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The Impact of Fin Surface Wettability on the Performance of Dehumidifying Heat Exchangers.


For air-cooling heat exchangers operating under wet-surface conditions, water from the air stream condenses and accumulates on the surface of the heat exchanger until it is removed by air-flow forces or gravity. The condensate retained on air-side heat transfer surface increases thermal resistance and decreases airflow passage, which lead to a smaller cooling capacity, higher fan power consumption, and maybe superfluous noise. Water on the heat-exchanger surface also provides sites for biological activity which adversely impacts the indoor air quality. The more water retained, the longer it takes for it to evaporate after the system is off. Therefore, good drainage performance of the heat exchanger is generally desired, and research has been conducted on improved drainage through wettability manipulation of the fins.

In 1987, Mimaki coated a group of round-tube-plain-fin heat exchangers with an organic film, which brought down the fin surface water contact angle from around 70 deg to around 10 deg. This treatment resulted in no impact on the air-side heat transfer coefficient, but due to the flattened water drop shapes on this highly wettable surface the pressure drop across the heat exchanger was reduced by 40 - 50% compared to the uncoated one. Wang and Chang 1998 and Hong and Webb 1999 followed the study by involving different fin geometries, including plain, louver, wavy, and lanced fins. The impact of the coating on the heat-transfer performance was also reported to be very small, and the pressure drop reduction varied from zero to 50% depending on the fin type. Min et al. 2000 investigated the long-term effectiveness of two types of coatings, and after 1000 wet/dry cycles a 9% and 12% reduction in core pressure drop were reportedly maintained for the ZN and AQ coatings studied. Unlike the previous work Wang et al. 2002 found that hydrophilic treatment of fin surfaces caused up to 20% degradation in the heat transfer performance. Ma et al. 2007 observed both an increase and decrease in heat transfer coefficient for a group of round-tube, wavy-finned heat exchangers after hydrophilic coating, and mentioned that [h.sub.coated]/[h.sub.uncoated] decreased as the relative humidity of incoming air went up. However, the article did not provide detailed explanation of these results. Heat exchangers tested in the work above were all relatively compact, with fin spacings from 1.2 - 2.1 mm (0.047 - 0.083 in).

Compared to investigations of thermal-hydraulic performance, very rarely have data been reported on the condensate retention on a heat exchanger, and even less has been published addressing the impact of fin wettability. Shin and Ha 2002 measured the amount of water hold-up on hydrophilic heat exchangers with three different fin shapes (plain fin, plain fin with a slant end, and discrete plain fins). Their work showed that a 57 - 75% reduction in condensate retention could be achieved by improving fin surface wettability. In 2009, Liu and Jacobi reported the real-time retention data for a group of round-tube-slit-fin heat exchangers with identical geometry and varying surface wettability. The specimens they studied had a tight fin spacing [F.sub.s] ~ 1.1 mm (0.043 in) and the retention reduction after hydrophilic treatment was recorded to be 20 - 35%. In the same paper, Liu and Jacobi also reported experiments on a specimen treated to manifest high advancing contact angle for water ([[theta].sub.A] ~ 110 deg), which displayed hydrophobic surface characteristics under wet-surface operating conditions. Because the droplets beaded up and blocked airflow passage, a 35% increase in pressure drop was recorded compared to the untreated baseline specimen, and it also caused a 25% increase in steady-state retention data.

Although some research has been conducted to evaluate fin wettability effects on heat exchanger performance, the variation of wettability was very limited. Most articles only investigated one hydrophilic coating at one time, or two with very similar wetting characteristics. Furthermore, data were reported in a separate manner, by only examining the heat transfer and pressure drop performance, or the amount of condensate retained on the studied specimen. Despite the close relationship between these two, little effort was attempted to link them. It is difficult to find retention data reported along with thermal-hydraulic performance, especially for heat exchangers with unconventional surface wettability. More importantly, it is expected that the wettability impact on heat exchanger performance will be affected by the fin spacing, because droplet bridging between adjacent fins plays a significant role in the overall retention and drainage. This factor, however, has not been carefully addressed before, as most of the results were obtained with very compact heat exchangers. It would be useful to understand the wettability influence under wider fin spacing conditions, when water drops grow freely and bridging is less a concern. The present work presents a study of two groups of heat exchangers with the same configuration but two different fin spacings [F.sub.s] ~ 2.0 mm (0.079 in) and [F.sub.s] ~ 5.2 mm (0.205 in). For each group of the specimens, four geometrically identical heat exchangers cover a wide range of fin wettability from completely wetting to very hydrophobic. The thermal-hydraulic data will be reported along with the retention data systematically, and the coupled relationship between them will be explored.


A total number of 8 heat exchangers (2 groups) with different surface wettabilities are studied in this work. The heat exchanger configuration is shown schematically in Figure 1, and their geometrical and wettability properties are listed in Table 1. The first group of heat exchangers contains four specimens (specimens 1 - 4) with identical geometry [F.sub.s] ~ 2.0 mm (0.079 in) and manifest completely wetting, hydrophilic, untreated aluminum, and hydrophobic fin surface wettabiliy, respectively. The second group (specimens 5 - 8) is identical to the first one except that it has a wider fin spacing [F.sub.s] ~ 5.2 mm (0.205 in). The corresponding specimens (specimens 1 and 5, specimens 2 and 6, specimens 4 and 8) share the same surface coating and wettability characteristics. The baseline heat exchangers (specimens 3 and 7) were cleaned using neutral detergent in order to eliminate the effects of fin press oil. In real systems, as Min and Webb 2011 demonstrated, the oil effects will be washed away as the number of dry/wet cycles increases. The advancing and receding contact angles for water ([[theta].sub.A] and [[theta].sub.R]) were employed to describe the surface wettability of different specimens. These angles were determined by capturing images of droplets at the point of incipient motion on an inclined surface (Liu and Jacobi 2008).

A closed-loop wind tunnel, schematically shown in Figure 2(a), was used to measure the performance of all 8 specimens. Air flow in the wind tunnel was driven by an axial blower, and it circulated through resistance heaters (conditioning the air temperature), a flow nozzle (measuring air mass flow rate), a mixing chamber, honeycomb flow straighteners, screens, and a 9:1 area contraction before it reached the test section. Steam generated by a boiler was injected into the system in order to achieve the desired relative humidity of air (regulated by a chilled-mirror dewpoint sensor and a PID controller). A mixture of ethylene glycol and water was supplied by a gear pump as the tube-side coolant. The coolant had its temperature controlled by a chiller system, and its mass flow rate measured by a Coriolis-effect flow meter. A six-junction, equally spaced thermocouple grid with uncertainty [+ or -]0.1 [degrees]C ([+ or -]0.18 [degrees]F) and another twelve-junction grid were used to measure the air temperatures upstream and downstream, respectively, of the heat exchanger specimen. Immersion RTD's with uncertainty [+ or -]0.03 [degrees]C ([+ or -]0.054 [degrees]F) were positioned at the tube inlet and exit to record the coolant temperatures. Upstream and downstream dewpoints of the air were measured using chilled-mirror hygrometers with uncertainty [+ or -]0.2 [degrees]C ([+ or -]0.36 [degrees]F), and pressure drop across the heat exchanger was acquired with an electronic pressure transducer with uncertainty [+ or -]0.2 Pa ([+ or -]0.0007 in[H.sub.2]O).

Schematically shown in Figure 2(b) is a magnified picture of the test section, which allowed a transient measurement of the amount of condensate accumulating on the heat exchanger specimen. During an experiment, the heat exchanger was suspended in the wind tunnel with a non-absorbing strap, which was grid-structured with holes allowing water to pass through and be drained. An electronic balance with uncertainty [+ or -]0.1 g ([+ or -] 0.0035 oz) was placed under the strap to record the mass of condensate retention on the heat exchanger. The heat exchanger was mounted in the test section with special care, in order to achieve a good insulation as well as allowing an accurate measurement of the condensate accumulation (Liu and Jacobi 2009). At the begining of every experiment, a set of calibrated weights were gradually added onto the heat exchanger and readings from the balance were recorded. For most of the experiments the deviation from balance readings and the calibration weights were within 5%. In order to enhance the measurement accuracy, a linear calibration curve was fitted from every pre-experiment calibrating activity and then applied for the subsequent experiment.

The experimental conditions employed in the present study are summarized in Table 2. An experiment was initiated by circulating the airflow while bringing it to the desired temperature, velocity, and relative humidity. Coolant flow which had also been preconditioned to the desired temperature was then started to begin the test. After a 30 - 40 min transient period of condensate accumulation, a steady state would be acquired. Thermal-hydraulic data would then be sampled using a LabView program for subsequent analysis.


For the air and coolant flows, the heat transfer rates at each side were calculated based on the mass flow rate and enthalpy/temperature change:

[Q.sub.a] =[m.sub.a] ([i.sub.a,i] - [i.sub.a,o]) (1)

[Q.sub.c] =[m.sub.c][c.sub.p,c] ([T.sub.c,o] - [T.sub.c,i]) (2)

For the purpose of combined uncertainty minimization (Park et al. 2010), a weighted average of the air-side and coolant-side heat transfer rates was calculated as:

[Q.sub.ave] = [[omega].sub.a][Q.sub.a] + [[omega].sub.c][Q.sub.c] (3)

where [[omega].sub.a] and [[omega].sub.c] are the weighting factors that depend on the uncertainties from each side. A log-mean-enthalpy-method (LMED) was employed to solve for the air-side heat transfer coefficient [h.sub.a] (Threlkeld 1970). During the solution, the fin efficiency was determined using modified equations for plane fins, as suggested by Wang et al 2000.

Data interpretation followed the methods detailed by the ARI Standard for fin-and-tube heat exchangers. Given the results of [h.sub.a], the Colburn j factor was calculated as

[mathematical expression not reproducible] (4)

where the mass flux G corresponded to the minimum free flow area. All the fluid properties were evaluated at the average values of inlet and outlet temperatures under steady-state conditions. The non-dimensional pressure drop across the heat exchanger was expressed as a friction factor f (entrance and exit losses neglected):

[mathematical expression not reproducible] (5)

where [sigma] is the ratio of minimum free flow rate to the frontal area of the heat exchanger.

Through a standard error-propagation calculation as described in NIST Technical Note 1297 (Taylor and Kuyatt, 1994), the average uncertainty in j factors was found to be [+ or -] 13.1% (primarily due to the uncertainty of upstream and downstream dew-point measurements) and the average uncertainty in f factors was [+ or -] 4.3%. The uncertainty in retention mass measurements was estimated to be less than [+ or -] 5% under steady state conditions, mainly due to small fluctuations caused by discrete shedding behavior (Liu 2011).


The steady-state mass retention data for different heat exchangers are shown in Figure 3. Even though the experiments were conducted at the same face velocity range for the two specimen groups, Figure 3(a) and 3(b) show different air-side Reynolds numbers because these two groups have different hydraulic diameters which are directly related to their fin spacing. It can be seen from Figure 3 that the amount of condensate retention consistently decreases with air-side Reynolds number, for all the specimens studied. This is because a higher air velocity increases shear at the liquid-vapor interface, which causes an increased flow of the condensate toward the exit face of the heat exchanger. It is also clear that the fin wettability plays an important role on the retention amount. Under similar operating conditions, the hydrophilic specimens always retain much less water than specimens that are relatively more hydrophobic. A completely wetting fin surface (specimens 1 and 5) resulted in a 32.4% and 35.3% averaged retention reduction for the [F.sub.s] = 2.0 mm (0.079 in) and [F.sub.s] = 5.2 mm (0.205 in) specimen, respectively. A very uniform water film was observed on the air-side surface of these heat exchangers during experiments.

One may also notice that specimens with a hydrophobic coating (specimens 4 and 8) retain slightly less water than their untreated counterparts (specimens 3 and 7, respectively), which is attributed to their high receding contact angle and consequent reduced drop sizes. The untreated aluminum specimens retain the largest amount of condensate under most conditions tested. This echoes the idea in the literature that either very hydrophilic or very hydrophobic surfaces could help with water drainage. Nevertheless, the hydrophobic coating used in this study does not possess strong enough water repellant characteristics and hence its effect is not as significant as the hydrophilic ones. It should also be noted that the Reynolds numbers presented in Figure 3 are the Reynolds numbers under wet conditions, i.e. after the steady state condensation has been established. Take Figure 3(a) as an example, the first data point of each curve was acquired under the same initial face velociy (same Reynolds number under dry conditions, [Re.sub.dry]). However, as water started to build up on a heat exchanger during the operation, the air-side Reynolds number continuously decreased from [Re.sub.dry]. The more condensate retained, the more the reduction. That is why the curves for specimens 3 and 4 shift to the left in Figure 3(a). As a result, in order to acquire the same Reynolds number (or air velocity), one needs to set a higher initial velocity (pay more fan power) for specimens with more condensate retention.

Air-side heat transfer and pressure-drop results under fully wet conditions are presented in the form of j and f factors versus air-side Reynolds number for each group in Figures 4 and 5. It can be seen that for heat exchangers studied in this research, surface treatment almost always caused degradation in air-side heat transfer performance under wet conditions. For the hydrophilic heat exchangers in each group (specimens 1 and 2, 5 and 6) this degradation was probably due to the filmwise mode of condensation, which builds up an insulating liquid film and provides little portion of bare surface area for heat transfer. This effect was more clear for group 2 specimens, which has a wider fin spacing [F.sub.s] = 5.2 mm (0.205 in). Specimens 5 and 6 exhibit j factors about 15% lower than the untreated specimen 7. A quantitative comparison issummarized in Table 3.

Heat transfer decrease was also observed on the hydrophobic heat exchangers. Wet j factors for specimens 4 and 8 were measured to be 27.4% and 7.9% lower than their untreated counterparts, respectively. Degradation of heat transfer performance on these two specimens was expected, based on the observation of very uniform droplet growth on the surface during tests. The coating eliminated a lot of the texture and imperfections that are typical on an aluminum fin surface, causing much more uniform nucleation and droplet development on the fin surface. As a result, droplets of similar sizes existed all over the heat-transfer surface, forming an equivalently much thicker layer of thermal resistance. For specimen 8 which had wide fin spacing, the dropwise mode of condensation still provided higher roughness on top of the condensate, which contributed to bringing up the convective heat transfer coefficient. The combined effects of these two mechanisms resulted in slightly decreased wet j factors. For specimen 4, however, its tight fin spacing caused condensate bridging and the advantage of dropwise mode was diminished, which resulted in the larger heat transfer degradation.

Wet f factors for different heat exchangers compared to the baseline are presented in Figures 4(b) and 5(b), as well as in Table 3. It can be seen that hydrophilic treatment consistently helped reduce core pressure drop across the heat exchanger, especially for group 1 specimens with a higher fin density. This enhancement can lead to reduced fan power and reduced fan noise, or at fixed pressure drop a filmwise condensation will allow higher air-side velocities. The difference in f factor after hydrophobic treatment for specimen 8 was probably attributed to the droplet distribution on the surface, as the mass retention was shown to be similar between specimen 8 and baseline specimen 7 in Figure 3(b). On the other hand, the relatively tight fin spacing for specimen 4 limited this distribution variability, and therefore f factors more similar to baseline numbers were observed.

It can be concluded from Table 3 that fin surface wettability profoundly impacts condensate retention on a heat exchanger, whether it has wide or narrow fin spacing. This benefit can hence be exploited on all similar types of heat exchangers, which provides a better drainage performance and shorter evaporating time after the system is off. Its impact on pressure drop is a little different, which is more pronounced for compact heat exchangers and less significant for heat exchangers with wide fin spacing. Compared to the numbers shown in Table 3, most of the previous studies in the literature involved slits and louvers on fins with tighter fin spacing, and thus resulted in larger pressure drop reduction up to 50%. This is because the existence of water retention on fin surface substantially decreases the free flow area for specimens with high fin density while not making as big difference for wide fin spacing. This finding suggests that hydrophilic treatment significantly impacts heat exchangers with higher fin density (and fin interruptions), when condensate bridging is the main contributor for high amount of retention. Lastly, when the fin spacing is large, the degradation in heat-transfer performance by the filmwise mode of condensation is more pronounced. As mentioned above, wide fin spacing allows the specimen to take advantage of the enhanced interface roughness under dropwise mode of condensation, and a larger portion of bare surface for heat transfer, while tight fin spacing causes condensate bridging and deteriorates the function of these mechanisms. Therefore, for compact heat exchangers, hydrophilic surface treatment is the best due to the fact that the pressure drop and pumping power consumption can be significantly decreased without decreasing the heat transfer performance. For heat exchangers with wide fin spacing, the pressure drop is not reduced as much while heat-transfer performance will be sacrificed more.


In this study, experimental data for condensate retention and thermal-hydraulic results for fin-and-tube heat exchangers with identical geometry and differing surface wettability were reported. Under the same operating conditions, hydrophilic heat exchangers retained much less water than heat exchangers that are more hydrophobic, due to the more filmwise manner of retention and reduced possibility of bridging. The amount of condensate retention consistently decreased with increasing air flow rate, due to air-shear effects. Whether the heat exchanger is very compact or not, hydrophilic fins always help significantly improving drainage performance. This benefit can result in reduced fan power and reduced fan noise, or allow higher air-side velocities at fixed pressure drop. Hydrophilic treatment causes heat-transfer degradation due to the filmwise mode of condensation. The degradation is larger for specimens with wide fin spacing, because benefits of dropwise condensation are more pronounced under conditions of little or no bridging. Surface wettability has a profound impact on condensate retention for all heat exchangers, but its impact on pressure drop is more prounced for compact heat exchangers and not as much when the fin spacing is large.


This work was financially supported by the Air Conditioning and Refrigeration Center (ACRC) at the University of Illinois. The authors are also grateful for assistance from Luvata for providing the heat exchanger samples, and for surface coating assistance from Circle Prosco, Inc.

A             =     area
[c.sub.p]     =     specific heat
f             =     Fanning friction factor
[F.sub.s]     =     fin spacing
G             =     mass flux based on minimum flow area
h             =     heat transfer coefficient
i             =     enthalphy
j             =     Colburn j factor
m             =     mass flow rate
[N.sub.u]     =     Nusselt number
[P.sub.r]     =     Prantdl number
Q             =     heat transfer rate
[Re.sub.dh]   =     Reynolds number based on hydraulic diameter
[rho]         =     density
[sigma]       =     surface ratio [A.sub.min]/[A.sub.front]
a             =     air
c             =     coolant
i             =     in
o             =     out


Hong, K. and Webb, R. L., 1999, Performance of dehumidifying heat dxchangers with and without wetting coatings, Journal of Heat Tranfer, 121, p. 1018-1026.

Liu, L., 2011, Effects of Air-side surface wettability on the performance of dehumidifying heat exchangers, Ph.D. Dissertation, University of Illinois.

Liu, L. and Jacobi, A. M., 2008, Issues affecting the reliability of dynamic dip testing as a method to assess the condensate drainage behavior from the air-side surface of dehumidifying heat exchangers, Experimental Thermal and Fluid Science, 32, p. 1512-1522.

Liu, L. and Jacobi, A. M., 2009, Air-side surface wettability effects on the performance of slit-fin-and-tube heat exchangers operating under wet-surface conditions, Journal of Heat Tranfer, 131, No. 051802.

Ma, X., Ding, G., Zhang, Y., and Wang, K., 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, p. 2525-2532.

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Liping Liu, PhD

Associate Member ASHRAE

Anthony M. Jacobi, PhD


Liping Liu is an assistant professor in A. Leon Linton Department of Mechanical Engineering, Lawrence Technological University, Southfield, MI. Anthony M. Jacobi is a professor in the Department of Mechanical Science and Engineering, University of Illinois, Urbana-Champaign, IL.
Table 1. Geometric and Wettability Description of the Studied Heat

Specimen  Fin Spacing, [F.sub.s]  [[theta].sub.A]  [[theta].sub.R]

1         2.0 mm (0.079 in)         0 [degrees]     0 [degrees]
2         2.0 mm                  102 [degrees]    22 [degrees]
3         2.0 mm                   85 [degrees]    42 [degrees]
4         2.0 mm                  116 [degrees]    66 [degrees]
5         5.2 mm (0.205 in)         0 [degrees]     0 [degrees]
6         5.2 mm                  102 [degrees]    22 [degrees]
7         5.2 mm                   85 [degrees]    42 [degrees]
8         5.2 mm                  116 [degrees]    66 [degrees]

Specimen  Surface Treatment

1         Hydrophilic polymer sealer
2         Hydrophilic polymer sealer (*)
3         Bare clean aluminum
4         Hydrophobic polymer sealer
5         Hydrophilic polymer sealer
6         Hydrophilic polymer sealer (*)
7         Bare clean aluminum
8         Hydrophobic polymer sealer

(*) Specimens 2 and 6 were observed to be completly wetted during
experiments. The high [[theta].sub.A] only manifests when placing
isolated droplets on an initially dry surface. Once a water film forms
on the surface the film does not rupture. For this reason this coating
is classified as "hydrophilic".

Table 2. Test Conditions


Inlet dry bulb temperature  23.9 [degrees]C (75.0 [degrees]F)
Inlet relative humidity     71%
Frontal air velocity         1.6 - 5.0 m/s (5.2 - 16.4 ft/s)


Inlet temperature  4.4 [degrees]C (40.0 [degrees]F)
Flow rate          0.05 - 0.17 kg/s (7.2 - 22.1 lb/min)

Table 3. Wettability Impact on the Performance of Studied Heat

Specimen                   Steady State Retention, M

Sp. 1, completely wetting      - 32.4%
Sp. 2, hydrophilic             - 22.0%
Sp. 3, clean aluminum              -
Sp. 4, hydrophobic              - 8.6%
Sp. 5, completely wetting      - 35.3%
Sp. 6, hydrophilic             - 17.0%
Sp. 7, clean aluminum              -
Sp. 8, hydrophobic              - 2.3%

Specimen                   Air-Side Heat Transfer, j

Sp. 1, completely wetting       - 4.2%
Sp. 2, hydrophilic              - 9.7%
Sp. 3, clean aluminum              -
Sp. 4, hydrophobic             - 27.4%
Sp. 5, completely wetting      - 14.8%
Sp. 6, hydrophilic             - 15.5%
Sp. 7, clean aluminum              -
Sp. 8, hydrophobic              - 7.9%

Specimen                   Friction Factors, f

Sp. 1, completely wetting      - 19.3%
Sp. 2, hydrophilic             - 19.9%
Sp. 3, clean aluminum              -
Sp. 4, hydrophobic              - 0.1%
Sp. 5, completely wetting      - 11.2%
Sp. 6, hydrophilic             - 18.1%
Sp. 7, clean aluminum              -
Sp. 8, hydrophobic              - 7.6%
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Author:Liu, Liping; Jacobi, Anthony M.
Publication:ASHRAE Conference Papers
Date:Dec 22, 2014
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