Appliance Design - July 2008 - (Page 34)

MOTORS & PUMPS on the application. If the application requires exposure to condensed moisture, piston cup materials used for general purpose applications will rapidly disintegrate. Pump manufacturers have conducted extensive life testing under various combinations of temperatures, pressures, strokes, and humidity conditions with hundreds of different composite blends, resulting in development of specialized materials for each type of application. Make sure that these environmental factors are communicated to the application specialists when selecting a pump to ensure that the appropriate seal materials are selected for the application. 7. Sound and Vibration. Sound and vibration can be a significant challenge when implementing a pump into a system. In some cases, the proper application of inlet mufflers and isolation mountings will help limit the noise in the end-use application. Noise and vibration transmission through a device can be complex. It often requires specialized skills and tools to determine the best ways to prevent transmission of unwanted vibrations and acoustic noise that can result from gas pulsations, valve action, rolling element bearings, and various flow-path restrictions encountered downstream of the pump. With the appropriate design remedies taken, noise can be reduced significantly. Solutions may be as simple as addressing stiff hose connections that can transmit vibration to other components. System resonances can greatly amplify the vibrations and resulting noise level of a device even with a well-balanced compressor. Take advantage of the compressor supplier’s noise and vibration reduction services to ensure that noise and vibration levels are reduced to satisfactory levels. 8. Agency Requirements. UL, CSA, TUV, FDA, ISO, and other regulatory agencies require that pumps and the motors used with them meet rigorous safety standards. Suppliers with certified labs can test and evaluate their products quickly and work closely with these agencies to ensure that their products comply and are properly labeled to demonstrate compliance with the appropriate standards. For custom-designed products, make sure that the compressor supplier is aware of all the agency requirements for the application so that regulatory issues can be addressed in parallel with the development of the product. 9. Altitude. Altitude above sea level, barometric pressure and temperature all affect air density – which is a major variable in sizing vacuum and pressure pumps to meet flow requirements. Day-to-day changes in barometric pressure can affect pump performance, but generally these are within tolerance ranges. Altitude, or more specifically its constant effect on diminishing the atmospheric pressure, is therefore the most critical factor. Fig. 5 and Fig. 6 illustrate how the effect of altitude on pump flow is magnified at higher pressures or vacuums. A compressor at free flow (high CFM) isn’t affected much at all, while a compressor operating closer to its full, deadhead pressure can lose virtually all of its flow at a very high altitude. Altitude affects vacuum applications even more than it does pressure applications. As the pump gets closer to maximum vacuum, the flow drops off steeply. In general, the clearance-volume to total-volume ratio or compression ratio will determine how sensitive the pump or compressor will be to altitude effects. Make sure that the application specialist is aware of any requirement to work at varying altitudes so that the pump can be properly selected or designed to meet the application requirements. 10. Cost Considerations. As with the specification of all components and materials, cost plays a role in determining what kind of pump to buy, and who to buy it from. Sometimes the role of cost is secondary, sometimes primary – but it’s always present and should be one of the key factors considered. When analyzing cost, consider all elements of it – including first-cost, lifecycle operating and maintenance cost, cost of a unit failure, and so on. Inform the pump supplier which of these elements is the top priority. No matter how much an equipment designer might know about vacuum and pressure technology, it’s a safe bet that the resident experts at pump manufacturers know even more, so product design engineers should fully utilize this resource. While pump suppliers offer a wide array of standard pumps, they can also provide modified versions or even new designs that are specific to the application requirements. A close involvement between the design engineers and pump supplier can help avoid the specification of a pump that is either overdesigned and too expensive, or under-designed and a recipe for failure. A careful consideration of all available technologies and application factors, combined with advice from the supplier, gives engineers a better chance of specifying the optimal pump at the lowest possible cost.< For more information, email: tpd.leads@gardnerdenver.com Fig. 4. Composite photo shows the characteristic tilt of a wobble pump’s piston at mid-stroke, compared with the piston’s perpendicular orientation at the top and bottom of the stroke. 34 applianceDESIGN July 2008 www.applianceDESIGN.com http://www.appliancedesign.com

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Appliance Design - July 2008

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