Humidity percentage

The percentage humidity is the ratio of the amount of water vapour present per unit mass of dry air to the amount the air could hold if... 

The relationship between the percentage humidity of a gas and the percentage relative humidity may be derived as follows ... 

The percentage humidity, by definition = 1 OOJf yjf o Substituting from equations 13.1 and 13.2 and simplifying ... 

When (P - Pwq)/(P — Pw) 1, the percentage relative humidity and the percentage humidity are equal. This condition is approached when the partial pressure of the vapour is only a small proportion of the total pressure or when the gas is almost saturated, that is as Pw -> PiM. 

Equations 13.16 to 13.18 give the enthalpy in terms of the temperature and humidity of the humid gas for the three conditions 0 = 0a, 0 > 0O, and 0 <0a respectively. Thus, given the percentage humidity and the temperature, the humidity may be obtained from Figure 13.4, the enthalpy calculated from equations 13.16, 13.17 or 13.18 and plotted against the humidity, usually with enthalpy as the abscissa. Such a plot is shown in Figure 13.7 for the air-water system, which includes the curves for 100 per cent humidity and for some lower value, say Z per cent. 

The distinction between percentage humidity and percentage relative humidity is of significance though, the difference in the values of the two quantities does not usually exceed 7 to 8 per cent. Reference may be made here to Volume 1, Section 13.2.1. 

On the humidity chart of Figure 2.5, temperatures are plotted as abscissas and humidities as ordinates. Any point on the plot represents a specific mixture of air and water. The curve marked 100% humidity refers to saturated air and is a function of air temperature. Any point to the left of the saturation curve represents a mixture of saturated air and liquid water (this portion of the plot is useful in determining fog formation). Any point to the right of the saturation curve represents undersaturated air. Any point on the temperature axis represents bone-dry air. The curves between the two limits (saturated line and the temperature axis) represent mixtures of air and water of definite percentage humidities. Linear interpolation between the saturation curve and the temperature axis locates lines of constant percentage humidity. 

The diagonal lines that run upward and to the left of the saturation curve in Figure 2.5 are referred to as adiabatic cooling curves. Lines for the specific volume of dry air and the saturated volume are also shown. Coordinates of points on these lines are computed by use of Equation 2.38. The humid volume of unsaturated air can be determined by linear interpolation between the two lines, based on percentage humidity. 

The temperature and dew point of the air entering a certain dryer are 130 and 60°F (328 and 289 K), respectively. Using a humidity chart (Fig. 19.9), find the following properties of the air its humidity, its percentage humidity, its adiabatic-saturation temperature, its humidity at adiabatic saturation, its humid heat, and its humid volume. 

Related Calculations. Do not confuse percentage humidity with relative humidity. Relative humidity is the ratio of the partial pressure of the water vapor to the vapor pressure of water at the temperature of the air, this ratio usually being expressed as a percent. Percentage humidity is the ratio of the actual humidity to the saturation humidity that corresponds to the gas temperature, which is also usually expressed as a percent. At all humidities other than 0 or 100 percent, the percentage humidity is less than the relative humidity. 

Humid air at 75°C, 1.1 bar, and 30% relative humidity is fed into a process unit at a rate of 1000 m /h. Determine (1) the molar flow rates of water, dry air, and oxygen entering the process unit, (2) the molal humidity, absolute humidity, and percentage humidity of the air, and (3) the dew point. 

The latest weather report includes the following statement The temperature is 78°F, barometric pressure is 29.9 inches, and the relative humidity is 87%." From this information, estimate the mole fraction of water in the air and the dew point ( F), molal humidity, absolute humidity, and percentage humidity of the air. 

Percentage humidity is the ratio of the actual humidity to the saturation humidity at the gas temperature, also on a percentage basis, or... 

At all humidities other than 0 or 100 percent, the percentage humidity is less than the relative humidity. 

Humidity, mass of vapor per unit mass of vapor-free gas at top of contractor at bottom of contactor at gas-liquid interface saturation humidity saturation humidity at wet-bulb temperature Percentage humidity,... 

---