Mass absolute humidity

In many respects the molal ratio is the more convenient, thanks to the ease with which moles and volumes can be interrelated through the gas law, but the mass ratio has nevertheless become firmly established in the humidification literature. The mass absolute humidity was first introduced by Grosvenor [7] and is sometimes called the Grosvenor humidity. 

The absolute humidity is the mass of water vapour per unit mass of dry air. 

The absolute humidity of air refers to the mass of water vapour actually present in unit vol., and it is determined by passing a known vol. of air over a suitable drying agent—calcium chloride, phosphorus pentoxide, etc. (1. 7, 2)—and... 

Any increase of the suspension injection, in order to realize an improved separation of the gas component which can be removed, is not possible. The maximum liquid and/or solvent injection into a fluidized bed is limited by the capacity of the fluidization gas. The maximum of the capacity is reached with the state of saturation of the gas. Figure 16.6 illustrates the dependence of the adjusting wetted surface and the absolute humidity on the spraying liquid mass flow. Clearly, a small deviation of... 

Spanel, P., Smith, D. On-line measurements of the absolute humidity of air, breath and liquid headspace samples by selected ion flow tube mass spectrometry. Rapid Commun. Mass Spectrom. 15, 563-596 (2001)... 

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... 

The psychrometric chart (oT humidity chart) conlam values of a number of process variables for air-water vapor systems at 1 atm. The values listed on the chart include dry-bulb temperature (the temperature measured by common temperature-measurement instruments), moisture content or absolute humidity (mass ratio of water vapor to dry air), relative humidity, humid volume (volume per mass of dry air), wet-bulb temperature (the temperature reading on a thermometer with a water-saturated wick around the bulb immersed in a flowing stream of humid air), and enthalpy per mass of dry air. If you know the values of any two of these variables for humid air at or near 1 atm, you can use the chart to determine the values of the other four, which can greatly simplify material and energy balance calculations. 

Absolute humidity Y Mass of water (or solvent) vapor carried by unit mass of dry air (or other carrier gas). It is also known as the mixing ratio, mass ratio, or dry-basis humidity. Preferred units are Ib/lb or kg/kg, but g/kg and gr/lb are often used, as are ppm and ppb (parts per million/billion by weight) ppm = 10 Y, ppb = 10 Y. 

Specific humidity Yw Mass of vapor per unit mass of gas-vapor mixture. Also known as mass fraction or wet-basis humidity, and much more rarely used than dry-basis absolute humidity. Y, = (Y/(l -I- Y) Y = Y /... 

Volumetric humidity Y Mass of vapor per unit volume of gas-vapor mixture. It is sometimes, confusingly, called the absolute humidity, but it is really a vapor concentration preferred units are kg/m or Ib/fT, but g/nV and gr/ft are also used. It is inconvenient for calculations because it depends on temperature and pressure and on the units system absolute humidity Y is always preferAle for heat and mass balances. It is proportional to the specific humidity (wet basis) Yy = Yy/pg, where pg is the humid gas density (mass of gas-vapor mixture per unit volume, wet basis). Also... 

Method (viii) allows the calculation of all the main parameters if the absolute humidity is known, e.g., from a mass balance on a process plant. 

Method (ix) converts the volumetric form of absolute humidity to the mass form (mixing ratio). 

Example 7 Determination of Moist Air Properties An air-water mixture is found from the heat and mass balance to he at 60 C (333 K) and 0.025 kg/kg (25 g/kg) absolute humidity. Calculate the other main parameters for the mixture. Take atmospheric pressure as 101,325 Pa. 

Absolute humidity is the mass ratio of water vapor (or other solvent mass) to dry air. 

Given Incoming wet sheet mass flow rate is 100 kg/h. It enters with 20 percent water on a wet basis and leaves at 1 percent water on a wet basis. The airflow rate is 1000 kg/h, with an absolute humidity of 0.01 g water/g dry air. The incoming air temperature is 170°C. The sheet enters at 20°C and leaves at 90"C. 

The mass flow rates of the dry air and incoming water vapor can be calculated from the overall airflow rate and the incoming absolute humidity ... 

Application of mass balances Plugging in the numbers gives the relationship between absolute humidity and moisture in the solids at any position. 

If the absolute humidity, h, is known, the molecular weights of water (18) and bone dry air (29) must be used to convert fi om a mass ratio to a molar ratio. The molar ratio of water vapor to BDA is... 

Relative humidity of air is defined as the vapour pressure of water vapour in a given volume of air divided by the saturated vapour pressure. The absolute humidity at saturation (in g m ) and the saturated vapour pressure of water are shown in Table 8.1. Thus a cubic metre of saturated air can hold 17 g of water vapour at 20°C, 130 g at 60°C and 600 g at 100°C. In the wider drying literature, absolute humidity is normally given as a mass ratio of water vapour to the mass of dry air (kg kg dry basis), since the flow rates of dry air and dry solids through any process are normally constant. 

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. 

Equilibrium data for moist solids are commonly given as relationships between the relative humidity of the gas and the liquid content of the solid, in mass of liquid per unit mass of bone-dry solidf Examples of equilibrium relationships are shown in Fig. 24,3. Curves of this type are nearly independent of temperature. The abscissas of such curves are readily converted to absolute humidities, in ma of vapor per unit mass of dry gas. 

Calculate the concentration (in terms of absolute humidity and molal absolute humidity) and enthalpy per unit mass of dry gas of saturated and unsaturated gas-vapor mixtures. 

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