Your questions answered: Efficient pump selection and control

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

04/13/2015


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 1 2 Next > Last >>

No comments
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.
Combined heat and power; Assessing replacement of electrical systems; Energy codes and lighting; Salary Survey; Fan efficiency
Commissioning lighting control systems; 2016 Commissioning Giants; Design high-efficiency hot water systems for hospitals; Evaluating condensation and condensate
Solving HVAC challenges; Thermal comfort criteria; Liquid-immersion cooling; Specifying VRF systems; 2016 Product of the Year winners
Driving motor efficiency; Preventing Arc Flash in mission critical facilities; Integrating alternative power and existing electrical systems
Putting COPS into context; Designing medium-voltage electrical systems; Planning and designing resilient, efficient data centers; The nine steps of designing generator fuel systems
Designing generator systems; Using online commissioning tools; Selective coordination best practices
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.
click me