It's a rare plant that doesn’t use compressed air in some capacity. Whether it’s facilitating a process or just powering tools, compressed air is both a useful and ubiquitous utility. Even if you’re already using compressed air, you may need more if you’re increasing automation or planning an expansion.
That typically means adding a new system.
Because these systems are so vital to key processes, the plant’s operation and your organization’s productivity, you need to get them right. Whether you have designed and scoped a compressed air system before or not, the goal of this post (through some helpful background, big-picture wisdom and veteran suggestions) is to help you install a successful system.
The basics of compressed air in facilities
You may already have experience with compressed air systems, but for those new to the process or wanting a refresher, let’s start with the basics. Industrial compressed air systems provide pressurized air throughout the facility to operate automated valves, diverters, slide gates, dust collectors, instruments, and any number of other devices. Air compressor systems are also required for industrial aerobic fermentation processes.
A compressed air system operates on pressure. Compressors are a motor-driven mechanical device used to pressurize the air. The motor can be variable speed or shut on and off. The compressed air then passes through a dryer to reduce humidity and is distributed through pipes out to the plant.
In most cases these systems provide about 80-90 pounds of compressed air across the facility. They’re not overly complicated, but they can get that way with facility expansions or when you’re operating more than one.
How to choose the proper compressor for your process
There are a variety of compressed air system to choose from – all have their advantages and disadvantages. Here are the major types:
Centrifugal compressors
These air compressors are ideal for plants with large, steady air demands. They can easily handle high capacities and are actually better and more efficient in that range. Their simple, oil-free design also requires less maintenance, but their controls and monitoring systems are a little more intricate. Because they have limited speed control, they offer limited air-flow flexibility. However, if you’re running and remaining at a high capacity (for a fermentation process, for example) they’re a great choice.
For facilities with multiple compressors, a centrifugal compressor is a good choice for the lead compressor. Centrifugal compressors are also an excellent choice for aerobic fermentation processes that require a constant volume of oil-free, sterile air.
Rotary screw compressors
Rotary screw compressors are the compressors of choice for medium-to-large-capacity plants with unsteady air requirements. The compressor itself is fairly compact and provides smooth, pulse-free output. It can also be set up to run off/on or with a variable frequency drive (VFD) to provide flexible modulation depending on your requirements.
Standard rotary screw compressors are not oil-free. Oil is injected into the screw to lubricate the screws and maintain an air seal. That oil mist then needs to be caught before it enters the plant, which presents an additional challenge. The air ends on these compressors are also not field serviceable, which complicates maintenance. As a variable-output option with good capacity, though, it’s a solid choice.
Oil-free models are available at a significantly higher cost.
Scroll compressors
This type of compressor is similar in design to rotary screw-type compressors, but they’re oil-free. Because they’re not good for high capacities, they’re best for small plants with unsteady air requirements, or when compressed air is introduced directly into the process. They’re extremely quiet and don’t require a lot of maintenance, but their ability to keep oil completely out of the process (when you’re working with kosher food products, for example) makes them more expensive.
Piston-type air compressors
These are very common, but not typically used for plant air. They can provide constant pressure over a wide range, but they’re better for light duty (tools, maintenance shop air, etc.). Positive Displacement and Reciprocating compressors are both piston-type compressors. Because of their limitations, high-wear and high-maintenance, though, they probably won’t be on your list if you need consistent, reliable plant-wide air.
Commercial vs. industrial air systems
When you’re investigating compressed air systems, you’ll probably see and hear the words commercial and industrial used in describing them. It’s an important distinction to understand so you get the appropriate system for your particular process.
As a rule, commercial compressed air systems aren’t built for dirty environments like manufacturing facilities. They’re not capable of using tower or process water for oil cooling and they won’t have some of the monitoring that’s standard on most industrial systems.
Most plant-wide air compressors fall into the industrial category and are designed to handle robust manufacturing processes. Just make sure you’re clear about the type of compressor you’re looking for when you talk to vendors.
Compressed air system design basics
Plant air is a relatively expensive utility, so proper system design is critical to ensuring efficiency. Here are some suggestions for getting those key design items right from the start:
Size it right
While compressed air systems don’t take up a lot of room, they will need some dedicated space inside the plant. The size of your system will be driven by the size of the plant and the breadth of the need. The best way to size the system right is to determine the sum need for air from all users (including valves, equipment and miscellaneous items). That starts with a “valve count” and a clear understanding of your airflow requirements. It’s also important to make a determination about what may be added on to the system in the future (planned expansions, increases in scope, etc.). You’re better off scoping the system to handle the future capacity up front.
Another critical note on sizing: When looking at manufacturer spec sheets, be sure you know whether their compressors are sized in scfm or acfm. Different vendors use different measures. You’ll want to make sure you’re comparing apples to apples.
Choose your piping to fit your plant environment
There are lots of choices for piping and fittings, from plastic coated carbon steel, galvanized and stainless steel to copper tubing. There are also a number of end connections including victaulic, press-fit, threaded or socket-weld. The key is to choose the material that best fits your process, your plant environment and your budget. The cost differences can be significant, so that will necessarily be a consideration. You just can’t let it outweigh the other two factors.
(Insert air piping design diagram from PowerPoint)
Understand and invest in proper controls
As with piping, it will be easy to try to save money on controls. Don’t take the bait. It’s important to study and understand the controls for the system you choose then invest what it takes to get the most suitable ones to ensure performance and safety.
Be sure to specify the accessories. That includes:
- Filters (air, oil, water – everything)
- Cooling systems
- Cooling water pressure switches
- Oil coolers
- Lube oil circulation pumps and pressure switches
- Separators (air-oil and oil-water for non-oil free compressors)
- Bearing temperature monitors
- Dryers
- Condensate drains
- After-coolers
- End connections
- NEMA-rated dust enclosures
The list is long and their functions are crucial. Some of them will be “included” with the system. Some will not. Whatever the case, make sure they’re accounted for when designing and speccing your system.
5 common compressed air system design mistakes
Ignoring any of the design basics above would be a mistake when it comes to putting your compressed air system together. There are a number of others that can happen during the design phase and after the fact that can cause trouble, too. Here are a few of the bigger ones:
- Going cheap on the compressor. You get what you pay for, so be sure to buy the best compressor you can afford. You may as well invest in a quality compressor at the start, because if you don’t you’ll likely wind up making up the difference in frustration and maintenance in the end.
- Making the header piping too small. We’ve talked about the importance of piping in terms of materials, but size matters, too. From air velocity and pressure to friction loss and future needs, there are lots of factors to consider when determining the size of your piping. Be sure your piping matches your capacity.
- Not keeping the air clean. Plant air typically needs to be filtered, oil-free (for most applications) and dry. Not meeting any of those demands can have a major impact on your system and your process. Be sure your system is delivering on all three at all times.
- Neglecting to include surge tanks. Having enough surge capacity in your system is essential, yet too many systems are installed without surge tanks. It can be a dangerous mistake. Don’t make it.
- Ignoring maintenance. There are enough parts, controls, devices and switches in a compressed air system to ensure things will go wrong. But those problems don’t have to be inevitable. Regular and preventative maintenance like oil changes are the keys to keeping your system (and your process) up and running. Plan for that maintenance and don’t neglect it. In fact, find out where your nearest service rep is before you make your purchase decision. Having them nearby may be your safest hedge against significant down time.
Where to start for your system
Designing, installing and operating a compressed air system shouldn’t give you too terrible a time. It just takes solid planning, smart questions and a thorough process.
If this is your first time designing a system like this, you might consider starting off by talking to an engineer who knows and understands them. There are just enough small details to make it tricky, and the potential impact of mistakes can be significant. Some seasoned advice might make your first time through a little easier. You’ll probably improve your process and save some money, too, and that's just one of the benefits of working with an innovative, practical and proven engineer..