Flow-rate and pressure-drop Simulator

Click here for full page version   and here for a YouTube explanation

All ‘wet’ heating systems involve ‘pushing’ heated water through pipework circuits. A circulation pump is used to create the ‘push’, and the bore and length of all the sections of pipe and fittings is what it’s working against.

For pipes and components in series (i.e. one after another), the pressure needed for each (one-after-another) part is added up to give the total.

For parallel circuits, the flow can be considered as splitting. E.g., if a pump is delivering 10litres/minute to 5 similar-sized radiators, you can assume each radiator has about 2litres/minute flowing through it.

On this page we are only considering lengths of pipe in a circuit, and it is this detail that should be calculated (looked up). In brief, the smaller bore and/or longer the pipe, the greater pressure required.   Or to think of it another way, you need to select the appropriate pipe bore-size required for a given pipe length and flow requirement.

This Pipe Flow Simulator was devised to help assess what pipe sizes might be needed.  It was never planned to be a design tool, rather a teaching aid, nonetheless, given that many pipes are simply guessed, this can help greatly in making a choice.

Description of the Simulator

This Simulator has a section of pipe that can be any length between 1 and 50m.  A variety of sizes, copper and plastic can be selected.  The inlet pressure is variable from 0 to 5metres ‘head of water’.  The outlet pressure is zero (atmospheric pressure), so the actual pressure drop across the pipe length is the same as the inlet pressure reading.

When designing a system, you generally have pipe runs of known length, and are trying to achieve a certain flow rate.  For heat pumps, the flowrate is generally around 3litres/minute for every kW heat quantity

Most domestic circulator pumps could deliver a pressure of around 5 or 6 metres head, though some may go up to 8.  But at the maximum limit, the flow rate is minimal.  Ideally the pump will operate best at mid-rage.   E.g., you may aim for around 3m head total pressure drop in your system.  This pump will need to overcome ALL the pressure drops in every section of pipe and every bend, filter, heat-exchanger etc., and they soon add up!

It should be noted that circulation pumps have become far more energy-efficient in recent years. This could simply mean that you save energy, however, it also can mean that you could choose smaller bore pipes at times.   This can have some advantages in that smaller pipes contain less water. In certain circumstances this could save energy. E.g., for intermittently used hot water pipes.

So, if you have pipe length and pressure in mind, you can enter these into the simulator.  Now you can change the pressure so that you achieve the flow rate required.

The volume of liquid in the pipe is also given.  This can be relevant to saving energy when long pipe runs are being considered.

Any pipe circuit involves bends and fittings.  All these add to the pressure drop in the circuit and this should be allowed for.  As a guide, for 15, 22 and 28mm copper elbows, you could add 0.4, 0.6 and 0.9 metre equivalent length of pipe respectively for each elbow in the circuit.

Accuracy……. for mid-range, the accuracy is good. But for the extremities and unusual scenarios, the accuracy can be up to 5% out.  You can verify your findings using any online pressure drop calculator.

Why have I made the pressure a variable input and not the flow rate?
In some ways, as a design tool it might be better to input the required flow rate.  However, as a learning tool, I consider it better to firstly think of the pipe and the pressure drop across it.  The flow rate is a secondary, and is a result of the pipe size/length and the pressure.  I think that this gives a better intuitive feel to what is going on.

For further reading, John Hearfield’s page is great.