Diy-bulb temperature

Trim Coolers Conventional air-cooled heat exchangers can cool the process fluid to within 8.3°C (15°F) of the design dry-biilb temperature. When a lower process outlet temperature is required, a trim cooler is installed in series with the air-cooled heat exchanger. The water-cooled trim cooler can be designed for a 5.6 to 11.1°C (10 to 20°F) approach to the wet-biilb temperature (which in the United States is about 8.3°C (15°F) less than the diy-bulb temperature). In arid areas the difference between diy- and wet-bulb temperatures is much greater. 

Approach temperature. The approach temperature, which is the difference between the process-fluid outlet temperature and the design dry-bulb air temperature, has a practical minimum of 8 to 14°C (15 to 25°F). When a lower process-fluid outlet teiTperature is required, an air-humidification chamber can be providea to reduce the inlet air temperature toward the wet-bulb temperature. A 5.6°C (10°F) approach is feasible. Since typical summer wet-bulb design temperatures in the United States are 8.3°C (15°F) lower than diy-bulb temperatures, the outlet process-fliiid temperature can be 3°C (5°F) below the dry-bulb temperature. 

The relative humidity is obtained by dividing the calculated partial pressure by the vapor pressure of water at the diy-bulb temperature. Thus ... 

A given humidity chart is precise only at the pressure for which it is evaluated. Most air-water-vapor charts are based on a pressure of 1 atm. Humidities read from these charts for given values of wet- and diy-bulb temperature apply only at an atmospheric pressure of 760 mmHg. If the total pressure is different from 760 mmHg, the humidity at a given wet-bulb and dry-bulb temperature must be corrected according to the following relationship. 

Determination of the Temperature of the Evaporating Surface in Direct-Heat Tray Dryers When radiation and conduction are negligible, the temperature of the evaporating surface approaches the wet-bulb temperature and is readily obtained from the humidity and diy-bulb temperature. Frequently, however, radiation and conduction cause the temperature of the evaporating surface to exceed the wet-bulb temperature. When this occurs, the true surface temperature must be estimated. 

Psychometric methods. A very common method of measuring the humidity is to determine simultaneously the wet-bulb and dry-bulb temperatures. From these readings the humidity is found by locating the psychrometric line intersecting the saturation line at the observed wet-bulb temperature and following the psychrometric line to its intersection with the ordinate of the observed diy-bulb temperature. 

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 diy-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. 

Average moisture content (%) Diy-bulb temperature fC) Wet-bulb temperature CO Relative humidity (%)... 

Enthalpy The (relative) enthalpy of a vapor-gas mixture is the sum of the (relative) enthalpies of the gas and of the vapor content. Imagine unit mass of a gas containing a mass Y of vapor at diy-bulb temperature t. If the mixture is unsaturated, the vapor is in a superheated state and we can calculate the... 

Dlustntkm 7.12 In the cooler of Illustration 7.11, to what temperature would the water be cooled if, after the tower was built and operated at the design V and Gg values, the entering air entered at diy-bulb temperature (fjf 32 C. wet-bulb temperature 28 C ... 

A sample of a porous, manufactured sheet material of mineral origin was dried from both sides by cross circulation of air in a laboratory drier. The sample was 0.3 m square and 6 mrn thick, and the edges were sealed. The air velocity over the surface was 3 m/s, its diy-bulb temperature was 52 0, and its wet-bulb temperature 2I C. There were no ratUation effects. The solid lost moisture at a constant rate of 7.5 X 10 kg/s until the critical moisture content, 15% (wet basis), was reached. In the falling-rate period, the rate of evaporation feU linearly with moisture content until the sample was dry. The equilibrium moisture was negligible. The dry weight of the sheet was 1.8 kg. 

Enthalpy data are given on the basis of kilojoules per kilogram of diy air. Entbalpy-at-saturation data are accurate only at the saturation temperature and humidity. Enthalpy deviation curves permit enthalpy corrections for humidities less than saturation and show how the wet-bulb-temperature hues do not precisely coincide with constant-enthalpy, adiabatic cooling hnes. 

However, the complete reaction mechanism of the hydrogen oxidation reaction is much more complex, both in its number of reaction steps, number of intermediates (OOH and H2O2), and observed behavior. A mixture of H2 and O2 can sit in a diy bulb for many years with absolutely no H2O detected. However, if water is initially present, the reaction will begin, and if a spark is ignited or a grain of platinum is added to the mixture at room temperature, the reaction wiU occur instandy and explosively. 

The wet-and-dry bulb hygrometer consists of two thermometers that are adjacent to each other. The diy-bulb thermometer measures the temperature of the air. The bulb of the other thermometer is kept wet by a wick dipped in water. As the water evaporates due to the air flowing across, the wet bulb is cooled. 

The temperature limit of the heated up water is the wet bulb temperature of the smoke. For all cold water temperatures above 40 deg.C, the hot water temperature remains fairly stable at about 72 deg.C. Weth a smoke temperature between 120 and 150 deg.C and a water content of about 0.3 kg water vapour per kg diy gas, such an exchanger one cubic meter in size, is able to warm np 40 m3/h of water from 40 deg.C to 72 deg.C. This type of heat recovery is best applicable to the heating of buldings or workshop. 

Inlet air, 24.0 C diy-bulb, 15.6 C wet-bulb temperature Outlet air, 37.6 C, essentially saturated... 

During the constant rate period, evaporation is taking place from the fabric surface. The rate of diying is essentially that of the evaporation of the liquid component under the conditions of temperature and airflow during the process. High air velocities will reduce the thickness of the stationary gas film on the surface of textile material and hence increase the heat and mass transfer coefficients. In commercial forced convection dryers the effects of heat transfer by conduction and radiation may be appreciable, due to the fact that the material surface temperatures are higher than the wet-bulb temperature of the diying air. 

Experiment.—Determine the Absolute Density of Dry Air. The volume of the bulb and its weight when exhausted are first determined, as explained above. The diy bulb is then clamped in a bath of water kept at a constant temperature, say 25° (see Chap. IV.), so that the bulb is entirely immersed, but not the stopcock (Fig. 10), and a T-tube a is attached to it by means of pressure tubing. The side tube of a is connected with a Fleuss or mercury pump by means of pressure tubing on which there is a screwclip, I and to the end of a tube of calcium chloride is attached, also by means of pressure tubing furnished with a screw-clip, II. This clip is kept closed,... 

Prepare a psychromctric chart for the mixture acetone-nitrogen at a pressure of 800 mmHg over the ranges —15 to 60 C, T = 0 to 3 kg vapor/kg diy gas. Include the following curves, all plotted against temperature (a) 100, 75, SO, and 25% humidity (b) dry and saturated humid volumes (c) enthalpy of dry and saturated mixtures expressed as N m/kg dry gas, referred to liquid acetone and nitrogen gas at — 15 C (d) wet-bulb curves for 25 C (e) adiabatic-saturation curves... 

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