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Achieving high pump efficiency
BY GEORGE TABER
contributing writer
As energy costs rise and as achieving energy efficiency becomes more and more important in the operation of hvac systems, continuing attention by equipment suppliers and operators will center on maximizing the efficiency of the system components in order to conserve energy and reduce operating expenses.
In designing a hydronic heating or cooling system, we 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:
In a piping system, a pump’s energy is consumed by the friction of the piping and fittings, 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 using on/off valves and balancing valves, we can pipe the system in a primary/secondary configuration with the coils in the secondary and turn on a low energy pump only when needed. Because it does not have to overcome the friction of all the control valves in a system, the main circulation pump could be a low energy pump.
The pump is one of the elements that will use more or less energy, so we need to look at the elements that affect its energy consumption. The major influences on centrifugal pump selection efficiency are specific speed (ns), pump size, NPSHA&R 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 ns. Ns is a dimensionless number calculated from the formula ns = NxQ.5/H.75, where n=rpm; q=flow (gpm), h=head (Ft. of water).
A circulator producing 20 gpm, 20 ft. head at 1725 rpm has a ns = 816. A pump producing 5,000 gpm, 150 ft. head at 1750 rpm has a ns = 2887. The pump efficiency at optimum ns at 816 = 30% and the efficiency correction chart would be 5%. The predicted efficiency = 30% - 5%= 25%. The normal deviation is +/-16%, so this pump predictive efficiency would be between 9% and 14%.
The 5,000-gpm pump has a ns = 2887 pump efficiency at optimum ns efficiency and would be 89% with no correction factor. The deviation from attainable efficiency is 3%. From these calculations we 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 by 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. This is 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 than 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, and erosion due to cavitation could result, along with an increase in noise and vibration.
Pumps are variable torque machines that follow the Affinity Laws. These laws explain the change in performance of a pump when the speed is changed or the impeller diameter is changed. They can be used to predict the performance of a pump 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)xGPM1=GPM2;
(RPM2/RPM1)2xH1=H2;
(RPM2/RPM1)3xBHP1=BHP2
You can see from the Brake Horse Power (bhp) formula that the bhp changes with the cube of ratios of the speeds, which is a big 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 we get away from full size diameter there will be a drop in efficiency, but the reduction in horsepower due to a lower head should offset this efficiency drop.
The bhp can be calculated from the formula BHP=QxFxSp Gr./3960xpump efficiency. This formula can also be used to predict the operating cost. The electric motor driving the pump also has an efficiency factor, so to determine the operating cost we would factor in the motor by BHP 5.746/efficiency of the motor = Pump kw.
As a heating-cooling system operates 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 than worrying about a few +/- points on pump efficiency. As mentioned earlier, proper impeller trim, pump size and operating point are all important for best operational efficiency.
George Taber is an applications engineer - technical services supervisor, Taco Inc.
