Centrifugal Compressors

Centrifugal compressors range in capacity from 50 to 10,000 tons (175 to 35,000 kWf) under ARI standard conditions. Economic factors relating to manufacturing limitations and efficiency losses generally dictate that most applications start above 200 tons (700 kWr ). Custom-built units may exceed 20,000 tons (70,000 kWQ. They offer smooth, almost vibration-free operation, and, because they have so few moving parts, offer long service life with limited maintenance requirements.

Whereas reciprocating and screw compressors are positive displacement machines that physically squeeze gas with pistons or rotors to compress it, centrifugal compressors are variable displacement machines that impart centrifugal force to the gas to produce velocity energy that is converted to pressure. Centrifugal compressors operate at speeds of 3,000 to 20,000 rpm and produce compression with a high-speed impeller(s). Vapor is fed through the suction line into a housing near the center of the compressor. A disk with radial blades (impellers) spins rapidly in the housing.

W hereas the piston and the helical rotors are at the heart of the reciprocating and screw compressors, respectively, the impeller is at the heart of the centrifugal compressor. Figure 37-10 shows an impeller. The center, or eye, of the impeller is fitted with vanes that draw gas into radial passages, which are internal to the impeller body. The rotation of the impeller increases centrifugal force and moves the vapor to the outside of the impeller, where it slows down and expands into a diffuser tube, causing pressure to increase. Figure 37-11 shows an impeller, volute, and diffuser passage.

Fig. 37-10 Centrifugal Refrigeration Compressor Impeller. Source: The Trane Company

Fig. 37-11 Impeller, Volute, and Diffuser Passage in Centrifugal Compressor. Source: The Trane Company

through the discharge baffle into the condenser. In this system, a subcooler is shown directly downstream of the primary condenser, and an oil cooler is shown directly upstream of the cooler.

The forces that act on the impeller can be broken down into two components: A radial velocity component, vr, acts to move the gas away from the impeller in a radial direction. A tangential velocity component, vt, acts to move the gas in the direction of impeller rotation. Both of these forces act to generate the resultant velocity vector, R, the length of which is proportional to the kinetic energy available for conversion to static pressure. For a given

Fig. 37-10 Centrifugal Refrigeration Compressor Impeller. Source: The Trane Company

Figure 37-12 illustrates the refrigeration cycle for a chiller system featuring a centrifugal compressor. Notice the suction flow of low-pressure vapor into the eye of the impeller and the subsequent flow of high-pressure vapor outside of the impeller in the volute casing and down

Fig. 37-12 Illustration of Refrigerant Flow in a Centrifugal Compressor-Based Chiller. Source: York International

compressor, vr is directly proportional to the mass flow of refrigerant handled, and vt is proportional to the impeller diameter times the rotational speed. Thus, changes in speed or impeller diameter for a given mass flow rate will result in increased or decreased static pressure producing capability.

The process described above is called a stage of compression. A centrifugal compressor using one impeller is called a single-stage machine. Multi-stage units use a series of impellers and provide higher compression ratios because they increase vapor pressure with each stage. The limiting factors of compression are tip speed and the number of stages. Tip speed is a function of the impeller's size and rpm. Single-stage machines may be limited by their surge line and may have to run under hot gas bypass at loads less than 15 or 25% of full load. Most centrifugal compressors have internal oil pumps that are driven by the compressor shaft or by an internal motor.

Figure 37-13 is an open-case view of an open-drive, horizontally split casing centrifugal compressor revealing the impellers in this integral multi-stage compressor. On the far right is the mechanical contact seal for drive-line shaft connections. Figure 37-14 provides a cutaway view of this type of open-drive centrifugal compressor. Notice that the compressor shaft extends through the casing to facilitate connection to a number of different types of drives.

Figure 37-15 is a cutaway illustration of a three-stage centrifugal compressor of hermetic design. Figure 37-16 shows a large-capacity, multi-stage, open-drive centrifugal compressor. This unit has a cast-iron casing that is horizontally split for accessibility to internal components. This compressor can be designed with two to eight stages, depending on the intended application.

Many centrifugal compressors use lower-pressure refrigerants such as CFC-11 (R-11) or HCFC-123

Fig. 37-13 Open-Case View of Open-Drive, Horizontally Split Casing Centrifugal Compressor. Source: Carrier Corp.
Fig. 37-14 Cutaway View of Open-Drive Centrifugal Compressor. Source: Carrier Corp.
Fig. 37-15 Cutaway View of Three-Stage Centrifugal Compressor of Hermetic Design. Source: The Trane Company
Fig. 37-16 Large-Capacity, Open-Drive Multi-stage Centrifugal Compressor with Horizontally Split Cast-Iron Casing. Source: York International

(R-123), and the evaporator operates at below-atmospheric pressure. Larger units of several thousand tons and some of the newer smaller units are designed to operate with higher pressure refrigerants, such as HCFC-22 (R-22). The trend in refrigerant replacement in centrifugal compressors is HCFC-123 for CFC-11 and HFC-134a for CFC-12 and HCFC-22.

Negative- (low-) pressure systems operate under a vacuum, which draws leaks into the chiller during operation. Noncondensables, such as air and water, must be eliminated from the system by a purge unit. These noncondensables can cause problems, compromising chiller performance and causing unplanned shutdowns, if left untreated. Purge units regularly vent refrigerant to the atmosphere along with the noncondensables. These purge systems must be maintained, and current environmental regulations prohibit the direct venting of refrigerants. Also, negative-pressure chillers may require use of a vacuum prevention system that heats the refrigerant to increase the pressure and prevent air from entering the unit. This is intended to decrease the amount of purging required as a result of leaks into the system during shutdowns.

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