Humidity chart example

For practical purposes, the properties of humid air are recorded on psychrometric (or humidity) charts such as those of Figures 9.1 and 9.2, but tabulated data and equations also are available for greater accuracy. A computer version is available (Wiley Professional Software, Wiley, New York). The terminal properties of a particular adiabatic humification of air are located on the same saturation line, one of those sloping upwards to the left on the charts. For example, all of these points are on the same saturation line (7)//) = (250,0.008), (170, 0.026) and (100,0.043) the saturation enthalpy is 72 Btu/lb dry, but the individual enthalpies are less by the amounts 2.5, 1.2, and 0, respectively. 

The adiabatic cooling lines are lines of almost constant enthalpy for the entering air-water mixture, and you can use them as such without much error (1 or 2%). However, if you want to correct a saturated enthalpy value for the deviation which exists for a less-than-saturated air-water vapor mixture, you can employ the enthalpy deviation lines which appear on the chart and which can be used as illustrated in the examples below. Any process that is not a wet-bulb process or an adiabatic process with recirculated water can be treated by the usual material and energy balances, taking the basic data for the calculation from the humidity charts. If there is any increase or decrease in the moisture content of the air in a psychrometric process, the small enthalpy effect of the moisture added to the air or lost by the air may be included in the energy balance for the process to make it more exact as illustrated in Examples 4.47 and 4.49. 

EXAMPLE 4.46 Properties of Moist Air from the Humidity Chart... 

Example 23.2. The temperature and dew point of the air entering a certain dryer are 150 and 60°F (65.6 and I5.6°C), respectively. What additional data for this air can be read from the humidity chart ... 

Effect of gas humidity. If the gas humidity H is decreased for a given T of the gas, then from the humidity chart the wet bulb temperature will decrease. Then using Eq. (9.6-7), Rc will increase. For example, if the original conditions areRci> fVi, H, and... 

Hs from the humidity chart at saturation isf 0.032. Substituting into Eq. (9.8-10) while assuming that does not change appreciably, a value of Ts = 32.8°C is obtained. Hence, the final value is 32.8°C. This is 3.9°C greater than the wet bulb temperature of 28.9°C in Example 9.6-3, where radiation and conduction were absent. 

This concrete example illustrates the various uses to which tire humidity charts may be put We choose moist air with a relative humidity of 25% and a (dry-bulb) temperature of 50dC and proceed to calculate various properties of interest using the chart shown in Figure 9.4. 

If the cycle is being traced out on a psychrometric chart, the enthalpy can be read off for the coil inlet and outlet conditions. In Example 24.1, the enthalpy increase as measured on the chart is 7.35 kj/kg dry air (taken at any value of humidity), giving... 

The dew point of humid air at a given point on the psychrometric chart can easily be determined. For example, locate the point on Figure 8.4-1 corresponding to air at 29 C and 20% relative humidity. Cooling this air at constant pressure ( = 1 atm) corresponds to moving horizontally (at constant absolute humidity) to the saturation curve. Tjp is the temperature at the intersection, or 4 C. (Verify this statement.)... 

To determine the volume of a given mass of wet air using the psychrometric chart, you must first determine the corresponding mass of dry air from the absolute humidity, then multiply this mass by Vh- Suppose, for example, you wish to know the volume occupied by 150 kg of humid air at 7 = 30°C and = 30%. From Figure 8.4-1, = 0.0080 kg H20(v)/kg DA... 

This result (which is far from obvious) allows us to perform adiabatic cooling calculations with relative ease using the psychrometric chart. First locate the initial state of the air on the chart then locate the final state on the constant wet-bulb temperature line that passes through the initial state (or on the 100% humidity curve if cooling below the adiabatic saturation temperature takes place) and finally perform whatever material and energy balance calculations are required. Example 8.4-7 illustrates such a calculation for an adiabatic humidification operation. 

The concentration of water vapor in air is called the humidity of the air. However, humidity may be expressed in several ways. To understand the interrelationships among temperature, vapor pressure, heat energy, and humidity, one may consult psychrometric charts that are found in most chemical engineering handbooks (15-17). Charts may be differentiated for certain conditions of temperature and pressure. For example, charts are designated for low, medium and high temperature as well as for conditions of pressure. A particularly lucid discussion of the use of the psychrometric chart may be found in Ref. 18. 

Absolute Humidity—the ratio of mass of vapor (moisture) to mass present in the carrier gas stream. Example 0.02 pounds of water per pound of air. This number can be used to find the relative humidity on the psychrometric charts. It is also useful for cumulative quantities in a stream due to such items as products of combustion (when a gas fired heater is used), and evaporation and ambient quantities. This is necessary for calculating condenser or venting amounts. 

There are many forms of psychrometric charts available from various technical sources as well as many manufacturers of process equipment who have tailored the chart for use with their equipment. These charts are useful for determining moisture content in the air at a given temperature and relative humidity or wet bulb temperature. The type of information obtainable from these charts depends upon which chart one uses, because each is designed differently. Usually, accompanying the chart is a set of instructions for use. Please see the references for examples of these charts and the various forms in which they exist. 

Figure 18-1 is a psychrometric chart. To find the relative humidity, follow the lines for a dry-bulb temperature and a wet-bulb temperature to the point of their intersection. Then read or interpolate the relative humidity from the percent relative humidity lines. For example, the 80 °F dry-bulb temperature and the 75 °F wet-bulb temperature lines intersect at the 80% relative humidity line. 

---