The following is an excerpt from my recent article Blown Away for Water & Wastes Digest. The full version can be read online or in the October issue of the magazine.
Modern variable-speed drives (VSD) have been used with positive displacement equipment for more than 20 years. VSD technology originated in the 1960s for small DC motors and evolved for use on large induction motors. Pulse width modulation (PWM) technology using insulated-gate bipolar transistors led to smaller, more economical options and wider use. Inverter duty (VSD duty) motor designs soon followed, along with widespread use of VSD technology for variable load applications. Positive displacement (PD) blowers often are used in these variable load applications, and VSD technology can provide additional energy savings.
PD lobe blowers have been used since their invention in 1856 by the Roots brothers. Like any PD machine, the delivered volume flow is roughly proportional to the blower’s operating speed. A majority of rotary lobe blowers are belt-driven, and the blower’s operating speed is set by the size ratio between the motor pulley and the blower element pulley. As a result, blowers were sized to the application needs, with the correct pulley ratios in belt-driven units, for many decades.
The only way to adjust the output flow of an operating machine was through the use of a relief valve to blow off some of the discharge air flow or start/stop machines on the same header pipe. These methods supply a variable flow output for applications that required it, including wastewater treatment aeration, aerobic fermentation, and cooling and combustion air. However, this provided limited savings in energy efficiency.
VSD Efficiency
In the 1990s, new VSD technology and inverter duty designs for three-phase induction motors made it economical to provide variable speed packages. Instead of using a blow-off valve or starting/stopping, the VSD varied the output frequency of the blower directly. Varying operating speed adjusts the pressure vs. flow curve, with lower operating speeds displacing less volume flow.
This is significant in that slowing down the blower can be more efficient than blowing off air. A blower running at nominal speed on a VSD compared to a fixed speed will have the same flow output and require a higher amount of power input due to electrical losses. With modern VSD equipment, this loss is typically less than 3%. However, with more flow reduction, more energy can be saved. A lobe blower operating with VSD has a turndown of 60 to 70%, meaning that the minimum flow is 60 to 70% lower than the maximum flow.
For a blower operating with a blow-off valve, it still produces the same amount of flow in the blower, and a potential slight decrease in the system pressure is the only energy benefit. Therefore, a VSD can offer almost 60 to 70% in energy savings in a process requiring 60 to 70% less flow than the blower’s nominal output. For any application with a large, variable flow demand, these savings can easily pay for the investment cost of the VSD.
The ability to increase the operating speed beyond the base frequency—60 Hz in the U.S.—is an additional VSD benefit. Older equipment can be retrofitted with VSD and a larger motor to increase the performance output. The investment costs for this retrofit are considerably lower than purchasing new equipment to get a slightly higher flow output.
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You can read the full article on online or in the October of Water & Wastes Digest. For more articles like this, make sure you subscribe to our blog.
