Should my pumps be 4 pole or 2 pole?

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The ongoing question in HVAC pump selections…


Should my pumps be driven at 4 pole or 2 pole speed?

Traditionally, consulting engineers specify 4 pole (1450rpm nominal). This is usually good practice, but 2 pole (2900rpm nominal) pumps also excel in different applications. To ensure the best results possible from your HVAC system, make sure the choice of speed is made based on efficiency and practicality rather than price.


When your project demands…

1. Pumps with minimal downtime – 4 pole

The faster you run your pump, the sooner and more often it will need to be repaired. Single pump units will require an entire system shutdown when needing repairs/replacements of mechanical bearings and seals. Therefore, when planning a single-pump plantroom, choosing a 4 pole pump would decrease the total time spent on repairs and increase the productivity of your system long-term.

2. Pumps with a low Life Cycle Cost – choose 4 pole

Clients expect equipment with a reasonable lifespan and minimal maintenance. Speed + operating hours OR the total revolutions in the life of the pump must be considered to achieve this. For example, a 2 pole pump on a long cycle might be running intermittently because of its speed, and is already going to have a lower lifespan due to it being prone to repairs. Insert a 4 pole pump into the same cycle and it’s able to run continuously, and also offers a longer lifespan and fewer repairs needed due to a lower operating speed. In the case of long operating hours, a 4 pole pump is usually a superior option over a 2 pole.

3. Pumps that are noise-sensitive – 4 pole

Because 2 pole pumps are double the speed of 4 pole pumps, they are also much louder (especially as the size of the motor increases over 4kW). Therefore, when noise is a dealbreaker, go with 4 pole pumps.

4. Pumps for condenser water – 4 pole

For condenser water systems, where pumps are mounted adjacent to an open cooling tower, the typical net positive suction head (NPSH) available is minimal due to the tower being mounted low. The NPSH is also reduced by friction losses in suction strainers at towers and/or at pumps, as well as the tees, elbows, valves and reducers at the pump inlet.

These losses can be reduced significantly by decreasing velocity or increasing pipe diameters – HOWEVER – a 4 pole pump usually has a lower NPSH required, so adjusts well to the situation. For this reason, we highly recommend using a 4 pole pump with condenser water systems.

5. Pumps for a small flow rate and low head – 2 pole

If your flow rate is small (under 4L/s) and your pressure is relatively high (greater than 150kPa), a 2 pole pump selection will usually offer superior performance compared to what’s offered by a 4 pole. This is common for heating water applications where flows are low, pressure is reasonably high and the run time is less than for chilled water or condenser water pumps.

If 4 pole pumps are considered for these low flow/high pressure situations, the selection is often very inefficient, beyond the acceptable range of the Best Efficiency Point and poses potential issues, such as cavitation.

The selection curves below illustrate the comparison.




So what?

We share the engineers’ preference for 4 pole pumps, especially for Condenser Water applications.

However, when there are low flows/high heads involved (as is mostly the case on Heating Water systems), 2 pole selections offer the best result.

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