How VSD screw blowers can save energy & increase reliability
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. Operating pressure varies the flow output of lobe blowers as higher discharge pressures increase the leakage within the blower. 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 or 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.
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 pay for the investment cost of the VSD. A quick example of these savings can be seen in Table 1 below.
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.
In addition to the energy savings offered by VSD driven blowers, their use improves the equipment reliability. They reduce starting torque, lower bearing loads and tighten flow control. Without VSD, the motor is started using either a direct-on-line (DOL) starter, wye-delta starter or soft starter. The DOL starter has higher torque than the blower’s required load torque. The wye-delta starters reduce this starting torque, and soft starters provide smoother starting. However, a VSD can be thought of as the “ultimate” soft start. A VSD can directly vary the voltage and current required during motor startup to match the required torque. This reduces loads on the motor and the drive system between the motor and blower to increase lifetime use.
Reducing blower speed instead of using a blow-off valve also means that the bearing loads are reduced. A bearing’s expected lifetime is a function of these loads and the operating speed, allowing the bearings to last longer using speed variation. Finally, a VSD can be controlled more finely than a blow-off valve. This does not have a direct effect on machine reliability except in one area: reducing the number of stops and starts in certain cases. This is true in comparison to blowers that only operate with on/off functions, and it is true compared to blow-off control. The VSD offers wider operating range, leading to tighter process control and fewer requirements for starting or stopping multiple pieces of equipment.
Rotary Screw Blowers
As discussed, VSD use with traditional positive displacement blowers such as rotary lobe machines can show significant benefits. Combining VSDs
with PD technology takes efficiency and reliability to the next level. Rotary screw blowers are improvements in PD technology that use internal compression within the blower element to increase energy efficiency. For a detailed look at this difference, see the diagram below:
The lobe blower moves air from the blower inlet to the outlet along its casing walls. It is trapped at the outlet and therefore forced into the discharge pipe, which means all the developed pressure is created in the system piping. This principle works but is inefficient. In the screw blower design, an uneven number of lobes and flutes reduce in clearance volume from the inlet to the outlet. This internal compression is more efficient than the lobe principle. On average, the screw blower is 30% more efficient than a lobe blower of the same flow, with higher improvements at higher pressures. The screw blower can turn down even further (80 to 85%) on VSD operation because the required torque is lower at these flows.
Additionally, gear drive screws gain efficiency over belt drive options as the gear losses of high-precision helical gears are in the range of 2%. Mechanical losses from a belt drive initially are 3 to 5% and approach 10% as the belts wear. These savings will not normally be seen on project evaluations, but will be present in real-world operating conditions.
A 4-million-gal-per-day wastewater plant in northern Kentucky purchased three 100-hp high-speed turbo blowers in 2010 to improve the plant’s energy efficiency and prepare for potential future expansion. Steady growth in local industry was expected to increase demand. The turbo blowers aimed to offer energy savings. After a few years, however, the local industry growth did not occur and a large industrial manufacturer moved out of the region. As such, the plant’s average demand fell well below projections and below the operating range of the purchased blowers. The blowers have a turndown of 50 to 60%, and the plant was forced to blow off the excess air into an unused basin, wasting energy. The operating budget relied on lower energy costs, but due to the wasted energy from blowers that were too large and could not be turned down, the plant could not cover its costs.
In 2015, a rotary screw blower with gearbox drive was installed to combat this situation. High-speed blowers typically are 10 to 15% more efficient than rotary screw blowers over the turbo blower’s operating range. However, because of the high turndown of over 80% for the screw blower, the plant could meet its permit and save approximately 20% of energy from producing lower flows. With these savings, the plant could achieve the originally projected energy savings expected from the turbo blowers.
Permanent Magnet Motors
Permanent magnet (PM) motors are coming to rotary screw blowers and compressors. This technology already is found in high-speed turbo blowers and other rotating equipment, and the implementation on rotary screw blowers with VSD drives pushes the efficiency and reliability further. Permanent magnet motors are more efficient than induction motors because there is no extra electrical energy required to induce a magnetic field in the rotor because it is created with magnets fixed to the rotor shaft.
PM motors also have a near-constant power factor at partial load, so the efficiency gains versus induction motors are higher at lower loads. Finally, PM motors are more compact and often are cooled with water or oil. This means they can
carry an IP66 rating, making them suitable for extreme environments.
VSD technology provides improvements in energy efficiency for traditional PD blowers used in variable-load applications. The advancement of PD technology with the introduction of screw blowers using internal compression also has reduced the power consumption in these products. The turndown of these units allows flexibility in processes requiring a wide flow range to maximize realized energy savings. Additional features such as new motor designs and drive systems will offer a path to more efficient and reliable equipment in the future.