Humidity wet 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 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. 

Use the psychrometric chart to estimate (1) the absolute humidity, wet-bulb temperature, humid volume, dew point, and specific enthalpy of humid air at 41°C and 10% relative humidity, and (2) the amount of water in 150 m of air at these conditions. 

YOURSELF the absolute humidity, wet-bulb temperature, dew point, humid volume, and specific enthalpy... 

An open vessel containing 0.205 Ibm of liquid water is placed in an empty room 5 ft wide, 4 ft deep, and 7 ft high, which initially contains dry air at 90 F. All the water evaporates without changing the room temperature. Use the psychrometric chart to estimate the final relative humidity, wet-bulb temperature, humid volume, dew-point temperature, and specific enthalpy of the room air. Take the molecular weight of dry air to be 29.0. and for simplicity, assume the mass of dry air in the room stays constant at its initial value. 

The calculations of absolute humidity, relative humidity, wet-bulb temperature, dew-point temperature, humid volume, humid heat, and humid enthalpy are based on information found in Pakowski and Mujumdar (2006) and Moyers and Baldwin (1997). 

The dry bulb temperature (DBT) is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. The wet-bulb temperature (0w) is the temperature a parcel of air would have if it were cooled to saturation (100% relative humidity) by the evaporation of water into it, with the latent heat being supplied by the parcel. In other words, wet bulb temperature is the temperature reached by water surface if the air is passed over it. Wet bulb temperature is a function of dry bulb temperature and humidity. The chart shows dry bulb temperature on the x-axis and moisture content on the y-axis. Any point below the saturation line represents air that is unsaturated, therefore, the chart has relative humidity cxirves going up to 100% relative humidity. Wet bulb temperature lines are constant enthalpy or adiabatic cooling lines. The change in composition of... 

Thus, a measurement of the wet-bulb temperature, and the temperature T, allows the molal humidity, Y, to be calculated because is known. 

Hours in kiln Dry-bulb temperature, °C Wet-bulb temperature, °C Relative humidity, %... 

In Figure 2 the lines, volume, m /kg dry air, indicate humid volume, which includes the volume of 1.0 kg of dry gas plus the volume of vapor it carries. Enthalpy at saturation data are accurate only at the saturation temperature and humidity however, for air—water vapor mixtures, the diagonal wet bulb temperature lines are approximately the same as constant-enthalpy adiabatic cooling lines. The latter are based on the relationship ... 

Experimentally it has been shown that for air-water systems the value of Tj /Zc c, the psychrometric ratio, is approximately equal to 1. Under these conditions the wet-bulb temperatures and adiabatic-saturation temperatures are substantially equal and can be used interchangeably. The difference between adiabatic-saturation temperature and wet-bulb temperature increases with increasing humidity, but this effect is unimportant for most engineering calculations. An empirical formula for wet-bulb temperature determination of moist air at atmospheric pressure is presented by Liley [Jnt. J. of Mechanical Engineering Education, vol. 21, No. 2 (1993)]. 

Example 1 Compare Wet-Bulb and Adiabatic-Saturation Temperatures For tne air-water system at atmospheric pressure, the measured values of dry-bulh and wet-hulh temperatures are 85 and 72 F respectively. Determine the absolute humidity and compare the wet-bulb temperature and adiabatic-saturation temperature. Assume that h /k is given by Eq. (12-4). 

Relative humidity = 15 percent Wet-bulb temperature = 51.5 F Dew point = 25.2 F... 

Example 4 Evaporative Cooling Air at 95 F dry-bulb temperature and 70 F wet-bulb temperature contacts a water spray, where its relative humidity is increased to 90 percent. The spray water is recirculated makeup water enters at 70 F. Determine exit dry-bulb temperature, wet-bulb temperature, change in enthalpy of the air, and quantity of moisture added per pound of dry air. 

Since the Schmidt number, Prandtl number, latent heat of vaporization, and humid heat are all essentially independent of pressure, the adiabatic-saturation-temperature and wet-bulb-temperature hues will be substantially equal at pressures different from atmospheric. 

Example 8 Determination of Air Properties For a barometric pressure of 25.92 inHg (Ap = —4), a dry-bulb temperature of 90 F, and a wet-bulb temperature of 70 F determine the following absolute humidity, enthalpy, dew point, relative humidity, and specific volume. 

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. 

Tti e wet-bulb temperature is established by a dynamic equilibrium between heat and mass transfer when liquid evaporates from a small mass, such as the wet bulb of a thermometer, into a veiy large mass of gas such that the latter undergoes no temperature or humidity change. It is expressed by the relationship... 

Humidity charts for other solvent vapors may be prepared in an analogous manner. There is one important difference involved, however, in that the wet-bulb temperature differs considerably from the adiabatic-saturation temperatures for vapors other than water. 

Figures 12-37 to 12-39 show humidity charts for carbon tetrachloride, oenzene, and toluene. The lines on these charts have been calculated in the manner outlined for air-water vapor except for the wet-bulb-temperature lines. The determination of these hnes depends on data for the psychrometric ratio /j Z/c, as indicated by Eq. (12-22). For the charts shown, the wet-bulb-temperature hnes are based on the following equation ...

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. 

Wet bulb temperature, °F, three digits Relative humidity, percent, three digits Clouds and obscuring phenomena Total amount, tenths, one coded alphanumeric character"... 

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... 

---