Summary:

Direct evaporative cooling is one of the most efficient techniques used in various air conditioning applications such as cooling towers, humidifiers and evaporative coolers. In this process, water and air are in contact with cross-flow arrangement, i.e., vertical channels for water flow and horizontal channels for air. First, warm air is drawn by fan into a dwelling through a porous wetted material or pads. Then, the low temperature water absorbs heat from higher temperature air and evaporates from porous wetted medium resulting low temperature air leave the system (Fig. 1). During the cooling process, the wet-bulb temperature of the air remains constant and the porous pad are wetted continually by spraying water on the surfaces of pads or by dripping water onto the upper edge of vertically mounted pads.

Fig. 1. Schematic of evaporating cooling process.

The efficiency of evaporative pad systems is affected by many factors including surface area and thickness of the pad, the type of material used to fabricate pad, the size of perforations, flow rate and relative humidity of air passing through the pad, and volume of water used. Evaporative pads have made from different materials such as metal, cement, wood, plastic, and glass. Manufacturing of commercial pads made of these materials are complicated and costly. Therefore, evaluating the locally available cheap materials, particularly in rural agricultural areas is essential. In this work, we introduced a new type of evaporative pad made of corrugated cellulosic papers (Fig. 2) and experimentally assessed its thermal and hydrodynamics performances. The proposed pad is inexpensive, visually attractive, locally available, and can be easily constructed into the required shape. Furthermore, it is energy efficient, compact in size and light in weight, and pollution free. These advantages make the cellulosic evaporative pads interesting candidates for wide range of applications including industrial and residential sectors, swine building, poultry, greenhouses, storage warehouses, agriculture market, combustion turbine inlet air-cooling system, household appliances, and in horticulture.

Fig. 2. Air flow channels formed by the alternate layers of cellulosic papers.

We experimentally studied thermal performance of two types of cellulosic pads (5090 and 7090) which were made from corrugated papers. Samples were tested in a subsonic wind tunnel made from polyethylene. The pads areas are 0.5* 0.5 m2 with 75, 100 and 150 mm thicknesses. Figure 3 show the configuration and dimensions of pads. Figure 4 shows the schematic of the experimental setup. Pressure drop, humidity variation, evaporated water and effectiveness have been measured for several inlet air velocities. As seen in Figure 5, the overall pressure drop and the amount of evaporated water increase by increasing the inlet air velocity and thickness of pads. On the other hand, effectiveness and humidity variation decrease by increasing inlet air velocity.

Fig. 3. Configuration and dimensions of cellulosic pads.

Fig. 4. Schematic of experimental set up.

Fig. 5. Pressure drop, evaporated water, Humidity variation, and effectiveness for pad 7090.