Increased energy and environmental demands support the case for premix combustion systems. These systems deliver a measured air/gas fuel mixture in a gas-fired burner system that optimizes combustion and reduces nitrous-oxide (NOx) and carbon-monoxide (CO) emissions. Coupling a variable-speed brushless blower with a venturi manifold mixing unit and a regulation gas valve produces a simple burner system that meets new government emission regulations and can reduce gas consumption. This technology is growing in popularity with commercial-boiler manufacturers because of its design simplicity, compact envelope, and expanded turndown capabilities.
An estimated half of all of the commercial buildings in North America are heated with hot water (usually assisted by air handlers, humidifiers, and air-conditioning systems to move heating and cooling around large spaces). However, the boilers in such systems — especially in mature buildings — often are older and relatively inefficient, unless upgraded; additionally, they typically generate undesirable levels of NOx and CO emissions.
As a result, commercial-property owners, with support in the form of financial incentives available from the government, have been prompted to consider how newer technology involving forced-draft, higher-efficiency hydronic heating can be applied in retrofits or new construction. In turn, commercial-boiler manufacturers have focused on how blowers can help fulfill a “wish list” of improved operational requirements.
First, manufacturers seek blowers that “plug and play” and integrate easily with European-style gas venturi, burner, and boiler controllers. The blowers must be able to exceed British-thermal-unit performance requirements to handle varying flue lengths in commercial buildings. High pressures and flows are essential, and blower size truly matters, as design envelopes for commercial boilers continue to shrink, and output requirements continue to grow.
Combustion engineers often must strike a balance between performance requirements and operating constraints. Until recently, they were limited to only a few blower alternatives, most notably fixed-speed or two-speed blowers, intake damper systems on fixed-speed blowers, and inverter-driven variable-speed blowers. All of these alternatives have tradeoffs that can be mitigated substantially by variable-speed brushless direct-current (BLDC) premix gas blowers.
Historically, most higher-efficiency boilers have provided a 5-1 turndown ratio of heating range. However, manufacturers have determined that a 10-1 turndown ratio can be more advantageous by further reducing the amount of required combustion air and gas during low heat-demand days and allowing more flexibility in packaging boiler sets.
Motors Hold the Key
At the heart of a premix-gas-blower assembly is the motor. The motor is critical to whether performance and service expectations will be fulfilled. Not all motor types are created equal. Historical alternatives have included shaded-pole motors, brush-commutated DC motors, alternating-current (AC) motors, and BLDC motors.
Shaded-pole motors benefit from a relatively simple design and construction. However, they operate at a low efficiency (20 to 40 percent), lack variable-speed capability (only one or two speeds are available), and require complex shutter systems to control airflow. Their life expectancy is limited to approximately 25,000 hr.
Brush-commutated DC motors can operate at significantly higher speeds and often can be speed-controlled with a separate voltage controller. However, the internal brushes (usually copper graphite) handling the commutation process limit motor service life to less than 10,000 hr because of the brushes' natural wear. These types of motors also can exhibit limitations in terms of performance and flexibility relative to size and speed.
AC motors provide a low-cost, longer-life solution for blower manufacturers, but are much larger and heavier than a comparative brushless DC motor. Also, they require a variable-frequency drive (VFD) to allow blowers to modulate in high-efficiency hydronic boilers. An AC blower, VFD, and interim cabling add size, weight, and complexity.
BLDC motors achieve commutation electronically via a permanent-magnet rotor, wound stator, and rotor-position sensing scheme. As a result, they can operate with a high efficiency (up to 85 percent) and longer life expectancy (up to 40,000 hr). A BLDC motor's electronic commutation technology also promotes accurate control and rapid transient response time for faster power availability.
The ability to adjust a BLDC blower's speed allows users to change temperature quickly, maximizing combustion efficiency, minimizing the amount of gas used, and potentially saving a substantial amount of energy. Available control schemes assist by contributing to the industry-preferred 10-1 turndown ratio.
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The simplest type of speed control relies on a potentiometer located on the blower. Speed can be adjusted without the need for an external controller. This control scheme is common for fixed-speed applications.
One alternative is a two-wire control scheme that includes a simple controller “on board” the motor and wires that serve as DC power-supply connections. The blower's speed is directly proportional to the supply voltage, and a separate speed command signal becomes unnecessary.
More advanced control schemes incorporate a more sophisticated “on-board controller.” The controller provides a variety of speed commands for a blower, whether via a DC signal, a pressure sensor in the air circuit (to adjust the blower-speed command according to the pressure), pulse-width modulation with tachometer feedback (to confirm blower speed), or mechanically with a potentiometer.
When Size Matters
Commercial boiler systems have been trending toward smaller design envelopes; BLDC premix blowers have been traveling along the same path. (Compact combustion systems especially make the grade when retrofitting older buildings that may provide access only through typically sized door openings.)
A blower's size generally will be specified relative to the size of the combustion system. The limiting factor usually is not the size of the blower, but the size of the venturi/gas valve required to keep the gas mixture on target. Advanced BLDC blowers utilize a venturi/gas valve to mix the air and gas to the desired 10-1 turndown ratio and mate the outlet to a cylindrical burner used to heat the hot water.
BLDC premix blowers ultimately offer practical airflow delivery in a compact package. For burner applications, 500,000 Btu requires 100 cfm of airflow. By using a BLDC premix blower, the airflow needs for a 400,000-Btu system can be satisfied with a 6-lb assembly, just a quarter of the typical weight of traditional blowers. For example, one company has been able to reduce the width of its 2-million-Btu boiler by switching to a blower half the size. Additionally, a manufacturer has been able to integrate the same size blower into its mid-size boiler line, simplifying production and service and reducing the parts count, which also reduces costs.
Some advanced blower types benefit from an outside rotor design that allows for higher speeds than conventional inside rotor designs. Faster rotational speeds, along with fan-design optimization, can deliver higher performance (pressure and flow) while taking up less real estate. This is a plus in combustion applications in which high system impedance occurs and more performance is needed to make full use of the heat exchanger. Additionally, higher pressure overcomes the challenges presented by longer flue lengths often found in larger commercial buildings.
Looking ahead, numerous technological improvements are on the horizon. For example, one advanced blower design uses a digital-signal-processing motor chip that can enable advanced programmable control of a blower. The blower's performance, inputs, and outputs can be tailored to meet specific system needs. Option cards also broaden the potential for blower customization.
As BLDC motor technology is applied to more combustion and industrial motor applications, the overall cost of these blowers will decline. Additional volume has helped BLDC blowers stay competitive with more traditional alternatives, including AC/VFD blowers.
The push toward greener products coincides with the introduction of BLDC motors to the commercial boiler market. The quick response time of BLDC blowers when used in a premix combustion system minimizes the flue-gas-emission problem that occurs with traditional slower-reacting blowers. Using a gas valve/venturi/boiler controller with a BLDC blower provides an economical and environmentally friendly solution for current boiler and water-heater manufacturers.
Every application, of course, will carry its own set of requirements and demands. Various factors will influence blower specification, including appropriate pressure and flow rate, available design envelope (governing blower size), desired service life, input voltages, and control scheme, among others. Choices will broaden with the capability to customize.
By consulting with a manufacturer early in the specification stage or selection process, users can arrive at the best-equipped BLDC-premix-blower solution for a commercial boiler application.
Global combustion market manager for AMETEK Technical & Industrial Products, Jodie McLay has 10 years of experience selling brushless direct-current motors and blowers and 24 years of experience in technical sales. He earned his bachelor's degree in commerce and engineering from Drexel University in Philadelphia. He can be contacted at email@example.com.