Reciprocating Air Compressor

Which Reciprocating Air Compressor Do I Need?

Which Reciprocating Air Compressor Do I Need?

The good news is you have plenty of choices for almost any application. The bad news is, you have plenty of choices for almost any application. Making the right decision seems overwhelming, but it’s not. Let’s break it down and simplify. An excellent reference resource to introduce you to the CA, CT and ML lines is the application table below. For more details though keep reading and we’ll get straight to the nuts and bolts of what you need and what you don’t need in a reciprocating compressor.


Whichever compressor you choose or determine is right for you, make sure it’s heavy-duty cast iron construction—NOT aluminum. Second, if you need a steady supply of clean, quiet, 100% continuous-duty air, a rotary compressor might be a better fit for you.


CT – If, one of your primary concerns is price and you are not concerned about water vapor in your air, then the CT is the right compressor for you. And the 2-year warranty offers good peace of mind. However, if you need more air and horsepower (CT offers 10hp, 29.7cfm), you’ll want to step up to CA or ML.

CA – If, you value the added peace of mind provided by a 5-year warranty, require low moisture content in your air (e.g. for extended tool and equipment life, improved paint outcomes, etc.), you appreciate the lower maintenance provided by the UltraPack option, or want a duplex machine to provide 100% backup, then the CA is the series you should buy.  CA machines top out at 15hp (42.6cfm) for simplex and 10hp (68.4cfm) for duplex, so if you need more, it’s time to take a look at our ML.

ML – If, you don’t mind paying a little more for the best and you have a very demanding application, our ML series will stand up to the challenge. Its “targeted oiling” system provided by our patented Centro Ring pressure lubrication means less worry for you. As with the CA, the ML series offers both UltraPack and Duplex options for low maintenance and 100% backup. In addition, ML machines go all the way up to 30hp (102.2cfm simplex, 204.4cfm duplex), providing plenty of air, even for large shops.

Learn More About Our Recip Products

Rotary Air Compressors in Your Automotive Shop May be Costing You Money

There has been a trend in recent years to promote the use of rotary compressed air in automotive care applications because 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 Ultra Pack 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 a rotary compressor.  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.

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 misapplied.

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 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.

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 the issues outlined above come into play.  An oversized machine will almost surely short-cycle and lead to increased maintenance and service issues.

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.

Bleeding air to the atmosphere – Another solution to short-cycling is to bleed air to the atmosphere if there is not enough demand.  This requires extra equipment and 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.Automotive shop air compressor

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:

  1. 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.
  2. 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.
  3. 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. Learn more about what compressor you need for your automotive shop.

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If you have an FS-Curtis product and need help troubleshooting your equipment or if you need a service, you can get in touch with your local distributors.


Stable Air Pressure and Why It’s Important

Understanding stable air pressure is simplier that you think. For most people compressed air is like electricity in that they do not think about it until they don’t have it. For example, most people would never stop to think about the pressure fluctuations in their compressed air system. Likely, the only time this becomes an issue is when an operator squeezes his air gun and the hissing sound isn’t loud enough which indicates that the tool will not be able to turn the bolt. In this case, the most common answer is to go to your favorite store and buy another air compressor.

Why should anyone care about how much the pressure swings in their piping and tank system? As we will see it’s all about saving money and increasing reliability.

It’s important to understand that everyone thinks that they have stable air pressure that does not fluctuate. However, if you do not have a flow control device, such as the DemandSmart Flow Controller, then you can be assured of these facts:

  • The air pressure that your compressor must produce is higher than necessary
  • It is fluctuating at least 10 psi. Even if you have a regulator it will still “track” with system pressure. That means if your regulator is set to 90 psi and the upstream air pressure fluctuates between say 125 and 135, then the regulator will also fluctuate between 90 and 100 psi
  • The all-important question is how does stabilizing the pressure to say 1 or 2 psi (versus 10 psi) save money and increase reliability?
  • Less air is lost by leaks and therefore the compressor can be “off” for longer and not consuming electricity
  • The compressor does not have to make as high a pressure. A good rule of thumb is that for every 2 psi drop in pressure that the compressor must make results in a 1% decrease in energy
  • Air tools are designed to work at very specific pressures so if this is stable then they last longer because the air pressure is at their ideal operating parameters. Air tools that last longer and operate at premium efficiency cut down on maintenance cost and prevents work stoppages
  • Air compressors will last a lot longer. This is because the compressor controls do not have to directly respond to users of the air. For example, with no flow control device, the compressor must load or unload as an operator blasts his air gun. With a flow controller, the valve will use the air in the tank and piping system to supply the operators demand. The valve cannot “produce” air so the benefit to the compressor is longer on and longer off time which saves wear and tear on valves and components

With all these savings combined, it is very typical to have a payback in less than a year. The best proof for the importance of stable air is to ask anyone using a flow controller if they would get rid of them…. Most compressed air users would not.

ML Series- Rugged, Affordable and Smart


Masterline (ML Series) is the most robust, heavy duty reciprocating air compressor lineup available. The perfect combination of quality components and efficient design make the ML Series rugged, affordable and smart. Because every model is built tough to last a lifetime in demanding working conditions, the ML Series is an outstanding alternative to splash-lubricated reciprocating compressors and small rotary compressors. Learn more about the  ML Series

FS-Curtis…Our Reputation Has Stayed The Same

For over 160 years FS-Curtis has prided itself on being the best in air compression. Assembled in St. Louis, MO we are proud that our reputation for having quality products and great customer service has continued to be unwavering. Our Reputation Has Stayed The Same...


How Can an Air Audit Maximize My Savings?

An air audit may be in order for your compressor system, especially if it hasn’t been carefully examined recently. While many people focus on the initial cost of purchasing an air compressor, they often forget to factor in the cost of operating the machine over the long run. Any issue within a system can reduce its efficiency, wasting air and electricity, and thereby costing you more money.

A compressed air audit, done by qualified professionals, will indicate the opportunities an organization has to increase efficiency and save money on energy costs by examining the air supply and demand. According to the U.S. Department of Energy, on average up to 50% of the energy it takes to run one 100hp air compressor is wasted.


But what exactly can an Air Audit do for you? Learn more about how you can help you maximize savings. Download "What is a Compressed Air Audit"?

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How to Reduce Your Compressed Energy Costs

Compressed air can be one of the most expensive forms of energy for manufacturing plants, often using more energy than other equipment. One horsepower of compressed air requires eight horsepower of electricity. With many air compressors running at efficiencies as low as 10 percent, there’s often plenty of room for improvement. Fortunately, 50% of compressed air systems at small- to medium-sized industrial facilities have opportunities for low-cost compressed energy conservation.

Learn how you can save money and keep your air compression working at a premium by downloading our "10 Ways You Can Optimize Your Air Compression and Save Money".



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What Is A Compressed Air Audit?

An air audit may be in order for your compressed air system, especially if it hasn’t been carefully examined recently. While many people focus on the initial cost of purchasing an air compressor, they often forget to factor in the cost of operating the machine over the long run. Any issue within a system can reduce its efficiency, wasting air and electricity, and thereby costing you more money.

But what exactly can an Air Audit do for you? Learn more about how an Air Audit can help you maximize savings. Download "What is a Compressed Air Audit"?


Keeping Your Reciprocating Air Compressor Working at its Peak

Here are a few guidelines on how to keep your reciprocating air compressor working optimally. No matter what brand of oil-lubricated reciprocating compressor you own, doing the following three things on a regular basis will extend its working life helping to ensure a trustworthy tool for years to come:

  1. Change lubricant quarterly
  2. Purge water from tank weekly (at least)
  3. Change air filter quarterly

Oil (Lubrication)

Your reciprocating air compressor is basically an engine, and as such it requires constant lubrication to prevent excessive friction from damaging the moving parts and ultimately seizing the piston/s.  Non detergent lubricants are typically preferred for reciprocating air compressors, but make sure you respect the viscosity the manufacturer suggests for your particular compressor.

Regularly check the oil level to make sure you have the right amount of oil in the crankcase.  While the dangers of too little oil are obvious, it’s also important not to overfill a compressor with oil.  If the oil level is too high the oil can get whipped and it will foam up, losing some of its lubricating properties.  In addition, it will gain volume, further increasing your problems.  So keep checking with your dipstick/gauge as you refill with oil and make sure your unit is perfectly level to the ground when you do this.

These are just as easy to read as the dipstick variety.  The red dot represents the “OK” mark (I don’t know why they paint it red, should’ve been green).  Ideally you want to keep the oil level within the center of the dot, but as long as the level is within the dot, you are OK.

On these it’s also easier to judge the state of the oil by looking at it with a flashlight, you can often see whether it’s getting milky (water) or darker (regular wear) without having to remove a sample from the crankcase.

 Tip #1-Oil is regularly lost through the exhaust and the breather hole on the crankcase. Oil also ages and because of this, you should replace the oil after a certain amount of duty hours. A good rule of thumb that will help you comply with most manufacturer’s requirements is to change your reciprocating compressor oil on a quarterly basis.  The actual time depends on many factors and its best if you obtain the manual for your particular compressor for a clear indication on when to replace the oil.

To remove the old oil there’s going to be a bolt at the bottom side or the bottom of the crankcase in all reciprocating air compressors.  Simply remove this bolt to allow the old oil to drain into a bucket for proper disposal.  To speed up the process, make sure you remove the oil when it’s warm and remove the filling cap so you don’t draw a vacuum.=

Removing Condensate from the Receiver Tank

As the compressor’s intake happens to be ambient air, humidity in the air is sucked in on every cycle. The water vapor from ambient humidity will condense and accumulate in the tank/s.  Because of this dynamic, it’s imperative to drain the tank/s at least once a week.

Draining the tank is easy.  Locate the drain valve at the bottom of the tank and open it until moisture and air come out of the valve.  As soon as the flow of water slows to a trickle, close the valve.

If you don’t drain the tank the condensate will rust the tank’s envelope and ultimately corrosion and rust-through will occur.  The more the condensate sits in the tank, the worse it’ll get in time.

To keep condensate from building up in the receiver tank with minimal effort, include the automatic electric timer drain with your purchase of a new compressor. An electric timer drain will open at preset intervals to keep condensate from building up in the tank and finding its way down stream to the shop equipment. Zero-loss drain valves do the same thing, but they lose less compressed air pressure and also reduce the number of short cycles needed to keep the compressed air system at the pre-set level. This saves on your electricity bill.

Tip #2- Because eliminating condensate from the tank is so important to the life of your compressed air system, put a tickler on your calendar to drain the tank at least once a week –more frequently in humid environments.  Better yet, automate this task by investing in an automatic tank drain.

Intake Air Filter

The intake air filter is there to stop the compressor from taking in particles of dust in the ambient air which might cause damage to the metal surfaces inside the air compressor.  If the location of the compressor has a lot of dust and dirt in the air, this filter may need to be kept clean or changed more often than the manufacturer’s recommendations.  Use the manufacturer’s replacement intake air filter for proper fit and filtration.

Tip #3 – Refer to your manufacturer’s recommended intake air filter replacement schedule.  Replacement of your intake filter is important to keeping your air compressor in compliance with the manufacturer’s basic and extended warranty. Even if your air compressor is out of warranty,

Pros and Cons of Individual Compressor Controls

Previously we described the 6 different types of Basic Air Compressor Controls but what kind of control would you want for your compressor. Her are few pros and cons of the controls to help you assess the needs of your compressor.



  • Simple control using only a pressure switch
  • Motor and compressor operate only when needed which saves energy
  • Good for small compressors that are 25 HP or less (depending on application)


  • Frequent starting wears down motor and compressor
  • Pressure setting to stop must be higher than required system pressure to build storage and may increase energy use
  • Loses of pressure control in the range of 35 psi
  • Limited to small compressors



  • Motor compressor runs continuously which reduces wear  and tear associated with too many frequent motor starts
  • Tighter range of pressure (approx 10 psi)
  • Provides adequate storage and offers energy-efficient control of rotary screw, reciprocating and some centrifugal compressors


  • If applied incorrectly short cycles cause  premature wear and tear. There is minimal or no power savings on lubricant-injected rotary screw compressors
  • There needs to be proper blow down time and the storage capacity required for lubricant-injected rotary compressors to achieve energy savings and prevent lubricant foaming
  • Requires over-pressurizing to maintain minimum system pressure



  • The motor and compressor run continuously reducing wear
  • Tighter range of pressure control (10 PSI)
  • Steady progressive capacity control that matches demand


  • Pressure ratios increase as inlet pressure is throttled
  • Inefficient at lower loads(lubricant-injected rotary compressors limited to 40-60% capacity; centrifugal compressors limited by potential surge and may require discharge blow off)

Dual/Auto Dual


  • Combines features of modulating, load/unload. and start/stop
  • Shuts down compressors when unloaded for pre-set duration which in turn saves energy
  • Better selects operation mode for small reciprocating compressors


  • Makes the control complex
  • The Over-run timer must be set to limit premature starting and stopping

Variable Displacement


  • Energy-efficient control scheme that gets down to 50% of capacity
  • Matches displacement to demand without reducing inlet pressure or increasing ratios of compression


  • Makes the control complex
  • High initial cost
  • Only available for 50 HP+ compressors

Variable Speed


  • Energy-efficient and precise control
  • Various rotating speeds and giving more displacement and power. These are directly proportional to speed rotation


  • Makes the control complex
  • High initial cost
  • Reduced full load efficiency
  • Efficiency of rotary screw compressor ends drop at lower or higher speeds

The 6 Types of Basic Individual Compressor Controls

Compressor controls are designed to match compressor delivery with compressed air demand, by maintaining the compressor discharge pressure within a highly specified range.  This discharge pressure should be set as low as possible to minimize the energy usage.

Compressor systems are typically composed of multiple compressors delivering air to a common plant air header. The combined capacity of these machines is generally sized to meet the maximum plant air demand. System controls are almost always needed to orchestrate a reduction in the output of the individual compressors during the times of having lower demand. Compressed air systems are usually designed to operate within a fixed pressure range and to deliver a volume of air that varies with system demand. System pressure is monitored and the control system decreases compressor output when the pressure reaches a predetermined level. Compressor output is then increased again when the pressure drops to a lower predetermined level.

There are 6 basic types of individual compressor controls that a person has to take into account when looking into purchasing and using air compression:


  • Turns the motor which drives the compressor on or off in response to a pressure signal (seen on reciprocating and rotary compressors)


  • Allows the motor to run constantly but unloads the compressor when a predetermined pressure is reached. The compressor reloads at a predetermined lower discharge pressure. This is also sometimes referred to as constant speed or constant run control (seen on reciprocating, rotary, and centrifugal compressors).


  • Restricts passage of air to the compressor to progressively reduce compressor output to a specified minimum, when the compressor is then unloaded. This is also referred to as throttling or capacity control (seen on rotary and centrifugal compressors).

Dual/Auto Dual

  • This controller is commonly seen in small reciprocating compressors, allows the selection of either Start/Stop or Load/Unload. When used in a lubricant-injected rotary compressor it provides modulation or load/unload control to a preset reduced capacity. When unloading the addition of an over-run timer will stop the compressor after running unloaded for a preset time.

Variable Displacement

  • This controller allows progressive reduction of the compressor displacement without reducing the channel opening (seen on reciprocating and rotary compressors).

Variable Speed

  • This controller adjusts the compressor capacity by varying the speed of the electric motor driving the compressor in response to system signals.

Learn more about the these six basic compressor controllers and more about other FS Curtis products

Air Receiver Sizing

When specifying a compressed air system for optimum operation and energy efficiency, proper selection of the compressed air receiver tank(s) is one of the most critical decisions one can make.  There are several different “rules of thumb” and formulas that will assist you, but finding straightforward guidance that says “use this size air receiver” is difficult if not non-existent.

Air receivers can be used in several different ways in a compressed air system:

  • “Primary” receiver between the supply side (air compressor and ancillary equipment), and the demand side (your plant). Today’s air compressor controls (on/ off-line, modulation, and variable frequency) strive to maximize energy efficiency and smooth compressor operation by responding to demand side pressure changes sensed at the discharge of the  package.

A properly sized air receiver acts as a “buffer” and minimizes the effect of dynamic demand side pressure changes, allowing the compressor controls to operate smoothly and consistently.  The end result is less energy used, longer component life, and consistency in plant air pressure.

  • “Secondary” receiver typically on the demand side, at the point of use to minimize the effect large intermittent air demands have on the overall compressed air system.


Sizing a PRIMARY receiver for general FIXED SPEED APPLICATIONS:

  • The air compressor industry has widely accepted the general rule of thumb that a properly sized air receiver for a fixed speed compressor should be between 1-2 gallons per CFM output of the compressor.

Example: a 100 CFM fixed speed air compressor should have an air receiver between 100-200 gallons sitting next to it. Err on the high side if your budget permits.

Sizing a PRIMARY receiver for general VARIABLE SPEED APPLICATIONS:

  • When considering air receiver sizing on a variable speed drive application, the general rule of thumb is between 2-4 gallons per CFM output of the compressor.

Example: a 100 CFM variable speed air compressor should have an air receiver between 200-400 gallons sitting next to it. Err on the high side if your budget permits.

Useful Air Receiver Sizing Formulas (Primary and Secondary):

Use this formula when you have an existing air receiver

and need to know how long you can draw CFM greater than the output of the air compressor, from the receiver, while still maintaining system pressure:

  • T = R x P1 – P2 Qr – Qc      7

Use this formula to determine what receiver size to use to supply pressure for a given period of time, not allowing the system to drop below a minimum pressure. The demand of air is greater than the CFM output of the air compressor:

  • R = 7 x (Qr – Qc) x T

P1 – P2

Use this formula to determine how long it will take to recharge an air receiver to P1, after demand goes back to being below CFM output of the air compressor:

  • T = R x (P2 – P1)

Qr – Qc x 14.7

T = Time in minutes

R = Receiver in cubic feet Qr = CFM removed

Qc = Compressor output in CFM

P1 = Maximum air receiver pressure P2 = Minimum air receiver pressure

14.7 = Atmospheric pressure in PSI (sea level)

CUBIC FT to GALLON CONVERSION: 7.48 gallons in a cubic foot

Don’t Rotate from Recip & Don’t Vary from Fixed Speed

A Variable Speed Drive compressor sounds like a smart idea, but is it really?

Let’s address the rotary compressor question that auto care facilities ask us on a regular basis: Should I spend the extra money to put an energy-saving Variable Speed Drive (aka: Variable Frequency Drive) rotary screw compressor in my facility? The short answer to this question is: probably not.

The folks asking this question have heard that VSD/VFD machines may cost more up front but that they cost less to run and thus pay for themselves in the long run. In addition, many utility companies offer rebates for VSD/VFD machines that can help offset the larger up-front cost. Just like everyone else, automotive care facilities want to save money and energy, so this supposed value proposition sounds attractive. Right?

So why then is VSD/VFD probably not a good idea for most automotive care facilities?  The short answer is that most automotive care facilities typically have severe peaks and valleys in their compressed air usage due to the intermittent use of tools and equipment in the shop. This in itself does not rule out VSD/VFD. In fact, VSD/VFD is ideal for customers whose demand fluctuates, provided that it does not fluctuate too much. The problem with auto care facilities is that their valleys are too deep. Too much downtime. The rule of thumb is that when the valleys are 30% or less of the peaks, the compressor will have to shut down, essentially forcing it to run as a start/stop machine. There are several issues with running a VSD/VFD machine in this type of application:

  • More expensive – First of all, the customer is paying a 30% or more premium for a VSD/VFD machine to run just like a cheaper start/stop fixed speed machine. Perhaps the more important comparison is that the customer would be paying a whopping 70% premium compared to a fully-loaded UltraPack recip, which is probably the best application in most auto care shops.
  • Phantom Energy Savings – VSD/VFD machines are ideal for loads somewhere between 30 and 80 percent of full load. There are little to no energy savings otherwise. Even though very short payback periods are often cited, these are for machines in ideal scenarios with exactly the right load profile. Auto care facilities very seldom have an appropriate load profile. If one isn’t going to get the benefit of the VSD/VFD, why pay extra for it, and risk some of the potential headaches outlined below?
  • Increased Maintenance - 

o   VSD/VFD’s require minimal incremental maintenance on the drive compared to standard fixed speed machines. These increased costs should be considered in any decision to go with a variable speed machine.

o   VSD/VFD’s require a cleaner environment for the sensitive electronics of the drive. This can lead to increased maintenance costs and possibly drive replacement, which is very expensive. If the machine is going to be placed in a shop with dust and other contaminants floating in the air, then the cost of protecting the drive from these contaminants, or paying extra for maintenance and/or replacement, must be considered.

The bottom line is that most auto care applications are not ideal for VSD/VFD machines.  The severe valleys in demand profiles mean there probably won’t be energy savings to offset the increased acquisition and maintenance/replacement costs. In addition, most shops are too dirty to install a VSD/VFD without increased risk and cost.