## 1 Introduction  
The quest for data mining, artificial intelligence, online data streaming, and rapid progress in edge and cloud computing has recently driven the demand for datacenters of various scales globally. In order to make these datacenters reliable, their environmental conditions, especially the humidity and temperature, have to be well controlled through proper HVAC systems. Compared to domestic HVACs, the environment for datacenters is of less concern to human comfort in terms of airspeed or noise. However, the control of humidity and temperature is more critical as they could cause reliability issues in datacenters. For large datacenters, the environmental air is typically channeled through a series of HVACs. In contrast, for datacenters of medium and small sizes, precision air conditioning (PAC) units are usually adopted. A typical PAC unit consists of an outdoor part and an indoor part. The outdoor part includes mainly the condenser and the cooling fans; whereas the indoor part has compressors, expansion valves, evaporators, fans, humidifying chambers, and heaters. These latter components are usually arranged compactly so that the indoor space can be utilized effectively. In regions of both high temperature and humidity, such as Taiwan, if the datacenters were not hermetically constructed and the entrance doors were open frequently, the humidity of the respective indoor environment would be high and could lead to water-condensate splash at the air outlets. It is known that the splash of water condensate is detrimental to the reliability of electronic components and systems. On the other hand, an improper dry environment could cause unwanted electro-staticdischarges. Thus, proper controlling of the environmental humidity level is essential for the reliability of the associated electronic components and systems. Moreover, datacenters tend to be operated continuously throughout the year. Hence, their power consumption is also of great concern. Thus, the goal of this research is to examine the performance of different surface-treated fins of the fin-and-tube heat exchangers in terms of flow resistance, heat transfer rates, and water-condensate splash. The commonly adopted fin-and-tube heat exchanger of indoor PACs generally consists of a copper tube with aluminum fins coupled with cooling fans for heat exchange between the refrigerant/ice water inside the tube and the surrounding air outside the tube. In the process of heat exchange, the surrounding air could be cooled down to its dew point to result in water condensation on the fin and tube surfaces. The water droplet size of the condensate will grow as the heat exchange proceeds further. When the water droplet size becomes larger, the droplets can merge together to form a water bridge between the fins. Both condensate and bridging will increase the airflow resistance through the tube and fin surfaces and simultaneously decrease the heat exchange efficiency. Furthermore, the condensate of large water droplets can be easily carried away by the cooling air, leading to water splash which can cause damage to the nearby electronic equipment. The factors causing water splash are multiple, including the geometries and surface conditions of the fins and tubes. Among these influencing factors, the present research focuses on surface hydro conditions, fin pitches, and air speeds with the goal of reducing both water splash and energy consumption. The surface hydro conditions in terms of wettability were varied by surface coating. The fin-and-tube heat exchangers can perform either in a dry situation without latent heat exchange or a wet situation with latent heat exchange. When acting as an evaporator, they always perform in a wet situation because of the water-condensate formation on the fin and tube surfaces.