## 1 Introduction
\begin{table}
\begin{tabular}{c c c} \hline
**Raw Material** & **Research** & \(\eta\) **[\%]** \\ \hline Cyprus marble & [19] & 15.0–34.0 \\ Dry bulrush basket & [19] & 19.0–35.0 \\ Fabric PVC sponge & [26] & 76.7–91.6 \\ G galvanized metal sheets & [12] & 83.0 \\ Greenhouse shading net & [27] & 25.2–32.8 \\ High density polyethylene plastic & [8; 28] & 70.0–94.0 \\ Metal sheets & [24] & 71.8 \\ Non-woven fabric perforated & [13] & 48.3–89.3 \\ Polypropylene hollow fibres & [29] & 73.0 \\ Pottery tubular rods & [30] & 49.4–56.9 \\ Pumice stones & [27] & 56.1–91.6 \\ Stainless steel wire mesh & [11; 12] & 50.0–85.2 \\ Volcanic tuff & [27] & 68.0–77.9 \\ Ceramic pipes & [12; 19] & 72.0 \\ Yellow stones & [19] & 18.0–49.0 \\ \hline \end{tabular}
\end{table} 
Table 2: Cooling efficiency (\(\eta\)) of cooling pad research studies based on inorganic raw materials. Alternative materials with cooling efficiencies above 70% were found (Tables 1 and 2), which is similar for the cellulose pad. However, most of these experimental tests had been conducted in favourable thermal conditions for evaporative cooling, high dry-bulb temperature and low relative humidity [8]. Hence, this has led to wondering whether the alternative materials are suitable for a particular location or specific thermal conditions [3; 25; 26; 37; 38; 39; 40; 41; 42; 43], for example, in low dry-bulb temperatures and high relative humidity. The air pressure drop across the pad is generally omitted in the evaluation of alternative materials. Increasing the pressure drop is equivalent to incrementing the electrical power required by the exhaust fans of the facility, leading to high operating costs. Research studies have reported values between 9 Pa [27] and 365.5 Pa [21], higher than the reported values for the cellulose pad (\(<\)40 Pa) [36; 44]. For cooling efficiencies above 60%, some authors have reported pressure drops above 100 Pa [13; 15; 21; 27]. The change in the dry-bulb temperature and relative humidity across the pad allows us to characterize the pad cooling efficiency. The change of the relative humidity across the pad is frequently omitted in reports of research results, being one of the variables that most affects the thermal comfort inside the agro-industrial facilities. Using indexes that relate both variables can provide an idea of the impact that the cooling pad has over the microclimatic conditions within the installation. The Temperature-Humidity Index (THI) is an index used frequently to characterize the animal thermal comfort, which provides four levels of thermal stress according to the characteristics of the animals: mild (68 \(\leq\) THI \(<\) 72), moderate (72 \(\leq\) THI \(<\) 80), severe (80 \(\leq\) THI \(<\) 90) and emergency (THI \(\geq\) 90) [45]. A comparison between a commercial cellulose pad and three alternative materials that have not been previously studied in tropical climates is presented in this paper. The influence of the pad thickness, particle size of the material, and dry-bulb temperature, relative humidity and velocity of the inlet air, on the cooling efficiency, THI, dry-bulb temperature drop and the increase in the relative humidity across the pads was analysed, with the objective of finding alternative materials that provide a fluid-thermal behaviour similar to that of the cellulose pad. For this, experimental validation in a wind tunnel and a phenomenological mathematical model of the pads were employed. A methodology based on image processing was also developed to compute the specific wetted surface area of each alternative material. Finally, expressions to compute the convective heat and mass transfer coefficients and the pressure drop across the pads were obtained for each material.