Psychrometric Chart

Psychrometrics is the study of air and its properties, including temperature, pressure, moisture, and other characteristics. Control o f these properties is essential for human comfort, optimum equipment operation, and to prevent damage to home furnishings. Psychrometric charts show the relationships o f these air properties graphically. A brief description o f the psychrometric chart and a discussion of some of its more common uses in servicing HVAC systems will be provided here. For a review of psychrometric theory and instructions for using the psychrometric chart, refer to the many HVAC-related textbooks and other materials that are readily available.

The psychrometric chart (Figure 1-35) is used to determine how each of the following air properties varies as the amount of moisture in the air changes:
• Dry bulb temperature
• Wet bulb temperature
• Enthalpy at saturation
• Relative humidity
• Grains of moisture
• Specific volume
• Dewpoint temperature

Charts are available that graph the air properties at low temperatures (-20 to 50° F), normal temperatures (20 to 110° F), and high temperatures (60 to 250° F) at sea level and at various elevations to correct for changes in barometric pressure. They are also available in metric (SI) form.

Using the easily-measured values of dry bulb and wet bulb temperatures taken with a psychrometer, the psychrometric chart can be used to find the value for one or more of the remaining properties of a given sample of conditioned air. For field service, these temperatures are frequently used to find the relative humidity (RH) of a conditioned space. RH is the ratio of the amount of moisture present in the air to the amount it can hold at saturation. It is expressed as a percentage, and helps to determine the level o f indoor comfort that exists in the various rooms of a house or other conditioned space. High relative humidities create moist environments. Potential problems that can occur include loss of personal comfort, development of bacteria, viruses, fungi, and mites, and the warping of wood. On the other hand, low humidity levels may cause respiratory ailments, static electricity, and damage to electronic devices such as computers.

Another common use of the psychrometric chart is to check the capacity of air conditioning equipment. U sing the wet bulb temperatures of the airstream entering and leaving the heat exchanger, the enthalpy function of the psychrometric chart is used to find the total heat difference (change in enthalpy) between the two readings in Btu/lb. of air. This value of change in enthalpy, along with the value for the airstream velocity in cubic feet per minute, is used to calculate the equipment capacity in Btu/hr. using the total heat formula shown below.

Btu/Hr. = 4.5* x CFM x Change in Enthalpy (Btu/lb.)

*4.5 is a factor derived when the specific density for standard air is substituted for the specific density of the leaving air.

One major advantage of the psychrometric chart over other instruments such as a hygrometer (discussed below) is that the chart can be used as an estimating tool to determine the impact of planned modifications.

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