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, 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.
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.