The Ins and Outs of Vacuum Generators
As an auditor, compressed air vacuum generators are one of my favorite topics. These are devices that utilize compressed air to generate vacuum and have become an easy target in compressed air system studies. There are a number of factors that should be considered before these devices are either put in place or left in place. The reality is that there is an enormous population of vacuum generators being used successfully by industry. Applications range from pick & place to vapor extraction to bulk material handling and the number of installations is growing. Before proceeding with an installation utilizing these devices, there are two general issues to consider: efficiency and appropriateness.
The efficiency issue is very clear. While I cannot say that electric driven vacuum pumps are always a more energy efficient choice, I personally have yet to find a vacuum generator application that I couldn’t make more efficient by going electric. The pick-up in efficiency when switching from compressed air driven vacuum to electric-driven vacuum ranges from two to fifty times. A good rule of thumb average is that electric driven vacuum pumps are four times more energy efficient than compressed air driven vacuum generators. These kinds of numbers tend to make energy managers take serious notice of the potential savings and as the costs for energy continue to rise, any approach to trim energy cost is important.
There are a few general operating characteristics that tend to make efficiency better or worse for venturi-style vacuum pumps. The first is operating vacuum level. The higher the required vacuum, the lower the comparative efficiency. If you only need a couple of inches of mercury, you are in far better shape (using vacuum generators) than if you need 22” of mercury. The second characteristic is high flow at high vacuum. Those applications that require high flow rates at higher vacuum levels are going to be far less efficient than electric driven vacuum pumps that can deliver similar capacity. The efficiency gap closes somewhat in circumstances that require high flow at low vacuum. Also, any application that requires constant vacuum flow is typically better suited for electric vacuum pumps.
A significant advantage for electric vacuum pumps is found in the central compressed air supply area. Compressed air-driven vacuum pumps each use quite a bit of compressed air for proper operation and facilities with large numbers of these devices may have issues maintaining adequate supply pressure. These facilities will typically have several “extra” compressors online to maintain site pressure requirements. Replacement of vacuum generators with electric vacuum pumps will allow for large horsepower compressors to be taken off-line and be replaced with much smaller horsepower vacuum pumps. An average example would be taking a 100 horsepower air compressor off-line and replacing it with a 25 horsepower dedicated vacuum pump. The associated compressed air filters and dryers will also be shut down. Not only are there energy savings in this case, there are also savings for maintaining smaller pieces of supply equipment. As an added benefit, it is far easier to maintain site compressed air pressure with a lower demand requirement.
Appropriate in Contaminated Atmospheres
When considering the appropriateness of using vacuum generators there is not as much clarity. In applications where there are a large number of vacuum generators in a small floor area it is much easier and economical to use a centralized vacuum system. However, in applications where there are just a few vacuum generators in a large floor area, it is not as practical to use electric vacuum pumps especially if compressed air is the only pneumatic supply within a reasonable distance. If adequate electrical service is not locally available, adding sufficient electrical supply may be an expensive endeavor. In processes where mobility is needed, vacuum generators are as equally mobile as electric driven vacuum pumps and can be much smaller in physical size.
In applications that generate corrosive vapors, venturi-style vacuum pumps are a viable choice given their simplistic design and ease with which varied materials of construction can be used. Environments with heavy particulate loads, especially where particles are very small, seem to lend themselves more to these types of systems. This is in part due to the filtration requirements of many electric vacuum pump designs. When a majority of particulates are smaller than one to five microns in diameter filtration systems tend to become relatively large for an electric vacuum pump.
Designs for vacuum generators have been changing in recent years and they are definitely more efficient than they used to be. They have also become less sensitive to compressed air supply pressure. Older designs required high pressure for normal operation. When supply pressure dropped so did performance. New designs also incorporate energy saving controls to shut off the compressed air supply as needed and vacuum sensors to detect when parts are not present on robotic arms. These types of advancements open up more applications for vacuum generators and make them appropriate for other uses.
The end result is that if energy reduction is your aim, finding alternatives to vacuum generators is a prudent course. Otherwise, it seems that vacuum generators are one of those sinful conveniences that many owners know about but are not willing to do without. I believe the reason is that these devices can be so versatile and capitally inexpensive that the energy downside is swept aside or not measured at all. Whether efficiency or appropriateness is more important, each vacuum generator should be individually evaluated to ensure it is the best technology to do the job.
For more information contact Dan Bott, Dan Bott Consulting LLC, tel: (251) 609-1429