Machine Design

Cutting the cost of compressed air

Engineered nozzles cut air consumption and improve safety.

Roy Sweeney
Exair Corp.
Cincinnati, Ohio

Saving air and money

Engineered air nozzles can have a dramatic effect on plant air consumption, especially when used in place of homemade blowoffs. Consider an example where an Exair Model 1001 Safety Air Nozzle replaces a 1/8-in. open pipe. At 80 psig, both provide about 9 oz of force at a 12-in. distance.

The 1/8-in.-diameter open-pipe blowoff at 80 psig consumes 70 scfm of compressed air. A Model 1001 nozzle, on the other hand, consumes only 10 scfm, a 60-scfm savings. For noncontinuous applications, multiply this number by the duty cycle to determine actual air savings.

Managers of most large plants know their costs per 1,000 standard cubic feet of compressed air. If actual costs are not known, $0.25/1,000 scf is a reasonable average. Cost savings in this example are

60 scfm x 60 min x $0.25/1,000 scf = $0.90/hr. Annual savings amount to $1,872 for a single nozzle.

Calculating compressed-air savings shows that engineered nozzles, including filter and installation costs, often pay for themselves in a matter of weeks, not years.

Compressed air is commonly used in manufacturing plants for cleaning, cooling, drying parts, and removing metal chips.

But high-pressure air in these blowoff applications often creates problems due to the cost, noise, and potential danger. Compressed air ejected from open lines, copper tubes, and drilled pipes are a few of the common abusers.

One way to cut energy costs is through proper maintenance -- eliminating leaks and regularly maintaining filters. Replacing outdated motors and controls with high-efficiency models also saves energy, and replacements often pay for themselves in the first year.

But the most important factor to boosting efficiency is proper use. Engineered air nozzles and air jets, for example, cut operating costs by using only a fraction of the compressed air of typical blowoffs, particularly if they can be cycled on and off so air is used only when needed.

Air nozzles and jets use the Coanda effect (that is, high-velocity fluids exiting a nozzle will attach to an adjacent wall). It lets them amplify compressed airflow 25 times or more.

In nozzles, compressed air ejects out a thin ring on the outer perimeter. Air travels along the outer wall of the nozzle and entrains surrounding air into the stream. A center hole concentrates this air stream, generating a high-volume, high-velocity blast of air while minimizing compressed-air consumption.

Likewise, air jets throttle a small amount of compressed air through an internal ring nozzle at supersonic velocity. This produces a vacuum pulling large volumes of surrounding "free" air through the jet.

Engineered nozzles and jets eliminate another problem -- noise. Compressed-air noise often exceeds OSHA exposure requirements, and can damage the hearing of nearby personnel. Conventional blowoffs at 80 psig, for instance, often exceed 100 dBA. An equivalent engineered air nozzle operates at 74 dBA.

Air can also be dangerous when the outlet pressure of a hole, hose, or tube exceeds 30 psig. In the event a hand or other body part blocks the opening, air may pass through the skin and into the bloodstream, resulting in serious or fatal injury.

Engineered nozzles eliminate these concerns. In some designs, compressed air exits through holes in recessed grooves that cannot be blocked or dead-ended by hand. Others have a slotted end that vents air to the sides if the nozzle is blocked. In much the same manner, blocking the end of an air jet simply reverses flow out the back end.

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