Calculation Methodology
This sizing engine automates the rigorous hydraulic calculations required to align fluid physics with the statutory Deemed-to-Satisfy (DtS) energy efficiency limits set out by the Australian National Construction Code.
The engine iteratively evaluates the fluid dynamics of the pipe network against standard manufacturing dimensions. It extracts the maximum allowable pressure drop (Pa/m) from Tables J6D8a, J6D8b, J6D8c, or J6D8d based on the system's operational profile (Speed, Network Type, and Annual Hours). The engine systematically loops through standard pipe sizes from smallest to largest, selecting the first nominal diameter that generates a friction loss strictly lower than the NCC limit.
Water density (ρ) and kinematic viscosity (ν) are not static; they fluctuate significantly with operational temperature. Cold water is physically "thicker" (higher viscosity) than hot water, resulting in elevated friction factors and higher pressure drops. To ensure absolute compliance under worst-case operational bounds, the engine processes the user-specified design temperature through active thermodynamic polynomial equations to precisely determine the fluid properties in real-time before executing the friction logic.
Actual pressure drop is mathematically modeled using the fundamental Darcy-Weisbach equation:
ΔP = f · (L / D) · (ρV² / 2)
- ΔP = Pressure Drop (Pascals)
- f = Darcy Friction Factor
- D = Internal Pipe Diameter (m)
- ρ = Fluid Density (kg/m³) at Design Temp
- V = Velocity (m/s)
To accurately determine the friction factor (f) for turbulent flow, the engine employs the Swamee-Jain approximation of the Colebrook-White equation. This accounts for the precise absolute roughness (ε) of the selected material:
- Copper Type B: Assumed exceptionally smooth with an absolute roughness of ε ≈ 0.0015 mm.
- Steel Schedule 40: Calculated with an absolute roughness of ε ≈ 0.045 mm, reflecting heavier drag and subsequent elevated pressure drop compared to copper.
To ensure precision, the algorithm does not use nominal (DN) sizes for mathematical calculations. It cross-references the selected Nominal Size against standard manufacturing tables (AS 1432 for Copper, ANSI B36.10 for Steel) to derive the exact internal free area, compensating for varying wall thicknesses before determining the final fluid velocity and friction rate.
Reference Tables
Air / water / refrigerant
| System Component | Temperature |
|---|---|
| Heating Water – Entering Coil | 80°C |
| Heating Water – Leaving Coil | 65 to 70°C |
| Condenser Water – Entering Tower | 35°C |
| Condenser Water – Leaving Tower | 29.5°C |
| Chilled Water – Entering Coil | 7°C |
| Chilled Water – Leaving Coil | 16°C |
| Air Cooled Condenser – Air Entering | 40°C |
| Air Cooled Condenser – Air Leaving | 54°C |
| Mixed Air on Cooling coil | 26°C |
| Air off Cooling coil | 15°C |
| Refrigerant Temperature – Chiller evaporator | 4°C |
| Mains Water | 10 to 30°C |
(Values shown are maximums unless otherwise stated and lower values may be used as appropriate)
Hot water
| Application | Temperature |
|---|---|
| Storage temperature (to inhibit Legionella growth). All sanitary fixtures used primarily for hygiene purposes | 60°C (minimum) |
| Early childhood centres, primary and secondary schools and nursing homes or similar facilities for young, aged, sick or people with disabilities | 45°C (maximum) |
| All other buildings | 50°C (maximum) |
| Hot Water – Dishwasher, commercial | 60 to 77°C |
| Hot Water – Laundry, commercial | 82°C |
(Based on AS/NZS 3500.4:2018)
Typically water velocity in pipework should not exceed 2.4m/s to minimise erosion and should not be less than 0.75m/s to ensure entrained air is transported to venting points. Restricting water velocity to below 2.0m/s can help save pumping energy but increases pipe size.
| Pipework Application | Velocity (m/s) |
|---|---|
| Chilled Water/Heating Hot Water pipework | 1.8 - 2.4 |
| Condenser Water pipework | 1.8 - 2.4 |
| Risers and Droppers | 0.9 - 2.4 |
| Coils - Cooling | 1.0 - 1.5 |
| Coils - Heating | 1.0 - 1.5 |
| Condensers | 1.5 - 2.0 |
| Pump Discharge pipework | 1.5 - 2.7 |
| Pump Suction pipework | 1.2 - 2.1 |
| Drain pipe | 1.2 - 2.1 |
| Mains Pressure Water Service Pipelines | 1.0 - 2.1 |
| Gravity Flow Pipelines from Upper Level Storage Tanks - (Top 2 Floors Only) | 0.1 - 0.4 |
| Gravity Flow Pipelines from Upper Level Storage Tanks - (Below Top 2 Floors) | 1.0 - 1.5 |
Note: AS/NZS 3500.1:2018 states that the maximum velocity in piping shall be 3.0m/s. This velocity limitation does not apply to any piping that is exclusively used for fire services whether independently served by a main, or combined with a domestic water supply – refer to Section 6 Pipe Systems and DA16.
The National Construction Code (NCC) sets maximum requirements for pressure drops in pump systems.
| Component | Pressure Drop |
|---|---|
| Coils – Cooling | 25 to 75kPa |
| Coils – Heating | 15 to 45kPa |
Note: Ensuring adequate full-load water velocity ensures that a good turndown ratio is achievable, before the flow turns laminar in the coil.