3.2 Psychrometric Chart Fundamentals: Dry-Bulb, Wet-Bulb, Dew Point & Humidity

Psychrometric Chart Fundamentals: Dry-Bulb, Wet-Bulb, Dew Point & Humidity

Psychrometrics is the thermodynamic study of the physical, thermal, and vapor properties of moist air (air-and-water vapor mixtures). In HVAC engineering and field diagnostics, the psychrometric chart serves as the fundamental graphical tool for plotting thermodynamic state points, evaluating coil performance, calculating sensible and latent cooling capacities, and diagnosing airflow and moisture issues. Standard sea-level psychrometric charts are constructed for a barometric pressure of 14.696 psia (29.92 inches of Mercury / 101.325 kPa).

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The Seven Core Thermodynamic Air Properties

If any two independent thermodynamic air properties are known, the state point of the air can be pinpointed on the psychrometric chart, revealing the remaining five properties. The seven core properties plotted on the chart are:

1. Dry-Bulb Temperature (DB): Ambient air temperature measured by a standard dry thermometer uninfluenced by moisture or radiant heat. Displayed along the horizontal baseline axis as straight vertical lines (°F).
2. Wet-Bulb Temperature (WB): Temperature measured by a thermometer covered with a distilled water-wetted sock exposed to rapid airflow (approx. 1,000 FPM). It reflects evaporative cooling capacity. Displayed as diagonal lines slanting downward from top-left to bottom-right (°F).
3. Dew Point Temperature (DP): Temperature at which air reaches 100% saturation and moisture condenses from water vapor into liquid water during cooling at constant pressure. Displayed as horizontal lines extending to the right vertical axis or curved saturation line (°F).
4. Relative Humidity (RH): Ratio of actual water vapor pressure present in the air to maximum saturated water vapor pressure possible at that dry-bulb temperature, expressed as a percentage. Represented by curved lines rising from lower-left to upper-right (0% to 100% saturation curve).
5. Specific Humidity / Humidity Ratio (W): Actual mass weight of moisture contained per pound of dry air. Expressed in pounds of water per pound of dry air ($lb\ H_2O / lb\ air$) or in grains of moisture per pound of dry air. There are 7,000 grains in one pound of liquid water. Humidity ratio is plotted on the right vertical axis.
6. Specific Volume (v): Volume occupied by one pound of dry air plus its associated water vapor, measured in cubic feet per pound of dry air ($cu\ ft/lb$). Represented by steep diagonal lines. Standard sea-level dry air at 70°F has a specific volume of $13.33\ \text{to}\ 13.50\ cu\ ft/lb$, yielding a standard air density of 0.075 lb/cu ft.
7. Enthalpy (h): Total heat content of the air-water mixture above a 0°F reference datum, measured in BTU per pound of dry air ($BTU/lb$). Plotted on an outer scale along extended diagonal wet-bulb lines.

The Saturation Line Rule

On the curved boundary of the psychrometric chart—known as the 100% Relative Humidity line or Saturation Curve—air is holding its maximum capacity of water vapor at that temperature. At saturation:

$\text{Dry-Bulb Temperature} = \text{Wet-Bulb Temperature} = \text{Dew Point Temperature}$

When air is cooled below its dew point temperature, moisture condenses out of the airstream onto cold surfaces (such as an evaporator coil operating below the dew point of return air).

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Psychrometric Processes and Vector Lines

Thermodynamic state changes across HVAC equipment appear as directional vectors on the psychrometric chart:

• Sensible Heating: Horizontal movement to the right. Dry-bulb temperature increases, humidity ratio (grains) remains constant, dew point remains constant, and relative humidity decreases.
• Sensible Cooling: Horizontal movement to the left. Dry-bulb temperature decreases, humidity ratio (grains) remains constant, dew point remains constant, and relative humidity increases toward saturation.
• Cooling and Dehumidification (Standard DX Coil Operation): Diagonal vector slanting downward and to the left. Dry-bulb, wet-bulb, dew point, humidity ratio, and enthalpy all decrease simultaneously.
• Heating and Humidification: Diagonal vector slanting upward and to the right.
• Evaporative Cooling (Adiabatic Humidification): Downward movement along a constant wet-bulb / enthalpy line toward the saturation curve. Sensible heat converts into latent heat: dry-bulb drops, relative humidity rises, grains increase, but total enthalpy remains unchanged.

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Governing Psychrometric Heat Equations

Technicians compute sensible, latent, and total cooling capacities using three fundamental mathematical formulas derived from standard air density ($0.075\ lb/cu\ ft$) and minute-to-hour conversion ($60\ min/hr$):

Sensible Heat Formula

$Q_{sensible} = 1.08 \times CFM \times \Delta T$

Where $1.08 = 60 \times 0.075 \times 0.24\ \text{BTU/lb/}^\circ\text{F}$ (specific heat of air), and $\Delta T = DB_{return} - DB_{supply}$.

Latent Heat Formula

$Q_{latent} = 0.68 \times CFM \times \Delta W$

Where $0.68 = \frac{60 \times 0.075 \times 1061}{7000}$, and $\Delta W = W_{return} - W_{supply}$ in grains of moisture per pound of dry air.

Total Heat Formula

$Q_{total} = 4.5 \times CFM \times \Delta h$

Where $4.5 = 60 \times 0.075\ \text{lb/cu ft}$, and $\Delta h = h_{return} - h_{supply}$ in BTU per pound of dry air.