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Psychrometric Calculator

Reference: ASHRAE Fundamentals

🌡️ HVAC

Calculate wet bulb temperature, dew point, humidity ratio, enthalpy and specific volume from dry bulb and relative humidity. ASHRAE.

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📋 ASSUMPTIONS & BASIS

ASHRAE Fundamentals

Design criteria, governing codes and assumptions.

ℹ About This Calculator

Psychrometrics is the study of the thermodynamic properties of moist air. These properties are fundamental to every HVAC calculation - from cooling coil sizing to humidification load to ventilation design. The psychrometric chart is the HVAC engineer's most essential tool, and our calculator provides all the key properties from just two inputs: dry-bulb temperature and relative humidity.

ASHRAE Handbook of Fundamentals Chapter 1 provides the complete psychrometric equations. At sea level (101.325 kPa), the equations above are exact to within ±0.1°C. At altitude (e.g., Bangalore at 920m: ~90.8 kPa), the atmospheric pressure must be adjusted - humidity ratio and enthalpy change significantly. The psychrometric chart for US standard conditions (summer: 35°C DB/25°C WB, winter: 15°C DB/10°C WB) is essential for HVAC system design.

Psychrometric Equations (ASHRAE Fundamentals)

ASHRAE Fundamentals

Saturation Pressure (Magnus formula): Pws = 0.6108 × exp(17.27 × T / (T + 237.3)) [kPa] Humidity Ratio: W = 0.622 × Pw / (P_atm − Pw) [kg/kg dry air] Pw = φ × Pws (φ = relative humidity as decimal) Specific Enthalpy: h = 1.006 × T_db + W × (2501 + 1.86 × T_db) [kJ/kg] Dew Point (Magnus inverse): T_dp = 243.04 × ln(Pw/0.6108) / (17.625 − ln(Pw/0.6108)) Wet Bulb (Sprung): T_wb ≈ T_db − (1 − φ) × (T_db + 273.15) / 1556 Specific Volume: v = 0.287042 × (T_db + 273.15) / (P_atm − Pw)

Worked Example

Consider a hot afternoon in Chennai with dry-bulb temperature 30°C and relative humidity 60%. Using the Magnus formula, saturation pressure Pws ≈ 4.24 kPa, so vapour pressure Pw = 0.60 × 4.24 ≈ 2.54 kPa. Humidity ratio W = 0.622 × 2.54 / (101.325 − 2.54) ≈ 0.016 kg/kg dry air.

Specific enthalpy h = 1.006 × 30 + 0.016 × (2501 + 1.86 × 30) ≈ 71.0 kJ/kg. The dew point works out to about 21.4°C - any surface below this temperature, such as a chilled water pipe or window pane, will show condensation under these conditions.

Frequently Asked Questions

What is the difference between dry bulb and wet bulb temperature? +

Dry bulb temperature (DBT) is the standard air temperature measured by a thermometer shielded from radiation - what we mean by "temperature" in everyday life. Wet bulb temperature (WBT) is measured by a thermometer with a water-wetted wick - evaporation cools the bulb, and WBT depends on both temperature and humidity. WBT equals DBT only at 100% RH; lower humidity creates greater depression (DBT-WBT).

What is humidity ratio and why does it matter for HVAC? +

Humidity ratio (W) is the mass of water vapor per kg of dry air (kg/kg or g/kg). It is the true measure of moisture content - unlike relative humidity which changes with temperature. For HVAC coil sizing, humidification load calculation, and energy calculations, humidity ratio is used. Typical indoor design: 0.010-0.012 kg/kg (10-12 g/kg). Outdoor in US monsoon: 0.018-0.024 kg/kg.

How do I find enthalpy from the psychrometric chart? +

Specific enthalpy (kJ/kg dry air) combines sensible heat (in temperature) and latent heat (in moisture). h = 1.006 × T_db + W × (2501 + 1.86 × T_db). For Mumbai summer conditions (33°C DB, 28°C WB): W ≈ 0.022 kg/kg, h ≈ 89 kJ/kg. For indoor conditions (24°C DB, 50% RH): W ≈ 0.0093 kg/kg, h ≈ 47.5 kJ/kg. Enthalpy difference (89−47.5 = 41.5 kJ/kg) drives total coil duty calculation.

What is the apparatus dew point (ADP) of a cooling coil? +

ADP is the lowest temperature that supply air could theoretically reach if it passed through the coil with 100% contact (bypass factor = 0). It represents the coil surface temperature. The actual leaving air condition lies on the psychrometric chart between the entering air condition and the ADP, depending on the coil bypass factor (typically 0.05-0.15 for multi-row cooling coils). Lower bypass factor = air leaves closer to ADP = more dehumidification.

Why does humidity ratio stay constant during sensible cooling? +

When air is cooled without condensation (above dew point), no moisture is added or removed - only temperature changes. On the psychrometric chart, this appears as a horizontal line (constant W) moving left (lower temperature) at lower enthalpy. Dehumidification only begins when the air is cooled below its dew point - at which point moisture condenses on the coil. Both temperature and humidity ratio decrease.