Air Properties - Temperature and Relative Humidity
Air is a vital component of our everyday lives. Air properties are important whether we are determining relative humidity in the home, ventilating a livestock housing unit or drying grain. In a commercial building, temperature, moisture, odors, and toxic or noxious gases must be controlled. In the home, moisture can either be added to or removed from the air to change the relative humidity. In each case, air provides the link between the controls and the mechanical equipment, and air properties influence the results of the processes.
Psychrometrics refers to the properties of moist air. A
psychrometric chart graphically illustrates the relationships
between air temperature and relative humidity as well as other
properties. A better understanding of air properties and the
psychrometric chart can aid in the selection and management of a
home humidifier, livestock building ventilation system or a grain drying system.
Definitions of Air
- Atmospheric air contains nitrogen, oxygen, carbon dioxide, water vapor, other gases, and miscellaneous contaminants such as dust, pollen, and smoke. This is the air we breathe and use for ventilation.
- Dry air exists when all of the contaminants and water vapor have been removed from atmospheric air. By volume, dry air contains about 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases. Dry air is used as the reference in psychrometrics.
- Moist air is a mixture of dry air and water vapor.
The Psychrometric Chart
Air temperature is a measure of the heat content of air. Three different temperature measurements are used in the psychrometric chart:
- Dry Bulb Temperature is the air temperature determined by
an ordinary thermometer. Dry bulb temperature is given in weather
reports. The dry bulb temperature scale is located at the base of
the chart. Vertical lines indicate constant dry bulb temperature
- Wet bulb temperature reflects the cooling effect of
evaporating water. This effect is often used to cool
buildings and some homes. Wet bulb temperature can be determined by
passing air over a thermometer that has been wrapped with a small
amount of moist cloth. The cooling effect of the evaporating water
causes a lower temperature compared to the dry bulb air
temperature. The wet bulb temperature scale is located along the
curved upper left portion of the chart. The sloping lines indicate
equal wet bulb temperatures (Figure 3).
- Dew point temperature is the temperature below which moisture will condense out of air. Air that is holding as much water vapor as possible is saturated or at its dew point. Water will condense on a surface, such as a building wall or pitcher of ice water, that is at or below the dew point temperature of the air. The dew point temperature scale is located along the same curved portion of the chart as the wet bulb temperature scale. However, horizontal lines indicate equal dew point temperatures (Figure 4).
As the name implies, relative humidity is a measure of how much moisture is present compared to how much moisture the air could hold at that temperature. Relative humidity, which is expressed as a percent, is given in weather reports. Lines representing conditions of equal relative humidities sweep from the lower left to the upper right of the psychrometric chart. The 100 percent relative humidity (saturation) line corresponds to the wet bulb and the dew point temperature scale line. The line for zero percent relative humidity falls along the dry bulb temperature scale line (Figure 5).
ExamplesThe versatility of the psychrometric chart lies in the fact that by knowing just two properties of moist air, the other properties can be determined. The following examples will help to show how the chart is used.
Example 1--Question: Moist air is at 80oF (dry bulb) and 50 percent relative humidity. What are the other properties of this air?
Answer: First locate the intersection of the 80oF dry bulb temperature line and the 50 percent relative humidity line. From this intersection, follow the appropriate lines to the correct scales (Figure 6). Table I lists the determined properties.
|Dry bulb temperature (given)||80o F|
|Relative humidity (given)||50%|
|Wet bulb temperature||67o F|
|Dew point temperature||59o F|
Example 2--Question: The air in a plant nursery building has a dry bulb temperature of 80oF and is at 70 percent relative humidity. How warm do the walls have to be to prevent condensation?
Answer: In this example, we need to know the dew point temperature. Locate the intersection of the 80F dry bulb temperature line and the 70 percent relative humidity line. Proceed horizontally to the left until the dew point temperature scale is intersected. This gives the dew point temperature as 69oF. The wall temperatures must be warmer than this to prevent condensation.
Example 3--Question: A shrimp producer is considering installing evaporative cooling in a growing building. What is the lowest temperature that can theoretically be obtained from the air coming off the cooling pads if the outside air has a 90F dry bulb temperature and 35 percent relative humidity?
Answer: Locate the intersection of the 90oF dry bulb temperature line and the 35 percent relative humidity line (which is not shown on the chart, but is mid-way between the 30 and 40 percent lines). Since evaporative cooling is the same process that determines wet bulb temperature, follow the wet bulb temperature line upward and to the left until the wet bulb temperature scale is intersected. The lowest possible temperature for these conditions would be the wet bulb temperature of 69oF. However, due to the inefficiency of evaporative coolers, the temperature of the air coming off the cooling pads will be 3 to 5 degrees F above the wet bulb temperature. This producer can probably expect to obtain air at about 73oF from the evaporative cooler under these conditions. This air would also have a relative humidity of approximately 85%.
Example 4--Question: How can a homeowner check the home's relative humidity in order to adjust a humidifier?
Answer: Relative humidity can be determined by using a sling psychrometer to measure wet and dry bulb temperatures and a psychrometric chart.
A sling psychrometer consists of two mercury thermometers, one of which is equipped with a wick around the mercury bulb. The other is used to measure dry bulb temperature. To use this instrument, the wick is saturated with clean water and the psychrometer is whirled for approximately 10 to 15 seconds. Repeat this process two or three times until there is no further temperature drop on the wicked thermometer.
At this point, the wet bulb temperature (on the thermometer equipped with a wick) is noted to be 47F. The dry bulb temperature thermometer reads 70oF. On the psychrometric chart, determine the intersection of the 70oF dry bulb line and 47oF wet bulb line. This intersection falls on the line representing 10% relative humidity. The humidifier should be adjusted upwards to maintain the relative humidity at 25 to 35 percent.
Figure 7. Determining wet and dry bulb temperatures by using two thermometers, water and a fan (Example 4).
Another technique, which gives satisfactory results for most applications, is to use two standard thermometers and a fan. The bulb of one thermometer is equipped with an absorbent wick--a braided shoelace or heavy string will work. Wet the wick and place the end in a small cup of water. Place both thermometers on the intake side of the fan, where air is drawn across them (Figure 7). When the wick-equipped thermometer shows no further temperature drop, note the wet and dry bulb temperatures. Now use the psychrometric chart to determine relative humidity from these temperature measurements.
The psychrometric chart is a graphical presentation of air properties under a variety of conditions. Developing an understanding of the chart will allow you to use it as an aid in the design and management of any building ventilation system.
This page only provides a simplified or abridged version of the psychrometric chart. A full chart also includes scales for absolute humidity or the weight of water per unit weight of air, specific volume or the weight per unit volume of air, enthalpy or the relative energy content per unit weight of air, and a correction for varying atmospheric pressure. Psychrometric charts that cover higher and lower temperature ranges are also available.