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.

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.

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

They also have some disadvantages, depending on the specific type of compressor and its duty cycle:
• 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 types

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.

Figure 2

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.

Dynamic air compressors

Dynamic, or centrifugal compressors, Figure 3, are dissimilar to the positive-displacement machines already discussed because they raise the pressure of air by converting the energy of its velocity into pressure. First, rapidly rotating impellers (similar to fans) accelerate the air. Then, the fast flowing air passes through a diffuser section that converts its velocity head into pressure by directing it into a volute.

Because the centrifugal is a mass flow compressor, it has a limited stable operating range. This has a large effect on economic operation or bhp/100 cfm delivered at part load. Minimum turn-down capacities for centrifugals may vary from 20 to 30% of full load, depending on impeller design, number of stages, etc.

There are limits to the pressure rise that can be achieved in a single stage by a centrifugal compressor — due to both physical and economical restraints — so two- to four-stage units are built that incorporate one to three water-cooled intercoolers. Cooling the air between stages reduces the power required to compress the air further, resulting in more efficient operation. Intercooling actually may permit the desired compression to be accomplished in fewer stages.

The centrifugal compressor is definitely a continuous-duty unit because its service life is unaffected by full-load operation. However, it is also a relatively sensitive machine because it operates at high speeds — often as high as 50,000 rpm. Ambient factors which affect flow are altitude, inlet air temperature, and the relative humidity of inlet air. The operating life of this type of unit is primarily determined by the amount of entrained liquids and solids carried into the unit at the inlet — and the quality of the cooling water. As in all machinery, correct installation and maintenance is critical to the efficient production of compressed air and reaching a satisfactory operating life.

When a facility requires a continuous-duty, high-volume (2000 to 25,000 cfm) supply of non-lubricated air, the centrifugal compressor is one of the best choices. In fact, it is the only choice in sizes above 1000 hp. Whether or not it fits the installation best is another question to be answered after analyzing the job conditions. In any event, when correctly applied, installed, and maintained, a centrifugal compressor offers a reliable, continuous source of compressed air.