Choose the pipe diameter for a chilled water or glycol circuit from the flow rate or the cooling capacity, with velocity limits by pipe size and the pressure drop per metre.
Pipe diameter from flow and velocity
Inner diameters of medium-weight steel pipe. Pressure drop by Darcy-Weisbach with the Swamee-Jain friction factor, straight pipe only. Check the inner diameter of the pipe you actually use.
How the tool selects a pipe
The tool calculates the velocity in each nominal size for your flow and picks the smallest pipe in which the velocity stays below the upper limit for that size. For the recommended size and its neighbours it also shows the pressure drop per metre of straight pipe and the flow regime. You can enter the flow directly, or let the tool calculate it from the cooling capacity and the temperature difference.
The velocity is the flow divided by the inner cross-section:
v = V / (pi x d² / 4)
The pressure drop per metre follows the Darcy-Weisbach equation, R = f / d x rho x v² / 2, with the friction factor f from the Swamee-Jain approximation of the Colebrook equation for turbulent flow, and f = 64 / Re for laminar flow. Density and viscosity are taken at the mean fluid temperature, so glycol mixtures and cold fluids are handled correctly.
Velocity limits
There is no single correct velocity. Small pipes have a high pressure drop at the same velocity, so their limits are lower; large mains can run faster without excessive losses. The tool uses these ranges:
| Nominal size | Inner diameter (mm) | Velocity range (m/s) | Flow at that velocity (m³/h) |
|---|---|---|---|
| DN15 | 16.1 | 0.5 to 1.5 | 0.4 to 1.1 |
| DN20 | 21.7 | 0.5 to 1.5 | 0.7 to 2.0 |
| DN25 | 27.3 | 0.5 to 1.5 | 1.1 to 3.2 |
| DN32 | 36.0 | 0.5 to 1.5 | 1.8 to 5.5 |
| DN40 | 41.9 | 0.5 to 1.5 | 2.5 to 7.4 |
| DN50 | 53.1 | 0.5 to 1.5 | 4.0 to 12 |
| DN65 | 68.9 | 0.8 to 2.0 | 11 to 27 |
| DN80 | 80.9 | 0.8 to 2.0 | 15 to 37 |
| DN100 | 105.3 | 0.8 to 2.0 | 25 to 63 |
| DN125 | 130.7 | 0.8 to 2.0 | 39 to 97 |
| DN150 | 159.3 | 0.8 to 2.0 | 57 to 144 |
| DN200 | 206.5 | 1.0 to 2.5 | 121 to 301 |
| DN250 | 260.4 | 1.0 to 2.5 | 192 to 479 |
| DN300 | 309.7 | 1.0 to 2.5 | 271 to 678 |
| DN350 | 339.6 | 1.0 to 2.5 | 326 to 815 |
| DN400 | 388.8 | 1.0 to 2.5 | 427 to 1,069 |
Below the lower limit the pipe is oversized: it costs more, holds more water and lets air and dirt settle. Above the upper limit, noise, erosion at bends and valves, and pumping energy rise quickly. For pipes in occupied rooms or near offices, stay in the lower half of the range.
Pressure drop targets
In chilled water systems a pressure drop of 100 to 300 Pa per metre of straight pipe is a common design range. Fittings, valves and strainers add to this; for a first estimate, add 30 to 50 % to the straight pipe loss in a typical plant room. The pressure drop of the chiller evaporator, the coils and the control valves usually dominates and must come from the data sheets.
Glycol and low temperatures
A glycol mixture is more viscous than water, especially when cold. At the same flow, the Reynolds number falls and the pressure drop rises, by 20 to 60 % in typical cases. In small pipes at low temperature the flow can become laminar. Select the fluid and the concentration in the tool to see the effect; if you convert an existing water system to glycol, check that the pump still delivers the required flow.
Practical advice
- Size the main pipes for the full design flow, including any future extension that is already planned.
- Do not reduce the pipe size at the chiller connections just because the flanges are smaller. Use reducers right at the unit.
- Older steel pipes have a rougher wall and often scale or corrosion that reduces the inner diameter. Select “older steel pipe” when you check an existing installation.
- For plastic or multilayer pipes, use the inner diameter from the manufacturer, which can differ a lot from steel of the same nominal size.
- Insulate chilled water pipes with a closed-cell material and a vapour barrier. The dew point calculator tells you when condensation starts.
We design and modernise water and glycol circuits; see chilled water installations. To get the flow from the cooling load, the cooling capacity calculator does the same calculation with more detail.
Frequently asked questions
What velocity should chilled water pipes have?
As a guide, 0.5 to 1.5 m/s in pipes up to DN50, 0.8 to 2.0 m/s up to DN150 and 1.0 to 2.5 m/s in larger mains. Stay lower in pipes near occupied rooms to avoid noise.
Why is the recommended size different from the chiller connection size?
The connection size is chosen by the manufacturer for the heat exchanger and the unit layout. The pipe size should follow the flow and the length of the run. Long runs often need one size larger than the connection.
Does the pressure drop include fittings?
No, it is the loss in straight pipe. Add the losses of bends, tees, valves and strainers separately, or add 30 to 50 % as a rough allowance for a compact plant room.
Can I use the tool for heating water?
Yes, up to about 60 °C mean temperature. Water at higher temperatures has a lower viscosity, so the pressure drop is a little lower than at chilled water temperatures.
Planning a chilled water circuit?
We design, install and commission water and glycol circuits for chillers, with recorded flow and pressure values.