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

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 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.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.

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.