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Compressed air has become a vital utility in the day-to-day opertions of most companies. Businesses are well aware of the need for compressors, but there is considerable debate as to which of the two most popular types - rotary screw or reciprocating - works best in an application. It is important to look at the similarities and differences between them to select the best one.

Fig. 1.
Rotary screw compressors have an inherently smooth running design and 100% duty cycle.

Key Concepts

Rotary screw compressors are used extensively in applications above 30 hp and for air up to 150 psig.

Reciprocating compressors cover low horsepower and demanding applications where reliability is essential.

Oil-free or oil-less compressors cost more to buy and maintain.

Compressors are used with a wide variety of gases, but air compression is the largest application. Stationary rotary screw compressors account for about 40% of the air market,while reciprocating compressors have 21% of market share by dollars.

Reciprocating compressors are applied in many diverse applications. The era of using reciprocating compressors in standard 100-150 psig air applications is rapidly fading. However, improvements in reciprocating compressor technology, requirements for higher gas-pressure applications, and needs for equipment that operates in severe duty ambient environments allow the reciprocating compressor to remain a viable and sensible choice for many applications.

Above 30 hp

Rotary screw compressors (Fig 1.) have taken over nearly all of the standard plant air 100-150 psig market above 30 hp.

Improvements in the performance and reliability of these compressors, coupled with reduced maintenance and lower initial cost, are key factors driving this trend.

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Although a double-acting reciprocating is still the most efficient compressor, rotary screw models have narrowed the efficiency gap. Better rotor profiles, machining improvements, and design innovations are contributing factors.

For example, a slow-speed, direct-drive, rotary screw compressor can deliver 4.5-4.8 cu ft of air/brake horsepower at 100 psig, which is within 5-10% of a double-acting reciprocating. Unless energy costs for a particular plant are quite substantial, more efficient double-acting reciprocating compressors normally can't show a short enough payback period to justify the higher initial purchase price for most users.

A well-maintained rotary screw compressor provides a decade or more of dependable service. Also, screw compressor control systems with diagnostic and troubleshooting capability and ability to indicate oil change intervals based on operating temperatures increase reliability and longevity.

Fig. 2.
Reciprocating compressors have a high overall efficiency, but can require frequent maintenance.

Selection factors

• How much air does the plant system require?
• What is the optimum pressure level?
• What quality of air is required?

Maintenance

When it comes to maintenance costs, rotary screw compressors ahve an advantage over reciprocating. Double-acting reciprocating compressors typically require more periodic maintenance than rotary screws. Valves, piston rings, and other consumables on a reciprocating compressor need expensive routine maintenance.

Rotary screw compressor maintenance is limited mostly to oil, oil filter, and air/oil separator changes. At some point there is a sizable cost associated with a rotary screw air end replacement, but they often last 10 years or more.

Standard rotary compressor packages have a microprocessor-based or electro-pneumatic controller. These controls allow the rotary to remain loaded 100% of the time. One main function of a control on a rotary machine is to allow for the regulation of airflow so the unit can run efficiently at ful load, partial load, or unloaded.

Controllers on some rotary units provide many beneficial functions, including operating displays,

Total cost factors

• Initial capital investment
• Installation
• Maintenance
• Energy

shutdown functions with annunciation, and service and preventive maintenance reminders.

The plant with operating experience and maintenance knowledge of double-acting reciprocating compressors (Fig. 2.) would do well to continue with business as usual. A plant in this position can utilize a preventive maintenance program or service agreement with a distributor to achieve a highly successful compressed air system.

Lubrication

Reciprocating compressors are divided into two categories; lubricated and nonlubricated. In lubricated units, oil is introduced into the compression cylinder to minimize wear of the cylinder and piston rings. In an average application, lubricated rings should last for several years. Advances in new compression ring materials are extending ring life in nonlubricated units to more than 8000 hours.

The cost difference between lubricated and nonlubricated reciprocating compressors deserves consideration. Certain applications demand that air or gas be oil free. In initial cost, a nonlubricated unit costs 10%-15% more. Power consumption and efficiencies are similar. The real difference is in the maintenance required for each type of unit. Nonlubricated machine maintenance costs can be up to four times higher than for a lubricated machine.

Unbalanced forces and the significant weight of reciprocating compressors play a major role in installation costs. Consider that reciprocating units, in many cases, require a heavy, weighted base and/or substantial foundation. Compressor manufacturers can provide the necessary data to determine the foundation required for a given application.

Reciprocating

• Cost advantage as a single-acting, air cooled unit below 30 hp.
• Double-acting units used above 250 psig and in nonlubricated applications.
• Normally used for heavy-duty, continuouse service.
• High overall efficiency.
• Operates efficiently at partial loads.
• Saves horsepower in no-load conditions.
• High initial and maintenance costs.
• Large sizes require heavy foundations.

Rotary Screw

• Used more in 150 psig, lubricated air systems above 30 hp.
• Used for constant-volume, variable-pressure applications.
• Oil or water is used for sealing and cooling.
• Must vent reservoir to lower power consumption when unloaded.
• Delivers high air volume in a compact space.
• Smooth, pulse-free output.
• Easy to install and maintain.
• Low vibration.

Although the initial purchase and installation costs of a reciprocating compressor are greater than a rotary, the reciprocating unit, with proper maintenance, lasts 2-5 times longer than a rotary design.

Over the decades, the reciprocating compressor has proven to be a reliable workhorse. With advancements in material technology, reciprocating units are improving intervals between maintenance and providing quality air. When looking at compressor applications outside of the 100-150 psig range or compressing gases other than air, the reciprocating compressor is usually the best choice.

Below 30 hp

The popularity of small air-cooled reciprocating compressors dates back to the early 1900s. They have been used extensively in applications requiring pressures up to 175 psig. Large and small air-cooled units are well suited for use in harsh environments.

The most common small reciprocating compressor is the single-acting design. Operating temperatures can reach 380 F and most units operate at sound levels above 80 dBA.

For lower horsepower applications, reciprocating compressors are considered a good value because the initial purchase price is generally 40-60% less than a rotary screw compressor. It is important to note that optional components, such as aftercoolers, starters, and operational components, such as switches, are usually offered as price adders to a base machine.

Small reciprocating compressors are durable machines that provide reasonably good quality compressed air for many applications. Their simplistic design, wide range of operation, and overall proven reliability are the most significant strengths.

Although the rotary screw compressor initial purchase cost is greater than a reciprocating compressor, those in the 10-30 hp range are becoming popular. One reason is they come as a complete package. In most cases rotary screw compressor packages are equipped with a starter, aftercooler, and compressor controller with diagnostic capability as standard equipment.

Rotary screw compressors are available in smaller sizes ranging from 5-30 hp. One advantage over reciprocating compressors of the same horsepower is they operate at cooler temperatures. Screw compressors are designed to run at 100% duty cycle and , due to low oil carryover, provide good quality compressed air.

Installation

Small reciprocating machines should be used with an air receiver. The receiver stores compressed air and minimizes the loaded run time of the compressor. Some small reciprocating compressorshave limited duty cycle of around 66%.

It is particularly important to the life of these compressors to use an adequately sized receiver. No matter what the receiver size or configurationof the compressor and receiver, small reciprocating machines are relatively easy to install. Any reciprocating compressor should always be anchored to the floor due to unbalanced forces.

The majority of small rotary packages are designed to stand alone. Base-mounted units can be mounted on top of an air receiver. Air is discharged from rotary screw compressors without pulsations. However, it is a good idea to include a receiver in the system to smooth the control air signal back to the compressor controller and provide consistent operation.

Small rotary screw compressors offer the purchaser a good value for a complete package that can be used in applications demanding constant volumes of compressed air. Most enclosed rotary screw units operate at sound levels well below 80 dBA. Packaged rotary compressors are easily installed by using simple floor anchors, and generally have single-point hookups for electrical and air connections.

Selecting the correct installation space for an air-cooled compressor is an important factor for reliability and long service life. Good airflow over the compressor pump is essential for the unit to operate properly and extend service life.

Overall, compressed air quality from rotary screw compressors is good. Even though the rotary can be an oil-flooded machine, an efficient air/oil separator reduces oil carryover into the compressed air system to less than 5 ppm. Air quality is further improved by lower operating temperatures that are inherent to the rotary design. Most rotary compressors operate at approximately 100 degrees .F above ambient temperature.

Oil free, oil-less, or non-lubricated

Which compressor should be used in applications where air or gas is required to be free from oil? Oil-free compressors use some form of lubrication for bearings and gears, but the compression area is free from oil.

Oil-free  compressors are often rotary screw. The oil-free machines, because of the lack of cooling at the point of compression, can run at elevated operating temperatures. Newer technology injects water into the compression area to keep operating temperatures down and extend service life. Rotary screw, oil-free compressors range in size from 20-260 hp and are used in applications of 150 psig or less.

Oilless  reciprocating compressors are used primarily for applications from 1-15 hp. Oilless compressors do not use any oil in the entire machine.

Nonlubricated  reciprocating compressors are a viable selection for higher pressure or higher horsepower applications.

---- Edited by Joseph L. Foszez, Senior Editor, Plant Engineering
Reprinted from PLANT ENGINEERING, September 1998
© 1998 by CAHNERS BUSINESS INFORMATION