Psychrometry is the science dealing with the physical laws of air – water mixtures.
When designing an air conditioning system, the temperature and moisture content of the air to be conditioned, and the same properties of the air needed to produce the desired air conditioning effect, must be known. Once these properties are known, the air conditioning task can be determined. This analysis can be performed using the psychrometric chart. The psychrometric chart graphically displays several physical properties of air over a broad range of conditions. Knowing the relationship of these air properties aids the task of air conditioning system design and analysis.
Properties of Air
At first glance, the psychrometric chart appears to be an imposing network of lines. When properly used, however, it provides valuable information about the properties of air. During this session, the psychrometric chart and its use in solving many air conditioning problems will be explained.
The psychrometric chart contains five physical properties to describe the characteristics of air:
- Dry-bulb temperature
- Wet-bulb temperature
- Dew-point temperature
- Relative humidity
- Humidity ratio
Dry-bulb temperature
Dry-bulb temperatures are read from an ordinary thermometer that has a dry
bulb.
Wet-bulb temperature
Wet-bulb temperatures are read from a thermometer whose bulb is covered by a wet wick. The difference between the wet-bulb temperature and the dry bulb temperature is caused by the cooling effect produced by the evaporation of moisture from the wick. This evaporation effect reduces the temperature of the bulb and, therefore, the thermometer reading.
Consequently, the difference between dry-bulb and wet-bulb temperature readings is a measure of the dryness of air. The drier the air, the greater the difference between the dry-bulb and wet-bulb readings.
Dew-point temperature
The third property, dew-point temperature, is the temperature at which moisture leaves the air and condenses on objects, just as dew forms on grass and plant leaves.
When the dry-bulb, wet-bulb, and dew-point temperatures are the same, the air is saturated. It can hold no more moisture. When air is at a saturated condition, moisture entering the air displaces moisture within the air. The displaced moisture leaves the air in the form of fine droplets. When this condition occurs in nature, it is called fog.
Relative humidity
The fourth property, relative humidity, is a comparison of the amount of moisture that a given amount of air is holding, to the amount of moisture that the same amount of air can hold, at the same dry-bulb temperature.
Relative humidity is expressed as a percentage. For example, if the relative humidity of the air is 50%, it contains one-half the amount of moisture possible at the existing dry-bulb temperature.
Humidity ratio
Finally, humidity ratio describes the actual weight of water in an air – water vapor mixture. In other words, if one pound of air were wrung completely dry, comparing the weight of the water to the weight of the dry air would yield its humidity ratio.
Humidity ratio can be expressed as pounds of moisture per pound of dry air, or as grains of moisture per pound of dry air. There are 7000 grains of water in a pound. To appreciate the magnitude of these units of measurement, at sea level one pound of 70°F air occupies approximately 13.5 cubic feet, and one grain of water in that air weighs about two-thousandths (0.002) of an ounce.
When any two of these five properties of air are known, the other three can be quickly determined from the psychrometric chart.
Example
For example, let’s assume that the summer design conditions are 95°F dry bulb
and 78°F wet bulb.
- 95°F dry bulb (DB)
- 78°F wet bulb (WB)
What is the relative humidity, humidity ratio, and dew point?
Only one point on the psychrometric chart represents air with both of these conditions. This point is located where the vertical 95°F dry-bulb (DB) and diagonal 78°F wet-bulb (WB) temperature lines intersect.
From this intersection, the remaining three air properties can be read from the chart. Both the dew-point and humidity-ratio lines are horizontal and the values are shown on the right side of the chart. In this example, the humidity ratio is about 118 grains of moisture per pound of dry air and the dew-point temperature is approximately 72°F.
Notice that the point of intersection falls between two relative humidity curves: 40% and 50%. By interpolation, the relative humidity at this condition is approximately 47%.
FREQUENTLY ASKED QUESTIONS
The psychrometric chart enables engineers to analyze the properties of air to be conditioned and determine the required air conditioning effect. By plotting the initial and final conditions of the air on the chart, engineers can visualize the changes in temperature, humidity, and enthalpy, and select the appropriate equipment and control strategies to achieve the desired air conditioning effect. This graphical representation simplifies the complex calculations involved in air conditioning system design and analysis.
Dry-bulb temperature is the temperature of the air measured by a thermometer, whereas wet-bulb temperature is the lowest temperature that can be reached by a thermometer wrapped in a wet cloth and placed in an air stream. The wet-bulb temperature is a measure of the heat and moisture content of the air. On a psychrometric chart, the dry-bulb temperature is plotted on the x-axis, and the wet-bulb temperature is plotted on the y-axis, enabling engineers to visualize the relationship between these two properties.
Relative humidity plays a critical role in air conditioning system design, as it affects the comfort level of occupants and the energy consumption of the system. High relative humidity can lead to mold growth and discomfort, while low relative humidity can cause dryness and discomfort. The psychrometric chart enables engineers to determine the required dehumidification or humidification load to achieve the desired relative humidity, ensuring a comfortable indoor environment while minimizing energy consumption.
The dew point temperature is the temperature at which the air becomes saturated with water vapor and dew or frost begins to form. On a psychrometric chart, the dew point temperature is an important parameter, as it indicates the maximum amount of moisture that can be removed from the air. Engineers can use the dew point temperature to determine the required cooling coil temperature and airflow rate to achieve the desired dehumidification effect.
Yes, the psychrometric chart can be used for both heating and cooling system design. While the chart is commonly associated with cooling system design, it can also be used to analyze the properties of air in heating systems. By plotting the initial and final conditions of the air on the chart, engineers can determine the required heating effect and select the appropriate equipment and control strategies to achieve the desired indoor environment.
