During the purchasing decision process, it is common for prospects to compare compressors with some sort of utility criteria. In other words, how much air will they get for their money. Below, we address some common approaches we encounter:
Compressor cost per horsepower
This is a quick comparison that can be done using basic product literature, but it is a very superficial metric for comparing compressors. Since the requirements of air tools and equipment are not rated in compressor horsepower, and since the flows among compressors of the same nominal hp can vary by 20% or more, this doesn’t tell you how much air (cfm) you are getting or whether a compressor will meet your air demands (assuming you know them). Our experience with hundreds of thousands of systems has shown that without knowing your actual system needs, you are more likely to oversize your system, which leads to higher power and maintenance costs and reduced longevity (see our blog post on oversizing).
Compressor cost per cfm
This can also be done with literature and is a step forward for basic comparison, and if you know your actual flow demands it will help avoid sizing mistakes. Like the first method, its shortfall is that it only considers initial cost. It does not reflect energy efficiency, so it is not a predictor of the largest component of compressed air life cycle costs: electricity usage.
Compressor cost and specific power (kW/100 cfm)
Specific power is the true measure of a compressor’s efficiency, so combining this with unit cost is a better indicator of compressor value. Keep in mind, however, that specific power is based on a fixed set of conditions and assumes the compressor is running at maximum capacity, which they rarely do. Nonetheless, when choosing machines it is very useful to compare the specific power (AKA “specific performance) of the compressors. Most major manufacturers provide this information in CAGI data sheets on their websites or by request (see our blog post on how to read them).
System specific power
Because most compressors run partly loaded for a variety of reasons (demand fluctuation, oversizing, changes in production), the best metric for energy efficiency (and therefore compressor selection) is system specific power. This metric reflects the ability of the total system to maintain efficiency throughout the full range of production demand and is a far better metric for operational efficiency. This is not easy to assess for new plants (unless there is a similar sister plant in operation), but it is easily done for upgrades on existing systems with tools like ADA/KESS that data log parameters including compressor run time, system pressure, power consumption and flow, and then select the best mix of machines to meet the need. We strongly recommend assessing system performance anytime you are adding or replacing compressors — even if you plan to simply replace a compressor with another of same size. This is an ideal time to baseline the system and identify inefficiencies in pressure drop, storage, sizing, and controls.
Because compressed air demand changes as plants increase or reduce production levels or upgrade pneumatic equipment, it can be a challenge to maintain optimal system performance. The best approach in multi-compressor systems is a combination of proper sizing of compressors and the use of adaptive smart controls. These learn system dynamics and switch compressors on/off in the most efficient manner while maintaining desired system pressure, balancing load hours and minimizing idle time.
By: Michael Camber, Jeff Owen (Sales Manager for Kaeser USA’s Atlanta branch), and Frank Remsik (System Specialist)
We are in the business of selling rotary screw compressors, and we sell quite a few to users that have outgrown their two-stage reciprocating/piston compressors in the 5 to 20 hp range. Sometimes demand or duty cycle has increased beyond the practical range of their piston compressors. Or they need higher air quality. Sometimes noise and vibration are the issue. But there are many cases where a reciprocating compressor is still a very good, economical fit for the shop, but service issues lead them to think they need a different solution.
Heat is often the enemy
Most small shop recips are not designed to handle 100% duty cycle. In other words, they cannot run flat out for long lengths of time without sustaining heat-related wear or damage. Generally, these small two-stage units operate at relatively high temperatures (275-350°F), so they need to stop and cool down periodically. (This is why they are typically set at 145-175 psig, even though most tools only need 90 psig.) Duty cycles vary — we’ve seen 50% to 80% –depending on the design and quality of construction.
There are a number of heat-related problems, but first let’s talk about what can cause them.
First, the compressor’s environment plays a critical role in its reliability. If the room is too hot, or doesn’t get enough ventilation it will run hotter than designed. To reduce noise, many recips are installed in out of the way locations (e.g. utility closets). Ventilation is often poor, creating more heat and higher discharge temperature.
Second, excessive run time can result in heat-related problems. There are several reasons for excessive run time, and a system can suffer from any or all of them:
The compressor is undersized for the productive demand.
More users or larger tools have been added to the demand.
Leaks have developed (leaks in fittings, hoses and tools are just another type of air user—even if completely unproductive).
Lack of storage in the tank due to water. The air leaving the compressor pump is hot and contains moisture in vapor state. In the tank, the air cools and moisture condenses into liquid. Condensate can build up quickly, especially in warmer, humid climates (gallons per day). If the tank is not routinely drained, it will fill with water leaving less room for air. Less air storage => more run time => more heat =>more problems.
Potential heat-related issues
Below are some of the mechanical issues caused by overheating. Generally, these can be repaired economically.
Piston rings no longer seal properly against cylinder walls, thus losing compression. When this happens the pump may have to run longer (and even hotter) to meet demand. Lubricating oil breaks down faster and gets past the rings more easily, requiring more make-up oil to prevent further mechanical issues (and degrading air quality).
Failed intake / exhaust valves
Oil carry-over builds up and may prevent valves from properly seating, creating blow-by through valves. This can cause the intercooler safety relief to release, and also cause the voltage supply breaker / fuses to trip due to stalling out the pump. Over time this can burn out the drive motor.
failed check valves
Recips tanks have check valves to make sure they don’t start under load. Over time the elevated temperatures along with oil carry-over can distort the nylon piston in the valve, so the piston can’t seal properly. When this happens, back-pressure from the tank will create head pressure on the pump. When the compressor tries to start, the extra amps drawn by the motor can trip breakers and burn the motors out. If you have issues with belts breaking prematurely, it could be from a failed check valve on the tank. If the compressor pump is trying to start against head pressure, the crankshaft may not move even though the motor is. Motor goes, pump won’t -> belts slip/wear/break.
Motor burn out
Motors generate heat in normal operation but will cool themselves adequately unless they are in too hot an environment or are energized and try to turn something that doesn’t want to be turned, such as a pump with head pressure (see above) or one that is not properly lubricated. Over time, the insulation on motor windings will degrade and the motor will need to be rewound or replaced.
By design, reciprocating compressors vibrate. Vibration affect many things. Vibration can loosen piping connections, as well as any threaded nut or bolt. It can loosen up electrical connections and create electrical drop-out, sparking, tripping out breakers and blowing fuses. Pressure switches can also fail due to excessive vibration. Vibration can create cracks in welds and joints at the tank feet, platform and saddles. Excessive vibration also increases noise levels, loosens safety guards, and can even break up a concrete floor.
Replace missing or cracked vibration pads. While you are at it, if the discharge piping from the tank is hard pipe, swap it out for flexible steel braided hose. Same goes for electrical supply from the wall disconnect to the starters. Make sure the belt guard is secure. A missing or loose belt guard is not just a safety issue but is an OSHA violation.
Noise can often be abated with well-placed, insulated stud walls. The key is not to restrict airflow. If you are contemplating constructing a separate room to isolate a hot, noisy compressor, it is worth doing the math to see if a rotary compressor makes sense. The quieter rotary unit may cost less that permitted construction and almost certainly take less time to install. This assumes you have a good place with good ventilation and access, and that the unit will be run enough to gain some of the energy advantage.
A note about tanks
As explained above, storage is vital to the longevity of the compressor. It’s also important for meeting demand and system performance. Tanks don’t need much maintenance but you want to keep them dry. Not just for the storage, but to minimize rust. Over time, rust will build up in the tank and plug up the drain port. This makes the case for a quality automatic condensate drain that won’t get gunked up by the oil-water-rust mixture.
An ounce of prevention
The piston style compressor is simple and requires relatively little service, but it cannot be ignored. Here are some tips, whether you are installing new or want to keep ol’ faithful going:
Ventilate the compressor room to maintain positive air flow. If the compressor is in a confined space, install louvers and thermostatically controlled fans as needed.
Routinely drain the tank. Better yet, install an automatic drain (with test function)
Check oil levels routinely. Add make up oil as needed and perform oil changes on schedule with an oil recommended by the manufacturer. Avoid automotive motor oils, which contain a lot of detergents that leave deposits.
Replace the air inlet filter routinely. You may be able to vacuum it out to extend the replacement interval. Plugged air filters restrict the performance of the compressor and increase operating temperature.
Find and fix leaks on the compressor and in the system all the way to the fittings, hoses and tools at point of use. Listen for leaks and hissing sounds while the unit is off. On the compressor, check the intercooler and its SRV, pressure switch, and the liquid drain on the tank (which some people leave cracked open to avoid liquid build up).
The belt life on most recips can be very long if you take care of them, but excess heat will reduce belt life. Look for wear and cracks that might cause them to come apart. Damaged belts can create more vibration.
Check duty cycle. If the unit is running more than it used to, you could be using more air or there could be water build-up in the tank and you have less storage. Another possibility is ring wear. A pump up test will tell you if the machine is still making air to specification and point you toward the cause.
For more information on piston vs rotary screw compressors check out our infographic or read our blog post on the subject.
In a recent article titled “Calculating the value of avoided unplanned downtime” from Plant Services magazine, Burt Hurlock poses the question “How much have the avoidable catastrophic events of the 21st century cost us, and how much would we have willingly spent to prevent them?”
He goes on to talk about how downtime events that were avoided still have value and sophisticated plant operators know exactly how much value. Understanding that downtime, of any length, has rippling effects on the entire organization is valuable in itself. One small event can cause unproductive labor hours, costly incident investigation, wasted raw materials, product spoilage, etc. and the bigger the event, the higher the risk and higher the cost is.
Knowing that your compressed air system is not only critical to your production but also affects the quality of your products, would you invest to avoid downtime from your compressed air system? We often encounter customers who tell us “If my compressor goes down, the facility stops.” But in the next breath tell us a back-up compressor isn’t worth the investment. Customers are frequently more concerned with purchasing a larger than necessary compressor that accommodates future growth than planning for the present with a redundant solution.
Planning your compressed air system to have redundancy with a back-up compressor or using the 50-50-50 approach with three smaller compressors can help eliminate the risk of unplanned downtime in your compressed air supply. Not only will a multi-unit solution eliminate the compressed air system as a downtime risk for your production lines, it will also supply air more efficiently as your compressed air demands vary by shift and production level.
Ask yourself Burt’s initial question about your own company. “How much have the avoidable events of this year (or the past 2 years or 5 years) cost us, and how much would we have willingly spent to prevent them?”
As Burt Hurlock writes in the article, “all avoided unplanned downtime has value.” Knowing that your compressed air system is vital to your plant, realizing the value, and planning to have a back-up air supply will help you avoid downtime from an unplanned event or even downtime for scheduled maintenance. The value of a reliable second compressor may far outweigh the cost of the downtime it prevents.
Burt goes on to say “The single most important ingredient to the just-right balance between investment and risk is information. Companies that know the value of events that don’t happen also understand that reliability is nine-tenths information and only one-tenth perspiration. The opposite is true for companies that don’t know the value of events that don’t happen.”
When you have your next event, how much will you be sweating?
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.
We’ve done numerous blog entries about the benefits of compressed air audits and leak detection. We’ve talked about how they can baseline your system, build a demand profile, and be an invaluable tool for optimization projects. But, we haven’t discussed how to read an actual chart that you might get back from a reputable compressed air auditor. At first glance, the charts can be a bit intimidating, but if you know what to look for, you can learn a lot about your system. Being able to understand the charts will also put you in a better position to explain and justify any capital expenditures that might be needed when presenting the plan to upper management. Here are the basics of reading a compressed air audit chart. Continue reading “All I Really Need to Know I Learned from a Compressed Air Audit Chart”→