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issue: September 2006 APPLIANCE Magazine

Technology Report
Safety Sensor

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Bringing added safety to household dryers, a new pressure switch notifies end users of blockage issues and helps maintain appliance efficiency.

The Lint Alert pressure switch exhibits a contamination-free pitot tube pickup and a creative design that allows for a slide-and-snap in place insertion.

The new sensing device, called Lint Alert™, is an electromechanical pressure switch that has been mated with a pitot tube receptor that is installed on the exhaust conduit of clothes dryers. Measuring the pressure differential within the exhaust conduit and atmosphere, the device will detect and notify appliance users of any blockage issues that could pose a fire hazard.
In-O-Vate Technologies, Inc. of Jupiter, Florida, U.S., conceptualized the idea of porting the pressure switch’s use to make dryers safer. However, the company’s expertise was in dryer venting products, so it turned to switch supplier World Magnetics of Traverse, Michigan, U.S. to collaborate. “This required a custom or special modification to their current embodiment to properly mate to the exhaust pipe and adequately measure the pressure within the exhaust conduit,” explains Rick Harpenau, president of In-O-Vate. Together, the companies created the UL-approved switch, which is designed to slide and snap in place on the exhaust conduit of the dryer at or near its exit port.
According to Steve McLintock, chief pressure switch product engineer at DesignFlex, a brand of World Magnetics, the safety solution is unique in that it uses pressure sensing. Competitive sensors, he says, often use vacuum, which can create reliability problems and lint build-up on the switch contacts. “Most instruments block flow to some extent, which changes the fluid flow from a laminar flow to a turbulent flow,” McLintock explains. “Lint carried in force air steam is permitted to slow and tends to gather on sensing probes or deflector plates, creating potential blockage points themselves.”
The Lint Alert sensing port, however, is designed to limit fluid flow characteristics. “The sensor under normal use sees only negative pressure created by a vacuum venturi created by air flow moving around sensing port,” McLintock explains. “Unlike a pitot sensing tube, the sensing port in this design is facing downstream and does not directly see a static positive pressure, only a static negative pressure created by a venturi effect. Based on this design, switch contacts are open until a positive pressure is detected due to a blockage or restricted flow.”
At a predetermined set-point (approximately 0.91 water column inch), the switch triggers an audible or visual alert on the control panel that maintenance or attention should be directed to the exhaust conduit of the dryer. McLintock says this happens when the vacuum or negative pressure reduces and eventually becomes a positive pressure, which the pressure switch recognizes and in response, closes its switch contacts. “Hence, the dryer alarm is triggered, alerting the user to the problem,” he says.
Potential blockage problems include condensation and moist air in the pipe, the potential of the flex being crushed or kinked creating a “bottleneck” situation, bird or rodent nests near the exhaust termination cutting off airflow, and screens within the termination hoods allowing lint to accumulate over time and making dampers inoperative. In-O-Vate says detecting these hazards not only helps prevent fires and save lives, it can minimize warranty issues and service calls. It also helps dryer efficiency. “Clogged duct vents or hoses decrease efficiency in drying cloths, requiring more energy and longer cycles to dry clothes,” McLintock says. “Efficiency is important to the customer to save on energy costs.”


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