This section provides information on use of the psychrometric chart as applied to air conditioning processes. The chart provides a graphic representation of the properties of moist air including wet- and dry-bulb temperature, relative humidity, dew point, moisture content, enthalpy, and air density. The chart is used to plot the changes that occur in the air as it passes through an air handling system and is particularly useful in understanding these changes in relation to the performance of automatic HVAC control systems. The chart is also useful in troubleshooting a system.
For additional information about control of the basic processes in air handling systems, refer to the Air Handling System Control Applications section.
To use these charts effectively, terms describing the thermodynamic properties of moist air must be understood. Definition of these terms follow as they relate to the psychrometric chart. Additional terms are included for devices commonly used to measure the properties of air.
Adiabatic process: A process in which there is neither loss nor gain of total heat. The heat merely changes from sensible to latent or latent to sensible.
British thermal unit (Btu): The amount of heat required to raise one pound of water one degree Fahrenheit.
Density: The mass of air per unit volume. Density can be expressed in pounds per cubic foot of dry air. This is the reciprocal of specific volume.
Dew point temperature: The temperature at which water vapor from the air begins to form droplets and settles or condenses on surfaces that are colder than the dew point of the air. The more moisture the air contains, the higher its dew point temperature. When dry-bulb and wet-bulb temperatures of the air are known, the dew point temperature can be plotted on the psychrometric chart (Fig. 4).
Dry-bulb temperature: The temperature read directly on an ordinary thermometer.
Isothermal process: A process in which there is no change of dry-bulb temperature.
Latent heat: Heat that changes liquid to vapor or vapor to liquid without a change in temperature or pressure of the moisture. Latent heat is also called the heat of vaporization or condensation. When water is vaporized, it absorbs heat which becomes latent heat. When the vapor condenses, latent heat is released, usually becoming sensible heat.
Moisture content (humidity ratio): The amount of water contained in a unit mass of dry air. Most humidifiers are rated in grains of moisture per pound of dry air rather than pounds of moisture. To convert pounds to grains, multiply pounds by 7000 (7000 grains equals one pound).
Relative humidity: The ratio of the measured amount of moisture in the air to the maximum amount of moisture the air can hold at the same temperature and pressure. Relative humidity is expressed in percent of saturation. Air with a relative humidity of 35, for example, is holding 35 percent of the moisture that it is capable of holding at that temperature and pressure.
Saturation: A condition at which the air is unable to hold any more moisture at a given temperature.
Sensible heat: Heat that changes the temperature of the air without changing its moisture content. Heat added to air by a heating coil is an example of sensible heat.
Sling psychrometer: A device (Fig. 1) commonly used to measure the wet-bulb temperature. It consists of two identical thermometers mounted on a common base. The base is pivoted on a handle so it can be whirled through the air. One thermometer measures dry-bulb temperature. The bulb of the other thermometer is encased in a water-soaked wick. This thermometer measures wet-bulb temperature. Some models provide slide rule construction which allows converting the dry-bulb and wet-bulb readings to relative humidity.
Although commonly used, sling psychrometers can cause inaccurate readings, especially at low relative humidities, because of factors such as inadequate air flow past the wet-bulb wick, too much wick wetting from a continuous water feed, thermometer calibration error, and human error. To take more accurate readings, especially in low relative humidity conditions, motorized psychrometers or hand held electronic humidity sensors are recommended.
Specific volume: The volume of air per unit of mass. Specific volume can be expressed in cubic feet per pound of dry air. The reciprocal of density.
Total heat (also termed enthalpy): The sum of sensible and latent heat expressed in Btu or calories per unit of mass of the air. Total heat, or enthalpy, is usually measured from zero degrees Fahrenheit for air. These values are shown on the ASHRAE Psychrometric Charts in Figures 33 and 34.
Wet-bulb temperature: The temperature read on a thermom- eter with the sensing element encased in a wet wick (stocking or sock) and with an air flow of 900 feet per minute across the wick. Water evaporation causes the temperature reading to be lower than the ambient dry-bulb temperature by an amount proportional to the moisture content of the air. The temperature re- duction is sometimes called the evaporative effect. When the reading stops falling, the value read is the wet-bulb temperature.
The wet-bulb and dry-bulb temperatures are the easiest air properties to measure. When they are known, they can be used to determine other air properties on a psychrometric chart.