The psychrometric chart is a graphical representation of air properties including dry-bulb temperature, wet-bulb temperature, relative humidity, dew point, and enthalpy. HVAC professionals use this chart daily to analyze air conditions, calculate load requirements, and design dehumidification and humidification processes. It combines multiple air properties on a single diagram, eliminating separate calculations.
Chart Structure and Use
The chart plots dry-bulb temperature horizontally (32°F to 120°F range) and moisture content vertically, with curved lines representing relative humidity from 10 percent to 100 percent saturation. A point at 75°F and 50 percent relative humidity immediately reveals the dew point (55°F), enthalpy (28 BTU/pound), and wet-bulb temperature (63°F). Technicians trace air changes through HVAC equipment to visualize heating, cooling, and dehumidification effects.
System Design Applications
Load calculations require psychrometric chart reference to determine latent and sensible cooling needs. Indoor design conditions typically target 72°F to 78°F with 40 to 60 percent relative humidity. The chart shows that cooling 95°F outdoor air (75 percent relative humidity, enthalpy 48 BTU/pound) to 75°F indoor conditions (50 percent relative humidity, enthalpy 28 BTU/pound) requires total cooling of 20 BTU/pound per cubic foot of air.