Tài liệu HVAC Systems Design Handbook part 19 doc

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

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19

Engineering Fundamentals:

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

Source: HVAC Systems Design Handbook

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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

read on an ‘‘ordinary’’ thermometer When not otherwise deﬁned, tem-perature means the dry-bulb temtem-perature In this text, the Fahrenheit

scale is used

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.

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 same temperature and pressure

w

Engineering Fundamentals: Part 4

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Engineering Fundamentals: Part 4 471

19.2.4 Relative humidity

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:

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

ratios may not be used The enthalpy of a moist air sample is

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

(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

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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

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

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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

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 moistempera-ture at saturation

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Figure 19.3 Mixing of two airstreams.

Figure 19.4 Sensible heating and cooling.

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Figure 19.5 Cooling and dehumidifying.

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:

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Figure 19.6 Adiabatic saturation process.

Figure 19.7 Adiabatic saturation.

The evaporative cooling or air washer process creates a sensible cool-ing effect by lowercool-ing 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

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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

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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

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,

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

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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

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

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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.

Tiêu đề	Engineering fundamentals: part 4 psychrometrics
Chuyên ngành	HVAC Engineering
Thể loại	Chapter
Năm xuất bản	2004

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