When developing HVAC centrifugal pumps, designers have traditionally aimed at a pump’s highest point of efficiency. Given the wide variation in flow and pressure demands of HVAC systems, however, Xylem’s new white paper looks at how pump units can meet these varying conditions not just at the optimal point but at an optimal range, dubbed an “efficiency island.”
By Ann Törnkvist
“When redesigning our pumps, we realized that in most applications our customers have a varying demand,” says Chris Johnson, Global Engineering Director, Centrifugal Pumps, at Xylem. “Traditional pump design called for a high peak efficiency point, but with customers wanting to save energy by sizing their equipment to match varying demand, and further controlling this with variable speed drives, we realized that we needed to design pumps to operate at high efficiencies across a broad range.”
Shifting from optimal point to optimal range comes with a clear advantage, says Johnson, as it takes into account that most systems have varying loads and rarely operate at their most efficient point.
“We saw the need for the white paper as our design process progressed and we began to test prototypes,” Johnson says. “So we set out to design the pumps to have high efficiency across a broad range of operation. The traditional design with a peak efficiency point at constant speed is a thing of the past.”
What is an efficiency island?
An efficiency island refers to the optimal range for centrifugal pump operation. When shown on a graph, its boundaries are the:
– Maximum impeller diameter performance
– Minimum diameter performance
– Iso-efficiency lines
“Because the iso-efficiency lines of these broad efficiency bands on the performance curve appear as round and resemble islands, we came up with the term efficiency island,” Johnson says. “Basically, it is a bigger sweet spot for pump performance.”
This broad range of performance includes the ANSI/HI preferred operating region of 70 to 120 percent of the best efficiency point, and also maintains high efficiency even when trimming impellers.
Large efficiency islands lead to reduced costs
Using the examples of a hospital and a school, the white paper presents operating costs and finds a reduction in every example where a system was designed or upgraded to incorporate a deeper and wider efficiency island.
Lower operating costs stand out, of course, but the paper also notes that focusing on efficiency islands could also lead to installing smaller pump or drive sizes in certain scenarios. It could also open the door for variable speed units, which offer significant annual operating cost savings when compared to fixed speed units for the same service conditions.