For engineers designing pressurized air or vacuum systems, it’s important to understand that no single air-pressure or vacuum technology is optimal for all applications, as illustrated by the comparisons shown in Fig. 1 and Fig. 2. To help narrow down the choices, engineers can peruse charts for flow, pressure, and vacuum that are widely available from pump manufacturers. Before starting the process, however, it is helpful to be acquainted with the chief characteristics of the most common pump technologies used by equipment designers.

WOB-L piston: This technology possesses high pressure and vacuum capabilities relative to the compact size and light weight of the unit. It delivers moderate to high air flows, depending on the design, and is very efficient, especially compared with similarly sized diaphragm pumps. The pump is relatively quiet and easily serviceable, and has a dry-running (oil-less) design for very clean output. Modern seal materials and simple design contribute to long, service-free life, especially at lower pressures. The intake air must be filtered and should be generally dry. The pump is not suitable for full-pressure restarts.

Rotary vane: This pump has the highest air flow relative to physical size, but is not applicable to high-pressure applications. It can be oil-lubricated or oil-less, and provides the smoothest air flow that is free of pulsation. Its simple design contributes to long life, but it is less efficient than piston or diaphragm pumps. The pump exhibits a characteristic whine, especially in smaller sizes, and its vane debris can contaminate output air. It is not suitable for full-pressure restarts.

Articulated piston: This type is generally chosen for heavy industrial applications that require the longest life, especially where full-pressure restart is required. It offers high pressure and vacuum with high flows and can be oil-lubricated or oil-less. The pump’s noise can be an issue, and it is relatively heavy and higher priced. Its intake air must be filtered and dry.

Enlarge this picture Fig. 1. Characteristics for standard designs.

Diaphragm: This style works best for lower pressure or moderate vacuum at lower air flows. The design is tolerant of aggressive media, including liquids. It has quiet operation and may have lower pulsation than some piston pumps. The pump is available in many sizes and price points for application flexibility. It has a generally oil-less design for clean air 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.

Enlarge this picture Fig. 2. Comparison of common working points for standard designs.

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 applications includes:

• Vending machines: Dispensers for foods, drinks, and compressed air.
• Business machines: Copy machines, mail sorters, and vacuum frames.
• Environmental and safety devices:  Air dryers, particle counters, ozone generators, dry fire sprinkler systems, and floor cleaners.
• Medical devices: Nebulizers, aspirators, oxygen concentrators, blood analyzers, blood pressure instruments, pneumatic hospital beds, emergency vehicle suction carts, dental carts, autoclaves, and other sterilizers.
• Laboratory devices: Water aspirators, vacuum filters, dryers, mass spectrometers, electron microscopes, gas analyzers, and samplers.

Enlarge this picture Fig. 3. Key components of a wobble piston pump. Dual cylinder design shown.

Despite this versatility, however, WOB-L technology is not necessarily the best choice for every application. As noted earlier, every pump technology has its own set of advantages and disadvantages, so it is important for design engineers to carefully define the requirements of their applications. While such a list can be potentially very long, the following 10 considerations should yield more than 90 percent of the information a pump supplier would need to recommend or design the optimal pump for a specific 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.

Most applications do not require the maximum flow, pressure, or vacuum at all times, so 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 note this.

2. Motor Requirements. Define the power source. If direct current (DC), specify the voltage and the source – for example, rectified AC, battery, solar, DC generator. 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, and whether thermal overload protection is required. 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 note that.

Finally, determine the duty cycle of the application. Define it as either continuous or intermittent, and if the latter, indicate the pattern of minutes on/minutes off.

3. Unit Envelope. List height, width, and length maximums that can be accommodated, and 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 issues, 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.

In the case of reciprocating pumps, there is inherent vibration due to torque pulsations that result from the motor speeding up and slowing down on each revolution in response to the build up of pressure in each cylinder and the subsequent intake of air at atmospheric pressure. Care has to be taken to isolate the compressor so that minimal vibration is transmitted to the surrounding structures and enclosures to minimize noise and vibration of the device or system.

Enlarge this picture Fig. 5. Flow vs. altitude.

In larger pumps, an isolation system will typically involve elastomeric members. These provide some damping and snubbing capability to prevent impacting of metal parts during shipping and handling or under startup and shutdown conditions. In smaller pumps, standard elastomeric mounts are often used to provide a measure of isolation and to withstand shock loads. Often overlooked are vibration transmission paths created by piping or hose connections to the pump. These need to be flexible enough to provide isolation, just as the mounting system does.

In small OEM pumps where cost is critical, isolation systems may consist of simply an elastomer squeezed between recesses on each side of the pump housing and the inner surfaces of the enclosure in which the pump is mounted, thus avoiding the need for fasteners. The elastomeric members must be carefully designed so that they are soft enough to provide adequate isolation and yet strong and stiff enough to withstand shock loading from handling and dropping the product.

Pumps, such as the WOB-L, that do not use liquid lubricants, can run in any orientation. 

This gives the product designer leeway in placing the pump inside a product. Low-cost mounting and isolation can be achieved by capturing the pump inside an enclosure with elastomer components and/or springs. This works well when pumps are ordered in a high enough volume to allow the tooling of custom mounting features. For lower production volumes, standard, off-the-shelf isolators are available that can be threaded into mounting feet for most pumps.

Enlarge this picture Fig. 6. Flow vs. altitude.

4. Operating Life. Be straightforward about the expectations for a pump’s service-free life. Custom pumps may be designed for anywhere from 500 to more than 30,000 hours of service-free life, depending on the ambient temperature, operating speed, type of motor used, and a variety of other factors. Often, larger units are serviceable to further extend unit life. Be sure to let the pump manufacturer know if serviceability is required so that the pump can be chosen or designed accordingly.

5. Environment. Typical ambient temperatures for pumps range from 50 DegF to 104 DegF (10 DegC to 40 DegC). However, special designs can operate in temperatures as low as -40 DegF or as high as 212 DegF (-40 DegC to 100 DegC). Describe the air surrounding the pump, whether it is clean, dusty, or gaseous, etc. Also determine the relative humidity. This information will help determine the type of filters required, as well as recommended 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, especially of the sealing materials. For example, the piston cups on the WOB-L are compression molded and may include a variety of materials depending 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.

Enlarge this picture 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.

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 over-designed 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