Appliance Design - July 2008 - (Page 30)

MOTORS & PUMPS Pump Technology Compression Ratio Price/Power Ratio Size/Power Ratio Noise/Power Ratio Vibration/Power Ratio Operating Efficiency Operating Life Tightness (lack of leakage) Tolerance of Aggressive Media WOB-L Piston Rotary Vane Articulated Piston Diaphragm Linear EXCELLENT GOOD FAIR POOR Fig. 1. Characteristics for standard designs. Pump Technology Maximum Flow (cfm) Maximum Pressure (psi) Maximum Vacuum (" Hg) WOB-L Piston Rotary Vane Articulated Piston Diaphragm Linear 7 21 15 3 8 430 15 175 45 6 29.5 29.6 27.0 29.0 24.0 Fig. 2. Comparison of common working points for standard designs. flow and can be designed to allow full-pressure restarts. Linear: This device is a fit for applications requiring moderate flow with low pressure or vacuum. It delivers long life and efficient operation with extremely low power consumption, and exhibits low pulsation on pumps with large integral exhaust volumes. Some types have liquid-pumping capability. Among those approaches, the WOB-L piston pump represents one of the most versatile — and popular — pressure and vacuum technologies available. The reasons are simple: A WOB-L pump combines the primary benefits of a conventional piston pump (pressure, vacuum, and flow performance) with the key advantages of a diaphragm pump (compact size, quiet operation, and clean airflow), and beats them both in efficiency and manufacturability. This combination has allowed WOB-L technology to capture a large — in some application areas, dominant — share of the pressure and vacuum market since its introduction 30 years ago. Ongoing engineering enhancements have further opened applications to WOB-L pumps. One of the most important enhancements is the option for variable-speed motors. This includes the choice of brushless DC motors to deliver long life and, with the addition of closed-loop control, extremely precise speed regulation. The key components of a WOB-L piston pump are depicted in Fig. 3. In simple terms, a WOB-L pump is mechanically a cross between a diaphragm pump and an articulating piston (or reciprocating piston) pump – the shared mechanical element being an eccentric connecting rod. Applying this design to a piston pump eliminates the need for a connecting wrist pin, which significantly reduces the size, weight, and mechanical complexity of the pump. As its name implies, the approach of direct coupling a unitary piston rod to the crank without a wrist pin introduces a characteristic wobbling motion to the piston, as shown in Fig. 4. At the bottom of its stroke, the piston is precisely perpendicular to the cylinder wall; as it moves upward, the piston tilts proportional to the ratio of the stroke to the overall rod length, and reaches perfect alignment again at the stroke’s top. The down stroke produces the reverse motion. In order to guide the piston in the cylinder bore, and also to provide a seal (similar to piston rings) between the wobbling piston and the stationary cylinder walls, the piston rides within a flanged polymer cup. Air pressure on the upward stroke of a pressure pump or the downward stroke of a vacuum pump expands the cup against the cylinder wall, increasing its sealing properties while compensating for the wobble action. Made of a composite containing polytetrafluoroethylene (PTFE), this cup produces a minimum of friction, requires no lubrication, and generates relatively little heat. The WOB-L pump was originally designed to supply compressed air for mobile applications, but the universe of applications for it has since become substantially larger. A sampling of appliance industry applicawww.applianceDESIGN.com 30 applianceDESIGN July 2008 http://www.exmek.com http://www.exmek.com http://www.appliancedesign.com

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Appliance Design - July 2008 Contents Editorial Shipments/Forecasts News Watch Heating Elements Gas Technology Motors & Pumps Controls & Sensors Design Marts Association Report: AHAM Advertiser's Index

Appliance Design - July 2008

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