Achieving High Pump Efficiency

Achieving High Pump Efficiency:

George Taber

George Taber, a 38-year veteran of Taco, Inc.

By George Taber – Applications Engineer-Technical Services Supervisor, Taco, Inc.

As energy costs rise and achieving energy efficiency becomes more and more important in operating pumping systems, continuing attention by equipment suppliers, system designers and operators will focus on maximizing the efficiency of HVAC system components in order to conserve energy and reduce operating expenses. The introduction of variable speed pumping adds another dimension to the quest for energy efficiency.

This blog series will focus on the factors affecting pump efficiency. We invite your comments to extend the discussion.

Examine the System

In designing a pumping system one should certainly look at the efficiency of the pumps to achieve a certain amount of energy savings. However, system pumps are only one element of the many components in a system that will determine the overall efficiency of that system. Additional components and elements of importance are:

  • the boiler, chiller or heat exchanger
  • proper pipe size and piping design (to reduce loses)
  • valves loses
  • radiation efficiency
  • in a heating system, water temperature (which should vary with outside temperature to reduce heating /cooling costs)
  • motor efficiency, pump size, and use of variable speed controls to match the system load conditions
  • operating point
  • piping design controls to monitor and operate the system to
  • match the load changes.

In a piping system a pump’s energy is consumed by the friction of the piping and fittings, heat exchangers, heating /cooling coils, control valves, balance valves (manual and automatic), and the use of constant speed pumps. If we eliminate all the friction wasting devices in a system we can reduce the pump size and reduce energy.

Instead of the use of on /off valves and balancing valves, one type of piping used in a heating/cooling system can be designed in a primary /secondary configuration with the coils in the secondary and turn on a low energy pump only

when needed. The main circulation pump could be a low energy pump due to the fact it does not have to overcome the friction of all the control valves in a system.

Elements that Affect a Pump’s Operational Efficiency

The pump is one of the elements that will require more or less energy usage in a system, so we need to look at the elements that affect its energy consumption. Most pump manufacturers specify centrifugal pump performance based on pumping clear 68º F water. The major influence on centrifugal pump selection efficiency are specific speed (NS), pump size, NPSHA& R, viscosity of the fluids being pumped, temperature, specific gravity and the type of pump selected to meet the system conditions. The Hydraulics Institute has charted the expected efficiency of different types of pumps at different Specific Speeds (NS). NS is a dimensionless number calculated from the formula NS= N X Q.5 / H.75      N=RPM, Q=Flow (GPM), H= Head. (Ft. of water)

Figure 1

A circulator producing 20 GPM and 20 ft. of head at 1,725 RPM has an NS = 816. A pump producing 5000 GPM and 150 ft. head at 1,750 RPM has an NS = 2887. Pump efficiency at optimum NS at 816 = 30%; the efficiency correction chart would be 5 %. The predicted efficiency = 30%- 5%= 25%. The normal deviation is +/-16% so this pump’s predictive efficiency would be between 9 to 14%.

The minimal optimum general attainable efficiency of the low specific speed pump is 30% minus 5%= 25%. The normal deviation is +/- 16% so this pump’s predictive efficiency is also 9 to 14%. The high flow and head pump would have an optimum general attainable efficiency of 89% with no correction factor. The deviation from attainable efficiency is 3%. From these calculations you can see that the low head and flow pumps have low efficiency, and the high flow and head pumps have high efficiency. It is also to be noted that the deviation is a lot smaller in high NS pumps.

Factors that affect the deviation from attainable efficiency are: surface roughness, internal clearances, mechanical losses such as bearings, lip seals, mechanical seals, and packing, high suction specific speed, impeller trim, and viscosity of the fluid pumped.  The low NS pumps are affected most by surface roughness, internal clearances and mechanical losses. High NS pumps are affected more from high suctions speed requirements, impeller trim and viscosity. All pump manufacturers for the same NS pump can only tweak the pump design variables to get close to the attainable efficiency.

Next Up: Centrifugal Pumps and BEP

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