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ASHRAE Psychrometric Moist Air & HVAC Coil Calculator
Thermodynamic State Properties & Cooling/Heating Coil Energy Balances (ASHRAE Fundamentals Ch 1)
State 1 (Entering / Room Air Condition)Inlet Dry Bulb & Moisture
State 2 (Leaving Air Condition / Supply Air)Discharge from Cooling / Dehumidification Coil
Total Cooling Load
11.8 Tons
41.5 kW (141,600 Btu/h)
Sensible Heat Ratio (SHR)
0.76
Comfort Cooling Match
Moisture Condensate Rate
3.8 gal/h
14.4 L/h (31.7 lb/h)
Coil Face Velocity
471 FPM
OK (<500 FPM Safe)
Full Thermodynamic Moist Air Properties Comparison
Property
State 1 (Entering)
State 2 (Leaving)
Difference (Δ)
SI Metric Units
Dry Bulb Temperature (Tdb)
80.0 °F
55.0 °F
-25.0 °F
26.7 °C → 12.8 °C
Wet Bulb Temperature (Twb)
66.7 °F
53.6 °F
-13.1 °F
19.3 °C → 12.0 °C
Dew Point Temperature (Tdp)
59.8 °F
52.3 °F
-7.5 °F
15.4 °C → 11.3 °C
Humidity Ratio (W)
77.3 gr/lb
58.4 gr/lb
-18.9 gr/lb
11.0 g/kg → 8.3 g/kg
Specific Enthalpy (h)
31.3 Btu/lb
22.4 Btu/lb
-8.9 Btu/lb
72.8 kJ/kg → 52.1 kJ/kg
Specific Volume (v)
13.88 ft³/lb
13.18 ft³/lb
-0.70 ft³/lb
0.866 m³/kg → 0.823 m³/kg
Air Density (ρ)
0.0729 lb/ft³
0.0765 lb/ft³
+0.0036 lb/ft³
1.168 kg/m³ → 1.225 kg/m³
Vapor Pressure (Pw)
0.518 in Hg
0.392 in Hg
-0.126 in Hg
1.75 kPa → 1.33 kPa
Coil Load Breakdown
Sensible Load (qs):107,800 Btu/h (31.6 kW)
Latent Load (ql):33,800 Btu/h (9.9 kW)
Total Load (qt):141,600 Btu/h (41.5 kW)
Apparatus Dew Point (ADP):50.8 °F (10.4 °C)
Mass & Volume Flow
Dry Air Mass Flow:17,290 lb/hr (2.18 kg/s)
Moist Air Mass Flow:17,480 lb/hr (2.20 kg/s)
Leaving Air Volume:3,798 CFM (6,450 m³/h)
Altitude Density Derating:1.000 (Sea Level Baseline)
Psychrometric Quality
Enthalpy Drift:0.00% (Balanced)
Moisture Extraction:2.70 grains/cu.ft
Face Velocity:471 FPM (2.39 m/s)
Carryover Risk:Low / Safe (<500 FPM)
5 Fatal Traps & Engineering Pitfalls in Psychrometric Design
1. The Sea-Level Standard Air Density Constant Trap
Relying on standard sea-level shortcuts like (q_s = 1.08 imes ext{CFM} imes Delta T) and (q_t = 4.5 imes ext{CFM} imes Delta h) on high-elevation projects (e.g. Denver at 5,280 ft or Calgary at 3,500 ft) causes 15% to 22% severe equipment undersizing. Standard constants assume sea-level air density of (0.075 ext{ lb/ft}^3) ((1.204 ext{ kg/m}^3)). At 5,000 ft elevation, air density drops to (0.062 ext{ lb/ft}^3). The multiplier drops from 1.08 to 0.89. HVAC systems designed with sea-level equations will chronically fail to maintain indoor temperature setpoints on hot days.
2. Entering Wet-Bulb Neglect in Cooling Coil Selection
Specifying cooling coils based strictly on entering dry-bulb temperature and CFM while ignoring entering wet-bulb temperature is a catastrophic mistake. A seemingly negligible 2°F (1.1°C) rise in entering wet-bulb temperature swings total cooling coil load by 15% to 20% due to the steep exponential rise in enthalpy. Chiller compressors will overload, chilled water return temperatures will rise, and cooling towers will reach approach limits.
3. The Condensate Carryover Velocity Blowout
When cooling and dehumidifying moist air, moisture condenses on coil fins. If the airflow velocity across the coil face exceeds 500 to 550 FPM (2.54 to 2.8 m/s) without moisture eliminator baffles, aerodynamic shear strips water droplets off the aluminum fins and flings them into the supply ductwork. Saturated acoustic fiberglass liner becomes an incubator for Stachybotrys mold, downstream HEPA filters dissolve, and ceiling drywall collapses under waterlogged duct weight.
4. Sensible Heat Ratio (SHR) Mismatch & Part-Load Humidity Runaway
Designing VAV systems for peak design sensible load without checking part-load sensible heat ratio creates toxic indoor air. On rainy or humid shoulder days with high outdoor humidity but mild temperatures, the sensible load drops to 30%, causing VAV boxes to throttle airflow. Because air volume drops but indoor occupants continue transpiring moisture, the space SHR collapses to 0.40. Space relative humidity climbs above 70%, creating musty sick building syndrome.
5. Ignoring Coil Bypass Factor in Cold Supply Air Selection
Assuming an idealized 100% contact efficiency (bypass factor (BF = 0)) when targeting cold supply air (e.g. 52°F / 11°C) produces leaving air that is too warm and too wet. In an actual 4-row or 6-row coil, 8% to 15% of air passes between fin tubes without touching cold surfaces. The mixed leaving air will leave at 55°F with higher moisture content, requiring colder chilled water (40°F instead of 44°F), degrading chiller COP and wasting megawatts of power.
ASHRAE First-Principles Mathematical Formulations
Per ASHRAE Handbook — Fundamentals (Chapter 1), moist air is treated as a binary mixture of dry air and water vapor behaving as an ideal gas:
Why does barometric pressure and altitude significantly alter psychrometric properties?+
Atmospheric pressure decreases with increasing elevation (e.g. from 101.325 kPa / 14.696 psia at sea level to 82.9 kPa / 12.02 psia in Denver at 5,280 ft). Because the humidity ratio W = 0.621945 * [Pw / (Patm - Pw)], a lower total atmospheric pressure Patm means a given partial pressure of water vapor Pw represents a significantly higher proportion of moisture per pound or kilogram of dry air. Furthermore, specific volume increases and air density drops (from 1.204 kg/m3 to under 1.0 kg/m3), requiring higher volumetric airflow (CFM) to achieve equivalent mass flow and sensible/latent heat removal.
What is the Sensible Heat Ratio (SHR) and why is it critical in HVAC coil selection?+
The Sensible Heat Ratio (SHR) is the ratio of sensible cooling load (temperature reduction) to the total cooling load (sensible plus latent moisture condensation): SHR = qs / qt. Comfort air conditioning systems typically have an SHR between 0.70 and 0.85, whereas dedicated outdoor air systems (DOAS) handling humid ventilation air may have an SHR as low as 0.40 to 0.55, and data centers have an SHR near 0.95 to 1.0. Selecting a cooling coil whose apparatus dew point (ADP) and sensible heat ratio do not match the space sensible and latent load slope leads to uncontrolled high indoor humidity or excessive energy-wasting subcooling and reheat.
How is thermodynamic wet-bulb temperature calculated from dry-bulb and relative humidity?+
Thermodynamic wet-bulb temperature Twb is defined by an adiabatic saturation energy balance. At steady state, heat transferred by convection from the unsaturated air to an evaporating water film equals the latent heat required to vaporize water into the air stream. In psychrometric equations, Twb is solved iteratively such that the energy balance between sensible enthalpy drop and latent moisture gain reaches zero: W = [(2501 - 2.326 * Twb) * Ws(Twb) - 1.006 * (Tdb - Twb)] / [2501 + 1.86 * Tdb - 4.186 * Twb], where Ws(Twb) is the saturation humidity ratio at the wet-bulb temperature.
Why do cooling coil face velocities above 500 FPM (2.54 m/s) cause major HVAC failures?+
When an air stream cools below its dew point, moisture condenses on the aluminum coil fins. If the face velocity across the finned coil exceeds 500 to 550 feet per minute (FPM) or 2.54 m/s, the aerodynamic drag of the passing air overcomes the surface tension holding water droplets to the fins. Liquid water is stripped from the coil face and blown downstream into the supply plenum and ductwork (moisture carryover), flooding duct insulation, breeding toxic mold/bacteria, and causing structural ceiling collapse.
What is the coil bypass factor (BF) and apparatus dew point (ADP)?+
The apparatus dew point (ADP) is the effective average surface temperature of the cooling coil tubes and fins. In an actual physical coil, not all entering air makes direct physical contact with the cold fin surfaces; a small fraction bypasses the heat transfer surfaces unconditioned. This fraction is the bypass factor (BF), typically 0.05 to 0.15 in commercial 4-to-8 row coils. The condition of the air leaving the coil is the psychrometric mixture of the bypassed unconditioned air and the fully saturated air at the ADP.