# 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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## Nội dung Text: HVAC Systems Design Handbook part 19

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.