Determination of Relative Humidity from Wet and Dry Bulb

DETERMINATION OF RELATIVE HUMIDITY FROM WET AND DRY BULB TEMPERATURES... 

Determination of Relative Humidity from Wet and Dry Bulb Temperatures.15-32... 

Example 3 Air Heating Air is heated by a steam coil from 30 F dry-bulb temperature and 80 percent relative humidity to 75 F dry-bulb temperature. Find the relative humidity, wet-bulb temperature, and dew point of the heated air. Determine the quantity of heat added per pound of dry air. 

A comparison of wet and dry bulb readings allows the relative humidity to be determined from a psychrometric chart. The wet bulb temperature is always lower than the dry bulb value except when the air is already saturated with water - 100% relative humidity. This is when the wet and dry bulb temperatures are the same. Tlie air will no longer accept water and the lack of evaporation does not allow the wetted bulb to reject heat into the air by evaporation. This situation would be... 

Relative humidity is difficult to measure reliably and instead it is determined indirectly from the wet- and dry-bulb thermometers of a hygrometer. The wet-bulb thermometer is kept moist with a fabric sleeve whose other end is in a reservoir of clean water. As air passes over the wet sleeve water is evaporated and cools the wet-bulb thermometer the drier the air the greater the cooling effect. The dry-bulb measures the air temperature there is no cooling effect on the dry-bulb thermometer. The difference between the dry-bulb and wet-bulb temperatures, the wet-bulb depression (AT), and the dry-bulb temperature are the parameters used to... 

Relative humidity may be determined by comparing temperature readings of wet and dry bulb thermometers. The following table, extracted from more extensive U.S. National Weather Service tables, gives the relative humidity as a function of air temperature... 

Psychrometry has to do with the properties of the air-water vapor mixtures found in the atmosphere. Psychrometry tables, published by the US Weather Bureau, give detailed data about vapor pressure, relative humidity and dew point at the sea-level barometer of 30 in Hg, and at certain other barometric pressures. These tables are based on relative readings of dry bulb and wet bulb atmospheric temperatures as determined simultaneously by a sling psychrometer. The dry bulb reads ambient temperature while the wet bulb reads a lower temperature influenced by evaporation from a wetted wick surrounding the bulb of a parallel thermometer. 

For example, suppose you wish to determine the wet-bulb temperature of air at 30°C (dry bulb) with a relative humidity of 30%. Locate the point on Figure 8.4-1 at the intersection of the vertical line corresponding to T = 30°C and the curve corresponding to h, = 30%. The diagonal line through the point is the constant wet-bulb temperature line for air at the given condition. Follow that line upward to the left until you reach the saturation curve. The temperature value you read on the curve (or vertically down from it on the abscissa) is the wet-bulb temperature of the air. You should get a value of 18°C. This means that if you wrap a wet wick around a thermometer bulb and blow air with T = 30°C and h, = 30% past the bulb, the thermometer reading will drop and eventually stabilize at 18 C. 

Alden et al. extended the method by estimating the equilibrium relative humidity based on the wet-bulb temperature (TJ, the partial pressure of water at the solid s temperature, and the saturated vapor pressure of water at 7 [46]. Thus, the moisture content of a given material can be inferred from the estimated temperature difference (AT) and the equilibrium relative humidity. This method has been successfully implemented to control the endpoint in batch fluid-bed dryer for aqueous and aqueous-alcohol granules. An pirical method was used to determine the desired AT value by frequent sampling of the dried material and moisture content determination. The realtime AT was estimated using a computer program based on the temperature measurements this value was then compared with the desired value and used as the drying-endpoint indicator. 

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