Industrial Utility Efficiency    

Air Compressors

By far the most important development in the world of screw type air compressors has been the introduction of variable speed control using electronic variable frequency drives (VFD’s). Systems that run with at least one air compressor at part load can almost always operate more efficiently if a well-controlled VFD is added to the system. But what if a system has two or more VFD units? This article discusses the challenges in controlling multiple VFD air compressors with some suggested solutions.
This article is going to identify two air compressor control situations that will preclude translating air use reduction in the production area into lower input energy into the air compressor.
The plant produces both molded and blow molded plastic parts on a 5 day per week, three shift schedule. Production and maintenance sometimes occurs on weekends, occasionally requiring the air compressors to run on a 24 x 7 basis so the practice was to leave the compressed air system always pressurized. The system consisted of three modulating lubricated screw compressors one sized at 150 hp and the others 125 hp (3 units), each controlled with their local compressor controllers.
A spectacular vision is gradually becoming reality in Cadarache in the south of France. Modeled on the sun, the ITER (International Thermonuclear Experimental Reactor) fusion system uses nuclear fusion to generate energy in order to secure humanity’s supply of electricity. One of the biggest challenges is the high temperature inside the reactor. Technology by Sauer Compressors is a key factor in cooling the reactor. The manufacturer has supplied the world’s largest system for helium recovery.
Compressed Air Best Practices interviewed Timo Pulkki (CEO), Hannu Heinonen (President, Tamturbo Inc.) and Mike Batchelor (Director of Sales Americas) from Tamturbo. Since the 1960’s, the Tampere region in Finland has been a birthplace of several air compressor innovations – many of which involved Kimmo Laine, a co-founder of Tamturbo. Mr. Laine was a leader in R&D in the air compressor business for many years since the 1960s. This included bringing a high-speed turbo air compressor to market later at Tamturbo. Working together in the 1980’s in a division of Tamrock, called Tamrotor-where Hannu Heinonen also worked, Mr. Laine met a gentleman named Jaakko Säiläkivi.
Today’s industrial manufacturing environment is extremely competitive, requiring companies to constantly search for cost saving opportunities and better efficiencies. In many cases, manufacturers find that centrifugal air compressors are a successful method for reducing the overall plant costs involved in supplying compressed air.
The air we breathe and the air compressors ingest is a mixture of gases, aerosols, biological material, and particulates. It’s a real mess! Particulate, for instance, is very harmful to humans, because lungs are complex oxygen separators, not filters. They tend to load up with particulate, this is harmful over time. There isn’t a sufficient “pre-filter” to prevent all harmful particulate from entering the lungs. However, humans prefer water in the air, gas, aerosol, and to a certain level, liquid form. A de-humidifier would not typically be a healthy addition to our built environments.
There are many choices of compressor technology and types of controls that can be used for variable demands. Some examples are rotary screw compressors with inlet valve control: variable speed drives: load/unload control; or centrifugal compressors with variable inlet guide vanes. However, in many cases, the efficiency of the overall compression process can be reduced significantly during lower flow demands, leading to more power per unit of air flow being delivered. It is very important to evaluate different available options and see how a plant can run most efficiently.
Larger air compressors, typically over 500 hp, in refineries, pulp and paper plants, chemical and other processing plants often have high-speed, multi-stage air compressors called “centrifugal” air compressors. As seen from a total system perspective, they are not much different than screw air compressors. They compress air to plant pressure from atmospheric conditions, and deliver it to the dryer. These types of air compressors have no internal wearing parts, besides bearings and seals, and are very reliable and efficient, at their best efficiency point. 
Knowing when to overhaul a unit is important, and there are certain signs indicating a unit needs attention. Performing routine fluid checks, taking oil samples and routinely checking for bearing vibration can unveil indicators suggesting an upcoming failure. Oil contamination with metal fragments usually indicates parts are wearing. It’s also important to take notice of airend temperature increases. If internal air compressor temperatures go up, it’s a good indicator the cooler may be failing.
It is becoming a “best practice” to install a variable frequency drive (VFD) air compressor whenever one is replacing an old air compressor.  As a result, real systems have fixed-speed and VFD air compressors, mixed.  I have observed several VFD compressor sizing methods.  In my last article, I referred to a common method: size one VFD compressor for the whole system.  This can work.  However, if it doesn’t meet a higher peak demand, one or more of the old compressors will be started, and a mixed system results.   Another method is to replace a compressor with the same size, but with a VFD.  If the compressor that was replaced is large, a big VFD is installed.  If small, a small one.