Ahu Psychro

Equations (at sea level)

Saturation Pressure (Hyland-Wexler):

ln(Pws) = C₁/T + C₂ + C₃T + C₄T² + C₅T³ + C₆ln(T)

Humidity Ratio:

W = 0.62198 × Pw / (P - Pw)

Enthalpy:

h = 0.240×Tdb + W×(1061 + 0.444×Tdb)

Specific Volume:

v = 0.370486 × (Tdb + 459.67) × (1 + 1.6078×W) / P

Altitude Correction

Barometric pressure at altitude:

P = 14.696 × (1 - 6.8754×10⁻⁶ × altitude)^5.2559

Process Calculations

Mixing Two Airstreams

Given: Outdoor air (OA) and Return air (RA) at known states

W_mix = (CFM_OA × W_OA + CFM_RA × W_RA) / CFM_total
h_mix = (CFM_OA × h_OA + CFM_RA × h_RA) / CFM_total

Solve for Tdb_mix from h_mix and W_mix.

Cooling Coil Process

Sensible Cooling (above dew point):

Q_sensible = 1.08 × CFM × ΔT

Total Cooling (with dehumidification):

Q_total = 4.5 × CFM × Δh

Sensible Heat Ratio:

SHR = Q_sensible / Q_total

Apparatus Dew Point: Intersection of process line with saturation curve.

Heating Process

Sensible only (constant humidity ratio):

Q_heating = 1.08 × CFM × ΔT

Humidification

Steam injection (constant dry-bulb):

lb_steam/hr = CFM × ρ × ΔW × 60

ASHRAE Design Conditions

Use ASHRAE Fundamentals climate data:

Input Requirements

Read from:

• state/request.json: Location, conditions
• state/concept.json: CFM, configuration
• state/constraints.json: Load requirements

Analysis Procedure

1. Establish Outdoor Conditions

   • Look up design conditions for location
   • Summer: 0.4% or 1% DB/MCWB
   • Winter: 99.6% or 99% DB

2. Define Return Air State

   • From request or default (75°F, 50% RH)

3. Calculate Mixed Air

   • Based on outdoor air fraction
   • Verify mixed air conditions

4. Determine Coil Requirements

   • Cooling: from mixed to supply
   • Calculate SHR to verify coil selection
   • Heating: winter preheat and reheat

5. Verify Load Balance

   • Compare calculated loads to constraints
   • Flag discrepancies

Output Specification

Write to state/psychro.json:

{
  "design_id": "from concept.json",
  "altitude_ft": 0,
  "barometric_pressure_psia": 14.696,
  "states": {
    "outdoor_summer": {
      "db_temp_f": 95,
      "wb_temp_f": 78,
      "rh_percent": 45.2,
      "humidity_ratio": 0.0167,
      "enthalpy_btu_lb": 41.8,
      "specific_volume_ft3_lb": 14.3,
      "dew_point_f": 72
    },
    "outdoor_winter": {
      "db_temp_f": 28,
      "rh_percent": 50,
      "humidity_ratio": 0.0023,
      "enthalpy_btu_lb": 9.1
    },
    "return_air": { },
    "mixed_air_summer": { },
    "mixed_air_winter": { },
    "off_coil_cooling": { },
    "supply_air": { }
  },
  "loads": {
    "cooling_total_btuh": 450000,
    "cooling_sensible_btuh": 340000,
    "cooling_latent_btuh": 110000,
    "sensible_heat_ratio": 0.76,
    "heating_btuh": 180000,
    "preheat_btuh": 95000
  },
  "coil_requirements": {
    "cooling": {
      "entering_db_f": 82,
      "entering_wb_f": 68,
      "leaving_db_f": 54,
      "leaving_wb_f": 53,
      "apparatus_dew_point_f": 50
    },
    "heating": {
      "entering_db_f": 55,
      "leaving_db_f": 72
    }
  },
  "verification": {
    "supply_conditions_met": true,
    "load_balance_check": "PASS",
    "notes": []
  }
}

Common Issues

1. Mixed air too cold in winter: Add preheat coil
2. High latent load: May need lower ADP, more coil rows
3. Humidity not achievable: Add humidification
4. SHR mismatch: Adjust coil bypass factor