As energy costs rise and energy efficiency becomes more important in operating pumping systems, equipment suppliers, system designers and operators will maximize the efficiency of system components to conserve energy and reduce operating expenses.
In designing a pumping system, pump efficiency should achieve a certain amount of energy savings. However, system pumps are only one element that will determine the overall efficiency of that system. Additional components and elements of importance are:
- The boiler, chiller or heat exchanger
- Piping size and design (to reduce losses)
- Valve losses
- Radiation efficiency
- Water temperature in a heating system (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
- 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. Eliminating all the friction wasting devices in a system can reduce the pump size and energy usage.
Instead of the use of on/off valves and balancing valves, piping in a heating/cooling system can be designed in a primary/secondary configuration with the coils in the secondary. A low energy pump can be turned on only when needed. The main circulation pump could be a low energy pump since it does not have to overcome the friction of all the control valves in a system.
The pump will require more or less energy usage in a system, depending on the elements that affect its energy consumption. Most pump manufacturers specify centrifugal pump performance based on pumping clear 68 deg F water. The major influences on centrifugal pump selection efficiency are specific speed (NS), pump size, NPSHA and R, viscosity of the fluids pumped, temperature, specific gravity and the pump type selected to meet the system conditions. The Hydraulic Institute has charted the expected efficiency of different types of pumps at different specific speeds. NS is a dimensionless number calculated from the formula:
NS= N X Q.5 / H.75
Where:
N=RPM
Q=Flow (GPM)
H= Head (feet of water)
A circulator producing 20 gpm, 20 ft head at 1,725 rpm has a NS = 816. A pump producing 5,000 gpm, 150 ft head at 1,750 rpm has a NS = 2,887. The pump efficiency at optimum NS at 816 equals 30 percent, and the efficiency correction chart would be 5 percent. The predicted efficiency equals 30 % - 5 %= 25 %. The normal deviation is +/-16 percent, so this pump predictive efficiency would be between 9 to 14 percent.
The 5,000 gpm pump has a NS=2,887 pump efficiency at optimum NS efficiency and would be 89 percent with no correction factor. The deviation from attainable efficiency is 3 percent. These calculations show that low head and flow pumps have low efficiency, and high flow and head pumps have high efficiency. The deviation is much smaller in high NS pumps.
Factors that affect the deviation from attainable efficiency are surface roughness; internal clearances; mechanical losses like bearings, lip seals, mechanical seals and packing; high suction specific speed; impeller trim and viscosity of the fluid pumped. Low NS pumps are affected most by surface roughness, internal clearances and mechanical losses. High NS pumps are affected more from high suction 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.
A centrifugal pump is designed for best performance at a head and flow at a certain speed called the Best Efficiency Point (BEP). A pump should be selected so that it will always operate near its BEP. Operating a pump at less or more than the BEP will lower the operational efficiency and place additional stress on the pump shaft and bearing due to increased thrust and radial load. Higher flows will increase the NPSH required, erosion due to cavitation, noise and vibration.
Pumps are variable torque machines that follow the Affinity Laws. These laws explain the change in a pump's performance when the speed is changed or the impeller diameter is changed. These laws can be used to predict a pump's performance at a reduced speed or smaller diameter impeller. The energy saving can be calculated. If a pump has excess performance, a greater energy savings can be achieved by using a variable speed drive or correcting the impeller trim to match the system resistance. Throttling the pump adds additional resistance to the system to control the pump and is not as efficient as reducing the speed or diameter of the impeller.
The Affinity Laws are:
(RPM2 / RPM1) X GPM1 = GPM2
(RPM2 / RPM1)2 X H1 = H2
(RPM2 /RPM1)3 X BHP1 = BHP2
The Brake Horse Power formula shows that the BHP changes with the cube of ratios of the speeds, which is an energy savings for a small change in speed. Replacing the RPMs with the impeller diameter will follow the same rules. Decreasing the diameter of the impeller from full size does reduce the head, flow and BHP. The further away from full size diameter, the greater the drop in efficiency. The reduction in horsepower due to a lower head should offset this efficiency drop.
The BHP can be calculated from the formula BHP = Q X F X Sp Gr. / 3960 X pump efficiency. This formula can also be used to predict the operating cost. The electric motor driving the pump has an efficiency factor, so to determine the operating cost, factor in the motor by BHP X .746 / efficiency of the motor = Pump kW. (Note: With fluids other than water, the fluid's specific gravity affects the BHP.)
As a process or heating-cooling system may operate at full load for only a small portion of a given day, if the pump speed can be changed, more energy savings can be achieved (as opposed to worrying about a few +/- points on pump efficiency). Proper impeller trim, pump size and operating point are all important to best operational efficiency. It may be economical to stage different size pumps to carry the load instead of a single large pump.
The viscosity of fluids pumped influences the pump's performance and efficiency. Viscosity influences the friction loss of all the components of the system and the heat transfer rate of heat exchangers in the system. It is the responsibility of the system designer to supply the pump manufacturers with the true flow and head requirements of the system operating with fluids other than water.
Many engineering handbooks can facilitate calculating the friction loss in pipes and fittings with different fluids. The Hydraulic Institute (www.pumps.org) has an engineering data book available with information on fluids and methods for calculating losses.
Once flow, resistance (head), fluid temperature and the fluid type-and if it is a mixture of water and fluid, the concentration-are calculated, the pump manufacturer can select the properly sized pump, materials of construction and motor. The viscosity of the pumped liquid is a critical factor to consider, since it affects pump performance and horsepower required. Centrifugal pumps normally use pumping liquids with viscosities below 3,000 SSU (660 centistokes, CST). They may be used up to at least 15,000 SSU (3,300 CST). The higher the viscosity, the more significant the reduction in capacity, head and efficiency.
The effects of viscosity on performance of a centrifugal pump operating at the Best Efficiency Point can be seen in Figure 3.
In the 1960s, the Hydraulic Institute published a chart that was used to determine the performance of pumps pumping different viscosity fluids. Since its initial publication, additional data has been collected from pump manufacturers, and ANSI/HI 9.6.7-2004, The Effects of Liquid Viscosity on Rotodynamic (Centrifugal and Vertical) Pump Performance, has been published. The new guideline allows the engineer to calculate the performance of a pump more accurately.
Many factors affect a pumping system's efficiency. It is not just the pump but the whole system that has to be analyzed. Ask whether the system is steady state or cyclic, and how it operates in off-peak periods to achieve the best efficiency.