How to do compressor selection?

For the purposes of this discussion, we will be talking in general about the transfer of all types of commodities in industrial applications via a reciprocating piston compressor.

Know The Landscape

Commodities as wide-ranging as industrial gases, whether petrochemical, liquefied gas, inert or others, are commonly transferred for process boosting, loading, unloading, and/or vapor recovery. Because all of these substances have their own unique handling characteristics, the operator must perform due diligence in order to ensure that the proper style of compressor is being selected for the proper operation. Operational parameters to consider include:

Before selecting a compressor, the operator must know the value of six critical operational parameters. These include compression ratio, heat of compression, duty cycle, packaging, location and lubrication system. Knowing their importance, Blackmer® has designed its HD Series Reciprocating Gas Compressors with the versatility needed to be a good choice for a wide range of operating conditions.

1. Compression Ratio
   This is a measurement that determines the amount of work a compressor has to do in order to transfer a certain amount of product, and is defined as absolute discharge pressure divided by absolute suction pressure. It is represented as a ratio of XX:X. In general, a higher compression ratio means more work for the compressor so decisions have to be made on the number of stages required and whether air-cooled or liquid-cooled equipment is the best choice. As previously stated, there is no “one size fits all” rule when selecting a compressor. So, if the compression ratio is between 1:1 and 5:1, a single-stage compressor is likely the best choice. At 5:1 to 8:1, a single-stage machine may still work, but a two-stage compressor may need to be considered. A two-stage unit is the best choice from 8:1 to 12:1, and anything higher than 12:1 usually requires a three-stage or higher compressor.
2. Heat of Compression
   The specific heat ratio of the product being compressed plays a big factor in compressor selection. This ratio dictates how a gas behaves with respect to the heat of compression and ultimately determines the number of stages required and whether air- or water-cooled equipment should be used.
3. Duty Cycle
   This illustrates how often the compressor will be required to operate. For instance, will it be running 24/7 in continuous-duty operation, or just intermittently throughout a period of time?
4. Packaging
   What is needed, required or desired besides the actual compressor itself is a big decision to consider in all applications. What type of base/skid is needed? What about drivers and controls to meet the needs and safe operation of the equipment? If the unit is lubricated, what type of lubrication system is required and what are any associated filtration needs? These are just some of the many considerations to take into account before a final selection can be confidently made.
5. Location
   In other words, where will the compressor be working? Ambient temperatures and elevation can also affect the compressor’s operation, so a model that is just right for colder climates like those in northern Russia or Canada, may not be the best choice in the hotter operating conditions found in desert countries or those situated along the equator.
6. Lubrication System
   There are two primary types of reciprocating piston compressors: lubricated and non-lubricated (also known as oil-free). If the product-handling system can handle small traces of oil, a fully lubricated unit should be considered. If the system is incapable of handling small traces of oil, then an oil-free unit is the best choice. Note that “oil-free” means that no oil is present in the area of the machine where the product flows, though oil may be used to lubricate other parts in the machine that do not come in contact with the transferred commodity.

Also, another lubrication-related thing to consider is the compressor’s leakage-control capability. In an age where the reduction or elimination of fugitive emissions to the atmosphere is becoming increasingly important, some compressor designs that require

lubrication in the upper cylinder and valve area do not have a gas-sealing section, known as a “distance piece.” This method of design can produce a compressor with significantly higher leakage rates, which can be viewed as an environmental hazard.

Good – And Getting Better

All things considered, oil-free reciprocating piston compressors have proven to be the best choice when transferring a wide range of liquids and gases. In fact, they significantly outperform their lubricated reciprocating piston cousins, as well as various models of rotary vane, screw and scroll compressors, in a number of important operational areas.

Not content with resting on their laurels, the developers and manufacturers of reciprocating piston compressor technology have introduced a number of enhancements to the standard design of the compressor. These enhancements include:

• Improved Valve Designs. Larger valves offer improved product-flow patterns, with polyetheretherketone (PEEK) valve plates now standard. All of the unit’s unloaders are now constructed of stainless steel, with stainless steel also an option for the valves. Competitive models with smaller valves have a restricted flow path with PEEK valve plates only optional and unloaders constructed of ductile iron or steel.
• One-Piece Piston Design. The piston is simply threaded to the piston rod and secured with one lock nut with no need for piston screws. Legacy designs feature a slip-fit piston platform that is clamped to the rod with a lock nut and roll pin, while the piston is attached via many small screws.
• Long-Life Piston Rings. The ring’s increased radial thickness provides longer service life.
• Improved Oil Control. For applications that require maximum leakage control, S3R oil-control seals provide more precise oil containment.
• Improved Bearing Design. Features a larger, more robust steel wrist pin needle bearing instead of bronze bushings. The larger wrist pins lower bearing loads, which equates to longer service life. The needle bearings are also lubricated under all rod-load conditions, which also improves service life.
• Improved Crankcase Design. A large main roller bearing is located on the crankshaft, which eliminates the need for bronze bushings at the oil pump bearing carrier. Older compressors models feature smaller main roller bearings and a shaft adapter between the pump and crankshaft, which can be a failure point.

Knowing the needs, parameters and peculiarities of any type of product-transfer application is imperative for any industrial operator. With the recent upgrades in their design and operation, Blackmer® HD Series Reciprocating Gas Compressors now provide additional benefits and peace of mind for the operator.

Conclusion

Knowing the needs, parameters and, yes, peculiarities of any type of product-transfer application is a must for anyone who wants to optimize their overall operation. Oil-free reciprocating piston compressors have proven to be the most effective machine in this realm, with the recent upgrades to their design and operation providing additional benefits and peace of mind for the operator.

Table of contents

Compressed air from a compressor efficiently powers your facility's tools and equipment. When you need a new unit for your operations, you need to understand how to size an air compressor. Determining the correct air compressor size can help minimize downtime and keep your operations efficient, ensuring you select a model that isn't overpowered or too small for your requirements.

In This Article

6 Things to Consider When Choosing an Air Compressor

Physical size is not the key factor to consider when purchasing an air compressor — a larger compressor isn't necessarily a more powerful one. You should instead evaluate your needs when it comes to:

1. Air pressure: Measured in pounds per square inch (PSI), this pressure describes how much air the compressor delivers. Higher PSI measurements mean the unit produces more air volume.
2. Air delivery: Expressed in cubic feet per minute (CFM), air delivery shows how much air a compressor can pump in one minute. Air delivery is also known as volume flow rate. The larger your application or the more complex your needs, the higher CFM you'll require.
3. Power: Air compressor power considerations include horsepower and the power supply. Horsepower shows how much power the unit's engine provides, and your air compressor power supply options include electric, hydraulic and gas. Electric motors often have simpler maintenance needs, lowering their operating costs. Hydraulic compressors are also cost-effective and can integrate into your existing power sources. Gas-powered models provide consistent pressure.
4. Tank size: Short tanks are suitable for intermittent compressor use, often in quick bursts. If you'll continuously use your compressor, you'll need a larger tank that can store more air and keep up with demand.
5. Duty cycle: Duty cycles are how often a compressor can run before pressurizing more air. This measure is often expressed as a percentage — a 50% duty cycle would require two minutes to cycle in new air after one minute of operation.
6. Configuration and air capacity: Consider your operating needs to determine the air compressor type and capacity you require. For example, reciprocating compressors, available in single-stage and two-stage configurations, have lower capacities, making them well-suited for workshops, construction sites and other needs that require intermittent use. Rotary screw compressors offer higher capacities and are designed to operate continuously, so they are often used in industrial applications.

How to Calculate Air Compressor Requirements

With those factors in mind, you'll need to note a few numbers and make your compressor sizing calculation following these steps:

1. Check your tool CFM requirements: Collect the CFM requirements for all the air tools you use at once in your operations.
2. Add the CFM requirements: Get the total CFM requirement by adding each tool's CFM together.
3. Multiply the CFM: To ensure the air compressor you select can meet your requirements and any potential new tools you add down the line, multiply the total CFM by 1.3 to increase it by 30%.
4. Calculate air storage needs: To determine your optimal storage tank size, multiply the CFM you calculated by 5. The result will be the ideal size of your storage tank in gallons.
5. Determine max PSI: To evaluate the maximum PSI your tools require, note each pressure required and select the highest one.

How to Choose the Right Air Compressor for Your Needs

After your calculations, you can determine how big of an air compressor you need. To choose among the various air compressor sizes, make these considerations:

1. Air Pressure

Understanding your maximum required operating pressure helps you select a compressor size. You'll typically find air compressors rated between 100 PSI to 175 PSI. The highest PSI requirement for your tools or machinery will determine what you'll select for your air compressor.

2. CFM Requirements

The larger your application, the higher CFM you'll need. By considering all the tools you use at once and increasing their total CFM ratings by 30%, you'll have a buffer that can accommodate factors such as leaks or added tools. Whenever you run more than one air tool or piece of machinery at once, you'll want to ensure the total CFM requirements do not exceed what your compressor can provide.

With your calculated CFM noted, you can compare your air compressor options using standard cubic feet per minute (SCFM). SCFM is the airflow converted to a standard that assumes these conditions:

• 36% humidity at sea level
• 68 degrees Fahrenheit
• 14.5 pounds per square inch absolute (PSIA)

Note that the compressor's CFM rating will be lower than the SCFM calculation. CFM values are measured when the air supply is pressurized — typically at 90 PSI.

3. Horsepower

While larger horsepower (HP) does not always equate to a more powerful compressor, it is a factor worth considering. A higher HP often means the air compressor can power heavy workloads. Air compressors typically range between 1.5 HP and 6.5 HP.

4. Air Capacity and Tank Size

Physical size matters most when it comes to your compressor's tank size. As an industrial operation, your facility will likely require a large tank size. Compressors that have larger tanks and suitable motors provide higher levels of air pressure for sustained periods. Larger tanks require fewer duty cycles, meaning your teams can continue working with little downtime. Because your facility will likely need a larger air compressor tank, keep your space limitations in mind.

5. Duty Cycle

Duty cycles describe a compressor's on-off cycle. Off-time is a momentary drop in pressure when the compressor is recharging. For applications that require bursts of air, off-time is acceptable. Uses that require a constant air pressure, such as painting vehicle parts, need a steady flow and a smaller duty cycle.

For uses that need constant air pressure and industrial applications, a 100% duty cycle is often necessary. These air compressors can work continuously and are often equipped with engine-cooling features to maintain efficient and safe operations.

You may also see the duty cycle described in active-idle time. A 70/30 duty cycle means the compressor is active for 70% of the time it's powered on.

6. Compressor Style

Other than size or air pressure and supply values, you'll have various compressor designs to choose from:

• Oil-free compressors: Choose oil-free air compressors if you require a contaminant-free yet steady power supply.
• Centrifugal compressors: This dynamic air compressor option turns kinetic energy into high air pressure and is suitable for clean environments.
• Rotary screw compressors: Rotary screw compressors are cost-effective and have little maintenance needs, and they're beneficial for 24/7 operations and continuous air needs.
• Reciprocating compressors: Facilities of all sizes can use reciprocating air compressors, and these units can supply more pressure than other models.
• Rotary vane compressors: With their compact design, rotary vane compressors are suitable for smaller spaces with low capacity needs.
• Pressure-lubricated compressors: A pressure-lubricated air compressor provides long-term operation for industries such as aerospace and automotive.

Fluid-Aire Dynamics can help you select the right commercial compressor size for your operations. Serving Chicago, Milwaukee, Minneapolis, Detroit and Philadelphia, we are established experts offering a range of air compressor solutions and services. We have helped businesses of varying sizes and industries find the perfect compressor systems for their needs, and we can help yours next.

Browse our air compressors, and call us at 610.913.9100 or contact us online for assistance in choosing the right compressor size.