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VARIABLE SPEED DRIVE CENTRIFUGAL BLOWERS
CAN SAVE ENERGY AND INCREASE RELIABILITY
By Deepak Vetal
There are various blower technologies available for low-pressure applications, including lobe, screw, multistage, integrally geared, direct drive turbo and claw. With each technology comes benefits depending upon the application and process where these blowers are used.
To choose the right technology, users should consider energy consumption, performance, process requirements, and life cycle cost. Variable speed drive (VSD) centrifugal blowers, for example, are relevant in applications, such as wastewater treatment, flue gas desulfurization and lead recycling.
The VSD centrifugal air blower is an approach to low-pressure compressed air applications. These blowers require non-contact bearings, either air bearings or magnetic bearings. The bearings are driven by a permanent magnet motor and raise efficiency by eliminating bearing losses in motors and blowers.
Air bearings have limited starts and stops due to bearing contact upon startup, shut down and under surge conditions. They are not protected against surge conditions. Magnetic bearings, on the other hand, are protected by their ability to have unlimited stops by way of electromagnets. Therefore, magnetic bearings are preferred in VSD centrifugal blowers due to reliability.
VSD centrifugal blowers use an internal closed loop air circuit to cool the motor housing and the integrated VSD converter. By using a direct drive system, there is no need for speed-increaser gears and the accompanying oiling system. Vibration, noise, required floor space and weight are reduced. A high-speed permanent magnet synchronous motor (PMSM) with VSD offers greater energy efficiency at full and part load compared to conventional motors.
Water treatment is one of the most important industrial segments for blowers and low-pressure compressors. Industrial wastewater generally contains higher concentrations of contaminants, such as biochemical oxygen demand (BOD), suspended solids, as well as fats, oil and grease, and in some cases, contains heavy metals and toxic materials that make disposal difficult.
Selecting an adequate treatment process is a function of load, speed, contaminant type and discharge standards. Treatment can vary from a simple single stage to a complex multistage with many steps and processes in which the optimal objective is to meet government and environmental regulatory compliance.
To gain efficiency and reliability in the process, a wastewater treatment plant installed low-pressure VSD centrifugal compressors. Due to oxygen requirements, four machines were installed, of which two or three operate around the clock depending on contamination levels. The fourth compressor serves as a contingency reserve.
The facility runs 365 days a year in 24-hour operation. The water depth and the membrane pipe system at the bottom of the basin required high pressure. Normal rotary lobe blowers could only provide 14.5 psi. Therefore, centrifugal technology was required.
The turbo compressors are single-stage, speed-controlled VSD machines. The motors have a contactless electromagnetic bearing system. The impeller is mounted on the motor shaft itself. This drive design does not require any extra lubricant so there is no risk of oil getting into the blower. Compared with rotary lobe blowers, this type of drive achieved energy savings of 30% to 40% and saved on space.
Flue gas desulfurization
Flue gas desulfurization (FGD) involves the removal of sulfur dioxide contained in gases produced by the combustion of fossil fuels, such as coal, oil, bitumen-based fuels, municipal solid waste, automobile tires, and for the processing of petroleum, cement, paper, glass, steel, iron and copper.
A power plant burning anthracite, for example, had to add a precisely defined amount of oxidation air to desulfurize the flue gas in two-circuit absorbers. Otherwise, the sulfur would stick together and could cause the reaction to take place either too early or start at the wrong location in the installation.
The process is currently under proper control using three VSD centrifugal blowers, which allow them to produce the exact amount of oxidation at any given moment. The blowers are speed controlled to allow volume flow to be adjusted to meet demand. This has saved 18.1% on energy costs compared to earlier installed blowers. The power plant also uses less coal per kilowatt hour produced.
Another user recycles lead from battery scrap and battery paste using VSD centrifugal blowers in a compressed air station. With a total of six blowers in operation, the facility uses flow-controlled blowers instead of pressure-controlled blowers.
Air from the blowers is needed for three shifts, around the clock, for process and after-burning air. The process air is blown together with natural gas and oxygen through a lance immersed in the molten slag, which has a variable consistency. The colder it is, the stiffer it is. The lance will also deteriorate, resulting in immersion depth variation. This and other effects cause the counter pressure to fluctuate at the tip of the lance.
To keep the metallurgic process securely under control, it was necessary to provide a constant flow volume at any time despite rapidly changing operating conditions. VSD centrifugal blowers with flow control were able to maintain the metallurgic process in the immersion melting furnace. In addition, after-burning air was fed into the furnace through the molten bath to ensure no reactive components, such as non-combusted natural gas ended up in the exhaust.
Implementing VSD centrifugal blowers with proper controls can save energy and make processes work more reliably in a variety of industries.
Author: Deepak Vetal is National Sales Manager for industrial blowers and low pressure compressors at Atlas Copco. He can be reached at firstname.lastname@example.org.