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.

Current Technology

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.

Burner-Configuration Options

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.