Regardless of the blower application for a gas-fired burner system, industrial combustion engineers typically must strike a balance between performance requirements and operating constraints. In striving to optimize blower control, increase efficiencies and reduce emissions, consideration has flowed naturally to premix blowers as practical solutions. Within the premix category, much interest has been generated in variable speed brushless DC (BLDC) gas blowers.
The technology behind BLDC premix blowers can deliver full-speed modulation and monitoring, effective turndown ratios, reduced power consumption due to higher motor efficiencies, and lower burner emissions with less excess air and optimized air-fuel controlled mixing.
Today’s increased energy and environmental demands make an especially strong case for considering a premix combustion system. Such a system serves to deliver a measured air-gas mixture to the burner along with a variety of accompanying benefits.
The standard combustion air requirement for an air and natural gas (methane) mixture is a volumetric ratio of 10 parts air to 1 part gas. This is traditionally referenced as stoichiometric air, or the exact proportion of air needed to provide the amount of oxygen necessary for complete combustion. During combustion, flue products include carbon dioxide, water vapor and nitrogen. Should incomplete combustion occur, flue products can expand to include carbon monoxide, hydrogen and aldehydes.
Much attention lately has focused on reducing emissions of nitrous oxide (NOX), which originates from two basic sources: nitrogen bound to the fuel, or “fuel NOX,” and nitrogen from high-temperature combustion, or “thermal NOX.” In general, for natural gas burner systems, fuel NOXgeneration will be far less significant than thermal NOX.
Targeting the reduction of thermal NOXemissions, engineers have been presented with several options: steam or water injection (into the flame); flue-gas recirculation; modification of the burner design (lowering the flame temperature); or actively controlling the air-fuel mixture to limit excess air through a premix combustion system.
Although all these avenues can help to reduce NOXemissions, they may not be suitable for all applications or settings. For example, incorporating injection and recirculation systems usually will require on-site engineering. Likewise, modifying the burner assembly may not be practical within an existing design envelope. Also, integrating modulating gas valves to control the air-fuel mixture and limit excess air may solve only part of the equation. Premix blowers can solve the other part.
Until recently, users were limited to a few blower alternatives, most notably fixed- or two-speed blowers; intake damper systems on fixed-speed blowers; or inverter-driven variable-speed blowers. Each style has benefits and drawbacks. The variable-speed brushless DC (BLDC) premix gas blowers mitigate these drawbacks and meet industry demands and expectations for performance, efficiency and service life.
Heart of the Blower AssemblyVarious factors will influence blower specification such as the appropriate pressure and flow rate; the available design envelope (governing blower size); the desired service life; and the input voltages and control scheme. Choices abound and will broaden the user’s ability to customize; at the same time, the requirements for a particular heat processing application will help narrow the field. Most importantly, though, the motor at the heart of a premix gas blower assembly is highly influential in how the blower operates and whether expectations will be fulfilled. Shaded-pole motors, brush-commutated DC motors, and BLDC motors represent the most popular alternatives.
While shaded-pole motors benefit from a relatively simple design and construction, disadvantages exist. The motors operate at a low efficiency (20 to 40 percent), lack variable-speed capability (built with only one or two speeds) and require complex shutter systems to control airflow. Their life expectancy is limited to approximately 25,000 hours.
Brush-commutated DC motors operate at significantly higher speeds and often can be speed-controlled with a separate voltage controller. However, their internal brushes limit the motor life to less than 10,000 hours due to the natural wear of the motor’s brushes (usually graphite with metal content) handling the commutation process. Performance limitations also can exist in terms of flexibility in size and speed.
Brushless DC motors achieve commutation electronically via a permanent-magnet rotor, wound stator and rotor-position sensing scheme. As a result, they can operate with high efficiency (up to 85 percent) and up to 40,000 hours, delivering a much longer life expectancy. Additionally, a BLDC motor’s electronic commutation technology promotes accurate control and rapid transient response time for faster power availability. The inherent ability to infinitely adjust a BLDC blower’s speed allows users to change the temperature quickly, which maximizes the efficiency of combustion, minimizes the amount of gas used, and can result in substantial energy savings.
Numerous improvements in technology are always on the horizon. For instance, a blower design has been introduced that uses a digital signal processing (DSP) motor chip to enable advanced programmable control of the blower. The blower performance, inputs and outputs can be tailored to meet specific system needs. In addition, option cards broaden the potential for blower customization.
In conclusion, by consulting with an experienced manufacturer early in the specification stage or selection process, users can gain valuable expertise and insights to arrive at the best-equipped blower solution for an application.
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