How Variable Speed Pumping Systems Save Energy
As discussed above, there are three options to control the water flow provided by a pump: flow restriction, flow recirculation and variable speed. However, speed control is the most efficient option because the other two waste pumping power.
- When a pumping system is controlled with flow restriction, the pump consumes power to compensate the pressure loss across the control valve.
- A similar principle applies when the flow is controlled with recirculation since the unused water flow represents wasted power.
Speed control can be achieved by adding a variable frequency drive (VFD) to the pump motor. A VFD modulates the voltage and frequency of the power supply, and the rotational speed of the pump changes accordingly. If the demand for water is below the rated flow of the pump, the VFD slows it down until the flow provided matches demand.
Before adding speed control to a pumping system, a very important step is verifying that the pumps and the proposed VFDs are compatible. There are various pump designs and VFD models, and not all of them are compatible. Some VFDs are exclusive for centrifugal pumps, others are exclusive for positive displacement pumps, and certain models offer compatibility with pumps of both types.
Variable Speed Pumping Systems with High-Efficiency Motors
Variable speed pumping systems offer the most efficient configuration, but optimal performance cannot be achieved if the pumps still use inefficient motors. NEMA Premium Efficiency motors have the highest efficiency rating in the industry, and you can achieve the lowest possible pumping costs by combining these motors with speed controls.
NEMA Premium Efficiency motors are available for a wide range of applications, and their rated power goes from 1 hp to 500 hp. These motors can save thousands of dollars each year when they replace older and less efficient units. They are very useful in pumping systems since their continuous operation provides an excellent chance to save electricity.
When deploying speed controls and efficient motors for pumping systems, it is also important to have reliable electrical protections. Variable frequency drives and NEMA Premium Efficiency motors can have a high upfront cost, and protecting that investment is very important. A reliable electrical installation also provides safer working conditions for maintenance personnel.
Key Design Considerations for Variable Speed Pumping Systems
Like many other mechanical installations, pumping systems can experience noise and vibration issues due to poor design decisions and inadequate maintenance. Periodic inspections are recommended to make sure all system components are working correctly. Some common causes of vibration are inadequate equipment supports, unbalance and misalignment. Controlling vibration is very important; it can speed up the mechanical wear of system components, increasing maintenance expenses.
When variable speed pumping systems are deployed as part of an HVAC installation, the design process must consider the entire installation and not only the hydronic piping. To take advantage of a variable speed pumping system, HVAC installations must also be capable of adjusting the air supplied by ventilation systems, and the cooling output of chillers and boilers.
Variable speed pumping systems provide an excellent opportunity to save energy in applications where large amounts of water are used. However, the benefits can only be achieved if the system is designed correctly and compliant with local codes. Professional design services from MEP engineers are strongly recommended.
