2.1. Sample Preparation

Figures 2 and 3 depict a thorough description of the site where the room–AC was installed, and the samples were collected. The RAC was installed in one of the offices of the Lawrence Bunzel Building at the University of San Carlos Technology Center in Cebu City, Philippines.

During the collection of the foulant samples, nitrile gloves which are lint-free were worn, and a new, clean, and soft toothbrush was used to scrape it from the external surface of cooling fins on both evaporator and condenser of the RAC which being contained in a sealable plastic container immediately to avoid contamination. Then, it was placed inside a borosilicate glass dish for drying and put inside the laboratory oven.

FESEM sample requirements must be followed as stated:

The sample should be no larger than 12.0 mm × 12.0 mm × 10 mm (height), with the side opposite the side of interest flat (to facilitate sample mounting). Smaller sample heights are preferred. Samples should be dry and able to withstand high vacuums after primary fixation and dehydration. To achieve the best results, the samples’ surfaces should be clean and free of impurities. Wet biological samples cannot be evaluated using FESEM since the equipment is moisture-free. Drying the sample is necessary because FESEM requires a moisture-free sample; otherwise, if significant moisture content is detected, the FESEM would stop working. The drying process takes place in the Mechanical Engineering Laboratory of the University of San Carlos Technology Center. The drying process was carried out using instruments such as a Thelco laboratory oven, crucible tong, glass crucible, and digital weighing scale, as seen in Figure 4.

The dried samples were taken to the FESEM laboratory to be examined for elemental composition. Lint-free gloves were used for handling the specimen sample and the sample holder to prevent contamination from fingerprints. Figure 5 shows the procedures for preparing the sample for placement on the sample holder which was thoroughly discussed [11].

Samples of all types, including ceramics, metals and alloys, semiconductors, polymers, biological samples, and much more, can be imaged using FESEMs that can be seen in Figure 6(a). To obtain high-quality data, some sample types are more difficult to work with and call for an additional stage in the preparation process. This additional step is applying a thin layer of a conductive substance, such as gold, silver, platinum, or chromium, to your sample that is around 10 nm in thickness [12]. In this case, the samples were put into the sputter coater to be coated with a conductive material, gold (Au) that can be seen in Figure 6(b). For normal SEM applications, gold is a great and often utilized coating material to coat nonconductive materials. It coats very efficiently because of its low work function, and sample surface heating is minimal when thin layers and cool sputter coaters are used. With contemporary SEMs, the grain size is apparent at high magnifications. The foulant samples were coated with 8-nm gold to prevent surface charging using the arc deposition process. An excellent SEM image was obtained by using this method.

3.1. Image Analysis

The samples were identified as specimens from the evaporator and condenser, designated S1 and S2, respectively. The sizes and textures of the grains vary among samples. Experimental microscopic images from the collected foulants on the air conditioning coils were given in this contribution. Figure 7 shows the 32-megapixel images from an Android smartphone. The condenser sample exhibits a blend of lustrous grayish-white and black shade, while the evaporator sample (S1) displays compact small grain and a black tint. The tiny black granules that resembled flakes and were detected in the S1 were similar to dried soot. Additionally, the grains in the S2 resembled dry silt, dirt, or sand. Figure 8 displays a thirty-fold magnified FESEM image for the purpose of choosing the spectra for analysis. As can be seen in Figure 9, the S2 samples appear to have tighter grain sizes and almost identical shapes, whereas the S1 samples were found to have inconsistent grain sizes and shapes in Figure 8.

Particulate pollution can impact items in existing buildings due to sources such as gas and oil heaters, workrooms, and air conditioning systems that are not properly maintained or equipped with filters [13].

3.2. Elemental Composition Analysis

This step involved the analysis of various spectra for various samples. The cost of the FESEM with EDX analysis limits the number of spectra that may be obtained for each sample. As seen in Figure 8(b) and Figure 9(b), distinct areas were focused on for each sample during the EDX measurement and labeled with numbers. These areas will be referred to as Electron Images 1, 2, 3, 4 for evaporator samples (S1), while Electron Images 5 and 6 for condenser samples. To easily visualize the FESEM images and its corresponding EDX results, the FESEM images are displayed and followed by its corresponding EDX result. These were arranged accordingly, which can be seen in Figures 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, and 21. Peaks on an EDX spectrum often correlate to energy levels for which the greatest number of X-rays have been received. Since each of these peaks is specific to an atom, they all relate to one element.

A spectrum’s highest peak indicates how concentrated an element is in the sample [14]. Elemental mapping, which is the process of obtaining more precise data on an element’s composition, suggests that a greater spectrum be used. Two FESEM analyses were performed: one for the samples from the evaporator (S1) and the other for the condenser (S2). The magnification was increased to 1000x to examine the topography of each sample with a distinct spectrum and identify the elements that are present.

Table 1 displays a summary of the EDX spectroscopy results for each spectrum of the concentrated area of evaporator samples from the electron images. A histogram of all the EDX spectroscopy data for each spectrum of the targeted area of the evaporator sample was created for simple result viewing which can be seen in Figure 22. Fergusson and Kim, claim that street dust and house dust are mostly composed of dirt, although certain elements are more abundant in one form than the other. These are Au, Cl, Br, Zn, Cu, Cd, As, Sb, Cr, Ca, Na, and Zn. Their origins can be traced back to several sources of pollution. House dust contains some elements that are created within the house, including Cu, Co, As, Sb, Zn, Cd, Au, C1, C, and Pb [15]. Nevertheless, Fergusson and Kim’s conclusions are contradicted by the EDX spectroscopy results of evaporator samples, which are similarly thought of as dust that is deposited onto the heat transfer surfaces of the evaporator coil and fins. The emission spectra, as depicted in Figure 22, mostly include C, O, Si, Ca, Al, and Fe with different weight percentages, along with minor quantities of Na, Cl, K, Mg, and S. In addition, it shows that the evaporator sample contains a high concentration of O and C which can be seen in Figure 23, in terms of average weight percentage per element. It can be suggested that SiO₂ (silica) and CO (black carbon or soot) might be presented as the main compounds in the evaporator samples since Si and C have an average percentage of 7.3% and 47.8%, respectively. Since the sources of these substances can be found anywhere on Earth, they cannot be avoided. Over the course of millions of years, silicon dioxide crystallizes. Silicates (minerals containing silica) make up 30 percent of all minerals and geologists believe they may make up as much as 90 percent of the earth’s crust. Common silica-containing materials include limestone, coal, ore, cement, concrete, brick, mortar, stone, aggregate/rock, and soil. As a result, silica dust is prevalent everywhere and is easily carried by wind [16]. Additionally, some carbon is not entirely oxidized during incomplete combustion, which results in the production of soot and carbon monoxide (CO). Given the building’s proximity to the road and the sample collection location, there is a good chance that wind will carry these COs inside the offices and classrooms [17]. According to Hess-Kosa, the various dust sources in the area correlate to the various element’s compositions [2].

Table 2 presents a summary of the EDX spectroscopy data obtained from the electron images for each spectrum of the focused area of the condenser samples. Figure 24 shows a histogram of all the EDX spectroscopy data for each spectrum on the focused region in the condenser sample, which was created to make the results easier to observe. As can be seen in Figure 24, the emission spectra are mostly composed of different weight percentages of C, O, Si, Ca, Al, and Mg, with lesser amounts of Fe, Na, Cl, S, and P. Nevertheless, there is a significant concentration of O and C in the condenser sample. The amount of Al and Ca, at 10.9% and 6.9%, respectively, is greater than the quantity of Si, at 3.9% only, according to the average weight percentage per element displayed in Figure 25. It is possible that the primary constituents in the condenser samples are SiO₂ (silica), CO (black carbon or soot), and CaO (calcium oxide or quicklime). Although it is present in many common minerals (such as calcite, aragonite, limestone, and marble), calcium oxide is not found in nature in a pure form. The EDX spectroscopy result confirms the presence of CaO as the cause of the lustrous grayish-white color seen in the image analysis of the condenser samples shown in Figure 7(b). The forms of calcium oxide include granular powder, white or grayish-white lumps, and odorless crystals [18]. According to the EDX spectroscopy data obtained from the evaporator and condenser samples, several components that differ in weight percentage were found in the condenser sample but not in the evaporator sample. The adhesion qualities of these foulants are impacted by environmental factors, particularly the surrounding temperature. The van der Waals, liquid bridge, and interface reaction of diffusive and electrostatic forces are the three subtypes of adhesive forces [19]. As per Berbner and Löffler as the particles get smaller, F/G increases if the particle weight grows with the third power of the diameter. Therefore, even though large particles often have a stronger absolute adhesive force, small particles adhere more firmly than large ones [19].

3.3. Comparison of Results From Other Studies

The fouling materials obtained from ACs in Cebu City, Philippines, were examined using FESEM and EDX spectroscopy to determine their morphology and chemical composition. These findings are compared to those from similar research in Pusan, South Korea, conducted by Ahn and Lee [3] and Assiut, Egypt, by [4] to highlight differences caused by regional environmental conditions which can be seen in Table 3. The study focuses on differences in surface features, and elemental composition, offering insights into how climate, air quality, and installation conditions influence fouling characteristics. This work underscores the importance of region-specific assessments, as the environmental conditions in Cebu City have high humidity and pollution common in tropical metropolitan settings. The results may not be found in desert or subtropical regions.

While some elements, such as aluminum (Al), silicon (Si), potassium (K), calcium (Ca), and iron (Fe), are found in all three regions, others are unique to a specific location as can be seen in Table 3. For example, elements such as magnesium (Mg) and chlorine (Cl) were found in Cebu City, Philippines, but not in samples from Assiut, Egypt. Similarly, sulfur (S) was abundant in Pusan, South Korea, but was not consistently present in other areas. These findings show geographical variations in fouling material composition, which could be caused by variances in environmental conditions, industrial activity, or operational factors specific to each area where the samples are taken. In addition, throughout the year, the humidity of Cebu City, Pusan, and Assiut is 81% [20], 64% [21], and 38% [22], respectively, which shows that Cebu City has the highest humidity among the three regions. While this study did not include specific hygroscopicity measurement or compound identification, the observed changes in elemental composition indicate that humidity may have an impact on dust deposition and composition. As expected, Cebu City, which has the highest humidity, has a greater concentration of several elements with hygroscopic characteristics such as Cl and Na. Cebu City’s indoor samples had higher amounts of Cl (0.9%) and Na (0.9%), respectively, while the outdoor unit in Pusan located on the coast has higher Na and Cl concentrations than the outdoor unit in Cebu City, which is further inland. This is likely due to the influence of marine aerosols carried by onshore winds, which cause a larger deposition of sea salt Pusan outdoor unit. This finding is consistent with the knowledge that higher humidity might encourage water molecules to adhere to the surface of hygroscopic particles, potentially affecting their depositional behavior. For other hygroscopic elements including K and Ca, the trend is less clear. In comparison with Pusan and Assiut, Cebu City has lower K and Ca concentrations despite having higher humidity. This suggests that their abundance in the dust samples may be influenced by variables other than basic hygroscopicity, such as source variations, particle size distribution, and atmospheric transport. For instance, higher K and Ca concentrations in Assiut could be caused by agricultural activities, desert dust, or biomass burning, whereas industrial emissions and coal combustion might be the major sources of K in Pusan [23, 24].

The ANOVA results in Table 4 show significant regional variations in the concentrations of some elements. Iron (Fe), calcium (Ca), and potassium (K) showed statistically significant differences across the three regions (p < 0.05), suggesting that these elements are not uniformly distributed in the fouling material, while silicon (Si) and aluminum (Al) did not show significant differences (p > 0.05), suggesting a relatively consistent concentration of these elements regardless of location.

The relative consistency of silicon and aluminum concentrations throughout the multiple sites could be attributed to their widespread presence in the environment, stability, and possibly their interaction with larger, less mobile particles. Si and Al are major constituents of the Earth’s crust, and their compounds are abundant in soil and dust. These elements are naturally occurring in the environment and can be carried by wind, water, and human activities, resulting in extensive distribution [23]. Silicate and aluminosilicate minerals are also resistant to the weathering process, which means they are less prone to degrade or dissolve in the presence of water or air pollution. This enhances their stability and consistent presence in the surroundings [25]. Si and Al are frequently associated with larger dust particles which have reduced mobility in the environment. They settle out of the air more quickly, resulting in a more consistent distribution across different regions [26]. Of the elements analyzed, calcium had the highest F value (F = 15.73, p = 0.007), highlighting pronounced variability in its concentration between regions. This suggests that calcium deposition is especially sensitive to local environmental conditions or operation factors, such as air or water quality, or variations in the AC system’s exposure to external contaminants. Significant regional variations in an airborne particulate matter dust chemical composition, and operating parameters like temperature and humidity may all be connected to the notable variability seen for potassium (K) and iron (Fe). These findings highlight the significance of understanding site-specific circumstances to optimize maintenance techniques and fouling mitigation for AC systems.

The demand for ACs rises as the global temperature keeps on rising. According to a report presented by the International Energy Agency (IEA) that one of the primary drivers of global power demand will be the rising usage of ACs in homes and offices around the world, approximately 37% of energy goes to power AC units, highlighting the urgent need to improve cooling efficiency [27]. Additionally, fouling of ACU can reduce efficiency leading to an increase in energy demand and greenhouse gas emissions. The study supports the growth of sustainable solutions in different kinds of fields, including material science and engineering like what Raja et al. conducted. Raja et al. emphasized the development of palm-fiber boron carbide epoxy composites with improved mechanical and thermal stability [28]. Similarly, our research contributes to the development of sustainable solutions by giving insights into the compositions of ACU foulants, which can help future researchers develop more efficient and ecologically friendly AC systems like for instance developing a foulant-resistant coating on heat exchangers.

The concentration of several elements varied significantly among the three regions according to the Tukey HSD test shown in Table 5. The notable variation was seen in calcium (Ca), with values in Cebu City being much lower than those in Assiut (mean difference = −27.93, p < 0.05) and Pusan (−22.31, p < 0.05), indicating significant regional environmental differences. Cebu City had considerably higher potassium (K) concentrations than Assiut (mean difference = 1.63, p < 0.05), whereas Assiut had significantly higher iron (Fe) concentrations than Cebu City (−9.05, p < 0.05).