Worked example: a glycol cooling loop

A closed cooling loop has to do two jobs at once: move the coolant and carry the heat. This worked example is a 30 per cent ethylene-glycol loop solved with heat transfer switched on, so the hydraulics and the temperatures are solved together rather than one after the other.

The setup

The fluid is 30 per cent ethylene glycol, denser and more viscous than water and with a lower specific heat (density 1038 kg/m3, kinematic viscosity 2.1 x 10^-6 m2/s, specific heat 3650 J/kg.K). A pump circulates it from a cold header supplied at 15 degrees Celsius, with a head-flow curve from 25 m at shut-off down to 14 m at about 7.5 L/s and an efficiency of 65 per cent. The loop runs in DN50 pipe:

• A heat-exchanger drop, modelled as a fitting with a loss coefficient of 6, standing in for the cooler the coolant passes through.
• A process load that injects 8 kW of heat into the stream.
• A 40 m bare return line that sheds heat to a 15 degree ambient through a direct overall coefficient of 8 W/m2.K.

The physics and the method

This is the Advanced heat-transfer mode. Fluid Network Studio solves the loop hydraulics and the heat transfer together, so the flow that sets the residence time is the flow the network actually produces. The 8 kW load raises the coolant temperature where it is injected, and the bare return line then sheds part of that heat to the ambient along its length, using the direct overall coefficient you set rather than a built-up film. An energy balance is reported as a residual on every solve, so conservation is checked, not assumed.

Glycol changes the numbers against plain water in two ways at once. Its higher viscosity costs pump head, and its lower specific heat means a given heat load moves the temperature more for the same flow. Colouring the network by temperature shows both the rise across the load and the partial recovery down the return.

What you learn

Solving the example gives the pump duty on its curve, the flow around the loop, and the temperature at every node. Colour by temperature to see where the heat goes, then raise the flow or swap the fluid back to water and re-solve to see how the temperature swing changes. The physics of pipe heat loss is set out in full on the pipe heat loss application page.

Heat transfer is part of the Advanced plan, A$39/month or A$390/year. The glossary explains the overall heat-transfer coefficient and the resistances behind it.

Open this example in FNS and colour the network by temperature to trace the heat around the loop.