Examine all factors when choosing between a reciprocating piston or rotary compressor
Making the correct choice between a reciprocating piston and a rotary compressor can be greatly beneficial for operations
When it comes to compressing air, there is more than one way to skin a cat, so to speak. If you’ve been in the market for a new air compressor, you’ve likely noticed this. There are a variety of technologies available to produce the same basic thing: compressed air. The trick is to find the type of compressor that will most efficiently meet your air demands.
Unless you have extremely high air requirements, require pressure greater than 200 psi, or have other specialty needs, the choice will likely come down to a standard rotary (screw or vane) or reciprocating piston compressor. However, deciding between the two is not always easy, largely because there is often no cut-and-dried answer.
In addition, the consequences of choosing incorrectly can be extremely costly. An operation will waste a lot of money by using an oversized compressor, and it will suffer painful downtime if it uses an undersized compressor. As a result, it’s important to base your compressor purchase on a variety of individual considerations.
Often, people use horsepower to help determine which type of air compressor to purchase. If they need only 10 hp or less to produce enough air for their needs, they may automatically select a piston compressor. Likewise, they may feel a rotary screw works best for practically all applications requiring more than 10 hp.
Contrary to this school of thought, there is no definitive cutoff point when it comes to horsepower. That’s why some manufacturers offer rotary screw compressors with as little as 3 hp or piston compressors with up to 100 hp. Although these extreme examples aren’t very common, they do serve a purpose.
A more appropriate place to start for selecting a compressor is to determine the constant air demand, or duty cycle. For example, a job shop with a CNC machine may have fairly steady demand, whereas an automotive shop with an impact wrench is only going to use air for short, periodic durations.
Although some piston compressors on the market are liquid cooled, more typical ones are air cooled, so they normally have duty cycles up to 75%. Even though they have a fan built into the flywheel to help cool the pump, piston compressors must cool at least 25% of the time. This prevents overheating, since approximately 2,500 btu per hp is generated when air is compressed.
On the other hand, all rotary compressors are liquid cooled, so they can run continuously to achieve 100% duty cycle. Therefore, if an operation’s air tools require up to 100 cfm and the demand stays fairly constant at 100 cfm throughout the day, a rotary compressor is likely the only option. However, if demand fluctuates wildly from 100 cfm one minute to 10 cfm the next, a piston compressor may also be practical, since it will have time to rest in between periods of compressing air.
If duty cycle doesn’t become the sole determining factor in choosing between a piston and rotary compressor, then other considerations come into play—one being upfront cost. Piston compressors have a much lower purchase price than comparable rotary compressors. That’s because piston compressors have a very simple design. An electric motor turns the pump, and that’s about all there is to it. On the other hand, rotary compressors are equipped with additional components, such as a liquid cooling system and air/oil separator, that makes them more expensive. Depending on budget, this could be a big influence on the decision.
Of course, energy usage must also be factored into the cost of a compressor. One advantage of piston compressors is that they only use electricity when they’re compressing air. On the contrary, rotary compressors are designed to run continuously, even when they’re not compressing air. Granted, some rotary compressors automatically go into standby mode after idling for 5 or 10 minutes; nonetheless, that’s still 5 or 10 minutes of wasted energy, which can significantly impact the electric bill.
In addition to weighing the purchase price and energy savings, one should factor service costs into the decision. That’s because, similar to buying a foreign sports car, the investment in a rotary compressor isn’t just a one-time hit. Along with the conveniences of these machines comes more expensive maintenance, due to the additional filters and lubricants.
Typically, the most costly maintenance item on rotary compressors is the oil. These units generally require specialized lubricants, which, in some cases, may cost several hundred dollars for a 5-gal bucket. It’s possible for a rotary compressor to rake up a significant service bill every 3 to 6 months and, in fact, total service costs for a rotary compressor can surpass the initial purchase price after 5 years of use. Unfortunately, those who don’t think of this ahead of time are often blown away after the purchase has already been made.
Some recognize that the higher maintenance cost of rotary compressors comes with the territory and, in many cases, there is no alternative if 100% duty cycle is required. But for others, the added maintenance is enough to sway their decision toward purchasing a piston compressor instead.
Overall, maintenance on a piston compressor is simpler. As long as the operator regularly changes the air filter and pump oil, the compressor is likely to provide years of trouble-free service. Furthermore, the oil for a reciprocating piston pump is much cheaper, and the quantity is less. For instance, the pump may only hold 2 qt of oil, while a comparable rotary compressor takes 5 gal. Beyond this simple routine maintenance task, there aren’t many other components on a piston compressor to service.
Work environment can also help determine which type of unit to purchase. The simple, time-tested design of piston compressors is well known for its ability to withstand harsh conditions. Rotary compressors do not have as long of a track record and, due to their more complex design, are not as well suited for use in some of the toughest industrial environments.
Despite having the upper hand when it comes to durability, piston compressors are louder and produce more vibration. Therefore, if noise and vibration are concerns to the facility, a rotary compressor may be more practical for the application.
Although piston and rotary compressors may offer slight variances in the way they produce air, their basic purpose is the same. Nonetheless, a person shouldn’t purchase a rotary compressor because it’s trendier, nor should he purchase a piston compressor because it’s cheaper. Instead, the correct approach is to carefully consider all factors and purchase the compressor that best meets individual needs, whether it’s a reciprocating piston or rotary unit. In the end, nobody working in the facility will ever know the difference as long as it’s reliable and efficient.
Daniel Leiss is president of Jenny Products, Inc.