Your questions answered: Efficient pump selection and control

Several questions about variable speed pumps, paralleling pumps, and controls are covered in these responses.


Reece Robinson, Grundfos Pumps Corp.Reece Robinson, senior technical trainer at Grundfos Pumps Corp., tackled unanswered questions from the March 31, 2015, webcast on Efficient pump selection and control.

Question: How we can install and control variable speed pumps in an old hydronic circuit with constant speed pump?

Answer: In most cases the answer is yes. The existing pumps are operating according to their performance curve with higher head output as flow approaches zero. A new variable speed pump can be installed to maintain either a constant differential pressure, a proportional differential pressure at the pump or a constant differential pressure at a remotely installed sensor.

Question: What about the net positive suction head (NSPH) of option 4?

Answer: Yes, good catch! In this example the most efficient option also had the highest NPSH requirement at the DESIGN flow. If this pump were to be considered for a closed loop installation (hot or chilled water), the NPSH requirement of 27 ft would not be of major concern. If the pump were to be considered for a cooling tower or other application with a flooded suction, care would have to be taken before using this pump.

Question: How do you create a curve for parallel connected pumps?

Answer: This can be done with a simple spreadsheet file. Most pump performance curves can be very well displayed with 5 points. Figure 1 shows 5 points for flow and head. For parallel pumps you simply multiply the flow by two, using the same value for head.

Figure 1: This shows 5 points for flow and head. For parallel pumps you simply multiply the flow by two, using the same value for head. Courtesy: Grundfos Pumps Corp.

Question: Efficiency sequencing: Does this mean a flowmeter and kW board are now required for my controller?

Answer: Not necessarily. Advanced controllers today can have pump performance data loaded into the controller. The controller will need some feedback, usually the differential pressure of the pump and flow from a flow sensor. Modern controllers will use a calculated flow rate and/or pump head as well. For example, if you know the pump head differential, the flow can be calculated if the pump curve information is loaded into the control. Power can be measured directly off of the variable frequency drives (VFDs) or it can be measured using current transformers and voltmeters.

Question: For a typical hot or chilled water HVAC application, do you see designs incorporating three pumps, two operating together to maximize efficiency instead of the typical to (one running, one redundant)?

Answer: Yes, especially when part load conditions are evaluated and also when central plants are installed with future loads are to be added later on down the road.

Question: Why is the design point on the system curve below the design point on the pump curve?

Answer: The design point is what the designer calculated. The duty point (actual operating point) is where the pump would be running unthrottled. Usually what happens is that there will be a multi-function valve and/or balancing valve that will add artificial head to the pump and the actual pump flow will match the design flow.

<< First < Previous Page 1 Page 2 Next > Last >>

Consulting-Specifying Engineer's Product of the Year (POY) contest is the premier award for new products in the HVAC, fire, electrical, and...
Consulting-Specifying Engineer magazine is dedicated to encouraging and recognizing the most talented young individuals...
The MEP Giants program lists the top mechanical, electrical, plumbing, and fire protection engineering firms in the United States.
Exploring fire pumps and systems; Lighting energy codes; Salary survey; Changes to NFPA 20
How to use IPD; 2017 Commissioning Giants; CFDs and harmonic mitigation; Eight steps to determine plumbing system requirements
2017 MEP Giants; Mergers and acquisitions report; ASHRAE 62.1; LEED v4 updates and tips; Understanding overcurrent protection
Power system design for high-performance buildings; mitigating arc flash hazards
Transformers; Electrical system design; Selecting and sizing transformers; Grounded and ungrounded system design, Paralleling generator systems
Commissioning electrical systems; Designing emergency and standby generator systems; VFDs in high-performance buildings
As brand protection manager for Eaton’s Electrical Sector, Tom Grace oversees counterfeit awareness...
Amara Rozgus is chief editor and content manager of Consulting-Specifier Engineer magazine.
IEEE power industry experts bring their combined experience in the electrical power industry...
Michael Heinsdorf, P.E., LEED AP, CDT is an Engineering Specification Writer at ARCOM MasterSpec.
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me