The psychrometric chart is a convenient and useful tool for determin- ing statepoint moist air psychrometric properties and visualizing the changes of moist air properties in a sequence of psychrometric processes; e.g., (a) as the outside and return air mixes and (b) proceeds through heat- ing and cooling coils, (c) the supply fan, and (d) supply duct to the con- ditioned space. This chapter is intended to give the reader an overview of the chart to help in overall understanding and to add clarity and meaning as each psychrometric property is developed and discussed in following chapters.
The main reader objective in this chapter should be to become famil- iar with the look of the psychrometric chart, the names of the lines plotted on the chart, and their general orientation with respect to one another. It is not important at this point to know the definition of each term or even its pronunciation. If the reader is especially curious about one term, then he or she may go directly to the chapter on that property.
A psychrometric chart is based on a specified barometric pressure or elevation with respect to sea level. The most common chart is based on 101.325 kPa barometric pressure. This is the normal barometric pressure at sea level and 15°C. The sea level psychrometric chart can be used with- out significant error for elevations between 400 metres above and below sea level. ASHRAE publishes SI psychrometric charts for sea level (101.325 kPa), +750 metre altitude (92.66 kPa), +1500 metre altitude (84.54 kPa), and +2250 metre altitude (77.04 kPa). Barometric pressure is related to altitude using the equation shown in Chapter 14, “Barometric Pressure.”
Haines (1961) and Threlkeld (1970) provide instructions for calcu- lating and plotting a psychrometric chart for any specific altitude above or below sea level. Some psychrometric software programs will instantly develop and display a printable psychrometric chart for any altitude.
Statepoints and processes may be added and calculations made, and the calculation results and the displayed psychrometric chart may then be printed to paper or to a computer file.
Figure 5-1 illustrates a skeleton psychrometric chart including selected property lines and the saturation curve for water. Many users of psychrometric charts are only interested in two or three properties and nat- urally develop the habit of looking at the psychrometric chart and seeing only those property lines of interest. It is an unconscious action that many of us do everyday as we occasionally look at our automobile speedometer. Some see kilometres per hour and others see miles per hour and yet both values appear on most speedometers. The reader may be somewhat con- fused when examining a psychrometric chart for the first time, but as Pro- fessor Fellows’s quotation at the chapter heading implies, those who spend time with the chart become very comfortable using it. The proper- ties isolines plotted on the chart are:
• Dry-bulb temperature isolines inqC, tDB
• Humidity ratio isolines in kilograms (sometimes in grams) water vapour per kilogram dry air, W
• Thermodynamic wet-bulb temperature isolines inqC, tWB
• Relative humidity isolines in percent, RH
• Water vapour saturation curve plotted as humidity ratio versus tem- perature at which the gas phase of water and the liquid phase are in equilibrium, ~tDP
Figure 5-1—Basic psychrometric chart.
5·Basics of the Psychrometric Chart
For chapter simplification, these important property isolines are omitted:
• Specific enthalpy (an important energy content property) isolines in kilojoule per kilogram dry air, h
• Specific volume isolines in cubic metres of moist air per kilogram dry air, v
Dew-point temperature is an important property; however, dew-point temperature isolines are seldom plotted on a psychrometric chart. This is also true for water vapour pressure isolines. Both dew-point temperature and water vapour pressure isolines, if plotted, would be horizontal lines and could be confused with the horizontal humidity ratio isolines. Some charts display scales for dew-point temperature and water vapour pres- sure on the far right of the chart. Dew-point temperature isolines can always be easily drawn (or visualized) on any psychrometric chart because dew-point temperature and dry-bulb temperature are equal at the intersection of the water vapour saturation curve. All points on a hori- zontal line through an intersection of the saturation curve and a dry-bulb temperature isoline have the same dew-point temperature.
Knowing barometric pressure (or its equivalent in altitude) and any two of the properties in the previous lists will establish the psychrometric statepoint and thereby all other psychrometric properties. Dry-bulb tem- perature is easily measured and is normally used along with one other property to fix the statepoint location on the psychrometric chart and then determine all other properties. Relative humidity or psychrometer wet- bulb temperature are often the second property used with dry-bulb tem- perature and altitude to fix a statepoint.
The psychrometric chart conceived by Dr. Willis Carrier in 1904 used dry-bulb temperature and water vapour density as rectangular plotting coordinates. With these coordinates, dry-bulb temperature isolines are all exactly vertical and evenly spaced.
In 1908, William Grosvenor was the first to use humidity ratio as a plotting coordinate (in his case, the horizontal scale). This simplified cal- culations. A majority of later psychrometric charts followed the lead of Grosvenor. He also included auxiliary lines of (a) constant cooling; (b) specific heat, specific volume, and density of humid (saturated) air; (c) specific volume and density of dry air; and (e) latent heat of vaporization of steam.
In 1923, Richard Mollier of Dresden, Germany, proposed a chart that looked quite similar to the Grosvenor chart but used specific enthalpy as an oblique coordinate and humidity-ratio as the ordinate with both plotted using evenly spaced grid lines. It is possible that Mollier patterned the appearance of his 1923 chart after Grosvenor’s 1908 chart. Mollier’s sig- nificant contribution was the use of a skewed enthalpy coordinate replac- ing the vertical temperature coordinate. ASHRAE adopted the skewed
enthalpy coordinate in 1961. Mollier is more famous for other charts in the field of thermodynamics and most of these are also called Mollier charts. Mollier’s 1923 hx diagram (originally named “ix diagram”) did not resemble the Carrier chart; however, Keppler, by carefully selecting the oblique angle for specific enthalpy lines and using an evenly spaced dry- bulb scale along the zero humidity-ratio isoline, made the Mollier diagram appear at first glance identical to the original Carrier psychrometric chart. Careful examination of the current ASHRAE enthalpy-humidity ratio psychrometric chart reveals that only one dry-bulb isoline is vertical. All other dry-bulb temperature isolines are straight but not parallel to one another. They diverge slightly as they extend into regions of higher humid- ity ratios.
Some practitioners in Europe and Asia use the current Mollier hx dia- gram. Nevertheless, the ASHRAE and Mollier diagrams produce identi- cal results. To convert from one to the other, first rotate the chart 90° and then look at the rotated image in a mirror or, better yet, plot one on semi- transparent material and then flip it over and rotate.
Many psychrometric texts cover construction of the psychrometric chart. A most informative presentation by Professor W.F. Stoecker (1972) starts with a dry-bulb abscissa (horizontal) scale and a water vapour pres- sure ordinate (vertical) scale. Saturation water vapour pressure versus temperature is plotted, which makes it perfectly clear that the curved line at the left of the psychrometric chart is the saturation curve for water vapour. Stoecker then converts the ordinate scale to humidity ratio using the equation presented in Chapter 17, “Humidity Ratio.”
Prior to the advent of personal computer psychrometric software, the psychrometric chart was used (1) for determining statepoint properties, (2) as a graphical display of processes, and (3) as an extremely useful graphic tool for solving psychrometric process problems. The psychro- metric chart with oblique enthalpy-humidity ratio coordinates provides slightly more accurate graphical solutions of mixing processes than the older-style charts using dry bulb-humidity ratio coordinates.
In 1990, approximately 75% of practitioners used the psychrometric chart as a tool in solving psychrometric problems, and the balance used computer software. In the first decade of the twenty-first century, most practitioners will rely on psychrometric software based on algorithms with 99% or greater accuracy. The chart itself will no longer be used as a graphical solution tool. As a secondary function, the software program will generate a psychrometric chart showing the statepoints and the psy- chrometric process lines connecting the statepoints. It will be displayed to provide a visual picture of the cycle of psychrometric processes and state- points. Since the chart will no longer be used as a graphical solution tool by the practitioner, the plotting grid will probably revert to rectangular plotting coordinates of dry bulb and humidity ratio. All statepoints will
5·Basics of the Psychrometric Chart
either be original data entries or the results of accurate calculations—not the result of graphical plotting.
The demise of printed psychrometric charts will probably be similar to the demise of the slide rule (replaced by the handheld calculator) and the “K & E” log-log and other special plotting papers (replaced by plotting capabilities of computer spreadsheet software programs). It is only a mat- ter of time before the once familiar pads of psychrometric charts from major air-conditioning and dehumidification manufacturers will no lon- ger be available. Put a collection away for your grandchildren (some day they may have the value of baseball cards of the ‘40s and ‘50s).