Compressed air has a major impact on production, with downtime a chief concern for industrial plants. The move towards IoT connectivity offers opportunities to better monitor and manage mission critical equipment, such as compressed air systems. Incorporating predictive technologies into day-to-day operations has the potential to improve reliability, increase uptime, and reduce maintenance costs.
Join compressed air experts Neil Mehltretter (Engineering Manager for Kaeser Compressors, Inc.) and Timothy Hitzges (Product Engineer for Kaeser Compressors, Inc) as they discuss ways to proactively improve compressed air system performance such as:
Broadly speaking, industrial plants in the United States have been making great strides towards improving processes, reducing costs, and finding new ways to increase profit margins. The desire to reduce waste and better understand daily plant operations has spawned a number of strategies to accomplish these goals. One such strategy is Root Cause Analysis.
Root Cause Analysis (RCA) is a systematic approach to problem solving, commonly applied when there is a significant failure or issue with far-reaching impact. Its goal is to identify the factors of the negative event and determine what needs to change to prevent similar future occurrences. The spirit of RCA is investigative and collaborative in nature, whereby a team works together to discuss and carefully document the findings.
For compressed air, the common issue of wasted energy is a problem that warrants such analysis. According to a survey conducted by the US Department of Energy, approximately 10% of the electricity consumed at a typical industrial facility is for generating compressed air. In some facilities, this percentage can reach 30% or more. Much of this power is wasted in generation due to poor choices in compressor size and lack of controls. Additionally, it’s estimated that half of all compressed air generated is wasted. Despite the widespread waste of compressed air and the potential for optimizing a compressed air system, it is quite often neglected when plant efficiency initiatives are discussed.
In one example, a furniture manufacturer had grown its business and over the years, its compressed air system had grown into a 2400 hp system. Now, with a power bill for compressors over $1 million per year, management felt it was time to take a look at ways to improve the system. An in-depth assessment produced some eye-opening results. The total productive demand on the system could be satisfied with only half of the compressors currently being used. The manufacturer was losing about $500,000 per year on power, about $250,000 per year on maintenance, and had spent about $600,000 in capital for equipment that was not needed.
Unfortunately, this situation is all too common. The tendency in industrial plants is to throw equipment (and power) at compressed air system problems instead of trying to find the root cause of issues like pressure fluctuation. As it turned out for this furniture manufacturer, $500,000 of power costs could be eliminated for less money than they were spending every year on maintenance for equipment they did not need. Very few manufacturers would accept a 50% scrap rate for production inputs, yet it is quite common in compressed air.
This blog entry is an excerpt from our white paper, “Applying Root Cause Analysis to Compressed Air”. To learn more, download the full version of the white paper here.
Rotary lobe and rotary screw blowers utilize positive displacement. This means they pressurize air by trapping a fixed amount and forcing (or displacing) it into a discharge pipe. Industrial applications include fluid aeration (wastewater treatment, bioreactors, and flotation), process air, pneumatic conveying, as well as fluidization.
Although all of these applications generally work within a low pressure range (up to 14.5 psi), they have very different operating cycles and needs. Fluid aeration applications generally have variable flow rates, but at constant pressure.
Others, like pneumatic conveying, require a near constant flow rate with high pressure fluctuations. Sometimes the blowers are required to idle, running without back pressure from the process side. This happens when there are no bulk goods in the line to move.
Naturally it’s important to decide which blower technology is best suited to the application. Technical requirements must be taken into account, such as a broad flow rate curve during pressure fluctuations. Ultimately, the choice may hinge on the amount of energy savings achievable from different alternatives. In determining energy savings, the “power bill” is determined solely by output (kW) x time (h) x rate ($/kWh).
The big variable here is time, which significantly impacts energy costs. Unless the cost per kWh is very high, the more efficient blower may need to run a lot more hours to justify the higher investment.
External (isochoric) versus internal (isentropic) compression
To determine which of these blowers would be more cost effective for a given application, it is important to first understand in greater detail how each functions.
Rotary lobe blowers:
Image 1 shows a cross-section of the rotors and cylinders, running parallel in the longitudinal direction and illustrates how the volume enclosed between the housing and the rotor blade remains constant. In thermodynamics, this is referred to as isochoric compression. The pressure does not build until the air molecules are pushed beyond the blower into the connected process line. In this way, with rotary lobe blowers, pressurization occurs externally. Moreover, if the process line is free of resistance (e.g. no bulk goods in a pneumatic conveying line), there is virtually no back pressure. In this regard, the rotary lobe blower can also be seen as adaptive: it only produces the amount of pressure needed.
With screw blowers (image 2), the tried-and-true technology of the single-stage screw compressor has been optimized for low pressures.
The rotor geometry is based on the screw. The inlet air is initially captured within the cavity between the two rotors where its volume is gradually decreased along the length of the rotors and then pushed out through the discharge port. The geometry of the rotors and housing (i.e. contour of the discharge port) determine how much air is proportionally compressed within the screw blower and how much pressure is built up internally. This internal pressurization can also be called isentropic compression.
Pushing an already compressed volume of gas against the system back pressure requires less energy than pushing the un-reduced volume created in isochoric compression (rotary lobe blower). The result is significantly lower electrical demand, and in many cases the screw blower delivers great ROI over a lobe blower. However, the better specific performance of the screw blower may not pay off if the running hours and/or cost per kWh don’t out-weigh the additional cost of the screw technology. You must do the math.
This blog post is adapted from our white paper, “The Proper Application of Rotary Lobe and Rotary Screw Blower Technologies”. Download the complete whitepaper here.
#1 The Art of Dryer Sizing: This post has been rising in popularity since it was published in 2015 and is the most viewed post from 2017. Read this post to understand how temperature and pressure impact water content and to learn how to make sure dryers are properly sized.
Do you have a topic you’d like us to cover in 2018? Let us know in the comments.
Managing Editor of Maintenance Technology Jane Alexander recently interviewed compressed air expert Ron Marshall for the July 2017 issue of the magazine. The article, “Untangling Compressed Air Misconceptions” provides fair warning: what you may be doing in your plant, albeit with the best of intentions, could actually be doing more harm than good. Continue reading “Compressed Air Ideas Have Consequences”→