HVAC Psychrometric Chart Practice Questions Psychrometric Chart Quick Reference

Psychrometric Chart Essentials

1. Key Properties

Property Symbol Unit Chart Lines
Dry Bulb Temp DBT °C/°F Vertical (x-axis)
Wet Bulb Temp WBT °C/°F Diagonal (saturation)
Relative Humidity RH % Curved (0-100%)
Humidity Ratio W kg/kg Horizontal (y-axis)
Enthalpy h kJ/kg Diagonal (parallel to WBT)
Specific Volume v m³/kg Steep diagonal

2. Fundamental Processes

Process Direction SHF Example
Sensible Cooling ← Horizontal 1.0 Dry coil operation
Cooling + Dehumid ↙ Down-left 0.6-0.8 Standard AC
Heating → Horizontal 1.0 Furnace
Humidification ↑ Vertical N/A Steam injection
Evaporative Cooling ↖ Up-left 0 Swamp cooler

3. Key Formulas

Sensible Heat Ratio (SHR) = Qsens / (Qsens + Qlat)
Total Heat (Qtotal) = ṁ × (h2 - h1)
Bypass Factor = (Tout - ADP) / (Tin - ADP)
Dew Point Temp: Where W line meets 100% RH curve

4. HVAC Design Parameters

Application DBT (°C) RH (%) W (kg/kg)
Comfort Cooling 24-26 45-55 0.008-0.010
Data Center 21-23 40-50 0.006-0.008
Pharmaceutical 20-22 30-40 0.004-0.006
Supermarket 22-24 50-60 0.008-0.012

5. Troubleshooting Guide

Symptom Psychrometric Analysis Solution
High RH, normal temp Process line too flat (low SHR) Increase coil capacity or reduce airflow
Low RH, cold air Excessive dehumidification Add reheat or raise coil temp
Short cycling Operating near saturation curve Check refrigerant charge/airflow