Psychrometric Chart Use
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.
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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.
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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 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).
Example 2 Winter ventilation
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.
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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).
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Example 3 Winter ventilation
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).
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| 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. |
Definitions
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).
