Refrigerating Centrifugal Compressors

Centrifugal compressors are often used in place of positive displacement compressors for very large capacities, or for high-flow low-pressure difference applications, and are available, designed for refrigeration use, in the 300 kW-20 MW range (e.g. 400–10,000 tons). Centrifugal compressors are also appropriate to multi-stage refrigeration applications, where two or more compression stages may be incorporated within the same turbine housing with interstage gas injection between the rotors. These compressors produce compression by means of a high-speed impeller connected to an electric motor or gas engine. Figure 3.13a shows the cutaway view of a centrifugal compressor which uses hybrid bearings. The incorporation of hybrid bearings in compressor designs allows the refrigerant itself to be used as the lubricant. Figure 3.14b shows a chiller unit with a centrifugal compressor using hybrid bearings.

The centrifugal compressors available in the market use R-123, R-22 and R-134a. This usually calls for semihermetic designs, with single or multi-stage impellers. In refrigeration industry multi-stage centrifugal compressors are now manufactured with cast iron, nodular iron and cast steel casings for discharge pressures up to 40 bar. With up to eight wheels in a single casing, the compressor has a capacity of 42,000 m3/h and 9000 kW.

Note that refrigeration systems using ammonia as the refrigerant are not generally available with centrifugal compressors. Only open-drive screw or reciprocating compressors are compatible with ammonia, largely because of its corrosive characteristics and reactions with copper.

The selection of single stage, multi-stage, open or hermetic designs is largely a function of individual manufacturer preference and the application. For example, centrifugal compressors are limited in their compression ratio per impeller. Therefore, applications calling for high temperature lifts (such as with ice thermal storage) may require multi-stage designs.

The operating principle of a centrifugal compressor is the same as that of a centrifugal pump, but the refrigerant gas is pumped instead of a liquid. A rotating impeller imparts velocity to the gas, flinging it outward. The housing slows the gas flow, converting a portion of the kinetic energy (the velocity pressure) into a static pressure. These compressors are commonly used for large-capacity refrigeration systems (e.g. from 200 kW to over 10.000 kW of cooling) with low-pressure ratios and operate with adiabatic compression efficiencies of up to 80%. Evaporator temperatures may reach -100°C.

Packaged water cooled centrifugal compressors are available in sizes ranging from 85 tons to over 5000 tons. Larger sizes, typically 1200 to 1500 tons and larger, are shipped in sub-assemblies. Smaller sizes are shipped as a factory- assembled package (Figure 3.14).

Centrifugal compressors use one or more rotating impellers to increase the refrigerant vapor pressure from the evaporator enough to make it condense in the condenser. Unlike the positive displacement, reciprocating, scroll or screw compressors, the centrifugal compressor uses the combination of rotational speed (rpm) and tip speed to produce this pressure difference. The refrigerant vapors from the chiller evaporator are commonly prerotated using variable inlet guide vanes. The consequent swirling action provides extended part-load capacity and improved efficiency. The vapors then enter the centrifugal compressor along the axis of rotation.

The vapor passageways in the centrifugal compressor are bounded by vanes extending from the compressor hub, which may be shrouded for flow-path efficiency. The combination of rotational speed and wheel diameter combines to create the tip speed necessary to accelerate the refrigerant vapor to the high pressure discharge where they move on to the condenser. Due to their very high vapor-flow capacity characteristics, centrifugal compressors dominate the 200 ton level, where they are the least costly and most efficient cooling compressor design. Centrifugals are most commonly driven by electric motors, but can also be driven by steam turbines and gas engines. Depending on the manufacturer’s design, centrifugal compressors used in the packages may be 1-, 2-, or 3-stages and use a semihermetic or an open motor with shaft seal.

Figure 3.15 shows a new type of centrifugal compressor which has recently been developed by York International, providing completely oil-free compression using magnetic bearings, particularly for large-tonnage refrigeration and gas compression applications. Tested extensively using S2M magnetic bearing experience and technology, the magnetic bearing option eliminates all the negatives of purchasing and maintaining a lubrication system. In fact, the magnetic bearings enhance centrifugal compressor efficiency and operation. These compressors are available in all sizes and options.