Humidity Charts and Their Use

Your objectives in studying this secdon are to be able to  

Define humidity, humid heat, humid volume, dry-bulb temperature, wet-bulb temperature, humidity chart, moist volume, and adiabatic cooling line. 

Explain and show by use of equations why the slope of the wet-bulb lines are the same as the adiabatic cooling lines for water-air mixtures. 

Prepare a humidity chart given relations for the heat capacities of air and water. 

Use the humidity chart to determine the properties of moist air, and to caiculate enthaipy changes and soive heating and cooiing problems involving moist air. 

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Sec. 4.9 Humidity Charts and Their Use TABLE 4.13 Pertinent Data for Humidity Charts... 

The water removed by vaporisation is generally carried away by air or hot gases, and the ability of these gases to pick up the water is determined by their temperature and humidity. In designing dryers using air, the properties of the air-water system are essential, and these are detailed in Volume 1, Chapter 13, where the development of the humidity chart is described. For the air-water system, the following definitions are of importance ... 

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. 

Although a desiccant cooling systems can be used in any air-conditioning operation as it can provide precise control of temperature and humidity of the space, the high initial costs of such systems t5q)ically limit their use. Harriman [47] developed a chart (Figure 13) for initial screening and or evaluation of the... 

Software is available that will perform calculations of humidity parameters for any point value, and for plotting psychrometric charts. Moreover, British Standard BS 1339 Part 2 (2006) provides functions as macros which can be embedded into any Excel-compatible spreadsheet. Users can therefore generate their own tables for any desired combination of parameters as well as perform point calculations. Hence, the need for published lookup tables has been eliminated. However, this software, like the previous lookup tables, is only valid for the air-water system. For other vapor-gas systems, the equations given in previous sections must be used. 

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