Fig. 1. As male and female rotors turn inside case (top), dark gray atmospheric air fills pilot root from inlet port to case end. With further rotation, female tip passes inlet, sealing the rotor while simultaneously engaging male-rotor tip to begin compression. Just as intermeshing male tip has rolled far enough down female root to produce specified pressure, far end of female root uncovers discharge port.

Fig. 2. Typical rotary-vane compressor has oil injected during compression cycle to absorb some heat of compression. Air exiting from vane (and screw) compressors usually is delivered to a separator where liquid oil is removed.

Every compressed-air system begins with a compressor - the source of air flow for all the downstream equipment and processes. The main parameters of any air compressor are capacity, pressure, horsepower, and duty cycle. It is important to remember that capacity does the work; pressure affects the rate at which work is done. Adjusting an air compressor's discharge pressure does not change the compressor's capacity - even though many people seem to believe it will.

There are a number of basic air compressor designs - and variations of them - on the market today. They all fall into two general categories: positive displacement and dynamic. Although the operating specifications for two different types of air compressors may be very similar on the surface, other installation and performance factors can make one design superior to the other in a real-world application. Let's review some of the basic designs and terminology.

Reciprocating compressors

Reciprocating compressors are positive-displacement units that trap a charge of air and then physically reduce the space that confines it, causing its pressure to increase. Reciprocating units, commonly called piston compressors, use a piston, cylinder, and valve arrangement. Their operation is very similar to the familiar internal-combustion engine, but they simply trap and compress the air without adding fuel to explode it. Note that whenever air is compressed, heat is generated. Proper cooling of the internal parts of any air compressor is a critical part of its design.

There are three basic selection decisions that must be made about reciprocating compressors:

• single- or double-acting operation,
• single- or multi-stage configuration, and
• air or water cooling.

In a single-acting piston compressor, the piston only compresses air in one direction of its stroke. In a double-acting model, the piston compresses air with both directions of its stroke. Obviously, because both strokes perform work, a double-acting compressor is more efficient (in moving a volume of air per input hp) than a comparable-size single-acting unit.

A single-stage unit compresses air from inlet to discharge pressure in one operation. A multi-stage unit compresses from inlet to discharge pressure in two or more operations - generally passing the air through an intercooler to remove some of the heat of compression between each stage. This saves power and keeps the compressor's internal operating temperatures lower.

In air-cooled compressors, ambient air circulates around the compressor cylinders and finned heads to provide cooling. Heat transfers through the metal to the air. Air-cooled units are generally designed for 50% to 75% duty cycles, depending on the particular units and their application. In water-cooled compressors, integral water jackets surround the cylinders and heads. Heat transfers through the metal to the water - more effectively than through metal to air. Thus, water-cooled reciprocating units reduce internal temperatures more efficiently than comparable air-cooled units.

Most air-compressor manufacturers promote the two-stage compressor as the optimum machine for producing 100-psi class air - the base pressure level in most industrial plants - providing the best efficiency per dollar cost with adequate reliability of internal working parts. For a reciprocating compressor to be categorized as continuous duty, it is generally agreed that it must be double acting and water cooled. Double-acting, water-cooled reciprocating compressors are offered in a variety of styles that combine efficient air compression with durability and reliability. However, they also are heavy and bulky, making them relatively expensive to install. They generally have more-significant unbalanced forces, which combines with their size to require a special foundation and support.

When they meet selection criteria such as capacity, weight, size, and price, single- and two-stage single-acting reciprocating units are a good choice - particularly in the 50- to 150-psig pressure ranges. (Three-stage reciprocating units are offered, but generally are used for pressures above 250 psig.)

Oil-cooled rotary-screw compressors

The rotary-screw compressor is another positive-displacement machine. In an analogy with the reciprocating compressor, Figure 1, the male rotor is like a piston, pushing air along the female rotor, which is like the cylinder. The sealing strips are like piston rings, and air is compressed against the stationary end plate, which is like the bottom of the cylinder. This design has been around for about 50 years. However, until the mid 1970s, it was considered suitable only for engine-driven portables and small-horsepower electric-motor units because of low efficiency (the ratio of compressed-air delivery to power cost).

In the 1970s, development began on two-stage rotary screw compressors for pressures up to 250 psi. Rotor-profile development during the 1970s, 1980s, and early 1990s has led the oil-cooled rotary-screw design to become the significant choice in electric-motor-driven, lubricated, industrial air compressors, particularly in sizes from 20 to 300 hp.

Then, a significant breakthrough in air-end design occurred. The introduction of the unsymmetrical profile resulted in an efficiency improvement of approximately 15%. This improvement was significant enough to make the oil-cooled rotary-screw compressor competitive in the larger-horsepower sizes for continuous duty. It has almost the same efficiency as the single-stage double-acting units and smaller centrifugal compressors.

Two-stage rotary-screw compressors can approach and sometimes equal the full-load performance of two-stage reciprocating units in 100-psig class service. Today, two-stage oil-cooled rotary-screw compressors are frequently used in the 150- to 400-psia pressure range. They also are used for 100-psi service with significant power savings. Two stages offer advantages associated with lower compression ratio per stage. Reduced pressure differential across the rotors minimizes blow-by and significantly reduces thrust-bearing loads. (Obviously two-stage units require two air ends, which increase the initial cost.)

The unique characteristic of this compressor is that it is cooled by oil. Oil injected into the air stream absorbs the heat of compression while it is being generated. The heated oil then is taken to an air- or water-cooled heat exchanger for cooling. Because the cooling takes place right inside the compressor, the working parts are never subjected to extreme operating temperatures. The cooling oil never is cracked nor burnt. No matter what the load on the compressor is, there are no hot spots inside the airend. The resulting absence of wear produces trouble-free service and high efficiency. In other words, oil-cooled rotary-screw compressors can run at full load and full pressure -twenty-four hours a day, seven days a week. This compressor's useful life in operating hours and its maintenance cost per hour will be the same as under any other load condition.

Continuous duty

The availability of continuous-duty air-cooled compressors (particularly in large sizes) offers a great deal of flexibility for installing them. Such compressors can be mounted on any surface that will support their static weight. In many facilities, great savings also are available in piping cost, compared to other types of systems. These compressors lend themselves to either the central- or departmental-compressor system concept. Units are available with electric motor and engine drives - on bases, on skids, on wheels, etc.

Compared to other types of continuous-duty air compressors, oil-cooled rotary-screw compressors offer a number of advantages:

• Oil cooling holds internal temperatures to an optimum level. As a result, discharge air is relatively cool -no more than about 180° F higher than ambient.
• Discharge air is clean - free from burned oil or carbon.
• The rotary design lends itself to higher speeds, particularly in the larger sizes. Consequently, larger flow capacity is available from compressors with physically smaller envelopes - providing significant savings on floor space and foundation requirements.
• Because of their compact size and inherent quiet-running characteristics, it is relatively easy to suppress noise. Electric-motor-driven models are commercially available rated from 75 to 85 dB at one meter per the CAGI Pneurop Test Code.
• Most models have fewer moving parts, and those parts run under more ideal conditions - resulting in lower temperatures and less vibration.
• Fewer parts make it easier to stock them for the rotary designs, and the machines are easier to work on.

In summary, oil-cooled rotary-screw compressors offer users a continuous-duty source of compressed air in a neat, compact package that has low initial cost, maximum flexibility of installation, and easy maintenance.

Non-lubricated rotary screw and lobe

In addition to the non-lubricated reciprocating compressors that have become so common over the years, there are several versions of non-lubricated positive-displacement lobe or screw rotary compressors. These units are referred to as clearance-type compressors because the internal parts do not contact each other, so they require no lubrication in the compression chamber. Cooling is accomplished through the cylinder walls via water jackets.

The lobes or screws do not drive one another either; they are driven by some type of gear arrangement instead. This drive system also acts as a timing gear to maintain the rotor or lobe profile relationship accurately. Lubricant for the drive train must be confined to the bearing and gear area - and not allowed to get into the compression chamber.

In this basic design, there is a constant leakage rate for any fixed set of conditions. The critical internal clearances are between end covers and the rotor, between the rotor lobes, and between the rotor OD and the cylinder ID. These gaps, combined with no injected oil to help with sealing, are the main reasons why two stages are required for these units to produce acceptable efficiencies in 100-psi class applications.

Because these are rotary units, they enjoy all the advantages of rotaries over similar-sized non-lubricated reciprocating units:

• compact size,
• smooth delivery of cool air,
• ease of installation, and
• simple (but critical) maintenance

• more sensitive to dirty inlet air,
• lower efficiency - resulting in higher power cost, and
• any repair work is more sophisticated and requires specialized training, which the user may not have nor want to have. This means repair work will probably have to be performed by the distributor or the manufacturer.

Sliding-vane rotary compressors

Oil-cooled sliding-vane compressors, Figure 2, operate as other positive-displacement compressors do by trapping a charge of intake air - in this case, between the vanes. As the eccentric rotor turns, the vanes are forced into the rotor slots, shrinking the size of the cell holding the trapped air. The air is compressed to full discharge pressure when it reaches the outlet port. The heat of compression is removed by cooling oil sprayed right into the air while it is being compressed. The same oil helps with sealing the vane tips.

For decades, oil-cooled, sliding-vane rotary compressors have been popular for continuous-duty applications. Their design has a number of unique characteristics:

• light weight - yet continuous rating,
• integrated and compact configuration,
• efficient production of compressed air at relatively low rotary speeds,
• smooth operation with little vibration,
• extremely quiet operation,
• coolest possible discharge air, and
• few wearing parts, making the machine easy and economical to repair.

However, the oil-cooled rotary-vane design in its single-stage configuration is limited in capacity. Bending stress applied to the vanes is the problem. The speed, size, and weight of the vanes must be limited for the machine to be durable. Because of this, oil-cooled rotary-vane compressors generally are applied only in a size range between 2 and 100 hp.

Lubricated or lube-free?

Two fundamental groups of compressor types are lubricated and lube-free. Lubricated compressors use oil to reduce friction between moving parts. As a result, some oil is entrained in the air being compressed. The entrained oil must be removed from or tolerated by the downstream system.

Lube-free compressors use no oil in the airend, and thus add no oil to the compressed air they produce.

Power and efficiency

Brake horsepower is the input power required at the compressor input shaft for a specific speed, capacity, and pressure condition.

Motor or engine horsepower is the nominal rating of the prime mover.

The service factor is the additional power built into an electric motor above its nominal rating - expressed as percent. Within the service factor, the brake horsepower driving an air compressor can be higher than the motor's nominal horsepower.

The power efficiency of a compressor is the ratio of the air delivered by the compressor and its input electrical requirements. Efficiency usually is expressed as brake horsepower per 100 cfm of delivered air.

Water-cooled rotary screws

Another version of oil-free rotary-screw compressors is a single-stage design that uses water injection to cool and seal the rotors during compression. The bearings and drive gears are lubricated with oil and sealed from the compression chamber. These units serve a selected market and are a special design. In some applications, care must be taken to avoid the build-up of bacteria in the water.