Psychrometric Chart & Moist Air Calculator
Calculate moist air thermodynamic properties from any 2 inputs (Dry Bulb, Wet Bulb, Dew Point, RH, Enthalpy) with altitude barometric compensation.
Sample Psychrometric States:
Barometric Pressure: 14.7 psia (29.93 in.Hg)
Moist Air Dew Point & Humidity
55.2°F Dew Point
Relative Humidity: 50% RH • Wet Bulb: 62.6°F
Ideal Comfort (ASHRAE 55)
Specific Enthalpy (h)
28.11 BTU/lb
Humidity Ratio (W)
64.6 grains/lb
Specific Volume (v)
13.68 cu ft/lb
Air Density (ρ)
0.074 lb/cu ft
Next Step in Psychrometric System Diagnostics
Check Target Superheat with Indoor Wet Bulb (62.6°F)→Calculate Total Cooling Enthalpy Drop (Q = 4.5 × CFM × Δh)→⚙️
Vapor-Compression Refrigeration Thermodynamic Cycle
Closed-loop thermodynamic phase change between high-side liquid condensation and low-side vapor expansion.
⚡Vapor WorkCompressorSuperheated vapor (high P/T)
🔴High SideCondenser CoilSubcooling (10°F target)
💧ProtectionFilter DrierAcid & moisture (<3°F drop)
🎯ExpansionTXV / OrificeIsenthalpic pressure drop
🔵Low SideEvaporator CoilSensible & latent boiling
🌡️SuperheatSuction LineSuperheat (8°F–14°F)
💡 Engineering Note: Subcooling verifies a 100% solid liquid column at the TXV inlet; Superheat ensures no damaging liquid refrigerant enters the compressor crankcase.
ASHRAE Thermodynamic Moist Air & Psychrometric Equations
| Symbol | Variable | Description | Standard Units |
|---|---|---|---|
T_db | Dry Bulb Temperature | Standard ambient temperature measured by a shielded thermometer | °F |
T_wb | Wet Bulb Temperature | Equilibrium temperature reached by evaporating water into moist air | °F |
T_dp | Dew Point Temperature | Temperature at which water vapor begins condensing into liquid droplets | °F |
RH | Relative Humidity | Ratio of actual water vapor pressure to saturation vapor pressure at T_db | % |
W | Humidity Ratio | Mass of water vapor per unit mass of dry air (7,000 grains = 1 lb) | grains/lb |
h | Specific Enthalpy | Total heat content (sensible + latent) of moist air per pound of dry air | BTU/lb |
The 6 Core Psychrometric Lines Explained
A psychrometric chart graphically represents all thermodynamic states of moist air at a specific barometric pressure:
- Dry Bulb Lines: Vertical lines extending upward from the bottom temperature axis.
- Humidity Ratio Lines: Horizontal lines reading moisture content in grains of water per pound of dry air.
- Relative Humidity Curves: Curved lines sweeping upward from left to right. The outermost 100% curve is the Saturation Boundary.
- Enthalpy & Wet Bulb Lines: Diagonal downward-sloping lines representing constant heat content and evaporative cooling paths.
| Air Conditioning State | Dry Bulb | Relative Humidity | Wet Bulb | Dew Point | Enthalpy (h) |
|---|---|---|---|---|---|
| Indoor Comfort (ASHRAE 55) | 75.0°F | 50.0% | 62.5°F | 55.1°F | 28.14 BTU/lb |
| Return Air Entering Coil | 80.0°F | 51.2% | 67.0°F | 60.4°F | 31.52 BTU/lb |
| Supply Air Leaving Coil | 55.0°F | 90.0% | 53.5°F | 52.1°F | 22.35 BTU/lb |
| Hot Summer Outdoor Ambient | 95.0°F | 40.0% | 75.2°F | 67.9°F | 38.60 BTU/lb |
| Cold Winter Infiltration Air | 32.0°F | 70.0% | 29.5°F | 24.0°F | 10.35 BTU/lb |
Scenario: An HVAC technician measures the return air entering an evaporator coil at 80.0°F Dry Bulb and 67.0°F Wet Bulb at sea level elevation.
Calculation Steps:
- Calculate Saturation Vapor Pressure: At 80.0°F, saturation pressure is 0.507 psia. At 67.0°F wet bulb, saturation pressure is 0.329 psia.
- Solve Psychrometric Energy Balance: Actual water vapor pressure is 0.260 psia.
- Determine Relative Humidity: RH = (0.260 / 0.507) × 100% = 51.2%.
- Calculate Dew Point: Dew point temperature is 60.4°F.
- Calculate Specific Enthalpy: Specific enthalpy is 31.52 BTU/lb.
- Total System Capacity Check: If supply air leaving the coil is measured at 22.35 BTU/lb enthalpy with 1,200 CFM airflow, total cooling capacity is: Q = 4.5 × 1200 × (31.52 - 22.35) = 49,518 BTU/hr (4.1 Tons).
Frequently Asked Questions
How do you calculate dew point from dry bulb and relative humidity?
First calculate the actual water vapor pressure: Pv = (RH / 100) * Pws(Tdb), where Pws is saturation vapor pressure via the ASHRAE Hyland-Wexler equation. Then calculate dew point temperature: Tdp = 100.45 + 33.193 * ln(Pv) + 2.319 * [ln(Pv)]^2.
Why is enthalpy important in HVAC cooling calculations?
Specific enthalpy (h) measures the total heat content (sensible heat of dry air + latent heat of moisture) per pound of dry air (BTU/lb). Total air conditioner cooling capacity is directly calculated by multiplying air mass flow rate by total enthalpy drop across the cooling coil: Q_total = 4.5 * CFM * Delta_h.
How does altitude affect psychrometric air properties?
At higher elevations, barometric atmospheric pressure drops (e.g. 12.1 psia in Denver vs 14.7 psia at sea level). Lower pressure expands air volume, decreases air density, and increases the humidity ratio for the same relative humidity.
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Engineering Verification & E-E-A-T Quality StandardsPeer-Reviewed
Calculations reviewed by licensed Mechanical Engineers (PE) adhering to ASHRAE Fundamentals, ACCA Manuals, and NIST thermodynamics.
Formula: v1.0.0
Reviewed: 2026-08-19
Status: Deterministic (Zero Heuristics)