Screw Compressors

Screw (rotary positive displacement) compressors range in capacity from under 20 tons (70 kWT) to about 1,800 tons (6,300 kWr) under ARI standard conditions. Their rotation motion allows for smooth, quiet, almost vibrationfree operation and low maintenance requirements, as compared with reciprocating compressors. The two main types of screw compressors used today are single screws and twin screws. Similar to reciprocating compressors, currently applied screw compressors typically operate on refrigerants such as HCFC-22, HFC 134a, and ammonia.

Screw compressors have meshing male and female helical rotors that compress refrigerant vapor. As the rotors turn and mesh, they trap the vapor that has been introduced by suction in the interlobe space. Typically, the male rotor is driven by the compressor driver and, in turn, drives the female rotor. The vapor is further compressed by continued rotation of the meshing rotors. The compression space becomes progressively smaller until the compressed refrigerant vapor is discharged toward the condenser as the interlobe space becomes exposed to an outlet port. The screw threads form the boundary separating several compression chambers that move down through the compressor at the same time. Twin screw compressors consist of two helically grooved rotors of the same shape, mounted in a housing with inlet and outlet ports. One or both rotors may be driven, depending on the specific design.

Figure 37-7 is a cutaway illustration of a screw compressor. Figure 37-8 illustrates the compression cycle of a typical screw compressor. On top, the lobes and grooves are shown in the compressor housing. On the bottom, the refrigerant flow path is shown from suction to discharge. Notice the profile design is asymmetrical to reduce refrigerant leakage. This is a 4-lobe/6-flute configuration. For higher capacity, lower-pressure applications, a 3-lobe/4-flute configuration may be used. For lower capacity and higher pressure, a 6-lobe/8-flute configuration may be applied.

Fig. 37-7 Cutaway Illustration of a Chiller Featuring a Helical-Rotary Compressor with a Hermetic Motor. Source: The Trane Company

Figure 37-9 illustrates the refrigeration cycle for a chiller system featuring a screw compressor. Notice the change in refrigerant phase and pressure as it moves through the cycle, with high-pressure refrigerant exiting the screw compressor on the upper left before being condensed in the condenser on the lower right.

Many designs feature an oil injection system to seal the clearance between the rotors and the cylinder. An oil separator is used to remove oil from the refrigerant discharge. The oil absorbs heat, which limits refrigerant discharge temperature. The oil is then cooled through methods such as external heat exchange or injection of liquid refrigerant. Alternatively, oil injection-free screw compressors are also commonly used. Instead of injecting oil into the compression chamber, liquid refrigerant can be injected. This allows for the elimination of discharge oil separators and external coolers associated with oil injection operation.

Screw compressors are typically more expensive than reciprocating compressors in smaller capacities, but offer smoother, often less noisy operation and lower maintenance requirements. As capacity increases, capital cost decreases relative to reciprocating compressors. When comparing similar (load) capacity equipment, screw compressors are relatively smaller than their counterpart centrifugal or reciprocating compressors. Screw compressors are typically less thermally efficient than comparable capacity centrifugal compressors at full load, but, due to

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