1、Volume 16, Number 3, May 2010An International Journal of Heating, Ventilating,Air-Conditioning and Refrigerating ResearchAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.Volume 16, Number 3, May 2010HVAC accepted March 12, 2010In this study, the effect of salt spray cor
2、rosion on the air-side performance of finned-tube heat exchangers with hydrophilic coating under dehumidifying conditions was experimentally inves-tigated. Artificial accelerated method of salt spray corrosion on the hydrophilic coated heat exchangers was used for simulating the corrosion process of
3、 the actual heat exchangers. The experimental results show that the contact angles of hydrophilic coated aluminum fins increase with the increase of salt spray corrosion hours, which results in the degradation of hydrophilic-ity of fins; the heat transfer is enhanced at lower inlet air velocity and
4、degraded at high inlet air velocity for the pitting corroded heat exchanger with hydrophilic coating; compared 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 o
5、n 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.INTRODUCTIONFinned-tube heat exchangers are widely applied as evaporators of air conditioners. The alumi-num fins are usually coated with hydrophilic
6、 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 wate
7、r 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
8、 transfer and pressure drop performance. Salt spray corrosion is a corrosion caused by the deposition of a certain amount of Clon fin surfaces (Ahn and Lee 2005), and it often happens in high salt con-centration districts, e.g., coastal areas. Therefore, it is necessary to pay attention to the effec
9、ts 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.Hui Pu is a PhD candidate and Guoliang Ding and Haitao Hu are professors at the Institute of Refrigeration and
10、 Cryo-genics, Shanghai Jiao Tong University, Shanghai, China. Yifeng Gao is an engineer at the International Copper Associ-ation Shanghai Office, Shanghai, China. 2010 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in HVAC Min et al. 2000;
11、 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 d
12、egraded 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 wit
13、h ser-vice 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 hy
14、drophilic 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 se
15、nsible heat transfer coefficient is hardly affected by hydrophilic coat-ing, 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
16、 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 e
17、t 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
18、 influence on the sensible heat transfer coefficients. The impact of hydrophilic coating on the air-side pressure drop of finned-tube heat exchang-ers 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 conditi
19、ons,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 exc
20、hanger without hydro-philic 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 anticorros
21、ion 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). How-ever, there is no publication about the effect of
22、 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
23、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 PROCESSHeat Exchanger GeometryFour finned-tube heat exchangers made of aluminum herrin
24、gbone wavy fin and copper tube were used in the experiments, and the fins were coated with hydrophilic coatings. The employment 2010 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in HVAC Hao et al. 2007; Bao et al. 2008) on the finned-tub
25、e heat exchangers with hydrophilic coating was used for simulating the corrosionFigure 1. Coating structure of fin surface.Figure 2. Geometric details of the tested finned-tube heat exchangers. 2010 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Pub
26、lished in HVAC 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
27、degradation of heat transfer characteristics. 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.
28、 When the inlet air velocity is in the range of 0.52.0 m/s (5905.523622.0 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 Figure 8. Effect of SSC on the air-side heat transfer coefficient of finned-tube heat
29、exchangers. 2010 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in HVAC 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 su
30、rface can decrease the variation rate of the air-side heat transfer coefficient and pressure drop.CONCLUSIONAn 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
31、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 inl
32、et 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 exch
33、anger 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.N
34、OMENCLATUREA=area, m2Cp= specific heat, Jkg1K1Dc= fin collar outside diameter, mDCA = dynamic contact angle, fdp= the pressure drop fouling factorfh= the heat transfer coefficient fouling factorfi= friction factorFigure 11. Effect of SSC on the air-side heat transfer coefficient factor and the air-s
35、ide pressure drop factor. 2010 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in HVAC accepted December 15, 2009This paper is based on findings resulting from ASHRAE Research Project RP-1299.The use of high-efficiency HVAC filters is a com
36、mon strategy to control exposure to airborne particulate matter in residential buildings. However, high-efficiency filters generally have a higher pressure drop and are widely assumed to have large energy penalties. In this paper, we explore the underlying theoretical energy implications of high-pre
37、ssure-drop filters and we present the results of a four-month-long period of detailed energy monitoring of two air-conditioning systems in a test home in Austin, Texas. A theoretical analysis shows that the magnitude of potential energy impacts associated with high-efficiency filters are overall lik
38、ely to be small and can result in either a net savings or additional expenditure, depending on the system. The measured results in the test systems confirm these findings, and energy consump-tion generally did not differ with high-efficiency filters compared to low-efficiency filters. These results
39、suggest caution when assuming that high-efficiency filters require more energy than low-pressure-drop filters in residential HVAC systems.INTRODUCTIONHigh-efficiency filters are used in forced-air heating, ventilating, and air-conditioning (HVAC) systems to protect building equipment and occupants,
40、but are also known to influence HVAC energy consumption. High-efficiency filters with a high minimum efficiency reporting value (MERV) typically have a greater pressure drop than a filter with a lower MERV (ASHRAE 2007). In large commercial HVAC systems, where fan and motor controls typically mainta
41、in required airflow rates, a greater pressure drop will generally lead to increased energy consumption (e.g., Chimack and Sellers 2000, Fisk et al. 2002, and Yang et al. 2007). This relationship between energy use and filter pressure drop is widely assumed to hold true for smaller residential air-co
42、nditioning systems, but operational differences between small and large systems suggest very different energy consequences. This paper explores the theoretical impacts of filters on smaller air-conditioning systems and reports measurements taken with different levels of filter efficiency installed i
43、n an unoccupied test house in Austin, Texas, during the cooling season. This paper is a companion paper to similar measurements taken on 17 HVAC systems in occupied residential and light-commercial buildings (Stephens et al. 2010).Brent Stephens is a graduate research assistant, Atila Novoselac is a
44、n assistant professor, and Jeffrey Siegel is an asso-ciate professor in the Department of Civil, Architectural, and Environmental Engineering at the University of Texas at Austin. 2010 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in HVAC
45、 Proctor and Parker 2000). Almost all of the pressure potential is used to overcome friction introduced by different components of the system, outlined in Figure 1a. Figure 1b shows the static pressure distribution relative to the surrounding pressure in the occupied residence space. The static pres
46、sure drop across individual components of the system is proportional to the square of the airflow rate and a coefficient of proportionality, C (ASHRAE 2005). There are some component installations where either a combination of a linear and a squared term or an empirically derived exponent is used (e
47、.g., Yang et al. 2007), but we focus on the classic form Figure 1. Pressure distribution in a residential HVAC system: (a) system schematic, (b) static pressure distribution. 2010 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in HVAC more
48、 usually exist in real systems). Equation 1 represents the combination of the component coefficients in Figure 1a, resulting in a total equivalent coefficient of proportionality for the entire system, Ctotal.(1)whereCtotal= coefficient of proportionality for the entire systemCreturn= coefficient of
49、proportionality for the return ductCfilter= coefficient of proportionality for the filterCCC= coefficient of proportionality for the cooling coilCHC= coefficient of proportionality for the heating coilCS1= coefficient of proportionality for the first supply duct branchCS2= coefficient of proportionality for the second supply duct branchCS3= coefficient of proportionality for the third supply duct branchSince the pressure loss in each component can be presented as a square function of airflow through the component and the geometry of all components remains consta
