Page 144 - Root Cause Failure Analysis

132 Root Cause Failure Analysis

Lubrication System A V-belt-driven, force-fed oil lubrication system is used on
water-cooled compressors. Oil goes to both bearings and several points in the cylin-
der. Ten times as much oil is recommended to lubricate the rotary cylinder as is
required for the cylinder of a corresponding reciprocating compressor. The oil carried
over with the gas to the line may be reduced 50 percent with an oil separator on the
discharge. Use of an aftercooler ahead of the separator permits removal of 85 to 90
percent of the entrained oil.

Helical Lobe or Screw
The helical lobe, or screw, compressor is shown in Figure 10-7. It has two or more
mating sets of lobe-type rotors mounted in a common housing. The male lobe, or rotor,
usually is driven directly by an electric motor. The female lobe, or mating rotor, is
driven by a helical gear set mounted on the outboard end of the rotor shafts. The gears
provide both motive power for the female rotor and absolute timing between the rotors.

The rotor set has extremely close mating clearance (i.e., about 0.5 mils) but no metal-
to-metal contact. Most of these compressors are designed for oil-free operation. In
other words, no oil is used to lubricate or seal the rotors. Instead, oil lubrication is
limited to the timing gears and bearings outside the air chamber. Because of this,
maintaining proper clearance between the two rotors is critical.

This type of compressor is classified as a constant volume, variable-pressure machine
that is quite similar to the vane-type rotary in general characteristics. Both have a
built-in compression ratio.

Helical-lobe compressors are best suited for baseload applications, where they can
provide a constant volume and pressure of discharge gas. The only recommended
method of volume control is the use of variable-speed motors. With variable-speed

Figure 10-7 Helical lobe, or screw, rotary air compressor (Gibbs 1971).