Energy savings of 30 percent was realized after installation of five compact pumps, control/monitoring units and motors.
by Bryan Orchard
January 28, 2019
The two pumps for hot water circulation dutiesImage 3. The two pumps for hot water circulation duties

“The attraction of the combination is that there is no requirement for control panels,” Zanus said. “This means that we can deliver the complete package to site and simply connect it to the power and water infrastructure.”

However, making use of energy-efficient motors only makes sense if all other savings potential have been exploited. No energy-efficient motor can achieve reasonable levels of energy consumption with a pump that is operating far off its best efficiency point (BEP), which is where an advanced control/monitoring unit comes in to play.

Pre-programmed to meet the end user’s specific operational requirements, one advanced control/monitoring unit continuously analyzes the pump operating data and establishes a load profile. This makes the operator aware of energy-saving potential that could be leveraged by using a variable speed system.

Employing this technology, the building services team at the Palazzo Grassi can remotely monitor the pumps in real time as each control/monitoring unit is accessed through the building services management software. The unit enables users to operate the pumps in such a way that they run at the BEP at all times, which also extends the service life. This directly translates into lower energy costs.

For the Palazzo Grassi installation, which employs variable speed pumps, the five control/monitoring units are powered by a modular, self-cooling frequency inverter that features a digital signal via Modbus RTU interface.

The variable speed system has an input/output board that is accessed through the building services management software. In the case of variable speed pumps, the control/monitoring units only measure differential pressure that is compared with the setpoint/control value.

In order to keep differential pressure constant, the modular, self-cooling frequency inverter makes the motor—together with the pump—run at variable speed (increases or reduces it) to fulfill the hydraulic variable load.

The building services management softwareImage 4. The building services management software enables operators to see how the entire system is operating at any given time.

Efficient Motors

A pump’s power consumption can become expensive over time, so efficiency is one of the many criteria used to save money in a pumping system. Less efficient pumps and motors require more power to move the same amount of water because of losses in each.

Put simply, efficiency is a measure of how much of the energy put into a pump or motor is used directly to produce work output.

A high-efficiency motor would eventually provide a return on the initial capital investment through the cost savings from lower power consumption. However, if that same high-efficiency motor is coupled with an inefficient pump, then cost savings could be wiped out because the pump would require more power to operate at the same level.

In practice, a detailed analysis of a pump’s operating behavior often reveals it is not running at its optimal operating point. If the power input is not adjusted to demand via some form of system control, valuable energy is wasted.

By changing the motor speed, and thus the flow rate, the pump input power can be reduced. In the case of closed-circuit systems, this allows energy savings of up to 60 percent depending on the load profile.

The control/monitoring unit comprises pressure sensors together with an analyzing and display unit attached to the pump. It measures suction pressure, discharge pressure and differential pressure. The difference between the two pressures is used to calculate the pump head, including the dynamic head share. This interdependence can be used to calculate the operating point.

First, the precise speed of the pump set is derived from the pressure pulsation caused by the passing impeller vanes. Then, the speed is used as a basis to compute the torque and the pump input power. This method supplies the pump input power in addition to the measured pressures and the head to determine the operating point of the centrifugal pump.

Conclusion

The energy savings resulting from the collaboration between the lead contractor Siram SPA and the supplier shows the effectiveness of the pumping installation, with a savings of 30 percent after just one year. These savings are largely attributable to the selection of the pumps, their advanced control mechanisms and energy-efficient motors.

As part of the energy-efficiency upgrade, the pumps were rewound to bring them up to the latest efficiency standards.

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