Leaving air comes to the saturation temperature

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leaving air comes to the saturation temperature. Although many types of evaporative cooler media are available, the most common type consists of a rigid, cross-fluted fiberglass or cellulose commonly referred to by the trade names Glas-dek and Cel-dek respectively. As shown in table 5, the saturation effectiveness of the media increases with the thickness and decreases with higher airflow velocities.
Design Guide for Heating, Ventilating, and Air Conditioning Systems February 29, 2000 8:09 – Last Rev: September 21, 2006 24 Table 5.—Evaporative cooler media saturation efficiency Saturation efficiency — percent Evaporative media thickness Face velocity (fpm/sq. ft.) 6 inches 8 inches 12 inches 18 inches 24 inches 300 74.0% 84.0% 91.0% 98.5% 99.7% 350 72.0% 82.0% 90.0% 98.2% 99.6% 400 71.0% 81.0% 89.5% 98.0% 99.5% 450 69.0% 80.0% 89.0% 97.8% 99.3% 500 68.0% 79.0% 88.8% 97.5% 99.0% 550 67.0% 78.0% 88.5% 97.0% 98.9% 600 66.0% 77.0% 88.2% 96.8% 98.8% 650 65.0% 76.0% 88.0% 96.3% 98.7% Pressure drop — inches w.c. 300 0.04 0.06 0.08 0.10 0.14 350 0.05 0.07 0.11 0.14 0.19 400 0.06 0.09 0.14 0.19 0.26 450 0.08 0.12 0.17 0.24 0.32 500 0.10 0.14 0.21 0.29 0.39 550 0.12 0.17 0.26 0.36 0.47 600 0.14 0.20 0.30 0.42 0.56 650 0.16 0.24 0.35 0.49 0.64 Note: Data based on CELdek ® or GLASdek ® evaporative media. 500 FPM is standard. Media are available from 4 to 24-inches thick. The standard airflow velocity is 500 ft/min. The most common media thickness used on Reclamation plants is 12 inches; however, 8, 12, and 18 inch with saturation effectiveness of approximately 79, 88.5, and 97.8 per cent, respectively, at the 500 ft/min, have been used. Some have been designed with velocities ranging from 400 to 550 ft/min. Once the saturation effectiveness, outdoor dry and wet bulb temperatures, and indoor design temperature are known, the required supply air temperature can be calculated or determined from the manufacturers
Design Guide for Heating, Ventilating, and Air Conditioning Systems February 29, 2000 8:09 – Last Rev: September 21, 2006 25 published ratings. The following equation can be used to calculate the leaving air temperature: T lvg = T db – e s (T db – T wb ) Where: T lvg = air temperature leaving the media T db = outdoor dry bulb temperature entering the media e s = saturation effectiveness (from table 5) T wb = outdoor wet bulb temperature Example: Assume outdoor design conditions of 109 ºF dry bulb, coincident wet bulb of 71 ºF, 500 ft/min airflow velocity, the leaving air temperature would be: 109 – (0.79)(109 – 71) = 79 ºF for 8-inch media ; 109 – (0.89)(109 – 71) = 75.4 ºF for 12-inch media ; 109 – (0.98)(109 – 71) = 71.8 ºF for 18-inch media ; Since the fan and motor are normally installed in the airstream, the actual supply air temperature to the conditioned space would be slightly higher (2 to 3 ºF) depending on the supply fan and motor losses. (b) Freeze protection. —Freeze protection is another major concern with evaporative cooling systems. This is especially true for unmanned plants in remote locations where freezing is possible. For these locations and conditions, the water circuit for the evaporative coolers should be designed for automatic draining. Automatic draining usually requires thermostatically controlled solenoid valves in the supply and drain piping. The valves open when a preset temperature, usually 35 to 40 ºF, is sensed. All piping and the cooling unit sumps and other auxiliary equipment such as strainers, pumps, etc. should be drained.

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