There has been a trend in recent years to promote the use of rotary screw air compressors in autocare applications due to the fact that reciprocating technology is perceived as “dated” and rotary as “cutting edge.” Rotary technology is also much quieter than recip technology, which increases the perceived value of rotary machines. It’s true that rotary technology is newer and quieter than recip, however, the initial cost for a rotary machine is typically 50% more than an UltraPack recip machine, regardless of whether it’s right for the job or not. In addition, there are several hidden costs associated with rotary machines that need to be considered before deciding which one is the best fit for your application:
- Maintenance Costs – Some manufacturers have mistakenly led customers to believe that recip machines are more expensive to maintain than rotary machines by stating that there is “easier” or “less” maintenance involved in rotary. While it is true that rotary maintenance intervals (typically 2000 hours or one year, whichever comes first) are longer than recip maintenance intervals (typically 3 or 6 months), the cost of the consumables makes rotary maintenance 2 to 3 times that of recip over a 5 year period. See the table below for two typical machines which details this cost comparison:
- Misapplications – One of the great advantages of rotary technology is that it loves to run all the time. So if you have an application requiring a continuous use of air, and you size the compressor appropriately, a rotary machine will perform well without overheating. But, this strength becomes a weakness if a rotary machine is mis-applied.
o Intermittent demand – If a rotary machine is placed in an application where demand fluctuates greatly, it will be underutilized during periods of low demand. This will cause the rotary machine to “short cycle” – i.e., the machine will not run long enough to build up the temperatures required in the airend to burn off water vapor in the oil. When a machine short cycles, it can lead to: premature fouling of internal filtration and downstream components; to pre-mature airend failure; and potentially to motor failure if an electronic controller or timer is not used to ensure that the machine does not start and stop too often. All of these potential issues add up to extra maintenance and/or replacement costs for misapplied rotaries. In addition, these things may void a manufacturer’s warranty, leaving the end-user to bear the burden of all costs.
o Importance of sizing – If a rotary machine is oversized to account for future growth or simply as an insurance policy to make sure enough air is supplied to a customer, all of the issues outlined above come into play. An oversized machine will almost surely short-cycle and lead to increased maintenance and service issues.
o Cost of extra tank if flexibility is needed – If a rotary machine is misapplied, one of the ways to fix the application is to provide a larger storage buffer by adding a remote stand-alone tank, which will cause the compressor to run longer and achieve the required operating temperature. While this can be an effective solution, it adds extra cost to some rotary applications that should be considered up front.
o Bleeding air to atmosphere – Another solution to short-cycling is to bleed air to atmosphere if there is not enough demand. This requires extra equipment and also means that the customer is spending money to compress air that is literally vented into the air instead of being put to work in the shop. Although this can be an effective solution, it again adds costs that should be considered up-front.
- Re-builds – While rotary airends typically are designed to last twice as long as the best recip pumps, it costs a great deal to have them repaired or re-built, and in some cases, it is not possible to do so. So if a rotary machine is misapplied and the airend fails prematurely, the cost of the re-build will dwarf the cost of re-building a piston compressor.
Bottom line: While rotary machines are great in many applications, it pays to do some homework upfront to make sure a rotary makes sense in your application. At a minimum, check the following before buying a rotary machine for your shop:
- Make sure it’s the right size – A walk-through assessment of your shop is the best way to tell what size you need. If that option isn’t available, a simple pump-up test may suffice. If math is not your strong suit and you don’t have access to assessment equipment, work with a local air compressor expert to determine what is the appropriate size.
- Make sure your demand profile is appropriate – Basically this means ensuring that air demand is consistent enough to warrant a rotary machine. Again, a walk-through assessment is a great way to determine this, but engaging a local air expert is a good backup.
- And finally, once you buy the machine, make sure that it’s installed and maintained on a regular basis by a qualified rotary screw service center.
Rotary or Recip? The choice is yours. But an educated decision could save you some money.
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