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S Small Pumps, Big Considerations Straight Talk on WOB-L Piston Pumps

Far From Being Toys, Today's

Tiny Pressure and Vacuum Pumps

Demand Serious Engineering Study

and Careful Application Planning

By David C. Droege

Miniature Pumps Product Manager

Thomas Products Division, Gardner Denver, Inc.

Some are barely as large as your fingertip. But packed inside modern miniature vacuum and pressure pumps are surprising performance capabilities that can be unlocked through proper system design. The keys are recognizing, first, that the smaller the pump the more precise a designer needs to be and, second, that involving the pump manufacturer early on can help avoid substantial re-design or reverse-engineering later. It's much more than a flow and pressure (or vacuum) game. Different miniature pump technologies ­ or even subtle modifications within the same technology ­ can have big effects on an entire range of critical system design parameters including product size and shape, noise, operating temperature, power consumption, service life and reliability. Miniature pumps play critical roles in a wide range of products. In medical applications, you'll find miniature pumps at work in devices that monitor gases in surgical operating rooms, keep tabs on patient pulse rates, or check blood pressure. In laboratories, miniature pumps are found in devices that draw samples, dispense fluids, and wash or sterilize vials and slides. In industry, portable devices that measure carbon dioxide, hydrogen sulfide, oxygen and other gases are powered by miniature pumps. Miniature compressors even touch your taste buds ­ as critical components in the espresso-makers found in coffee shops as well as in the home.

WHAT DEFINES A "MINIATURE" PUMP? There is no standard industry classification that separates large pumps from small pumps, and small pumps from miniature pumps. Physical size is one guide, but that's a relative

thing, as the group photo above indicates. This sample includes everything from a tiny diaphragm pump the will fit in a 1 cu. in. envelope, to a WOB-L® piston pump requiring 20 cu. in. of space. Pressure and vacuum capabilities could be another guide, but these vary depending on the pump technology chosen. A more universal defining point, then, is maximum airflow. Here is a table suggesting the maximum air flows that the three most common technologies used in "miniature" pumps would provide:

Pump Technology Maximum Flow to Be Considered a Miniature Pump

Diaphragm WOB-L Piston Rotary Vane

13.8 lpm 24.5 lpm 20.0 lpm

ZEROING IN ON THE BEST TECHNOLOGY FOR THE JOB

No single air pressure or vacuum technology is best across the board. Flow, pressure and vacuum charts, commonly available in pump manufacturers' literature, will help get you in the ballpark. In addition, here are chief characteristics of the most common miniature pump technologies used by equipment designers:

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Diaphragm: The leading miniature pump technology as it is easily manufactured in very small packages. Ideal for lower pressure or moderate vacuum needs. Tolerant of aggressive media, including liquids. Quiet operation. Lower pulsation than some piston pumps. Many sizes and price points for application flexibility, including extremely small sizes. Oil-less design for clean air flow. Can be designed to allow full-pressure restarts. Rotary vane: Highest air flow relative to physical size, but not applicable to high-pressure applications. Smooth and pulse-free. Simple design contributes to long life. Characteristic "whine," especially in smaller sizes ­ though can be reduced through sound insulation. Not suitable for full-pressure restarts. WOB-L piston: High pressure and vacuum capabilities relative to the compact size and light weight of the unit. Efficient and relatively quiet. Oil-less for clean output. Modern seal materials and simple design contribute to long, service-free life, especially at lower pressures. Intake air must be filtered and should be generally dry. Not suitable for full-pressure restarts.

SOME NEW IDEAS Pump manufacturers are constantly at work within proven technologies, squeezing out additional performance, reducing size, or addressing myriad other application issues. Thomas' new Model 1101 pump is an excellent example of this. It uses a "rolling diaphragm" design that combines the best features of rotary and diaphragm pumps. Rather than moving up and down, the 1101's diaphragm rolls in the pump's cylinder. This increases airflow relative to size and provide an extremely precise flow with limited pulsation ­ ideal for the pump's major application of inflating blood pressure sleeves. Another development is demonstrated in the Thomas Model 4002 pump, which uses an elongated diaphragm having a larger surface area than a conventional round diaphragm. Because of this the Model 4002 can produce higher flows than a similarly sized pump with a standard diaphragm. Small and lightweight, the 4002 was originally designed for handheld glucose monitors. The pump is also a good fit for gas analyzers and eyesurgery equipment.

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The Thomas G09-LC takes dead aim at the noise that can be associated with small rotary vane pumps. This pump uses a rubber (EPDM) sound reduction jacket to reduce the pump's noise level to 44 dB(A), just 1/16th the sound emitted by a similar pump without the noise-reduction feature. The G09-LC is finding wide use in lumbar support systems for automotive, aircraft and home furniture seating. And the Thomas Model 1420 miniature diaphragm pump is built with an innovative twin-head design that allows the pump to achieve higher flows than conventional diaphragm pumps of the same physical size. The pump's heads can be channeled in either parallel or series to meet the flow and pressure/vacuum requirements of the application. Flow is rated at 6.0 to 11.0 lpm, when the heads are in parallel configuration, and up to 5.5 lpm in series configuration. In WOB-L piston pumps, recent developments focus on the adaptability and versatility of this technology in prebuilt systems that can also include solenoids, switches, wiring harnesses, integrated check valves, tubing, circuit boards or other electrical controls. More and more OEMs are looking to pump manufacturers to provide complete systems that simplify downstream assembly processes.

DETAILS ARE A BIG DEAL WITH SMALL PUMPS When coming up with a list of all the factors that might go in to specifying a miniature pump, it can be hard to know when to stop. The following "Top Ten List" should cover 90-plus percent of the information a pump manufacturer will need to know to recommend or design the best pump for your application:

1.

Working Range

Define the maximum pressure or maximum vacuum required by the application. When determining this, be sure to take into account the maximum allowable pressure or vacuum tolerated by all devices in the system. In this case, look especially at connecting lines and hoses to be sure they are specified properly. Define the airflow requirement at the maximum working point. It is very important that the airflow requirement be tied to the highest pressure or vacuum required, as standard airflow ratings for pumps are usually "wide open" or "full flow" ­ meaning at zero pressure and zero vacuum. Manufacturers usually have performance curves showing pressures or vacuum that can be delivered at various flows.

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Most applications do not require the maximum flow, pressure or vacuum at all times, so also define a typical working point and provide some idea of how frequently and for how long peak or maximum performance is required. This information can help prevent pump over-sizing and potentially reduce cost. If stop and restart under pressure or vacuum is a requirement, be sure to state this. 2. Motor Requirements

Define your power source. If direct current (DC), specify the voltage. If alternating current (AC) define the voltage as well as the frequency (generally 60 Hz in North America and 50 Hz in Europe and many other places in the world). List any power consumption and current draw limitations. If the motor is to run at a constant speed, define how precisely must this speed be maintained. If variable flow through an adjustable-speed motor and controller is required, be sure to state that. Finally, determine the duty cycle of the application. Define as either "continuous" or, if intermittent, indicate a pattern of "minutes on, minutes off." 3. Unit Envelope

This is often the deciding factor with a miniature pump. List height, width and length maximums that can be accommodated. Also, determine how much free air space will exist around the unit. Knowing this will help the pump manufacturer determine whether there is sufficient cooling air, or whether additional fans or other cooling mechanisms will be required. Pump mounting must also be considered. Mounting systems can be as simple or complex as needed to meet the specific needs of an application. When noise and vibration are important considerations, careful attention must be paid to both the mounting system and piping or hose connections to ensure that vibrations are not transmitted to surrounding structure inadvertently. 4. Operating Life

Be straightforward about your expectations for a pump's service-free life. Because of their application versatility, miniature pumps may be designed for anywhere from 500 to more than 10,000 hours of service-free life, depending on the ambient temperature, operating speed, type of motor used, and a variety of other factors. Serviceability is generally not an issue with miniature pumps, as it is almost always more economical to replace than to repair a small unit.

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5.

Environment

Typical ambient temperatures for miniature pumps are from 50º F to 104º F (10º C to 40º C). However, special designs can operate in temperatures as low as minus 40º F or as high as 212º F (minus 40º C to 100º C). Describe the air surrounding the pump: clean, dusty, gaseous, etc. Also determine the relative humidity. This information will help determine whether filters required, as well as recommended diaphragm or seal materials. 6. Media to Be Pumped Chemicals, volatile gases and moisture in the air, as well as the media temperature will affect pump sizing and construction. Pump manufacturers have conducted extensive life testing under various combinations of temperatures, pressures, strokes and humidity conditions with different diaphragm, piston seal and vane materials, resulting in development of specialized materials for each type of application. Make sure that these environmental factors are communicated to the application specialists. 7. Sound and Vibration

Sound and vibration can be significant challenges when applying miniature pumps. 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. Take advantage of the pump supplier's noise and vibration reduction services to ensure that levels are satisfactory. 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 pump supplier is aware of all the agency requirements for your application so that regulatory issues can be addressed in parallel with the development of the product. Failure to do so can delay getting the product into production. 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. Temperature will already have been defined in Point 5. Day-to-day

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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 ­ and more so with vacuum than with pressure applications. Especially, make sure your pump supplier 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

It's just a fact of life. 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. Let your manufacturer know which of these elements is your top priority.

INVOLVE YOUR PUMP SUPPLIER EARLY ON Don't underutilize the resources available through your pump supplier. While the manufacturer probably offers a wide array of standard pumps, modified or even all-new designs specific to the application requirements are also possible ­ given sufficient time for design, prototyping, testing and manufacturing. Point is, don't put yourself or the pump supplier in a box that can lead to a pump that's either over-designed and too expensive, or under-designed and a recipe for failure. Consider all technologies and all application factors, as a team. Then share your data, thoughts and concerns with your supplier, and ask for advice. The result is likely to be the perfect pump at the lowest possible cost.

David C. Droege is Miniature Pumps Product Manager of Thomas Products Division, Gardner Denver, Inc. Our thanks to Mr. Droege for his article. Thomas is the world leader in oil-free pumps and compressors for the OEM market. Combined Fluid Products Co. is a leading source of vacuum pumps, compressors, blowers, air knives, and filters and may be contacted at 800-521-2083 or [email protected]

© 2006 Gardner Denver Thomas, Inc. All rights reserved.

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