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Elusive Energy Savings:
Centrifugal Pumps and Variable Speed Drives - Part I

Copyright © 2002 Francis J. Martino


High energy savings are often expected through the application of
variable speed drives on centrifugal pumps. However, in many applications
the savings are often much lower than expected.

The lower realization of savings is often due to the omission of system
static head data when originally reviewing an application. A typical
application of a cooling tower pump will have the pump located in the
basement of a high rise facility and the tower located on the roof. The
pump and driving motor must then develop enough torque to act against
the weight of the column of water before developing additional torque to
actually move the water. It is the head caused by the column of water that
must be taken into account.

In order to reduce energy consumption, some processes will accept
the operation of a pump at a continuously reduced speed rather than at
full speed at all times. In addition, operating with a continuously reduced
speed rather than cycling the motor on and off may reduce demand
charges. The affinity curve of a centrifugal pump is used to calculate
energy savings in those applications. However, the system head
requirements are often overlooked.

The affinity curve is shown in Figure 1. Notice that at 100% rated
speed and a fully loaded pump the horsepower consumption is maximum.
If a 20% reduction in speed is allowable by the system process
requirements then the pump may be driven at 80% of the maximum motor
RPM with a nominal power consumption of 51.2% of the full load, full
speed consumption.

A typical pump is rated for 863 GPM at a head of 154 feet when
operating at its maximum efficiency point of 81% with a motor of 1750
RPM. Brake Horsepower is 41.9. (Crane-Deming pump, 4160 Series,
6 x 4 x 12).

Using the pump affinity curve, at 80% of rated speed the above pump
will be operating at 1400 RPM, 690 GPM, and a Brake Horsepower of 21.5
at a head of 99 feet, yielding a savings in power consumption of:

41.9 - 21.5 = 20.4 HP, or, 746 watts per HP x 20.4 HP = 15.22 KW

The static head will now be introduced into the graph. Figure 2. shows
System Head vs. GPM for three conditions: Curve A for a very low or no
system head which closely follows the affinity curve, Curve B for a
medium system head of 50 feet, and Curve C for a high system head of
99 feet. Figure 3 shows Brake Horsepower vs. GPM. The data for Figures 2
and 3 were taken from the pump manufacturer’s curves.

Curve A represents a static head of zero feet. When the pump is driven
at 1400 RPM, the 99 feet of head on the affinity curve represents the
frictional head, which is proportional to the square of the flow rate, plus
the energy required to move the fluid. If the system discharge diameter is
larger than the pump inlet diameter then the velocity head will be zero.

Curve B shows a system static head of 50 feet which, on the y-axis and
at zero GPM, represents a fixed static head for all flow rates. Curve B has
an upward parabolic curvature as it extends toward the maximum speed
point. The curvature represents an increase in head due to the frictional
head and the energy required to move the fluid.

As the pump operates along Curve B of Figure 2, increasing the pump
speed to 1750 RPM will place the pump at its normal full load, full speed
rating of 154 feet of head. Figure 3 indicates that a system flow of 690
GPM with an initial static head of 50 feet will be, from the pump
manufacturer’s curve, achieved at approximately 1518 RPM, 124 feet of
head, and a brake horsepower of 28.2. The operating points of Curves A, B
and C are shown on Figure 3.

We may now calculate the system energy savings at the desired flow
rate of 690 GPM with a 50 foot static head:

41.9 - 28.2 = 13.7 HP, or, 746 watts per HP x 13.7 HP = 10.22 KW

Curve C indicates that a system flow of 690 GPM with an initial static
head of 99 feet will be achieved at approximately 1607 RPM, 134 feet of
head, and a brake horsepower of 30.5.

We may now calculate the system energy savings at the desired flow
rate of 690 GPM with a 99 foot static head:

41.9 - 30.5 = 11.4 HP, or, 746 watts per HP x 11.4 HP = 8.50 KW

The affinity curve, when taken alone without the system static head,
showed an energy savings of 15.22 KW. With static heads taken into
account of 50 and 99 feet we have seen that the energy savings will be
10.22 KW and 8.50 KW respectively, and savings of 67.1% and 55.8%
respectively of what had been determined with the use of the affinity curve
and without consideration of static head.

The above systems are based on a constant flow requirement of 690
GPM at three static heads of 0, 50 and 99 feet. The systems below use the
same manufacturer’s pump curves and are based on a constant pressure of
80 feet above three initial static heads of 0, 30 and 50 feet.
For constant pressure systems, Figure 4 shows Feet of Head vs. GPM,
and Figure 5 shows Brake Horsepower vs. GPM.

Curve D of Figure 4 has a static head of 0 feet and represents points on
the affinity curve. Adding a variable speed drive to operate the system at
80 feet rather than at the pump’s maximum capability at 154 feet, the
pump will be operating at 1242 RPM, 618 GPM, and a Brake Horsepower of
17.69. Using the affinity curve, the savings in power consumption will be:

41.9 - 17.69 = 24.21 HP, or, 746 watts per HP x 24.21 HP = 18.06 KW

Curve E begins at 30 feet of head and extends upward to 154 feet.
Maintaining the system pressure at 80 feet above the 30 feet static
pressure gives a system pressure of 110 feet. The pump will operate at
110 feet, 1467 RPM, 680 GPM and 24.55 Brake Horsepower.

The savings in power consumption will be:

41.9 - 24.55 = 17.35 HP, or, 746 watts per HP x 17.35 HP = 12.94 KW

Curve F begins at 50 feet of head. Maintaining the system pressure at
80 feet above the 50 feet static pressure gives a system pressure of 130
feet. The pump will operate at 130 feet, 1590 RPM, 750 GPM and 31.60
Brake Horsepower.

The savings in power consumption will be:

41.9 - 31.60 = 10.30 HP, or, 746 watts per HP x 10.30 HP = 7.68 KW

The affinity curve, when taken alone without the system static head,
showed an energy savings of 18.06 KW. With static heads taken into
account of 30 and 50 feet we have seen that the energy savings will be
12.94 KW and 7.68 KW respectively, and savings of 71.7% and 42.5%
respectively of what had been determined with the use of the affinity curve
and without consideration of static head.

To determine monetary savings, the hours of use at maximum speed
must be determined and compared to the estimated hours of use at the
required reduced speed for either a constant flow or a constant pressure
system. The total reduction in kilowatt hours will allow the savings to
be calculated.

It will be found that variable speed drives will indeed reduce power
consumption. However, an accurate estimate to determine actual savings
will allow the user of the equipment to determine if the user’s available
financial resources are well invested in a speed control or if the available
resources are better invested in some other capacity within the facility.


Other Notes:

A variable frequency drive will introduce harmonic currents into the
motor windings which will cause a nominal increase of five per cent in
motor heating and therefore five percent higher energy losses. "Inverter
Duty" motors are manufactured with constant speed blowers in order to
dissipate the generated heat. As a rule, motor manufacturers do not
publish efficiency ratings of inverter duty motors that give motor
efficiencies at reduced speeds.

"Premium Efficiency" motors have a rated efficiency for operation at
sixty cycles of pure sinusoidal waveform. The waveform supplied from
variable frequency drives, however, is rich in harmonic content and is far
from being pure sinusoidal. Thus the nominal five percent additional
losses due to harmonic currents are not addressed in the nameplate
efficiency rating.

Fan and blower applications, which also utilize the affinity curve, must
be reviewed with the inclusion of static pressures. An example of a
changing system static pressure is the positive pressure air supply to a
hospital operating room. The pressure is subject to change with the
opening and closing of doors as people enter and exit the room.

With an oil filtration unit, the system operating head will slowly
increase as the filter collects material. It should be noted that brake
horsepower is directly proportional to the specific gravity of the fluid
being pumped.

In calculating payback on investment, some utilities use a life
expectancy for variable frequency drives of seventeen years. However,
within the first ten years of use, variable frequency drives will often
need replacement or will receive repairs that will cost over forty per
cent of the cost of a replacement drive.


References:

Joseph R. Pottebaum, “Optimal Characteristics of a Variable-Frequency
Centrifugal Pump Motor Drive,” IEEE Transactions on Industry
Applications, Vol. 1A-20, No. 1 January/February 1984, pp 23-31.

Ron Carlson, “The Correct Method of Calculating Energy Savings to Justify
Adjustable-Frequency Drives on Pumps,” IEEE Transactions on Industry
Applications, Vol. 36, No. 6 November/December 2000, pp 1725-1733.

Return to Payback Analysis for Variable Frequency Drives.

Return to Energy Savings: Solid-State Reduced Voltage Starters vs. VFDs.

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