HVAC Systems Design Handbook part 19

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HVAC Systems Design Handbook part 19

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Psychrometrics deals with the thermodynamic properties of moist air, which is the final heat transport medium in most air conditioning processes. The use of psychrometric tables and charts allows the designer to make a rational and graphic analysis of the desired air conditioning processes.

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  1. Source: HVAC Systems Design Handbook Chapter Engineering Fundamentals: 19 Part 4 Psychrometrics 19.1 Introduction Psychrometrics deals with the thermodynamic properties of moist air, which is the final heat transport medium in most air conditioning processes. The use of psychrometric tables and charts allows the de- signer to make a rational and graphic analysis of the desired air con- ditioning processes. The general use of psychrometric data and charts began with the publications of Dr. Willis Carrier in the 1920s. In the 1940s, a research project conducted at the University of Pennsylvania by Goff and Gratch [sponsored by American Society of Heating and Ventilating Engineers (ASHVE)] resulted in new, more accurate data, which re- mained definitive until the results of further research were published in the 1980s. This chapter deals with the subject rather briefly and simply, but in sufficient depth to provide an adequate background for HVAC de- sign. For further study see Ref. 1. 19.2 Thermodynamic Properties of Moist Air Moist air is a mixture of atmospheric air and water vapor. Dry air contains no water vapor. Saturated air contains all the water it can hold at a specified temperature and pressure. The properties of moist 469 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  2. Engineering Fundamentals: Part 4 470 Chapter Nineteen air can be evaluated by the perfect gas laws with only a small degree of error, which is not significant in most HVAC processes. The prop- erties of interest in this discussion are the dry-bulb (db), wet-bulb (wb), and dew point temperatures; humidity ratio; degree of satura- tion; relative humidity (RH); and enthalpy and density. 19.2.1 Temperatures The dry-bulb temperature Tdb is the temperature of the moist air as read on an ‘‘ordinary’’ thermometer. When not otherwise defined, tem- perature means the dry-bulb temperature. In this text, the Fahrenheit scale is used. The wet-bulb temperature Twb is measured by a thermometer on which the bulb is covered with a wetted cloth wick. Air is blown across the wick, or the thermometer is moved rapidly through the air (as in the sling psychrometer), resulting in a cooling effect due to water evap- oration. The amount of water which can be evaporated (and, therefore, the cooling effect) is limited by the humidity already present in the air. The temperature obtained in this manner is not the same as the thermodynamic wet-bulb temperature used in calculating psychro- metric tables, but the error is small. The difference between the dry- and wet-bulb temperatures is sometimes called the wet-bulb depres- sion. The dew point temperature Tdp of moist air is defined by cooling the air until it is saturated and moisture begins to condense out of the mixture. For saturated air, these three temperatures are equal, as shown by their intersection on the saturation curve of the psychro- metric chart. 19.2.2 Humidity ratio The humidity ratio w is the ratio of the mass of the water vapor to the mass of the dry air in a sample of moist air. The specific humidity is the ratio of the mass of the water vapor to the total mass of the moist air sample. Although the two terms are often used interchange- ably, they are not identical. 19.2.3 Degree of saturation The degree or percentage of saturation is the humidity ratio w of a moist air sample divided by the humidity ratio ws of saturated air at the same temperature and pressure. w (19.1) ws Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  3. Engineering Fundamentals: Part 4 Engineering Fundamentals: Part 4 471 19.2.4 Relative humidity The relative humidity is the ratio of the mole fraction of water vapor Xw in a moist air sample to the mole fraction of saturated air Xws at the same temperature and pressure. The relative humidity is ex- pressed as a percentage, from 0 percent (dry air) to 100 percent (sat- urated air). It can also be defined in terms of the partial pressures of the water vapor in the samples: Pw (19.2) Pws Relative humidity values differ from percentage of humidity except at 0 and 100 percent. 19.2.5 Enthalpy The enthalpy h is the total heat of a sample of material, in Btu per pound, including internal energy. However, in the ASHRAE tables and charts, the value of the enthalpy of dry air is arbitrarily set to zero at 0 F. This is satisfactory in terms of enthalpy differences, but enthalpy ratios may not be used. The enthalpy of a moist air sample is h ha whg (19.3) where ha enthalpy of dry air in sample w humidity ratio of sample hg enthalpy of water vapor in sample (as a gas) h total enthalpy of sample (all at temperature of sample) 19.2.6 Volume and density The volume of a moist air sample is expressed in terms of unit mass, in cubic feet per pound in this text. The density is the reciprocal of volume, in pounds per cubic foot. 19.3 Tables of Properties The above-described properties and others are tabulated in Table 19.1, which is abstracted from an ASHRAE table. Table 19.1 is calculated for moist air at the standard atmospheric pressure of 14.696 lb/in2 (29.921 inHg). At any other atmospheric pressure, these data will be different, because the partial pressure of water vapor is a function of temperature only, independent of pressure (see Sec. 19.7). It is possible to calculate new values for a table similar to Table 19.1 at a different atmospheric pressure, by starting from the standard Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  4. Engineering Fundamentals: Part 4 Thermodynamic Properties of Moist Air TABLE 19.1 472 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  5. Engineering Fundamentals: Part 4 473 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  6. 474 TABLE 19.1 (Continued ) Engineering Fundamentals: Part 4 Any use is subject to the Terms of Use as given at the website. Copyright © 2004 The McGraw-Hill Companies. All rights reserved. *Extrapolated to represent metastable equilibrium with undercooled liquid. SOURCE: Copyright 1997, American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc., www.ashrae.org. Abstracted by permission from ASHRAE Handbook, 1997 Fundamentals, Chap. 6, Table 2. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
  7. Engineering Fundamentals: Part 4 Engineering Fundamentals: Part 4 475 values in the table.2 More accurately, new tables should be calculated by using the basic psychrometric equations. 19.4 Psychrometric Charts The psychrometric chart is a graphical representation of psychrome- tric properties. There are many charts available from various equip- ment manufacturers and other sources. In this text, the ASHRAE chart in Fig. 19.1 is used. This chart is for sea level in a dry-bulb temperature range from 32 to 120 F. Charts for other temperature ranges and altitudes are available. (See Sec. 19.7.) The basic coordinate grid lines of the ASHRAE chart are the en- thalpy, which slopes up to the left, and the humidity ratio, which is horizontal. The slope of the enthalpy lines is carefully calculated to provide the best possible intersections of property lines. Dry-bulb lines are uniformly spaced and approximately vertical; the slope of the lines changes across the chart. Wet-bulb lines slope similarly to enthalpy lines, but the slope increases as the temperature increases and no wet- bulb line is parallel to an enthalpy line. This is because of the heat added to the mixture by the moisture as it changes from dry to satu- rated air. Spacing between wet-bulb lines increases with temperature. The enthalpy lines (except every fifth line) are shown only at the edges of the chart to avoid confusion. A straightedge is needed to determine a value of enthalpy within the chart. Volume lines are uniformly spaced and parallel. Relative-humidity lines are curved, with the 100 percent line (sat- uration) defining the upper boundary of the chart. These lines are not uniformly spaced. (Percentage of saturation lines would be uniformly spaced but are not used in HVAC design.) When any two properties of a moist air sample are known, a state point may be plotted on the chart (Fig. 19.2) that identifies the values of all the other properties. Typically, the known properties are those most easily measured, i.e., dry- and wet-bulb temperatures and rela- tive humidity or dew point temperature. 19.5 HVAC Processes on the Psychrometric Chart Any HVAC process may be plotted on the chart if the end state points are known and sometimes if only the beginning state point is known. 19.5.1 Mixing of two airstreams A very common HVAC process is the adiabatic mixing of two air- streams, e.g., return air and outside air, or hot and cold streams in a Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  8. Engineering Fundamentals: Part 4 Heating, Refrigerating and Air Conditioning Engineers, Inc., www.ashrae.org. Reprinted by per- Figure 19.1 The ASHRAE psychrometric chart. (SOURCE: Copyright 2001, American Society of mission.) 476 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  9. Engineering Fundamentals: Part 4 Engineering Fundamentals: Part 4 477 Figure 19.2 A state point on the psychrometric chart. dual-duct or multizone system. The ASHRAE chart is a Mollier-type chart. On a Mollier chart, a mixing process may be shown as a straight line connecting two initial state points (Fig. 19.3, points A and B). The mixture state point C will be on the line located such that it divides the line into two segments with lengths proportional to the two initial air masses. The mixture point will be closer to the initial point with the larger mass. In the figure, if the volume at point A is 7000 ft3 /min and the volume at point B is 3000 ft3 /min, then line AC will be 3 units long and line BC will be 7 units long. The state point values for C can then be read from the chart. They can also be calculated from the tables, but the graphical solution is much faster unless a high degree of accuracy is required. 19.5.2 Sensible heating and cooling The word sensible implies that the heating or cooling takes place at a constant humidity ratio. These processes are shown as horizontal lines—constant value of w—with the dry-bulb temperature increasing for heating (line AB in Fig. 19.4) and decreasing for cooling (line CD in Fig. 19.4). Note that although the humidity ratio remains constant, there is a change in the relative humidity. As the dry-bulb tempera- ture increases, the air will hold more moisture at saturation. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  10. Engineering Fundamentals: Part 4 478 Chapter Nineteen Figure 19.3 Mixing of two airstreams. Figure 19.4 Sensible heating and cooling. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  11. Engineering Fundamentals: Part 4 Engineering Fundamentals: Part 4 479 19.5.3 Cooling and dehumidifying Most refrigerated cooling processes also include dehumidification (Fig. 19.5). The process is shown as a straight line sloping down and to the left from the initial state point. As discussed in Sec. 9.7.2, the real process involves sensible cooling to saturation, then further cooling down the saturation curve to an apparatus dew point (ADP). Some air is ‘‘bypassed’’ through the cooling coil without being cooled. The final state point is therefore a mixture of the initial state and the ADP state, usually very close to the ADP. 19.5.4 Adiabatic saturation If an airstream is passed through a water spray (Fig. 19.6) in such a way that the leaving air is saturated adiabatically, then the process can be shown on the chart as a constant-wet-bulb process (Fig. 19.7), and the final wet- and dry-bulb temperatures are equal. In practice, this process is called evaporative cooling, and saturation is not achieved (Fig. 19.8). The efficiency, denoted eff, of an air washer or evaporative cooler is the ratio of the dry-bulb temperature difference from point 1 to point 2 to the initial difference between the dry- and wet-bulb temperatures: Figure 19.5 Cooling and dehumidifying. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  12. Engineering Fundamentals: Part 4 480 Chapter Nineteen Figure 19.6 Adiabatic saturation process. tdb1 tdb2 Eff (19.4) tdb1 twb1 The evaporative cooling or air washer process creates a sensible cool- ing effect by lowering the dry-bulb temperature, but increases the rel- ative humidity in so doing. 19.5.5 Humidification As noted above, moisture may be added and humidity increased by the evaporative cooling process. This usually requires reheat or mixing Figure 19.7 Adiabatic saturation. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  13. Engineering Fundamentals: Part 4 Engineering Fundamentals: Part 4 481 Figure 19.8 Evaporative cooling. for accurate temperature control. The more common humidification process involves the use of steam or sometimes a heated evaporative pan (see the discussion in Sec. 10.19). Humidification by means of steam humidifier is shown in Fig. 19.9 as a straight line sloping up- ward (increasing humidity ratio) and to the right (heat added by steam). The slope of the line can be calculated from the masses of the airstream and the added water vapor together with their heat con- tents, as shown in the examples in Secs. 10.19.2 and 10.19.3. 19.5.6 Chemical dehumidification This process is described in Sec. 11.7.2. 19.6 The Protractor on the ASHRAE Psychrometric Chart Figure 19.1 includes a protractor above and to the left of the main chart. For a full discussion of this tool, see Ref. 1. One of the most important uses of the protractor is in determining the slope of the condition line for the air being supplied to a space to offset sensible and latent cooling loads. First, the sensible heat/total heat ratio S/ I, based on design load calculations, is calculated. For example, if the total cooling load is 125,000 Btu/h and the sensible Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  14. Engineering Fundamentals: Part 4 482 Chapter Nineteen Figure 19.9 Steam or heated pan humidification. load is 100,000 Btu/h, the ratio is 0.80. Second, a line is plotted on the protractor from the origin to the value of the ratio, as shown in Fig. 19.10. The state point corresponding to the design room condition of, say, 76 F db and 50 percent RH is located on the chart. Then a line is drawn from this state point toward the saturation curve, parallel to the line on the protractor. The state point of the air supplied to the room must be somewhere on the line on the chart. In this example, there is an ADP at about 52 F, so this process can be accomplished without reheat. If the sensible/total heat ratio were 0.60, as shown by the dashed line on the protractor, then the process on the chart, also shown dashed, would have no ADP and would be impossible to accom- plish directly. An arbitrary ADP could be established, and reheat would be needed, as shown. The other scale on the protractor, based on the enthalpy divided by the humidity ratio, can be used to determine the slope of a humidifi- cation process. 19.7 Effects of Altitude The tables and the chart of Fig. 19.1 are based on a standard atmo- spheric pressure of 29.92 inHg. The partial pressure of water vapor is Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  15. Engineering Fundamentals: Part 4 Engineering Fundamentals: Part 4 483 Figure 19.10 Using the protractor. a function of temperature only, while the total atmospheric pressure decreases with altitude. The rule of thumb is that the standard chart and tables are sufficiently accurate up to about 2000 ft above sea level. At higher elevations, new tables and charts are needed.2 High-altitude charts are available from several sources. ASHRAE publishes charts for 5000 and 7500 ft. The U.S. Bureau of Mines pub- lishes a composite chart for various elevations below sea level, down to 10,000 ft. The general effect of increasing altitude is to expand the chart (Fig. 19.11). That is, for a uniform grid of enthalpy and humidity ratio, as the altitude increases (and atmospheric pressure decreases), the lines defining the other properties change as follows: 1. Dry-bulb temperature lines are unchanged. 2. Wet-bulb temperature lines expand up and to the right. 3. Relative-humidity lines, including saturation, expand up and to the left. 4. Volume lines expand up and to the right. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  16. Engineering Fundamentals: Part 4 484 Chapter Nineteen Figure 19.11 Effects of altitude. 5. For a given combination of dry-bulb and wet-bulb temperatures, the change in relative humidity is very small and for most air con- ditioning processes can be neglected. 19.8 Summary This discussion of psychrometrics has been very brief. The subject is very important to the HVAC designer, and further study of Ref. 1 and other sources is recommended. Every set of HVAC design calculations should include one or more psychrometric charts, reflecting the antic- ipated performance of the system being designed. References 1. ASHRAE Handbook, 2001 Fundamentals, Chap. 6, ‘‘Psychrometrics.’’ 2. R. W. Haines, ‘‘How to Construct High-Altitude Psychrometric Charts,’’ Heating / Piping / Air Conditioning, October 1961, p. 144. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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