Wet-bulb thermometer

For the air—water system, the humidity is easily measured by using a wet-bulb thermometer. Air passing the wet wick surrounding the thermometer bulb causes evaporation of moisture from the wick. The balance between heat transfer to the wick and energy requited by the latent heat of the mass transfer from the wick gives, at steady state,... 

For the case of flow past cylinders, such as a wet-bulb thermometer, Bedingfield and Drew [Ind. Eng. Chem., 42, 1164 (1950)] obtained a correlation for their data on siiblimation of cylinders into air and for the data of others on wet-bulb thermometers. For wet-bulb thermometers in air they give... 

The stream of gas over the liquid surface may be produced by a small fan or other similar means (Figure 13.10a), The crude forms of wet-bulb thermometer which make... 

One instrument for determining the relative humidity of the atmosphere. It consists of a pair of thermometers, the bulb of one of which is surrounded by a fabric wick kept moist from a small water reservoir. Due to evaporation the temperature indicated by the wet bulb thermometer is lower than that of the other. The difference in the readings compared to the dry bulb reading enables the relative humidity to be obtained from tables. 

To measure the internal flow velocity in the duct, dust sampling was taken at various points along the vertical diameter. A pitot static tube and magnehelic gauge, shown in Figure 1, was the equipment used for these measurements. The duct humidity, tempertaure, and static pressure were measured to calculate the gas density. In determining the humidity, the wet and dry bulb temperature of a continuous sample stream was used. To prevent dust buildup on the wet bulb thermometer, an inline metal filter was inserted into the line. 

As water travels down through the lower regions of the cooling tower, it will have a temperature of, or less than, the dry-bulb temperature of the air. Under these conditions, sensible heat and mass transfer are in opposite directions. This is analogous to the conditions of a wet-bulb thermometer (see Chapter 2). In fact, the wet-bulb temperature represents the limit to which the discharge water temperature can fall to in a cooling tower where adiabatic equilibrium exists between the water and inlet air. 

Temperature of ambient air, as measured with a wet-bulb thermometer... 

For the sake of simplicity and clarity of the presentation, we confine further considerations to the so-called constant drying rate period. In this period, the temperature of the dried body is constant and equal to the temperature of the wet bulb thermometer if the drying condition are stable, i.e.,... 

The balance between conduction and diffusion still operates for a much larger isolated wet object, provided radiation is excluded. This is the basis of the wet bulb thermometer method for measuring humidity. The actual rate of evaporation now is not as simply determined and is influenced by wind. The wet bulb temperature is almost independent of wind condition, owing to a convenient accident. Heat conduction is a diffusion process, and the diffusion coefficient for water vapor in air (0.24 sq. cm./sec.) is numerically close to the diffusion coefficient of temperature in air (thermal conductivity/specific heat = 0.20 sq. cm./sec.). Hence, the exact way in which each molecular diffusion process merges into the more rapid eddy diffusion process is not important because no matter how complex the transition is, it must be quantitatively similar for the two processes. 

The cover on a wet-bulb thermometer is soaked in benzene, and the thermometer is exposed to a stream of dry air. The thermometer indicates a temperature of 26°C. Calculate the free-stream air temperature. The vapor pressure of benzene is 13.3 kPa, and the enthalpy of vaporization is 377 kJ/kg at 26°C. 

Air at 1 atm and I00°F flows across a wet-bulb thermometer, producing a temperature of 70°F. Calculate the relative humidity of the airstream using Carrier s equation and compare with results obtained by methods of this chapter. 

Wet-bulb temperature the temperature indicated by a wet-bulb thermometer where the bulb is covered by a thin cotton or muslin sleeve, wetted with distilled water, and air is drawn over the bulb in a velocity of at least 107 meters/ minute (350 feet/min). 

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). 

Figure L Experimental apparatus A, air compressor B, tank C, silica gel trap D, dry and wet bulb thermometer system E, capillary meter F, constant temperature bath G, fluidized bed /, fines collector J, dry and wet bulb thermometer sys tern Tg, entrance temperature thermometer H, U tube manometer To, outlet temperature thermometer...

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 ... 

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... 

In order to use equation (8-15) for determination of T it is necessary to know the value of hGtkr known as the psychrometric ratio. Values of hG and ky can be estimated independently for the particular shape of the wetted surface by correlations like those presented in Chapter 2, using the heat- and mass-transfer analogy if necessary. Alternatively, experimental values of the psychrometric ratio can be used. Henry and Epstein (1970) have examined the data and methods of measurement and have produced some measurements of their own. For turbulent flow of gases past cylinders, such as wet-bulb thermometers, and past single spheres, the results of 18 gas-vapor systems are well correlated by... 

Main experimental apparatus include safety helmet, safety cover, dry bulb thermometer, wet bulb thermometer, thermocouple thermometer, stopwatch, gas chromatograph and so on. 

For turbulent flow past a wet cylinder, such as a wet bulb thermometer, the accumulated experimental data give... 

FIGURE 2.4 Schematic diagram of the arrangement based on measuring the wet bulb temperature (1) sample, (2) measuring chamber, (3) foil ballon, (4) dashpot, (5) air pump, (6) paddle wheel with magnetic clutch, (7) motor, (8) thermometer, (9) wet bulb thermometer, (10) thermostat. 

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