Appliance Design - November 2007 - (Page 46)

GAS TECHNOLOGY The DDM3 from Maxitrol controls valve pressure in minute increments. Maxitrol's GV60 e-flame remote ignition and control system fully modulates between low fire and high fire in a residential fireplace. taking down the turn down to 2:1, which is not a big deal, we are taking the flow from 100 percent down to 50 percent, and we have to take the pressure from 100 percent to 25 percent. Imagine where this goes when people start getting into 10:1 or 20:1. We could be down to the 1/100ths of the pressure that it was at a high fire rate.” CompuValve’s modulating valve system is also microprocessor controlled. The valve, which can control gas flow from down to 1 millionth of a liter with a potentially infinite variability for points between lowest and highest position. In the Caldera Arrow gas cooktop application, using the variable capacity allows the appliance to hold a pot of water to within +/- 0.5 DegF, according to the company. The Arrow’s control system relies on a number of different monitoring approaches including a thermocouple positioned in the flame of each burner to detect flame presence or absence and a thermistor implanted inside each burner body to measure burner temperature. This last sensing approach permits the Arrow to compensate for expansion of the gas, which occurs when the burner heats up. This alone can cause the loss of as much as 20 percent of energy output unless the flame is readjusted. An algorithm within the controller looks at the burner temperature, predicts gas expansion, and, as needed, opens the gas valve a little more to prevent the effect from occurring. Another variable-capacity burner control system that is beginning to make a name for itself is Varidigm. The company’s Variable Burner Control (VBC) concept began at the Gas Research Institute where a modulating furnace was under development in the 1990s. That technology was spun off into Varidigm, says Gambiana. Varidigm’s patented VBC is an integrated burner control system that modulates the firing rate from 10 to 100 percent in either furnaces or boilers. Varidigm’s modulating furnace technology includes premix burners, variable-speed motors, and combustion controllers. The combustion controller sequences, times and monitors the combustion process using a sensor located in the flame. “It controls the entire combustion process from proving air levels in the combustion chamber, mixing the air and gas, operating a feedback system and using recirculating blower motors,” says Gambiana. In addition to the typical advantages of energy efficiency and home comfort, Gambiana points to low emissions of harmful gases as well as low noise. Gambiana says that the units typically have a 25 to 30db reduction below standard furnaces. Varidigm’s technology is just one of many innovations that came out of GTI. While Johnson did not work on this process, he has worked on a wide variety of units including the pizza oven discussed earlier, a steam generator, and building humidifier. GTI’s system features three types of parameters, a High Firing Rate (HFR), a Low Firing Rate and a Modulation Bandwidth (MB). The high and low firing rates are based on the maximum output of the burner. “The way the system works is it looks at how far away from set point the temperature is at and automatically adjust the firing rate at that point,” says Johnson. For example, if the temperature in the oven is set at 350 DegF, if the oven is less than 300 degrees the system will run high fire full out, then once it gets to that defined range (MB), it will slowly start cutting back the gas flow in correspondence to how fast it is approaching that final temperature. “So, if I’m within 50 Deg, I’m full out, and within 40 degrees I might be 75 percent of full, so just as you get close to the temperature, you start dropping the burner firing rate,” says Johnson. Another way to achieve a variable-capacity gas fired system is to utilize a variable-speed premix combustion blower combined with a specialty gas valve and venturi to regulate the flow of the air-fuel mixture to the burner. In this scenario, burner output is regulated by decreasing or increasing the speed of the blower while the air-fuel ratio remains constant. One such product is a pre- The ebm-papst NRG118/0080-3612 premix combustion gas blower. 46 applianceDESIGN November 2007 www.applianceDESIGN.com http://www.appliancedesign.com

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Appliance Design - November 2007 Contents Editorial Shipments/Forecasts News Watch Today’s LEDs are Brighter and More Colorful than Previous Versions, Allowing Designers More Opportunities to Improve and Differentiate Their Products. When Converting from CCFL to LED Backlights in LCD Panels, New Driving Methods are Needed. Electrowetting Technology Allows Displays to be Designed in an Extensive Range of Colors. Noise Problems are Not Always Easy to Find or Fix, but Acoustic Labs Can Help Designers Ferret Out the “Good” Noise from the “Bad.” A New Sanitization Protocol for Clothes Washers Combines the Best of Three Well-Established Provisions to Ensure Efficiency, Quality and Safety. Stricter EMC Requirements Raise the Bar for Appliances with Electronic Controls and Require Testing to New Levels. Despite Added Cost and Complexity, Gas-Modulating Systems have Migrated from Large-Scale Boiler Applications to Smaller Residential and Commercial Furnaces and Appliances. A New Treatment Technology Hardens the Surface of Light Alloys such as Aluminum and Magnesium and Expands Their Potential Applications. DesignMart Advertiser’s Index Association Report: AHAM

Appliance Design - November 2007

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