Machine Design

Sorting out vacuum-pump technology

Here's how air-driven and mechanical systems compare.

Michael Tuohey
Piab USA
Hingham, Mass

The COAX system, developed by Piab, integrates the internal components of a multistage vacuum pump into a cartridge, producing a small, efficient system. The cartridges are up to twice as fast and provide three times more flow than conventional ejectors with the same air consumption. It is designed for low and fluctuating feed pressures (25 to 90 psi) with sustained vacuum. The units produce vacuum flow as soon as the pressure valve opens, maximizing compressed air use while reducing energy consumption.

Air-driven vacuum pumps work when compressed air passing through an orifice increases its velocity and lowers its air pressure. Ambient air is drawn in through channels in the generator, creating suction and vacuum. The air stream exits through the exhaust.

In high-pressure manufacturing environments where every second counts, it's ironic that so many processes rely heavily on low-pressure solutions. Vacuum systems are frequently the method of choice for keeping products moving and plants and equipment running efficiently.

These operations require vacuum pumps. But within the vacuum-pump arena are several options, each creating vacuum by greatly different methods. The two most widely used are compressedairdriven pumps, commonly referred to as generators, and electromechanical pumps.

Electromechanical vacuum pumps usually have an electric motor as the power source. Among the many variations, rotary-vane models are most commonly used in industrial vacuum applications. Highvelocity rotation of a rotor and vanes traps air entering the intake port and sweeps it through the pump, creating a partial vacuum behind it. Most vanes are made of carbon and operate within a housing that requires exacting tolerances for proper performance. Such systems are widely used and well suited for many applications but if looking to maximize efficiency and uptime, rotary-vane units are often not the best choice.

Compressed-air-driven vacuum pumps, on the other hand, operate on the Bernoulli Principle. There is a consistent relationship between pressure and velocity, with a high-velocity fluid creating low pressure. Air-driven pumps force compressed air through an orifice or nozzle, increasing velocity and lowering air pressure. Higher-pressure ambient air outside the system flows in through channels in the generator, trying to create equilibrium. This outside air mixes with high-speed air used to generate low pressure, and the combination exits through the exhaust. With this method, the vacuum level depends on the nozzle. In the narrowest part of the nozzle, the air stream reaches its greatest speed and creates the deepest vacuum and suction at the system's openings. Compressed-air-driven pumps can have one or more stages. All references within this article are to multistage ejectors. Here are some of the features of such systems.

Intermittent operation. A rotary-vane pump's electric motor cannot efficiently cycle on and off to break a vacuum. A vacuum valve can be added for intermittent use, but the motor must still run continuously. In such applications, multistage air-driven pumps tend to be more energy efficient. A pneumatic valve, which simply cycles the pump on and off, controls intermittent use. The pumps do not need to run continuously.

Energy savings. Air-driven pumps are compact and lightweight. They run efficiently and are ideal when designing an automation system or machine where small size and simplicity are important. They can be positioned close to where needed, which eliminates long vacuum lines and reduces flow losses. This, in turn, saves energy and reduces response time. They generate only the vacuum an application needs and can regulate pressure to the lowest level required. And they limit the potential for leaks common in long lines with numerous connectors.

Without careful attention to system demands it is easy to oversize a rotary-vane pump, which results in higher energy consumption. Also, the weight and size of a rotary-vane pump commonly limits installation to the plant floor, necessitating vacuum hoses that can be quite distant from the point where the work is performed.

Ambient temperature. Temperature extremes have relatively little affect on air-driven pumps, due to the internal multichambered design and pneumatic operation. Expanding airflow creates a refrigerant-effect within the body of the pump, keeping internal temperature low while emitting cool exhaust air.

Most manufacturers of rotary-vane pumps guarantee proper operation only within an ambient temperature range of 32 to 100°F. Lower temperatures hamper the unit's ability to start, and higher temperatures affect pump life.

Oilless operation. Multistage air-driven pumps have no rotating parts that create friction, so they require no lubrication. This environmentally friendly design comes in handy in applications that cannot tolerate trace amounts of oil, and it also reduces maintenance costs.

Some rotary-vane pumps require lubrication, and oil flow must be precisely controlled to prevent oil condensation or premature pump failure.

Downtime. Air-driven pumps are often a better option when an uninterrupted vacuum supply is a must. The only moving parts are flap valves that open and close internal chambers as the pump adjusts to varying loads. With few parts that can fail, they typically require little or no maintenance.

Most rotary-vane pumps need to be shut down from time to time for regular maintenance. Standard replacement parts for mechanical pumps typically include shaft seals, gaskets, O-rings, valves, bearings, vanes, springs, and other items. Wear and tear on the parts may require a total rebuild and, with it, the need to keep spare pumps on hand.

Life expectancy. Because compressed-air-driven pumps have few moving parts, they tend to last a long time. Standard pump life is about five years, or 43,200 hr.

In contrast, a number of variables negatively affect rotaryvane pump life. These include ambient temperature, duty level, operating cycle, and operating speed. The condition of the air handled plays a role, including cleanliness, humidity, temperature, and chemical vapors. How well the pump is maintained is another factor. For instance, rotary-vane pumps rated for 25,000 operating hours under controlled conditions have failed after several thousand hours in harsh operating conditions.

The advantages of vacuum generators come into play across many different industries. For instance, automotive manufacturers rely on the efficiency and reliability of air-driven pumps in a wide variety of applications, from stamping and press transfer to paint coating and parts handling. For example, in die casting, air-driven vacuum conveys particulates into the molds and also evacuates the die. They are the units of choice because the hot and dirty environment has little effect on pump performance and life.

The chemical industry uses vacuum in a range of applications, such as lowering the boiling point of liquids, increasing liquid flow rates through filters, and mixing different products. Most multistage air-driven pumps resist chemical attack and corrosion, ensuring reliable operation.

Air-driven pumps are often used in the packaging industry because they are easy to control, can operate intermittently, and readily fit into compact spaces. For instance, many food products are packaged under vacuum to extend shelf life. Vacuum conveyors transfer materials into hoppers and filler tubes, and vacuum is used to grip both sides of pouches for packaging. Rotary cartoner machines often rely on a series of vacuum bars and suction cups to move the cartons.

Piab USA,
(800) 321-7422,

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