Ahu Thermal

• 1-2 rows typical
• 8-12 fins per inch
• 180°F entering, 160°F leaving water

Steam Coils

• Distributing tube or non-freeze designs
• Steam pressure: 5-15 psig typical
• Consider condensate drainage

Electric Heating Coils

• SCR controlled for modulation
• Open coil or finned tubular
• Sizing by kW

DX Cooling Coils

• Direct expansion refrigerant
• Matched to condensing unit
• Face split for staging

Selection Methodology

Step 1: Establish Requirements

From psychro.json:

• Entering air: DB, WB, CFM
• Leaving air: DB, WB
• Total capacity (MBH)
• Sensible capacity (MBH)

Step 2: Determine Face Area

Face Area = CFM / Face Velocity

Target: 450-500 fpm for cooling coils
        500-600 fpm for heating coils

Step 3: Calculate Rows

Approximate rows needed:

Cooling: 4 rows for 10°F ΔT, +1 row per 2°F additional
Heating: 1 row per 40°F rise (hot water)

Step 4: Water Flow Rate

Cooling GPM = Q_total / (500 × ΔT_water)
Heating GPM = Q / (500 × ΔT_water)

where 500 = lb/hr per GPM × 60 min/hr / (BTU/lb-°F)

Step 5: Tube Velocity Check

Velocity (fps) = GPM × 0.408 / (N_circuits × tube_ID²)

Target: 3-8 fps (erosion limit ~10 fps)

Step 6: Pressure Drop

Approximate:

Water PD (ft) ≈ 0.05 × L × V^1.8 / D^1.2

Target: < 20 ft for most applications

Step 7: Air Pressure Drop

Air PD (in. w.g.) ≈ 0.08 × rows × (V/500)²

Typical: 0.3-0.5 in. w.g. per coil

Heat Transfer Fundamentals

Overall Heat Transfer

Q = U × A × LMTD

U = overall heat transfer coefficient
A = surface area
LMTD = log mean temperature difference

LMTD for Counterflow

LMTD = (ΔT₁ - ΔT₂) / ln(ΔT₁/ΔT₂)

ΔT₁ = T_air_in - T_water_out
ΔT₂ = T_air_out - T_water_in

Correction for Crossflow

LMTD_corrected = F × LMTD_counterflow

F = correction factor (0.7-0.95 typical)

Coil Configuration

Circuiting Options

Fin Spacing

Materials

Input Requirements

Read from:

• state/psychro.json: Thermal requirements
• state/concept.json: Face area, arrangement
• state/constraints.json: Water temps, limits

Output Specification

Write to state/coils.json:

{
  "design_id": "from concept",
  "coils": [
    {
      "tag": "CC-1",
      "service": "cooling",
      "type": "chilled_water",
      "face_area_sqft": 42,
      "face_velocity_fpm": 500,
      "rows": 6,
      "fins_per_inch": 12,
      "tube_od_in": 0.625,
      "circuiting": "half",
      "performance": {
        "total_mbh": 450,
        "sensible_mbh": 340,
        "entering_db_f": 82,
        "entering_wb_f": 68,
        "leaving_db_f": 54,
        "leaving_wb_f": 53,
        "gpm": 90,
        "water_velocity_fps": 5.2,
        "water_pd_ft": 12,
        "air_pd_in_wg": 0.45
      },
      "construction": {
        "tube_material": "copper",
        "fin_material": "aluminum",
        "casing": "galvanized"
      }
    },
    {
      "tag": "HC-1",
      "service": "heating",
      "type": "hot_water",
      "face_area_sqft": 42,
      "rows": 1,
      "fins_per_inch": 10,
      "performance": {
        "capacity_mbh": 180,
        "entering_air_f": 55,
        "leaving_air_f": 72,
        "gpm": 18,
        "water_pd_ft": 4,
        "air_pd_in_wg": 0.08
      }
    }
  ],
  "totals": {
    "air_pd_in_wg": 0.53,
    "chw_gpm": 90,
    "hw_gpm": 18
  }
}

Validation Checks

1. Face velocity 400-550 fpm
2. Water velocity 3-8 fps
3. Water PD < 25 ft
4. Air PD reasonable for rows
5. Leaving conditions meet spec
6. SHR achievable with selected rows