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 of environmental 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 the Definitions 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 which represents 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 from that 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 air temperature 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 a psychrometric 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 off seperate scales.
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 relative humidity 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 dry-bulb temperature is located along the bottom horizontal axis. Find the line for 78oF, which runs vertically through the chart. Wet-bulb temperature is located along diagonal dotted lines leading to scale readings at the upper, curved boundary marked "saturation temperature". The intersection of the vertical 78oF dry-bulb line and the diagonal 65oF wet-bulb line has now established a "state point" for the measured air. Now read relative humidity as 50 percent (curving line running from left to right up through the chart) and dewpoint temperature as 58oF (follow horizontal line, moving left, toward the curved upper boundary of saturation temperatures). This example is shown in Figure 3 so you may check your work.
What might we conclude from this information? The relative humidity of 50 percent is acceptable for most livestock and greenhouse applications. If we allowed the air temperature (dry-bulb) to decrease to 58oF (dewpoint) or below, the air would be 100 percent saturated with moisture and condensation would occur. The humidity ratio, as seen on the vertical, y-axis scale, is a reliable indicator of air moisture level since it reflects the pounds of moisture contained in a pound of dry air and does not fluctuate with dry-bulb temperature readings as does relative humidity. The humidity ratio for air in this example is about 0.0104 lb moisture/ lb dry air (move right horizontally from state point to humidity ratio scale).
Figure 3. Diagram of Example 1. Verify these values on the psychrometric chart (Figure 1).
Often air is heated before it is introduced into greenhouse or young-livestock building environments. Consider an application where outdoor air at 40oF (dry-bulb) temperature and 80 percent relative humidity is heated to 65oF (dry-bulb) before it is distributed throughout the building.
Find the state point for the incoming cool air on the lower left portion of the psychrometric chart (point A in Figure 4) Note that other properties of the 40oF air include a wet-bulb temperature of 38oF a dewpoint temperature of about 34oF and humidity ratio of 0.0042 lb moisture/ lb dry air. Heating air involves an increase in the dry-bulb temperature with no addition or reduction in the air's water content. The heating process moves horizontally to the right along a line of constant humidity ratio. See Figure 4 for this heating process between points A and B. Heating the air to 65oF (dry-bulb) has resulted in decreasing the relative humidity to about 32 percent. The heated air entering the building is dry enough to be useful in absorbing moisture from the plant or animal environment. (Verify that the heated air at point B continues to have a dewpoint of 34oF and humidity ratio of 0.0042 lb moisture/ lb dry air.) The heated air, with its lower relative humidity, can be mixed with moist, warm air already in the building. As fresh air moves through an animal environment, it will pick up additional moisture and heat before it reaches the ventilation system exhaust. We might measure the exhausted air conditions at 75oF (dry-bulb) and 70 percent relative humidity, represented by point C in Figure 4. Note that in this exhausted air, the humidity ratio has tripled to 0.013 lb moisture/ lb dry air. This means that a lot more water is ventilated out of the building in the warm, moist exhaust air than was brought in by the cold, high relative humidity incoming air. This is one of the major functions of a winter ventilation system: removal of moisture from the plant or animal environment.
The air surrounding us is a mixture of dry air and moisture and it contains a certain amount of heat. We are used to dealing with air temperature, relative humidity and, oftentimes, the dewpoint as weather conditions are discussed. All these properties and more are contained in a psychrometric chart. Chart shape and complexity take some getting used to. Refer to Figures 1 and 2. You will find that the upper curved boundary of the chart has one temperature scale yet can represent three types of temperature: wet-bulb, dry-bulb, and dewpoint. This upper curved boundary also represents 100 percent relative humidity or saturated air.
Figure 4. Diagram of Example 2. Outdoor air at 40oF,80 percent relative humidity (point A is heated to 65oF (point B) for use in ventilation. Exhaust air (point C) at 75oF and 70% relative humidity contains three times the moisture of the fresh air (point A and B).
Evaporative cooling uses heat contained in the air to evaporate water. Air temperature (dry-bulb) drops while water content (humidity) rises to the saturation point. Evaporation is often used in hot weather to cool ventilation air. The process moves upward along the line of constant enthalpy or constant web-bulb temperature, for example, from point D to point E in Figure 5. Notice that hot dry air (points D to E with a 24oF temperature drop) has more capacity for evaporative cooling than hot humid air (points F to G with only a 12oF temperature decrese).
|Figure 5. Diagram of Example 3. Evaporative cooling process with hot dry air from points D to E and with hot humid air from points F to G. Notice greater evaporative cooling capacity with dry air.|
Dry-bulb temperature is the commonly measured temperature from a thermometer. It is called "dry-bulb" since the sensing tip of the thermometer is dry (see "wet bulb temperature" for comparison). Dry-bulb temperature is located on the horizontal, or x-axis, of the psychrometric chart and lines of constant temperature are represented by vertical chart lines. Since this temperature is so commonly used, it can be assumed that temperatures are dry-bulb temperatures unless otherwise designated.
Relative humidity is a measure of the amount of water air can hold at a certain temperature. It is "relative" with respect to the amount of water that air, at that same temperature, can hold at 100 percent humidity, or saturation. Air temperature (dry-bulb) is important because warmer air can hold more moisture than cold air. Air at 60 percent relative humidity contains 60 percent of the water it could possibly hold (at that temperature). It could pick up 40 percent more water to reach saturation. Lines of constant relative humidity are represented by the curved lines running from the bottom left and sweeping up through to the top right of the chart. The line for 100 percent relative humidity, or saturation, is the upper, left boundary of the chart.
Humidity ratio of moist air is the weight of the water contained in the air per unit of dry air. This is often expressed as pounds of moisture per pound of dry air. Since the humidity ratio of moist air is not dependent on temperature, as is relative humidity, it is easier to use in calculations. Humidity ratio is found on the vertical, y-axis with lines of constant humidity ratio running horizontally across the chart.
Dewpoint temperature indicates the temperature at which water will begin to condense out of moist air. Given air at a certain dry-bulb temperature and relative humidity, if the temperature is allowed to decrease, the air is no longer able to hold as much moisture. When air is cooled, the relative humidity increases until saturation is reached and condensation occurs. Condensation occurs on surfaces which are at or below the dewpoint temperature. Dewpoint temperature is determined by moving from a state point horizontally to the left along lines of constant humidity ratio until the upper, curved, saturation temperature boundary is reached.
Wet-bulb temperature is determined when air is circulated past a wetted sensor tip. It represents the temperature at which water evaporates and brings the air to saturation. Inherent in this definition is an assumption that no heat is lost or gained by the air. This is different from dewpoint temperature where a decrease in temperature, or heat loss, decreases the moisture holding capacity of the air, and hence, water condenses. Determination of wet-bulb temperature on this psychrometric chart, follows lines of constant enthalpy but values are read off the upper, curved, saturation temperature boundary.
Enthalpy is the heat energy content of moist air. It is expressed in Btu per pound of dry air and represents the heat energy due to temperature and moisture in the air. Enthalpy is useful in air heating and cooling applications. The enthalpy scale is located above the saturation, upper boundary of the chart. Lines of constant enthalpy run diagonally downward from left to right across the chart. Lines of constant enthalpy and constant wet-bulb are the same on this chart but values are read off separate scales. More accurate psychrometric charts use slightly different lines for wet-bulb temperature and enthalpy.
Specific volume indicates the space occupied by air. It is the increase of density and is expressed as a volume per unit weight (density is weight per unit volume). Warm air is less dense than cool air which causes warmed air to rise. This phenomena is known as thermal buoyancy. By similar reasoning, warmer air has greater specific volume and is hence lighter than cool air. On the psychrometric chart, lines of constant specific volume are almost vertical lines with scale values written below the dry-bulb temperature scale and above the upper boundary's saturation temperature scale. On this chart, values range from 12.5 to 15.0 cubic feet/ pound of dry air. Greater specific volume is associated with warmer temperatures (dry-bulb).