients can be enhanced by the SSC at the lower inlet air velocity ([u.sub.a,in] = 0.5 m/s [5905.5 ft/h]). However, the air-side heat transfer coefficients can be degraded by the SSC at the higher inlet air velocity ([u.sub.a,in][greater than or equal to] 1.0 m/s [11811.0 ft/h]) for the corroded heat exchangers. The effects of SSC on the air-side heat transfer coefficients are approximately within the range of -20.5% ~ 8.7% under the present experimental conditions. The possible explanation is shown below. At lower inlet air velocity, the corrosion pits, the corrosion fouling, and the condensate water drops on the corroded fin surfaces can roughen the fin surfaces, and then the rough fin surfaces may cause the twisted airflow, which can enhance air-side heat transfer; at higher inlet air velocity, the dehumidifying capacity enhances compared with that at low inlet air velocity, and more and more condensate water drops and water bridges adhere on the fin surfaces as the receding angle increases (Min et al. 2000; Min and Webb 2002), resulting in the degradation of heat transfer characteristics.

[FIGURE 8 OMITTED]

Figure 9 depicts the effects of SSC hours on the air-side pressure drops of finned-tube heat exchangers with hydrophilic coating. As shown in the figure, the air-side pressure drops of the corroded heat exchangers increase with the increase of SSC hours. When the inlet air velocity is in the range of 0.5~2.0 m/s (5905.5~23622.0 ft/h), the increase of air-side pressure drops of the corroded heat exchangers is approximately 1.7%~13.1% compared with the uncorroded heat exchangers. With the increase of SSC hours, the increased air-side pressure drops are caused by the increased amount of water drops and water bridges.

[FIGURE 9 OMITTED]

The phenomena of increased water drops and water bridges with the increase of SSC hours on the fin surfaces can be observed by the CCD camera located at the outlet of the tested heat exchanger during the experiments. The inlet air velocity being 2.0 m/s (23622.0 ft/h) is taken as an example for showing the condensation pattern on the fin surfaces. Figure 10 shows the images of condensation phenomenon when the inlet air velocity is 2.0 m/s (23622.0 ft/h). For the uncorroded heat exchanger with hydrophilic coating, it can be seen that there are no water drops on the fin surfaces. The condensation water can be drained in the form of water film easily, as can be seen in Figure 10a. However, for the corroded heat exchangers with hydrophilic coating, there are water drops and water bridges found on the fin surfaces, which can be seen in Figures 10b through 10d. The water drops and water bridges can increase the air-side pressure drop of corroded heat exchangers.

[FIGURE 10 OMITTED]

Effects of SSC Hours on Fouling Factors

In order to analyze the effect of SSC on heat transfer and pressure drop, the heat transfer coefficient fouling factor [f.sub.h] and pressure drop fouling factor [f.sub.dp] are employed (Yang et al. 2007), as shown in Equations 17 and 18.

[f.sub.h] = [[100([h.sub.f] - [h.sub.n])]/[h.sub.n]]% (17)

[f.sub.dp] = [[100([DELTA][P.sub.f] - [DELTA][P.sub.n])]/[[DELTA][P.sub.n]]]% (18)

where [[DELTA]P.sub.n] and [[DELTA]P.sub.f] are the air-side pressure drop of heat exchanger without corrosion fouling (new heat exchanger) and heat exchanger with corrosion fouling, respectively, and [h.sub.n] and [h.sub.f] are the air-side heat transfer coefficients of heat exchanger without corrosion fouling and heat exchanger with corrosion fouling, respectively.

Figure 11 shows the effects of SSC on the air-side heat transfer fouling factor ([f.sub.h]) and the air-side pressure drop factor ([f.sub.dp]) at different inlet air velocities. As shown in Figure 11, the effect of SSC on the [f.sub.h] first decreases, continuously comes up to a minimum value, and then increases gradually with the increase of inlet air velocity. A reverse trend for the [f.sub.dp] was found in Figure 11, the effect of SSC on the [f.sub.dp] first increases, continuously comes up to a peak value, and then decreases with the increase of inlet air velocity.

[FIGURE 11 OMITTED]

The possible explanations for the variations of the [f.sub.h] and [f.sub.dp] are as follows: (1) When the inlet air velocity increases from 0.5 to 1.5 m/s (5905.5 to 17716.5 ft/h), it can be observed that the amount and the volume of the condensate water drops keep rising as the surface tension of the water drops is bigger than their gravity. The condensate water drops adhering on the fin surface can degrade the air-side heat transfer performance and increase the air-side pressure drop gradually. (2) When the inlet air velocity reaches up to 2.0 m/s (23622.0 ft/h), it can be observed that many small diameter water drops begin to converge into larger diameter water drops, and a small number of water bridges forming between the fin surfaces, and a large number of water drops begin to flow down the fin surface as the surface tension of the larger diameter water drops is less than their gravity; during the flowing down process of larger diameter water drops, other small diameter water drops are taken at the same time, so the water drops flowing down the fin surface can decrease the variation rate of the air-side heat transfer coefficient and pressure drop.

CONCLUSION

An experimental study on the hydrophilicity, heat transfer, and pressure drop performance of corroded heat exchangers with hydrophilic coating was carried out. Major conclusions of this study are summarized as follows.

1. The static contact angle, the advancing dynamic contact angle, and the receding dynamic contact angle of hydrophilic-coated aluminum fins increase with the increase of salt spray corrosion hours, which results in the degradation of hydrophilicity of fins.

2. At lower inlet air velocity, the heat transfer can be enhanced for the pitting corroded heat exchanger with hydrophilic coating; at higher inlet air velocity, the heat transfer can be degraded for the pitting corroded heat exchanger with hydrophilic coating.

3. Comparing with the uncorroded finned-tube heat exchanger with hydrophilic coating at the inlet air velocity ranging from 0.5 to 2.0 m/s (5905.5 to 23622.0 ft/h), the effects of salt spray corrosion on the air-side heat transfer coefficient and on the air-side pressure drop are approximately within the range of -20.5%~8.7% and 1.7%~13.1%, respectively.

NOMENCLATURE

A = area, [m.sup.2]

[C.sub.p] = specific heat, [J*kg.sup.-1]*[K.sup.-1]

[D.sub.c] = fin collar outside diameter, m

DCA = dynamic contact angle, [degrees]

[f.sub.dp] = the pressure drop fouling factor

[f.sub.h] = the heat transfer coefficient fouling factor

[f.sub.i] = friction factor

F = correction factor

[F.sub.p] = fin pitch, m

h = heat transfer coefficient, [W*m.sup.-2]*[K.sup.-1]

i = enthalpy, [J*kg.sup.-1]

[[DELTA]i.sub.m] = logarithmic-mean enthalpy difference, J*[kg.sup.-1]

k = thermal conductivity, [W*m.sup.-1]*[K.sup.-1]

m = mass flow rate, [kg*s.sup.-1]

N = number of longitudinal tube rows

[DELTA]p = pressure drop, Pa

[P.sub.l] = longitudinal tube pitch, m

[P.sub.r] = Prandtl number

[P.sub.t] = transverse tube pitch, m

Q = heat transfer rate, W

[Re.sub.Dc] = Reynolds number based on tube collar diameter

[Re.sub.Di] = Reynolds number based on tube inside diameter

SCA = static contact angle, [degrees]

SSC = salt spray corrosion

t = corrosion hours, hour

T = temperature, [degrees]C

UA = overall heat transfer coefficient, [W*m.sup.-2]*[K.sup.-1]

V = velocity, [u*s.sup-1]

Greek Symbols

[delta] = thickness, m

[[eta].sub.f, wet] = wet fin efficiency

[theta] = wavy angle, [degrees]

[micro] = dynamic viscosity of air, Pa*s

[rho] = density, [kg*m.sup.-3]

Subscripts

a = air

act = actual

exp = experiment

f = fin, fouling

i = inner

in = inlet

m = mean value

n = new

o = outer

out = outlet

t = tube

w = water

Hui Pu is a PhD candidate and Guoliang Ding and Haitao Hu are professors at the Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai, China. Yifeng Gao is an engineer at the International Copper Association Shanghai Office, Shanghai, China.

Received July 24, 2009; accepted March 12, 2010

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Author:Pu, Hui; Ding, Guoliang; Hu, Haitao; Gao, Yifeng
Publication:HVAC & R Research
Article Type:Report
Geographic Code:1USA
Date:May 1, 2010
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