Figure 1. Psychrometric Chart
Psychrometric Chart and Air Characteristics
A psychrometric chart presents physical and thermal properties of moist air in a graphical form. It can be very helpful in troubleshooting greenhouse or livestock building environmental problems and in determining solutions. Understanding psychrometric charts helps visualization ofenvironmental control concepts such as why heated air can hold more moisture, and conversely, how allowing moist air to cool will result in condensation. The objective of this fact sheet is to explain characteristics of moist air and how they are used in a psychrometric chart. Three examples are used to illustrate typical chart use and interpretation. Properties of moist air are explained in theDefinitions at the end for your reference during the following discussions.
Psychrometric charts are available in various pressure and temperature ranges. Figure 1, at the top of the page, is for standard atmospheric pressure (14.7 psi) and temperatures of 30o to 120 oF which is adequate for most greenhouse or livestock housing applications. Psychrometric properties are also available as data tables,equations, and slide rulers.
A psychrometric chart contains a lot of information packed into an odd-shaped graph. If we dissect the components piece by piece, the usefulness of the chart will be clearer. Boundaries of the psychrometric chart are a dry-bulb temperature scale on the horizontal axis, a humidity ratio (moisture content) scale on the vertical axis, and an upper curved boundary whichrepresents saturated air or 100 percent moisture holding capacity. The chart shows other important moist air properties as diagrammed in Figure 2: wet-bulb temperature; enthalpy; dewpoint or saturation temperature; relative humidity; and specific volume. See Definitions for explanation of these terms. Moist air can be described by finding the intersection of any two of these properties and fromthat point all the other properties can be read. The key is to determine which set of lines on the chart represent the air property of interest. Some practice with examples will help. Use Figures 2 and 3 with the psychrometric chart in Figure 1 to verify whether you can find each air property.
An understanding of the shape and use of the psychrometric chart will help in diagnosing airtemperature and humidity problems. Note that cooler air (located along lower, left region of chart) will not hold as much moisture (as seen on the y-axis' humidity ratio) as warm air (located along right side of chart). A rule of thumb, inside typical greenhouses or animal buildings during winter conditions, is that a 10oF rise in air temperature can decrease relative humidity 20 percent. Use of apsychrometric chart will show that this is roughly true. For example, to decrease relative humidity in a winter greenhouse during a critical time period, we could heat the air.
|Figure 2. Properties of moist air on a psychrometric chart. Wet-bulb |
|temperature and enthalpy use the same chart line but values are read |
|offseperate scales. |
Use of Psychrometric Chart in Greenhouse and Barn
Example 1 Find air properties
A sling psychrometer gives a dry-bulb temperature of 78oF and a wet-bulb temperature of 65oF. Determine other moist air properties from this information. Two useful air properties for environmental analysis in agricultural buildings would be relativehumidity and dewpoint temperature. Relative humidity is an indicator of how much moisture is in the air compared to desirable moisture conditions, and dewpoint temperature indicates when condensation problems would occur should the (dry-bulb) temperature drop.
Find the intersection of the two known properties, dry-bulb and wet-bulb temperatures, on the psychrometric chart, Figure 1. The...