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 temperatures are read from an ordinary thermometer that has a dry
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.
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.
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.
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.
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%.