To BPO or not BPO ? – that is the question

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Back Pull Out

When selecting centrifugal pumps for air conditioning circuits, it’s good to be “fluid” in your decision making – after all, that’s what we’re dealing with.

We would all agree that the Back Pull Out (BPO) pump is a versatile and preferred selection when it comes to chilled, heating and condenser water circuits; they offer flexibility in allowing impeller changes within the same casing without having to disturb the pipework or change the electric motor if originally sized correctly.

This always made the BPO the preference when designing air conditioning systems.

However, this should not be a given. There are times when BPO pumps are not the best option.

This is particularly so on small flow rates.

Take, for example, a duty of 2 litres per second @ 200kpA.

If we plot this duty on a BPO type pump curve, be it DIN or ISO standard, it will be found to be a relatively flat curve. This will make it difficult to balance the system. A steep curve is always preferable.

So in this case a smaller pump running at 2 pole speed such as the SV or HMS series (pictured) are a better selection.

They use less power, being smaller, yet have the same level of efficiency as a BPO (which at this low duty point will always have low efficiency as a function of centrifugal design).

The smaller pump will save space due to its smaller footprint. Installation costs will be less, making the process simpler and more economical.

The noise factor is often questioned with these smaller pumps, as being a higher-speed unit, it is often assumed they would be noisier than an average BPO.

Masterflow tested the SV4 on our testing rig in the presence of an acoustic engineer for the COM Project some five years ago.

The SV4 met the acoustic requirements on this sensitive project and as a result, a dozen were installed and have been pumping merrily ever since.


So, when deciding whether you need a BPO or a smaller pump, ask yourself these questions:

  1. What’s the flow rate? – a smaller pump is better suited to smaller flow rates.
  2. Is the plantroom on the tight side? – a smaller pump has a smaller footprint.
  3. Is noise something to think about? – a smaller pump at high speeds still meets acoustic requirements.


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