Two useful air properties for environmental analysis in agricultural buildings would be relative humidity and dewpoint temperature. Determine other moist air properties from this information. Use of Psychrometric Chart in Greenhouse and Barn Example 1 Find air propertiesĪ sling psychrometer* gives a dry-bulb temperature of 78☏ and a wet-bulb temperature of 65☏. For example, to decrease relative humidity in a winter greenhouse during a critical time period, we could heat the air. Use of a psychrometric chart will show that this is roughly true. A rule of thumb for inside typical greenhouses or animal buildings during winter conditions is that a 10☏ rise in air temperature can decrease relative humidity 20 percent. Note that cooler air (located along the lower, left region of the chart) will not hold as much moisture (as seen on the y-axis' humidity ratio) as warm air (located along right side of chart). Wet-bulb temperature and enthalpy use the same chart line but values are read off seperate scales.Īn understanding of the shape and use of the psychrometric chart will help you diagnose air temperature and humidity problems. Properties of moist air on a psychrometric chart. Use Figures 2 and 3 with the psychrometric chart in Figure 1 to verify whether you can find each air property.įigure 2. This is called a "state point." From the state point all the other properties can be read. Moist air can be described by finding the intersection of any two of these properties. See the Definitions Sidebar for an explanation of these terms. 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. 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 which represents saturated air or 100-percent moisture holding capacity. If we consider the components piece by piece, the usefulness of the chart will be clearer. Psychrometric properties also are available as data tables, equations, and slide rulers.Ī psychrometric chart packs a lot of information into an odd-shaped graph. Figure 1, is for standard atmospheric pressure (14.7 psi) and temperatures of 30° to 120☏, which is adequate for most greenhouse or livestock housing applications. Psychrometric charts are available in various pressure and temperature ranges. Properties of moist air are explained in the Definitions Sidebar for your reference during the following discussions. Three examples are used to illustrate typical chart use and interpretation. This article explains how characteristics of moist air are used in a psychrometric chart. Understanding psychrometric charts can help you visualize environmental control concepts, such as why heated air can hold more moisture or, conversely, how allowing moist air to cool will result in condensation. It can be very helpful in troubleshooting and finding solutions to greenhouse or livestock building environmental problems. Psychrometric Chart and Air CharacteristicsĪ psychrometric chart presents physical and thermal properties of moist air in a graphical form.
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