As energy costs rise, equipment suppliers, system designers, and operators focus increasingly on maximizing the efficiency of HVAC-system components to conserve energy and reduce operating expenses. This article discusses methods of achieving more-efficient pumping systems.

Examine the System

In designing a pumping system, one needs to look beyond the pumps, if energy savings are to be achieved. Also of importance are:

• The boiler, chiller, or heat exchanger.

• Pipe size and design.

• Valve losses.

• Radiation efficiency.

• In a heating system, water temperature, which should vary with outside temperature to reduce heating/cooling costs.

• Motor efficiency, pump size, and the use of variable-speed controls.

• Operating point.

A pump loses energy because of friction related to piping and fittings, heat exchangers, heating/cooling coils, control valves, and balance valves. If friction can be minimized, pump size can be reduced and energy saved. For instance, instead of using on/off valves and balancing valves, design piping in a primary/secondary configuration with the coils in the secondary loop, and turn on a low-energy pump only when needed. The main circulation pump can be low energy because it does not have to overcome control-valve-related friction.

Elements Affecting Pump Operating Efficiency

Most centrifugal-pump manufacturers specify performance based on the pumping of clear 68°F water. The major influences on centrifugal-pump efficiency are specific speed, pump size, net positive suction head (NPSH) available and NPSH required, viscosity of the fluids being pumped, temperature, specific gravity, and pump type. The Hydraulic Institute has charted the expected efficiency of different types of pumps at different specific speeds. Specific speed is a dimensionless number calculated from the formula:

N × Q^.5 ÷ H^.75

where:

N = revolutions per minute

Q = flow, gallons per minute

H = head, feet of water

A circulator producing 20 gpm and 20 ft of head at 1,725 rpm has a specific speed of 816. A pump producing 5,000 gpm and 150 ft of head at 1,750 rpm has a specific speed of 2,887. Pump efficiency at the optimum specific speed of 816 is 30 percent; the correction factor is 5 percent. The predicted efficiency of 30 percent minus 5 percent is 25 percent. The normal deviation is ±16 percent, so this pump's predictive efficiency is 9 to 14 percent.

The high-flow-and-head pump would have an optimum generally attainable efficiency of 89 percent with no correction factor. The deviation from attainable efficiency is 3 percent. From these calculations, one can see the efficiency of pumps with low head and low flow is low, while the efficiency of pumps with high flow and high head is high. In high-specific-speed pumps, the deviation is much smaller.

Factors affecting deviation from attainable efficiency are surface roughness; internal clearances; mechanical losses, such as those related to bearings, lip seals, mechanical seals, and packing; high suction specific speed; impeller trim; and the viscosity of the fluid pumped. Low-specific-speed pumps are affected most by surface roughness, internal clearances, and mechanical losses. High-specific-speed pumps are affected most by high-suction-speed requirements, impeller trim, and viscosity.