Most industrial systems like compressed air have essentially random demand if you look at the long-term life cycle of the system. Hundreds, even thousands of independent small and large subsystems require constant or varying flow. These demands are typically not timed or synchronized with each other, so they aggregate to a fairly random flow profile, within a range. That range changes significantly when production processes change. Certainly a 2-week audit might show some patterns that appear predictable for demand A (“production”) and demand B (“non-production”) or day type, but they change over time as the plant adapts to new production systems and removes old ones. If demand was that profile forever, a lesser experienced auditor might be tempted to size one set of compressors that work perfectly for that profile but not for alternates.
Air compressors need to be matched to load effectively and efficiently. If the air compressors’ range of variation can’t be matched to the system variation, instability and/or inefficiency can result. This article discusses the problem when it isn’t matched, which is called “control gap” and what to do to avoid it.
Have you ever wondered how to stay “in control” of an engineering organization with a fixed staff and a varying workload, where the engineers all have a mind of their own? “Herding cats” is what they call it. Of course, that’s normal, right? Well, controlling multiple centrifugal air compressors is pretty close to that model, which can lead to a condition known as “control gap.” This article discusses the reasons for control gap with centrifugal air compressors and solutions to help avoid it.
Maintenance is the customer of controls and energy engineering is the customer of monitoring. And I discussed potential problems that can occur when combining monitoring and control in the same system. In this article, I will get more specific about building practical systems that address both controls and monitoring.
Many OEMs of air compressors, dryers, sensors and master controls are integrating monitoring features and capabilities into their components. It would seem a no-brainer to keep it simple and use those sensors and systems for both control and monitoring. What could be simpler?
The steel mill in this article is a rolling “minimill,” a facility that melts scrap recycled steel and produces rebar for the construction industry. It fits in SIC code 3310. There are many plants like this all over the world, providing an environmentally sound service and product for their local community. They recycle waste steel from local sources and support local infrastructure projects with rebar, using electricity generated locally.
Many food processing plants are already using oil-free compressed air at a low-pressure dewpoint. This has required the use of two-stage oil-free screw air compressors, centrifugal air compressors and other technologies, as well as regenerative dryers of one type or another. The most common oil-free air compressor in industry is the two-stage “dry screw” machine and the most common regenerative dryer type is the heatless type. These are combined in many food processing, pharmaceutical, and high-tech plants.
Compressed air dryers need to get the ball handed to them on the 25 yard-line by a compressor providing low enough temperature, and high enough pressure for the dryer to take it to the design dew point. If not, the dryer is not able to work properly. Once the dryer gets the right moisture level, it needs to operate properly. Heat exchangers, drains, switching valves, etc., all have to work with the proper control sequence to provide reliable dew point to the plant.
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.
Load-sharing is an important part of a multiple centrifugal-compressor master control system. It minimizes blow-off based on the available turn-down. In addition, remote start-stop saves more energy if load floats between different ranges. Finally, adding a screw compressor and implementing a hybrid control system might save the most energy and provide the best back-up. In any case, a well-instrumented system allows engineers and operators to assess, optimize and tune the system.
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.
When a system has the right combination of VFD and base-load air compressors, how do you coordinate their control? What tells the air compressors to run and load, to have just enough (or no) base-load air compressors and a VFD running, all the time air is needed? Appropriate master controls are needed. These controls are often called “sequencers” or “master control systems”.
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.
As an end user, have you ever heard the message to put in the biggest VFD air compressor, and the system will always be reliable and efficient. Why do an audit? Just add up the compressors on site and put one VFD for that size or larger. Why have the complexity of multiple compressors, storage, sequencing, etc? Even better, put in two of them, one for the whole system, and one for back-up. If you could wave a wand, wouldn’t that be what every system should look like? Perfect peace and efficiency, with 100% confidence of reliability.
This article will talk about testing. I will assume a “typical” system, a screw air compressor mix with regenerative dryers. Testing has to be done at several stages and locations, due to the cobbled-together nature of a compressed air system.
A large pharmaceutical company needed huge flow rates of 30 psig air to aerate multiple fermentation processes which create food-grade materials. Flow could vary from about 12,000 scfm to 35,000 scfm. There were a variety of batch processes, mostly running independently. An hour-by-hour schedule for anticipated air flow is developed every afternoon for the next day. Based on that schedule, the boiler operators run the air compressors that can handle the load range for the whole day. In reality, the peak flow can be higher than anticipated.
Facility managers, how would you like the peace of mind from knowing the system you had installed or modified is thoroughly tested - to the same degree as a new production line? How would you like to be confident that the money you spent is still paying back benefits, year after year?
“Retro-Commissioning” (ReCX) of compressed air systems has become a trendy activity with many utility demand-side-management programs emerging in the last 5-10 years. This is intended to be the process of “tuning up” a compressed air system, getting low cost savings from mostly adjustments and repairs. The term was borrowed from the building/HVAC industry, where it means to get a system operating as it was originally “commissioned”.
As a reader of this journal, you are well aware that large compressed air systems often have significant wasted air — often from leaks — that represent tens of thousands of dollars of waste per year. However, it is our experience that the so-called “low-cost” measures identified often go un-repaired, while other more costly capital projects get funded. Why? With an ROI of a half year or less, they seem like IQ tests to many compressed air auditors.
There is a partly true idea floating around some plant maintenance circles that “compressed air is free.” Readers of this journal know that isn’t true. But, what if non-compressed air could be seen as “free?” Is there something we can get for free from nature to reduce the cost of our compressed air? What if lower temperature intake air was nature’s gift? What if all we need is a bit of tin to duct air from a different source?
A common adage that has been quoted many times in this journal is: “If you don’t measure it, you can’t manage it.” This is partly true. It assumes that managers are willing and able to manage the costs and reliability of their compressed air system. Without data, however, they can’t do an effective job. But because managers are at times already overwhelmed with data, more data doesn’t automatically make them a better manager. A better way of saying it is: “Appropriate measurement can make you a better manager.”
As readers of this publication know, there are many ways to save energy in industrial compressed air systems. One common supply side technology is the variable frequency drive (VFD) of the compressor. It is well-documented that positive-displacement compressors with VFDs provide cost-effective savings in comparison to inlet modulating, load-unload, and variable displacement control.
Recently, this major pulp & paper mill made compressed air optimization a mill-wide priority. At the request of the utility company providing energy to the mill, Compression Energy Services performed a comprehensive energy analysis that outlined the following four energy efficiency measures (EEM’s) for the mill to consider.
The goals of this article are to show why sequencers often have problems, and to demonstrate how avoid problems by proper system integration and controls design.
