Psychrometric Chart & Moist Air Calculator
Calculate moist air thermodynamic properties from supported input pairs (DB+RH, DB+WB, DB+DP) with barometric altitude adjustment.
Interactive Calculator & Visualizer
Engineering Methodology & Governing Equations
Vapor-Compression Refrigeration Thermodynamic Cycle
Closed-loop thermodynamic phase change between high-side liquid condensation and low-side vapor expansion.
Moist Air Thermodynamic Formulations & Psychrometric Equations
| Symbol | Variable | Description | Standard Units |
|---|---|---|---|
T_{\text{db}} | Dry Bulb Temperature | Standard ambient temperature measured by a shielded thermometer | °F |
T_{\text{wb}} | Wet Bulb Temperature | Equilibrium temperature reached by adiabatic evaporative cooling in moist air | °F |
T_{\text{dp}} | Dew Point Temperature | Saturation temperature at which water vapor begins condensing into liquid droplets | °F |
\text{RH} | Relative Humidity | Ratio of actual water vapor partial pressure (P_w) to saturation vapor pressure (P_ws) at T_db | % |
W | Humidity Ratio | Mass of water vapor per unit mass of dry air (7,000 grains = 1 lb dry air) | grains/lb (or lb/lb) |
h | Specific Enthalpy | Total heat content (sensible heat of air + latent heat of water vapor) per pound of dry air | BTU/lb dry air |
Moist-Air Thermodynamic Property Reference
Psychrometric calculations solve simultaneous mass and energy balances for dry air and water vapor mixtures. Key properties include:
- Dew Point Temperature (T_dp): Governed by partial vapor pressure P_w. In cooling systems, surface temperatures below T_dp initiate moisture condensation. Compare entering air T_dp with refrigerant evaporating temperature using the Refrigerant PT Chart to verify dehumidification conditions.
- Specific Enthalpy (h): Represents total heat content: sensible heat (0.240 × T_db) plus latent heat associated with evaporated water vapor (W × [1061 + 0.444 × T_db]). Enthalpy differences across a cooling or heating coil (Δh) quantify total thermal load transfer.
- Wet Bulb Temperature (T_wb): Reflects the combined dry-bulb and moisture evaporative potential. Entering wet bulb is the governing air-side variable for fixed-orifice target superheat diagnostics per ACCA field procedures.
Supporting Research & Technical References
• Refrigerant Saturation: Refrigerant PT Chart & Saturation Calculator — cross-reference evaporator saturation temperature against moist-air dew point.
• Airflow Measurement: CFM Airflow Calculator — calculate total volumetric supply air flow rate entering cooling coils.
• Field Diagnostics: Target Superheat & Subcooling Calculator — evaluate refrigerant charge balance using entering wet-bulb conditions per ACCA field procedures.
• Building Envelope Physics: Psychrometrics & Building Envelope Physics Guide — interstitial condensation risk modeling and vapor retarder mechanics.
Standard Engineering Reference Matrix
Standard HVAC Psychrometric Reference States (Sea Level: 14.696 psia)
Representative thermodynamic state points calculated across standard residential and commercial HVAC design conditions:
| Operating Reference State | Calculation Basis & Reference | Dry Bulb | Wet Bulb | Relative Humidity | Dew Point | Enthalpy (h) |
|---|---|---|---|---|---|---|
| Representative Indoor Summer Comfort State | ANSI/ASHRAE Standard 55-2023 Reference Point | 75.0°F | 62.5°F | 50.0% | 55.1°F | 28.14 BTU/lb |
| Cooling Coil Entering Air (AHRI Rating Condition A) | AHRI Standard 210/240-2023 Reference Point | 80.0°F | 67.0°F | 51.2% | 60.4°F | 31.52 BTU/lb |
| Cooling Coil Leaving Air (Supply Reference) | AHRI 210/240 Nominal Supply Benchmark | 55.0°F | 53.5°F | 91.2% | 52.5°F | 22.35 BTU/lb |
| Outdoor Ambient Air (AHRI Rating Condition A) | AHRI Standard 210/240-2023 Outdoor Condition | 95.0°F | 75.0°F | 40.0% | 66.5°F | 38.60 BTU/lb |
| Winter Heating Ambient Baseline | ASHRAE 90.1 / Fundamentals Ch. 14 Baseline | 32.0°F | 29.5°F | 70.0% | 23.7°F | 10.35 BTU/lb |
Worked Engineering Sizing Example
Scenario: An HVAC technician measures return air entering an evaporator coil at 80.0°F Dry Bulb and 67.0°F Wet Bulb at standard sea level barometric pressure (14.696 psia).
Calculation Steps (ASHRAE Fundamentals Formulation):
- Calculate Saturation Vapor Pressures (Hyland-Wexler): At 80.0°F, saturation pressure is P_ws(80°F) = 0.507 psia. At 67.0°F wet bulb, saturation pressure is P_ws(67°F) = 0.329 psia.
- Solve Psychrometric Energy Balance: Actual water vapor partial pressure is P_w = 0.260 psia.
- Determine Relative Humidity: RH = (0.260 / 0.507) × 100% = 51.2%.
- Calculate Dew Point Temperature: Dew point temperature is 60.4°F. If the evaporator coil surface operates below 60.4°F (cross-check via PT Chart), moisture condenses out of the airstream.
- Calculate Specific Enthalpy & Humidity Ratio: Specific enthalpy is 31.52 BTU/lb dry air; humidity ratio is 78.4 grains/lb (0.0112 lb/lb).
- Secondary Coil Capacity Illustration (Standard Air Density Approximation): If supply air leaving the coil is measured at 22.35 BTU/lb enthalpy with 1,200 CFM airflow (verified via CFM Calculator), total cooling capacity is evaluated under standard sea-level air density (ρ = 0.075 lb/ft³, where 60 min/hr × 0.075 lb/ft³ = 4.5):
Q_total = 4.5 × CFM × Δh = 4.5 × 1200 × (31.52 - 22.35) = 49,518 BTU/hr (4.13 Tons)
Note: The 4.5 multiplier is an engineering approximation strictly valid for standard air density (ρ = 0.075 lb/ft³ at sea level). For high-altitude installations, adjust density directly using local barometric pressure.
Frequently Asked Questions
How do you calculate dew point from dry bulb and relative humidity?
Why is enthalpy important in HVAC cooling calculations?
How does altitude affect psychrometric air properties?
Governing Research Monograph: Thermodynamic Formulations of ASHRAE Hyland-Wexler Moist Air Psychrometrics and Numerical Energy-Balance Solvers for Building Sizing and Field Diagnostics
Report: HL-TR-2026-PSY04 • DOI: 10.2139/ssrn.7430738 • Authors: HVACLogic Research Group, Miad S.
Calculation engines undergo software-level verification against documented equations, reference values, boundary conditions, and automated tests referencing ASHRAE, ACCA, SMACNA, and NIST publications.
⚖️ Engineering Reference & Regulatory Disclaimers
Engineering Reference Notice: HVACLogic.org is an independent computational reference and engineering design aid authored by Miad S. Calculations are based on consensus engineering formulations (including ASHRAE, ACCA, and SMACNA publications) and are intended solely for preliminary estimation, parametric analysis, and educational use. HVACLogic does not provide licensed professional engineering services, structural evaluations, or legally binding code determinations.
Professional Review & Permitting Notice: Where the applicable jurisdiction, project type, occupancy classification, permit process, or professional-practice law requires licensed professional review, certification, or a sealed/stamped calculation, the user must obtain that review from an appropriately licensed Professional Engineer (PE) or qualified mechanical contractor. Where a jurisdiction or Authority Having Jurisdiction (AHJ) requires specific calculation software, documentation, or permit submittal forms, users must follow the applicable local requirements.
Manufacturer Data Notice: Generic engineering formulas provide baseline theoretical approximations. Actual equipment performance, expanded cooling/heating capacities at specific outdoor temperatures, sensible-to-total heat ratios, fan airflow curves, and electrical characteristics (MCA/MOP) must be verified against manufacturer technical product data specifications.