Rotors

The rotor is the working portion of the compressor and, as was pointed out, is machined to both generate the helix and form the profile (see Figure 4-13). Some dry compressors are furnished with hollow rotors

Figure 4-13. A rotor set for an oil-free helical-lobe compressor. (Courtesy ofA-C Compressor Corporator!)

through which cooling fluid is circulated. As with the cooling jacket, the cooling is somewhat misnamed because the more important aspect of the fluid is to help stabilize the rotor. Materials of construction are steel in most applications. The material may be either a forging or bar stock, based on size availability of the bar stock in the quality needed. Other materials are used whenever carbon steel is not compatible with the gas being compressed. These range from stainless, either of the austenitic or 12 chrome type, to more exotic nickel alloys.

Some vendors furnish coatings for the rotors in order to keep the rotor from wearing and losing seal clearance. One such coating is TFE. There is wide diversity of opinion on the value of coatings in light of varying performance. For the purpose of renewing clearances at the time of maintenance, some vendors use a renewable seal strip. This is a very good feature on dry compressors where seal strip clearance is .0005 to .001 inch per inch of rotor diameter at 350°F. If the compressor is designed for 450°F and run in at 500°F, the loss in area would result in an efficiency loss of lA to 1%.

Rotor speeds are such that dynamic balancing is required for proper vibration control. Also, while the critical speeds are generally above the operating speed, review of the rotor dynamics should not be ignored, particularly for the dry type.

Bearings and Seals

For a general discussion of bearings and seals refer to Chapter 5. The coverage at this point will be limited to the identification of the various types used on the screw compressor.

In the larger, dry process compressors, the radial bearings are of the sleeve or tilting pad type. Bearing surfaces use a high tin babbitt on a steel backing. API 619 requires the bearings to be removable without removing the rotors or the upper half on the horizontally split machine. Thrust bearings are generally tilt pad type, though not necessarily symmetric. On standardized compressors for air or refrigeration, the bearings are normally the rolling element type. Some standardized dry compressors use a tapered land thrust bearing. Most of the flooded compressors and some of the standardized dry compressors use rolling element thrust bearings. In all cases, the bearings are pressure-lubricated with some compressors using the gas differential pressure to circulate the lubricant and thus pressurize the bearings. For difficult services mentioned previously, unusual bearings may be used, such as graphite with a sulfuric acid flooding medium.

In dry compressors, shaft end seals are generally one of five types. These are labyrinth, restrictive ring, mechanical contact, liquid film, and dry gas seal. The labyrinth type is the most simple but has the highest leakage. The labyrinth seal is generally ported at an axial point between the seals in order to use an eductor or ejector to control leakage and direct it to the suction or a suitable disposal area. Alternatively, a buffer gas is used to prevent the loss of process gas. Appendix D presents a calculation method for use with labyrinth seals.

Probably the most common seal is the restrictive ring type, normally used in the form of carbon rings. This seal controls leakage better than the non-floating labyrinth type, although it wears faster. The carbon ring seal does not tolerate dirt as well as the labyrinth seal. The carbon ring seal and the labyrinth seal may be ported for gas injection, ejection, or a combination of both. Any injection gas should be clean.

The mechanical contact seal is a very positive seal. The seal is normally oil-buffered. The mechanical seal, which is the most complex and expensive, is used where gas leakage to the atmosphere cannot be tolerated. This may be due to the cost of the gas, as in closed-loop refrigeration, or where the process gas is toxic or flammable. The mechanical contact seal requires more power than the other seals, which is a deterrent to its use on lower power compressors.

The liquid film seal uses metallic sealing rings and is liquid buffered to maintain a fluid film in the clearance area and thereby preclude gas leakage. It is not unusual in the screw compressor to find the radial bearing and seal combined.

The dry gas seal is a variation of the mechanical contact seal. It differs in that it uses a microscopically thin layer of gas to separate and lubricate the faces. The seal is configured in a tandem or double-opposed seal arrangement. More complete details are covered in Chapter 5 under Dry Gas Seals.

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