Premix gas blowers on modern furnaces are just one component of a collection of interconnected technologies that drive combustion in an efficient, safe and environmentally friendly manner.

Combining blowers with specialty gas valves, venturi, burner technologies and evolving electronic controllers, allow for a more accurate and optimal air-to-fuel mixture than ever before. This means that a burner’s output is determined by increasing and decreasing blower speed, while the air-to-fuel ratio remains constant—and not by varying or turning off the burner. Blowers are designed to consistently deliver the 10:1 air-to-fuel ratio that is considered the magic number for optimum fuel combustion.

Today’s gas premix blowers use improved electronics, mechanics and other features to reduce emissions of regulated chemicals and increase output capability, which allows smaller blowers to produce the same output requirements as their larger forebears.

Long gone are the days when a burner system would use a single-firing burner with an on/off method to heat output. This is a method in which the burner fires until it reaches the desired temperature, then shuts off until the temperature falls back below that level and it restarts—essentially, a temperature yo-yo. The on/off system made it simple to deliver the proper fuel mixture, but it also meant that the furnace would cycle on and off, wasting energy and fuel, and generating emissions including CO2, which can occur with each cycle start and stop.

Eventually, two-speed blowers became available, but that was about the extent of a designer’s choice. The two-speed blowers were a step forward that allowed burners to kick on high or slow as it approached the temperature set point. However, even with the increased modulation, additional cycling would occur, and, with each cycle, pre- and post-purge airflow removed heat from the boiler, up and out the stack—resulting in energy losses, slow response times to load changes, and additional stress on the boiler components.

In the recent past, turndown rates were around 4:1 and 5:1. A typical boiler utilizing a standard burner with these turndown ratios could cycle as frequently as 288 times a day, according to Boilers and Heaters: Improving Energy Efficiency from the CANMET Energy Technology Centre, Ontario.

Today, 10:1 ratios are readily available with the gas premix blowers. Brushless direct current premix gas blowers such as the Nautilair from Ametek Technical and Industrial Products, Kent, OH, can achieve that level. The Nautilair line is available in a range of sizes and capacities, including a new 12.3-in. model. The line can deliver full-speed modulation and airflow monitoring, according to Jodie McLay, east coast sales manager for Ametek.

The 12.3-in. blowers also can achieve a maximum sealed pressure of up to 25 in. of H2O, and maximum open flows to 800 cubic feet per minute, depending on the model. Versions can accept line voltage input of 120 Volts alternating current (VAC) and 240 VAC. Internal electronics convert the AC to DC. Brushless DC motors help achieve commutation electronically via a permanent magnet, wound stator and rotor-position sensing. These new generation blowers can operate with high efficiency of up to 50 percent, and operate for up 40,000 hours.

Improved Controllers

Blower control has improved dramatically in the last few years in part because of new controller technology. Previously, a common control method was a pneumatic, negative regulation system called “neg-reg.” With this method, as the blower speeds up and slows down, the intake pressure rises and falls. As it falls, negative pressure is created in the blower’s intake that pulls fuel through a valve. As pressure fluctuates, more or less fuel is drawn. With this configuration, the air-to-fuel ratio remains somewhat constant.

Some BLDC blower types were then engineered with a two-wire control scheme mounted on the motor. The wires serve as a DC power supply connection and the blower’s speed is directly proportional to the supply voltage, meaning that a separate speed command signal was not necessary.

More advanced BLDC blower types use more sophisticated controllers to control blower speed. This includes a DC-high and low voltage, pulse width modulation, mechanically with a potentiometer, and, most recently, blowers digital signal processing capabilities to enable more programmable control.

Some models such as Nautilair, adjust blower speed in multiple ways including the onboard potentiometer, an external PWM or the traditional DC control system. The controller actively varies blower performance and eliminates the need for complex air intake damper systems, says McLay. Available add-on expansion cards and other options can be incorporated to customize blower performance for specific application requirements.

One of the most sophisticated controllers to hit the market is the LambdaConstant from ebm-papst, Farmington, CT. The LambdaConstant is available on certain ebm-papst gas premix blowers, and independently controls the air and fuel flow, says Tom Costello, sales manager for blowers.

Each blower features a restrictor on the air intake side that diverts air to a digital mass flow sensor for measurement. A laminar flow element with an extremely low-pressure drop ensures a stable signal sensor over the entire modulation range. Fuel is also measured to determine its levels. Additionally, sensors on the burner measure flame temperature. The measurements are transmitted via a digital bus system from the blower to the burner controller. The measured values are used to regulate the fan exactly to the specified air volume. This is done in near- to real-time.

The low pressure drop in the laminar flow elements allows the same gas blowers to be used with higher air supply and exhaust pipe lengths, with the same pipe lengths, and the gas blower power consumption. Because of this, the air-to-fuel ratio can be held constant no matter the type of gas in use, and no matter the conditions in which the furnace is located.

For instance, a furnace at sea level would have different air and fuel values than at a 5,000-foot elevation. At the higher elevation, there is less oxygen per cubic foot so the blower must run faster to provide equivalent oxygen, says Costello. Likewise, the fuel’s heating value will drop and so the gas valve would need to be opened wider to allow for additional cubic feet per minute of gas flow.

Wider BTU Capabilities

One new trend in gas premix blowers is increasing voltage capabilities. The gas premix blower market started in small BTU sizes and has consistently gotten more powerful—rising from a top of 2.5 million BTUs, to 3 million BTUs. Now, soon-to-be introduced models will reach 4 million BTUs.

Bigger and more efficient blowers mean that smaller units can be used while still achieving the same heat outputs, according to McLay. A basic rule of thumb for burner applications is that 500,000 BTUs require 100 cubic feet per minute of airflow. While not an exact comparison, a BLDC blower used for a 400,000 BTU system could meet the airflow requirements with a 5-lb. blower—a quarter of the typical weight of previous units.

One model coming soon is ebm-papst’s MegaWatt blower, model G3G250-MW. It can provide heat outputs from 10 kW to approximately 1 MW or 4-million BTUs. The blower uses a three-phase brushless DC motor that accepts AC voltage input and then rectifies it internally to DC voltage. It has a low-voltage speed-control circuit that accepts either PWM or a 0 ~ 10 VAC signal. According to Costello, the MegaWatt is going to be available in two voltage ranges: 200 ~ 240 VAC and 380 ~ 480 VAC. Both models will feature a power factor controller so the operation of the motor will remain constant regardless of the voltage.

The Trade Off

Constancy in air-fuel mixtures is something that might help designers with an ongoing dilemma. In the past, designers were faced with a trade-off in terms of efficiency vs. emissions. Traditionally, to assure complete combustion, boilers were fired with excess air. Too much air is a main factor in the formation of nitrous oxide emissions (NOx). Excess levels can increase NOx formation because of the additional nitrogen and oxygen that is found in the air.

NOx emissions are considered culprits in the production of ozone and acid rain. They are regulated by federal and state agencies in the U.S., and governments throughout Europe. NOX is primarily formed in two ways: thermal NOx and fuel NOx. Thermal NOx is formed when nitrogen and oxygen in the combustion air combine with one another in the high temperatures of the flame. Thermal NOx makes up the majority of NOx formed during combustion. Fuel NOx is formed by the reaction of nitrogen in the fuel with oxygen in the combustion air. In fuel containing significant amounts of fuel-bound nitrogen, fuel NOx can account for up to 50% of total NOx emissions.

“It is an interesting argument,” McLay says. “Everybody was to have lower NOx levels, and they want higher operating efficiencies. But, one is the opposite of the other.”

In striving to optimize blower control, increase efficiencies, and reduce emissions, consideration has flowed naturally to premix blowers as practical solutions, he says. By working to ensure that the blowers provide the correct air-to-fuel ratio to the burner, better combustion occurs and fewer chemicals are emitted.