issue: April 2007 APPLIANCE Magazine
The Next Generation of Premix Gas Blowers
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by Tom Costello, product manager, ebm-papst Inc.
Advancements in gas blower technology can provide boiler manufacturers with improved performance, reliability and cost savings.
Continued increases in energy costs and consumer demand for improved thermal comfort have spurred the development of gas-fired condensing boilers. However, due to their complexity and high initial equipment costs, boiler OEMs are faced with a limited customer base unless they can meet the challenge of manufacturing a smaller, lower cost system that can be marketed to more consumers. One key component toward this goal would be a smaller, quieter, more reliable premix gas blower manufactured at a reduced cost.
This new modulating gas blower would be sized to deliver the air performance currently supplied by larger blowers installed on boilers rated up to 102,000 Btu/h (30 kW) without sacrificing efficiency. Its component parts would be designed for high automation assembly, yet offer the flexibility of custom inlet and outlet flange configurations, three electrical connector positions symmetrical about the motor axis, and supply voltages of either 24 V DC, 115 V AC or 230 V AC. It would also include a 10-percent reduction in scroll housing size, dynamic balancing in two planes for lower structure-borne noise and improved reliability with a 30-percent reduction of electronic drive components integrated into one IC module.
The next generation of premix gas blowers will provide boiler manufacturers with improved performance, reliability and cost savings that can lead to a more affordable high-efficiency heating system for all consumers.
Figure 1. Benchmark for new gas blower
Consumer demand in the residential heating market often drives new product development, and for high efficiency condensing boilers an input rate of 102 MBtuh (30 kW) is a popular size for many homes and condominiums. The premix gas blower used in these systems is based on an electrically commutated (DC) motor driving a 5.1-inch (130-mm) diameter backward-curved impeller. This radial gas blower (see Figure 1) is designed to deliver a measured air-fuel mixture at high static pressures to overcome the system impedance of an air-gas venturi, burner, heat exchanger, and venting system. However, with rising energy costs, there is increased consumer demand for a smaller, quieter, low-cost boiler that will depend, in part, on the next generation of gas blower technology.
This paper presents the results of a 2-year development program that yielded a smaller, quieter, more reliable premix gas blower manufactured at a reduced cost that still satisfied the combustion requirements of a 102-MBtuh (30-kW) boiler without sacrificing efficiency, or increasing noise or impeller speed. Reaching this level of product performance required innovative designs, improved reliability, standardization, and a smaller installation size. These targets yielded cost reductions, new impeller and scroll housing designs and automated assembly.
Combustion Air Requirements
To size a new impeller and housing, it was necessary to first identify the operating point (i.e., flow rate vs. static pressure) for a condensing boiler rated at 102 MBtuh (30 kW). The general guideline for the combustion air requirement for an air and natural gas (methane) mixture is a volumetric ratio of ~10 parts air to one part gas. This is also known as the stoichiometric or theoretical air, which is the exact quantity of air necessary to provide the required amount of oxygen for complete combustion.
In practice, complete combustion may also depend on an additional quantity of air required, called excess air. The excess air may range from 5 percent to 50 percent, depending on the fuel, burner system and method of air-fuel delivery; however, 25-percent excess air is usually optimum. The general relationship to calculate the theoretical air required for methane and propane is based on the volumetric ratio of these fuels with air: Methane requires 9.53 parts air to 1 part gas and propane requires 23.82 parts air to 1 part gas.
The equation to calculate the theoretical air required for combustion of gaseous fuels, where “i” is the percentage of each gas within the fuel and “k” is the air-fuel constant for the gas (e.g., methane is 9.53 and propane is 23.82) is:
Example: Calculate the theoretical and excess air required to size a combustion air blower to be installed on a natural gas-fired boiler, operating on 25-percent excess air, with an input rate of 102,000 Btu/h (30kW). Assume the natural gas is made up of 100-percent methane with a heating value of 1,000 Btu/cf.
Step 1: Determine the volume flow rate of gas in cubic feet per minute. This will be the 1 part gas required.
Step 2: Determine the volume flow rate of theoretical air in cubic feet per minute. This will be the 9.53 parts air required.
Step 3: Determine the volume flow rate of excess air in cubic feet per minute.
Step 4: Sum the theoretical air and the excess air. This will equal the total air required for combustion.
Step 5: If the gas will be mixed with the air before the blower inlet, then sum the total air and gas.
Identifying the static pressure required at a volume flow rate of 22 cfm (10.6 l/second) is the next step. (This can often be more challenging to calculate and is best determined empirically.) Therefore, a complete boiler system is connected to an airflow chamber with an auxiliary fan pulling air through the system at various flow rates. The corresponding pressure drops across a nozzle in the airflow chamber were measured and recorded. The data is plotted depicting an exponential system impedance curve of the boiler system. The operating point is defined as the point of intersection between the system impedance curve and an air-performance curve. Based on a representative sampling of various condensing boilers rated at 102 MBtuh (30 kW), it was determined a static pressure of 4.4-inches wc (1,100 Pa) was required to deliver an air-fuel mixture of 22 cfm (10.6 l/second).
Motor and Drive Electronics
Recent advances in motor and drive electronics provided a solid foundation for new product development. A third generation of brushless DC motors (see Figure 2) is clearly defined:
Figure 2. Exploded view of unit-bearing motor and drive electronics
• reduction in overall height
• reduction in structure-born noise through improved dynamic balancing in two planes
• reduction in component parts for improved reliability and automated assembly
To complement the unit-bearing motor design, the printed circuit board (PCB) containing the drive electronics was designed to mount to the end of the motor. This allowed for rotation of the electrical connections in increments of 120 degrees, providing three possible connector positions (see Figure 2). The design also allowed for ease of assembly with automated production equipment.
The circuit design was further improved with an IC module added to the PCB that increased reliability by replacing 30 percent of the electronic components. The IC module integrated multiple functions into one component—voltage regulation, over current protection, under voltage protection, over heating protection, and pulse width modulation signal conditioning. To maintain safe component temperatures, a radial impeller was placed in a hole located in the center of the circuit board with a 360-degree discharge pattern for optimum cooling (see Figure 2).
Heating units are increasingly installed in living spaces, which is why low noise during operation is critical. If a rotating system is balanced in two planes, a lower level of imbalance can be achieved. Therefore, dynamically balancing the primary impeller together with the cooling impeller for the PCB provided a means to reduce structure-borne noise (see Figure 2).
The objective for the new impeller design was to reduce the overall size, yet deliver the same air performance without increasing the speed or noise levels. Cost reduction and automated assembly would also be included in the design process. The result was a one-piece injection molded plastic backward curved impeller with dimensions of 4.7-inch diameter by 0.32-inch width (118 mm by 8 mm). The one-piece design reduced the diameter by 9 percent, reduced component parts by 50 percent and eliminated a step in the assembly procedure. However, the reduction in diameter and removal of the back plate lowered the overall efficiency. To compensate for this, the blade angle was changed and the housing geometry modified to align performance with the benchmark product.
The development of a smaller impeller provided an opportunity to reduce the overall size of the scroll housing by 10 percent. An in-house CFD program allowed for 3-D geometrical analysis in order to optimize the scroll housing design and compensate for a reduction in volume. To allow for ease of assembly and specific customer arrangements, the inlet/outlet flanges were combined into one scroll half. A liquid sealant applied to the two scroll halves during automated assembly ensured a premix ready connection.
Figure 3. Performance comparison of old and new gas blower
The primary goal to develop a smaller, quieter, more reliable, and lower cost gas blower that performed at the same level as its larger predecessor was reached at the end of the 2-year development program (see Figure 3). Creative design and engineering provided an opportunity to develop a new platform for the next generation of gas blower products. The benchmark has been raised to a new level of engineering design for the future of high-efficiency condensing boilers.
About the Author
Tom Costello is a product manager at ebm-papst Inc. He has 20 years of design, application, and product management experience in air-movement and combustion systems for residential, commercial and industrial applications. If you wish to contact Costello, please e-mail firstname.lastname@example.org.