Ambient wet-bulb temperature

Cooling-tower fan horsepower can be reduced substantially as the ambient wet-bulb temperature decreases if two-speed fan motors are used. Theoretically, operating at half speed will reduce air flow by 50 percent while decreasing horsepower to one-eighth of full-speed operation. However, ac tual half-speed operation will require about 17 percent of the horsepower at full speed as a result of the inherent motor losses at lighter loads. 

The variable-pitch arrangement at constant motor speed changes the pitch of the olades through a pneumatic signal from the leaving-water temperature. As the thermal load and/or the ambient wet-bulb temperature decreases, the blade pitch reduces air flow and less fan energy is required. 

Precise control of the outlet chilled-water temperature does not occur with thermocycle operation. This temperature is dependent on ambient wet-bulb-temperature conditions. Normally, during cold winter days little change occurs in wet-bulb temperatures, so that only slight water-temperature variations may occur. This would not be true of many spring and fall days, when relatively large climatic temperature swings can and do occur. 

Ambient Wet-Bulb Temperature The temperature in degrees Fahrenheit to which air can be cooled, making it adiabatic to saturation by the addition of water vapor, in practical terms, the wet-bulb temperature is the temperature indicated by a thermometer, the bulb of which is kept moist by a wick and over which air is circulated. 

Ambient dry bulb temperature for air-cooled condensers Available water temperature for water-cooled condensers Ambient wet bulb temperature for evaporative types... 

Heat from the water is transferred to the air, so the available heat gain by the air will depend on its initial enthalpy. This is usually expressed in terms of ambient wet bulb temperature, since the two are almost synonymous and the wet bulb is more easily recognized. This is used as a yardstick to describe performance in terms of the approach of the leaving water temperature to the ambient wet bulb. The two could only meet ultimately in a tower having an air flow infinitely larger than the water flow, so the term is descriptive rather than a clear indication of tower efficiency. 

Estimate the height and base diameter of a natural draught hyperbolic cooling tower which will handle a flow of 5000 kg/s water entering at 300 K and leaving at 294 K. The dry-bulb air temperature is 287 K and the ambient wet bulb temperature is 284 K. 

The difference between cold-water temperature and the ambient wet-bulb temperature (AWB). 

Determine the theoretically required KaV/L value for a cooling duty from 105°F inlet water, 85°F outlet water, 78°F ambient wet-bulb temperature, and an UG ratio of 0.97. 

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