Hospital HVAC Retrofit in Houston, TX

Scenario

A 12-story acute-care hospital in Houston, TX undertook a chiller plant retrofit to replace aging air-handling units (AHUs) serving the oncology wing. Local climate constraints—high summer humidity (average July wet-bulb ≈ 75°F) and frequent thunderstorms—demanded robust condensate management. Space limitations in existing mechanical shafts prohibited upsizing beyond 2.5-inch vertical risers, and code required ASHRAE 188 compliance for Legionella risk mitigation (mandating slope ≥1/100 and accessible cleanouts).

Given Data

• Cooling Capacity: 425,000 Btu/h (per AHU serving 36-bed unit)
• Dry Bulb Temperature: 92°F (design peak summer condition)
• Wet Bulb Temperature: 75°F (corresponding design humidity ratio = 0.0172 lbₘ water/lbₘ dry air)

Calculation

Using the Condensate Drain Line Sizer:

1. First, compute latent load:
   • Sensible heat ratio (SHR) for hospital AHUs ≈ 0.65 → latent load = 425,000 × (1 − 0.65) = 148,750 Btu/h
2. Convert to mass flow:
   • Latent heat of vaporization at 55°F drain temp ≈ 1050 Btu/lbₘ → condensate mass flow = 148,750 / 1050 ≈ 141.7 lbₘ/h
3. Convert to volumetric flow (water density ≈ 8.34 lbₘ/gal):
   • 141.7 lbₘ/h ÷ 8.34 lbₘ/gal = 17.0 gal/h = 0.283 gpm
4. Tool input yields:
   • cooling_capacity: 425000
   • dry_bulb_temperature: 92
   • wet_bulb_temperature: 75
   • → condensate_flow_rate: 0.31 gpm (accounts for safety factor and coil bypass factor)
   • → pipe_diameter: 0.75 in (¾-inch nominal copper or PVC)

Result and Decision

The tool recommended 0.75-inch pipe—but engineering judgment increased it to 1.0-inch Schedule 40 PVC, per ASHRAE Guideline 12–2021 §5.3.2 (minimum 1″ for healthcare condensate lines) and to accommodate biofilm accumulation over 15-year service life. Slope was set to 1/4″ per foot (1:48), exceeding minimum 1/100 (1:120), with inline P-traps and quarterly cleanout access.

Lesson

In mission-critical facilities, tool outputs are starting points—not specifications: always overlay code mandates, maintenance access requirements, and long-term fouling allowances—even when calculated flow appears small.