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Chilled Water Pipe Sizing Calculator

Size chilled water pipes for central HVAC systems. Calculate pipe diameter, pressure drop, and pump head for chilled water distribution per ASHRAE Handbook.

📐 Standard: ASHRAE Handbook / ISHRAE
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Chilled Water Pipe Sizing Calculator Calculator
Reference: ASHRAE Handbook / ISHRAE
🌡️ HVAC
Size chilled water pipes for central HVAC systems. Calculate pipe diameter, pressure drop, and pump head for chilled water distribution per ASHRAE Handbook.
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ℹ️ About This Calculator

The chilled water (CHW) pipe sizing calculator determines pipe diameters for the distribution network from the chiller plant to all air handling units and fan coil units. The calculator uses the equal friction method to maintain 100–250 Pa/m pressure drop, ensuring adequate flow to all terminal units without excessive pump energy per ASHRAE Handbook – HVAC Systems and Equipment.

Chilled water systems in Indian commercial buildings typically use a primary-secondary or variable primary flow (VPF) configuration. Primary pumps (in the chiller room) maintain constant flow through chillers; secondary pumps distribute to AHUs and FCUs at variable flow based on load. Insulation: 25 mm armaflex or equivalent on CHW pipes to prevent condensation. Variable speed drives on secondary pumps save 30–50% pump energy compared to constant speed systems.

📐 CHW Pipe Sizing

ASHRAE Handbook / ISHRAE

CHW Flow Rate:
  Q_chw = Q_cooling / (ρ × Cp × ΔT)
  ΔT = 6°C (standard: 12°C supply, 6°C return temperature difference)
  ρ = 1000 kg/m³; Cp = 4.187 kJ/kg·K
  Q_chw (L/s) = Q_cooling (kW) / (4.187 × 6) = Q_cooling / 25.1

Pipe Velocity Limit:
  Main headers: 2.5–3.5 m/s; branches: 1.5–2.5 m/s
  Low noise zones: ≤ 1.5 m/s

Pressure Drop:
  Use Darcy-Weisbach; target 100–200 Pa/m friction rate
  Total pump head = index circuit losses (highest resistance path)

🧮 Worked Example

Example: An AHU riser must carry 350 kW of cooling load at the standard ΔT of 6°C. Flow: Q_chw = 350/25.1 ≈ 13.9 L/s. For a main header velocity limit of 3.0 m/s, required area A = Q/v = 0.0139/3.0 ≈ 0.0046 m², giving diameter D = √(4×0.0046/π) ≈ 77 mm - so an 80 mm (3 inch) header is selected.

Checking actual velocity at 80 mm: v = 0.0139/(π/4×0.08²) ≈ 2.8 m/s, within range, and the friction rate at this flow and size is close to the 150 Pa/m target used for pump head calculation.

❓ Frequently Asked Questions

What temperature differential (ΔT) should I use for CHW systems? +
Standard ΔT: 6°C (12°C supply / 18°C return – common in India for 100% humidity control). High ΔT: 8–10°C (7°C supply / 15–17°C return) – reduces pipe sizes and pump flow by 25–40%, saving capital cost and pump energy. Low ΔT syndrome: poorly maintained AHU coils, short-circuiting, or bypassed three-way valves cause actual ΔT to be 2–4°C rather than 6°C – increasing pump flow and energy significantly.
How do I size the chilled water pump? +
Pump flow = sum of all AHU/FCU design CHW flows. Pump head = pressure drop through the index circuit (highest resistance path from pump through pipes, coils, and back). Index circuit: chiller → longest pipe route to most remote AHU → return. Include: pipe friction, fittings, isolation valves (2–3 m), control valve (3–5 m), chiller cooler bundle (6–10 m), AHU coil (3–5 m). Total head typically 15–35 m for commercial buildings.
What pipe insulation is required for chilled water pipes? +
CHW pipes must be insulated to prevent condensation (surface temperature must stay above dew point). Minimum insulation: 25 mm closed-cell foam (Armaflex type) for pipes up to 50 mm; 32 mm for 50–100 mm; 38 mm for 100–200 mm pipes. Insulation with aluminium cladding in exposed areas (plant rooms, roof). All valves, flanges, and fittings must also be insulated (use pre-formed sections). Uninsulated sections form condensation and cause damage.
What is a variable primary flow (VPF) chilled water system? +
VPF: variable speed pumps circulate water directly through chillers at variable flow (no secondary pumps). More energy-efficient than primary-secondary for systems with widely varying loads. Requires: chillers that can accept variable flow (minimum flow protection bypass valve); chillers with stable surge-free operation down to 20–30% flow; pressure differential control across the index circuit. VPF saves approximately 15% pump energy versus primary-secondary.
How do I balance a chilled water system? +
Hydraulic balancing ensures each AHU/FCU receives its design flow: install commissioning/balancing valves (STAD or similar) at each terminal; set them to create the design pressure differential. Pre-set pressure independent control valves (PICVs) combine balancing and control – simplest approach for new systems. For existing systems: use differential pressure controllers at risers and ensure control valve authorities are ≥ 0.5.

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