Improve energy efficiency by restructuring your vacuum generators

In many applications, electric motor-driven vacuum pumps can achieve the same performance as vacuum generators while using one-fourth to one-tenth the energy.

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By Dan Bott

PlantServices.com

Vacuum generators powered by compressed air represent one of the most inefficient uses of that valuable utility. While sales literature highlights the benefits of using vacuum generators, it fails to paint the entire picture. Behind every quiet, vibration-free, low-cost, environmentally friendly venturi vacuum pump is an expensive, energy-consuming, large footprint air compressor. In many applications, electric motor-driven vacuum pumps can achieve the same performance as vacuum generators while using one-fourth to one-tenth the energy. In fact, replacing compressed air vacuum generators might be one of the last methods remaining for increasing production energy efficiency and taking overworked air compressors off line.

Venturi-style vacuum pumps, also called vacuum generators or compressed air ejectors, produce vacuum by passing high-velocity compressed air through a venturi or nozzle. Performance depends on the nozzle’s shape and size, compressed air pressure and flow, and the desired vacuum level. The greater the vacuum being maintained, the lower the flow of induced air. Other motive fluids include steam, vapor, water and other liquids.

Compressed air vacuum generators are common in industry. Palletizers, material-handling systems, pick-and-place operations, drum-type vacuum cleaners and packaging applications are just a few examples. Each generator is mounted in close proximity to the point-of-use, with supply tubing connecting the vacuum device to a central compressed air system.

Vacuum generators are reliable, compact, lightweight and quiet. They have no moving parts and can be mounted directly on production machinery. Their maintenance requirements are minimal. They’re available in aluminum, plastic and corrosion-resistant construction for harsh applications. Replacement or repair is simple and requires no special tools or training.

Considering these seeming advantages, why use an alternative? The answer involves efficiency and energy savings. What at first appears to be a winning way to produce vacuum turns out to be a technology with inadequate performance.

A vacuum generator, by itself, is equivalent to an engineless automobile. Neither makes any noise nor requires maintenance. Neither has an operating cost. The drawback, of course, is that neither does any useful work. Without an air compressor “engine” operating under the equipment room “hood,” the vacuum generator can do no work. One can’t evaluate a vacuum generator without accounting for the air compressor in the calculus. An objective evaluation compares the relative efficiencies of electric-driven vacuum pumps and vacuum generator-compressor combinations.

Vacuum generator literature uses two key terms: induced airflow and air consumption. Induced airflow is the air being evacuated from inside the vacuum system. Air consumption refers to the compressed air the vacuum generator requires. These flows combine and discharge through an exhaust port.

A vacuum pump driven by an electric motor, on the other hand, uses a varying rotational swept volume to produce a suction that “induces” flow from inside the vacuum system. The rotor compresses the induced flow and discharges it to an exhaust port. Motor-driven vacuum pumps consume no compressed air.

Production demands dictate vacuum pump size and serve as the basis for evaluation. Continuous vacuum applications seek to maintain a fixed vacuum level. In cyclic applications, on the other hand, a chamber at atmospheric pressure is evacuated to a target vacuum level for a period of time and then vented. Given the two application types and the two vacuum technologies, which pairing is most energy efficient?

The answer lies first in determining the cost of compressed air and how much induced flow a vacuum generator develops. Table 1 illustrates a representative continuous application with 20 vacuum generators. The values represent the average performance of typical industrial units. Each generator requires 20 scfm of motive air to induce a vacuum flow that is a function of vacuum level.