How to calculate the pressurization unit pump flow ?

In this article, I will outline what a pressurisation unit is just to ensure we are all on the same page and then I will go on to explain some points to follow when selecting a suitable one for your requirements.

So, let’s start with answering the question of what a pressurisation unit is. A pressurisation unit, PU for short, is a mechanical piece of equipment used to pressurise a system, usually heating or hot and cold water services. It fills the system to the optimum pressure to allow the system to flow correctly; it uses one or two small peripheral pumps to maintain the pressure throughout the system. The PU is controlled via pressure switches which sense the system pressure and engage the pumps if pressure is lost for any reason. 

A PU is connected in to a closed system at neutral point. This is the point that is not affected by the duty of a system pump. This is normally on the return to the main circulation pumps.

If the PU is piped in the wrong place in the system, the pressure will fluctuate when the main pumps are switched on. Causing pressure problems, air and noisy.

A pressurisation unit usually has two main factors when being sized.

Firstly, the static head of the system. This is worked out the same way as a booster set:

     The static head of the building is calculated by counting the floors and then times them by 3 meters.     Adding 0.5 Bar on top so that the upper floors are always pressurised.     We should always remember that pressures are measured in the following scales: 1 Bar = 10m.      10 Floor building + basement = 11 x 3m = 3.3 Bar + .5 Bar = Static Pressure = 3.8 Bar.

So our pressurisation unit should have a fill pressure of 3.8 Bar.

The static head is always the pressure required to fill the system. If the plantroom is on the roof of a 11 floor building you only need 0.8 Bar to fill the system as the rest of the floors are below the unit.

Secondly, the expansion of a system. The vessels on a system are sized from the expansion factor. The system content has to be used along with the heat or chilling power and the static head of the system.

A heating system on a roof will have much smaller vessels than a system in a basement as the static head is much smaller. Also the vessels on a chilled water system only take up expansion when the chiller is switched off.

Also we should check that the system has a Pressure relief valve which would lift if the system was to over expand.

If the vessels in a system have been sized correctly the following setting should suit most applications.

     Cold fill pressure = Static pressure calculation from above.     Low Pressure cut out = 0.2 Bar below fill pressure.     High Pressure cut out = 0.2 Bar below the safety valve setting.     Vessel pre-charge = Same as fill pressure.

When Vessels are drained down and re charged the isolation valves should be opened slowly so that the pressure is maintained by the pressurisation unit.