From wet and dry bulb thermometer

Table 11.4 Solutions for Maintaining Constant Humidity Table 11.5 Concentration of Solutions of H2SO4, NaOH, and CaCi2 Giving Specified Vapor Pressures and Percent Humidities at 25°C Table 11.6 Relative Humidity from Wet and Dry Bulb Thermometer Readings Table 11.7 Relative Humidity from Dew Point Readings...

TABLE 11.6 Relative Humidity from Wet and Dry Bulb Thermometer Readings... 

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 detennined 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 (dry bulb) and the difference between dry and wet bulb temperatures. The data assume a pressure near normal atmospheric pressure and an instrumental configuration with forced ventilation. 

Figure 1 depicts the experimental apparatus used in the determination of heat and mass transfer coefficients.A compressor (A) feeds the air to a tank (B), to minimize pulse fluctuations in flowrate. The air is dried as it passes trough a bed of silica gel (C). Air flowrate is measured with a rotameter and in addition with a calibrated capillary meter(E). The inlet air moisture content is measured by means of a dry and wet bulb thermometer system (D) prior to its entrance to a coil submerged in a constant temperature bath (F). From here, the air enters the bottom of the fluidized bed (G) where its temperature is measured.The fluidized bed consisted of an insulated QVF glass tube 2 inches in diameter and 12 inches in length. A system for the collection of fines(I) was installed after the bed to evaluate entrainment, although at all experimental conditions used in this work entrainment was absent. A thermometer placed on top of the bed of solids was used to measure the temperature of the bed exit.Air moisture content was also determined at the outlet of the fluidized bed by means of a hygrometer and a wet and dry bulb temperature system(J). 

Both the heat and mass transfer coefficients are functions of air velocity. However, at air speeds greater than about 15 ft/s (4.5 m/s), the ratio h kgis approximately constant. The wet-bulb depression is directly proportional to the difference between the humidity at the surface and the humidity in the bulk of the air. In the wet- and dry-bulb hygrometer, the wet-bulb depression is measured by two thermometers, one of which is fitted with a fabric sleeve wetted with water. These thermometers are mounted side by side and shielded from radiation, an effect neglected in the derivation above. Air is drawn over the thermometers by means of a small fan. The derivation of the humidity from the wet-bulb depression and a psychrometric chart are discussed later. 

This instrument consists of two normal thermometers, one of which has its bulb exposed to the air while the other has its bulb surrounded by a wick connected to a water reservoir. Evaporation of moisture from the wick to the surrounding air causes the wet bulb thermometer to show a lower reading than the corresponding dry bulb thermometer. The difference between the wet and dry bulb temperature related to the dry bulb temperature defines the hygrometric state of the atmosphere. Tables have been produced, from large numbers of observations, which give the relative humidity corresponding to likely wet and dry bulb temperature combinations. 

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. 

The effect can be observed and measured by using two similar thermometers (Figure 23.3), one of which has its bulb enclosed in a wet wick. The drier the air passing over them, the greater will be the rate of evaporation from the wick and the greater the difference between the two readings. In the case of air at 25°C, 50% saturation, the difference will be about 6.5 K. The measurements are termed the dry bulb and wet bulb temperatures, and the difference the wet bulb dqrression. 

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. 

A sling psychrometer is a device to measure the humidity of air. A porous cloth (the wick) is wrapped around the bulb of a mercury thermometer, which is then whirled around in the air. As the water in the wick evaporates, the temperature of the thermometer bulb drops, and finally stabilizes at the wet-bulb temperature of the air. The dry-bulb temperature is read from a second thermometer mounted on the sling. 

The wet-ljulb temperature is the temperature attained by a fully wetted surface, such as the wick of a wet-bulb thermometer or a droplet or wet particle undergoing drying, in contact with a flowing unsaturated gas stream. It is regulated by the rates of vapor-phase heat and mass transfer to and from the wet bulb. Assuming mass transfer is controlled by diffusion effects and heat transfer is purely convective ... 

Curves showing the relative humidity (ratio of the mass of the water vapor in the air to the maximum mass of water vapor the air can hold at that temperature, i.e., if the air were saturated) of humid air appear on the psychrometric chart. (See Figure 2.) The curve for 100% relative humidity is also referred to as the saturation curve. The abscissa of the humidity chart is air temperature, also known as the dry-bulb temperature (T b). The wet-bulb temperature (7 yb) is another measure of humidity it is the temperature at which a thermometer with a wet wick wrapped around the bulb stabilizes. As water evaporates from the wick to the ambient air, the bulb is cooled the rate of cooling depends on how humid the air is. No evaporation occurs if the air is saturated with water hence WB and Fob are the same. The lower the humidity, the greater the difference between these two temperatures. On a psychrometric chart, constant wet-bulb temperature lines are straight with negative slopes. The value of 7 vb corresponds to the value of the abscissa at the point of intersection of this line with the saturation curve. 

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