There are two main types of compressors, positive displacement and Dynamic.
Positive displacement air compressors raise air pressure by reducing the volume of air within a confined space. The scroll compressors use two inter-meshing scrolls, where one scroll is moving, and the other scroll is stationary (reference photo below). Ambient air will get trapped at the inlet side, and as the orbiting scroll moves, the spiral volume gets smaller and smaller. When volume decreases, the pressure will increase. Rotary Scroll air compressors are less common in the rotary family, as they are limited in capacity.
The dynamic type raises the air pressure by using kinetic energy and velocity with rotating impellers that continuously bring in airflow. In this blog, I will cover the centrifugal type of the dynamic branch.
As mentioned, the centrifugal compressor works by transforming kinetic energy and velocity into pressure. Ambient air passes through guide vanes into the center of a rotating Impeller with radial blades and is then pushed outward by a centrifugal force.
With the increase in pressure, you will get an increase in heat. It is a natural occurrence with air compressors. Heat from the centrifugal compressor is dissipated by heat exchangers before moving onto the next stage. Multiple stages are required to raise the pressure to a sufficient level for typical industrial plant requirements. The most common centrifugal air compressors have two to four stages to generate pressures up to 100 to 150 PSIG. Centrifugal compressors are near the middle of the road regarding efficiency. Their typical operating cost is 16 to 20 kW/100 CFM.
Advantages:
Up to 1500 HP systems are available
Price per horsepower drops as system size increases
Supplies lubricant-free air
Special installation pads are not required for installation
Disadvantages:
Costs more Initially
Requires specialized maintenance
No matter the type of air compressor that you use, they are very costly to operate. To help you use them efficiently and safely, EXAIR offers a range of products that can clean, cool, blow, conserve, and convey. This would include our Super Air Knives, Super Air Nozzles, Safety Air Guns, Cabinet Coolers, and much more. If you want to save energy, increase safety, and cut costs no matter what size air compressor you have, contact an Application Engineer at EXAIR. We will be happy to help you make your use of compressed air as efficient and safe as possible.
One of the advantages to compressed air operated equipment is the ability to precisely “dial in” the performance by regulating the supply pressure. Consider an EXAIR Super Air Knife, for example. The flow & force can be adjusted from a “breeze to a blast” and any point in between, via a point-of-use Pressure Regulator. I know of users who operate them with a supply pressure as low as 5psig (that’s the “breeze”) and as high as 120psig (that’s the “blast”), depending on the requirements of the application.
EXAIR Stainless Steel Super Air Knives are popular in food processing applications (left to right): removing excess moisture prior to flash freezing of fish fillets, preventing clumping while packaging shredded cheese, and (my personal favorite) ensuring a consistent and even glazing of fresh, delicious doughnuts.
For a wide variety of typical industrial blowoff applications, a supply pressure of 80psig is a good place to start. So, it stands to reason that the compressed air header pressure will have to be at least 80psig. If the piping/distribution system is sized properly to carry the total amount of air flow you need to the points of use, though, it doesn’t need to be an awful lot higher than 80psig…and that’s a good thing. Here’s why:
Any fluid encounters friction as it flows through a pipe (or hose or tube) which causes a drop in pressure along every bit of the length of flow. The larger the pipe (or hose or tube) the lower the friction and hence, the lower the pressure drop. Now, that’s only important if you care about how much you’re spending on running your air compressor(s). Consider this:
We’ve got a customer that puts our Model 110042 42″ Aluminum Super Air Knives on machinery they make & sell to their customers. This Air Knife will use 121.8 SCFM when supplied at 80psig with the stock 0.002″ thick shim installed, and does the job quite well, most of the time. Some specific applications, however, need higher flow & force from the Air Knife, so our customer offers, as an option, the Super Air Knife with a 0.004″ thick shim installed. Since this doubles the air gap, it also doubles the air consumption. They’d plumbed the supply line to the Air Knife per the recommended in-feed pipe sizes from the Installation & Maintenance Guide:
Super Air Knife Kits include a Shim Set, Filter Separator, and Pressure Regulator.
Since the drop was less than 10ft long, they used a 3/4″ pipe, which was fine…until they installed the 0.004″ thick shim, which meant the air consumption doubled, to 243.6 SCFM. To get that much flow, at 80psig to the Air Knife, they had to increase their header pressure to 110psig, from the 90psig level at which they had been running. This was well within the operating parameters of their air compressor, but it made the compressor work harder, so it used more energy…and cost more to run. In fact, every 2psi increase in compressor discharge pressure results in a 1% increase in operating horsepower (source: Compressed Air & Gas Institute Compressed Air Handbook, chapter 4, page 8).
So, by increasing the discharge pressure by 20psi, the compressor’s power draw (and hence, operating cost) went up 10%. Now, I never found out what size their customer’s compressor was, but I DID look up prices for SCH40 black iron pipe, and for an 8ft length, the 1″ pipe was only $10-15 more than the 3/4″ pipe they were using. Since 243.6 SCFM is roughly 60HP worth of a typical industrial air compressor load (industry thumb rule says they use about 1HP to make 4 SCFM), we can assume that it’s at least a 75HP compressor. Using the following formula to calculate the operating cost while it’s drawing 80% of full load (while making a few reasonable assumptions):
Cost ($) = bhp x 0.746 x # of operating hours x $/kWh x % time x % full load bhp motor efficiency
bhp = motor full load horsepower (frequently higher than nameplate HP but we’ll use nameplate 75HP to be conservative) 0.746 = conversion from hp to kW # of operating hours (assume a month’s worth, 8 hours/day, 5 days/week, 4 weeks/month=800 hours) $/kWh (assume $0.08/kWh) % time = percentage of run time at this operating level (assume 85% of the time) % full load bhp = brake horsepower as percentage of full load bhp at this operating level (assume 60HP load, 85%) Motor efficiency = motor efficiency at this operating level (assume 95% fully loaded)
75HP x 0.746 x 800 x $0.08 x 0.85 x 0.85 = $2,723.29 .95
An additional 10% power draw changes the % full load bhp to 95%, and the cost for monthly operation is:
75HP x 0.746 x 800 x $0.08 x 0.95 x 0.85 = $3043.68 .95
That’s an extra $320.00 spent on running the compressor (per month) at 110psig discharge pressure, instead of an extra $15.00 spent on a larger pipe (one time cost) to run it at 90psig.
This is just one example of the effect of “artificial demand”, which is, essentially, wasted energy due to running your system at a higher pressure to compensate for undersized lines, leaks, intermittent high loads, etc. In addition to helping you specify the right supply line size for your compressed air operated products, we can assist with leak detection, intermediate storage, regulating supply pressures for differing loads, and replacing inefficient devices with engineered products. If you’d like to talk about any, or all, of that, give me a call.
Russ Bowman, CCASS
Application Engineer Visit us on the Web Follow me on Twitter Like us on Facebook
When you say Downtime in an industrial or manufacturing setting, it may easily carry a negative connotation. This means that the output of production is not happening and input to production has halted as well. If this is not planned, it is absolutely a worst-case scenario. In our personal lives, though, downtime generally doesn’t have a negative meaning behind it. That’s the time to disconnect and recharge to maximize your output after you return to production and that is exactly what I had the luxury to do recently. This is also a message I received from a person I look up to and trust in their experiences. Vacation time can be looked at similar to a preventative and planned downtime of equipment. Without it we just wear down and eventually productivity grinds to a halt. While hanging out at a lake with my daughters this past week, I helped them hone their fishing skills. They each baited their own hooks with worms and chose their spots. We completely slayed some bluegills, and released every single one of them.
The calm of a storm rolling in when you have nothing to do is serene.
Prescribed maintenance, preventative maintenance, vacations all help to build back into the production of whatever good or service the company provides. The entire production of a facility all starts with the utilities, energy, water, compressed air, steam, other compressed gases, and the personnel. If your power input isn’t maintained, monitoring connections and disconnects, you can find yourself with a lack of service, resulting in dangerous situations. City water is often required for processes or for the facility to function properly, even an office building needs it for plumbing, fire suppression, and drinking. Steam, compressed gases and compressed air may all be required by the processes.
Servicing the compressed air where it starts is one of the most critical steps in operating a compressed air system. Making sure that your compressor has the minimum downtime, all starts with the preventative and prescriptive maintenance. One of the first tasks should always be changing and monitoring the intake air filter. Like Russ Bowman said a while back in his blog, take a deep breath, if you sneeze or smell something that is from the intake air your nose just took in from the surrounding area. That’s even after your nose hair has already partially filtered air intake. Your compressor is no different. If you let it suck up debris, dust, and pollen, then it is eventually going to have a failure. Instead of sneezing, it may burn up a vane, valve, scroll or screw. That is going to be a considerably higher cost and longer downtime than just performing the manufacturer’s listed items to maintain optimal performance.
The compressor shown above according to the caretaker receives a regular change on the airfilter every month. This is just before the cleaning and changeover. Not only do they change the filter, they make sure to clean the entire housing inside and out. That’s one of the ways this compressor has lasted with minimal downtime over the past 20 years.
If you want to learn more about other key maintenance items in your compressed air system, please contact an Application Engineer today.
1 – Steve Martin & Edie Brickell – “When You Get To Asheville”
Over the past week, my amazing wife and I traveled to Asheville, NC for a long weekend away. This is our second year going down, and I can most certainly say that we will be going back. Our days consisted of going to a small mom-and-pop type diner for breakfast, loading the cooler with water, and then picking a hike to hit up. This time we hiked mostly in the Pisgah National Forest and while we did not hit the same elevation as last year, we still managed to double the first hike of the week on the second day and felt great once we reached the end. I also chose to make the hikes hard on myself by carrying my trusted GO-RUCK GR1 to carry our water, first aid kit, and a 30 lb. steel plate, because you should always choose the harder thing.
While we weren’t at elevations like Pikes Peak in Colorado, we still felt the difference in the air between being in Cincinnati and being in the mountains. Maybe it was just the fact it was cleaner. When we crested a hill on the trail and stopped to take a quick break, we looked around and realized that after all the switchbacks we had just gone through, we looked over the valley we had just climbed out of and were at the tree tops of the valley and still nowhere near the top of the mountain. This got me to thinking about how I was working harder because I had a steel plate, walking too many lunches where I just sit for 30 minutes instead of walking and that is immediately connected to the ACFM calculations for an air compressor and just how a compressor will have to work harder to produce the same volume of air when elevated because the air is thinner. This is going to change the air density, which results in a lower atmospheric pressure due to higher altitude.
Altitude is just one of the factors that matters in the calculation to determine a compressor’s output at different locations. The other factors include relative humidity, which was way better in the mountains than here in Cincinnati, and the actual temperature, again better in Asheville than Cincinnati.
In case you were wondering, the post-Ruck/Hike hydration is always better after events, it also always helps to have a good partner in crime to enjoy all the experiences with you. Thankful for the ability to connect all these hobbies and my knowledge of compressed air on top of sharing it with others. If you want to discuss how to calculate some ACFM or SCFM consumption and outputs of your compressor or application, or if you want to talk about rucking, hiking, or any of your favorite trails, give me a call, chat, or tweet.
