Centrifugal compressors use one or more rotating impeller to increase the refrigerant vapor pressure from the chiller 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 pre-rotated 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 form the compressor hub, which may be shrouded for flow-path efficiency. The combination of rotational speed and wheel diameter combine to create the tip speed necessary to accelerate the refrigerant vapor to the high pressure discharge where they move on to the chiller condenser. Due to their very high vapor-flow capacity characteristics, centrifugal compressors dominate the 200 ton and larger chiller market, 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 water chiller packages may be 1-, 2-, or 3-stages and use a semi-hermetic motor or an open motor with shaft seal.
Due to their very high vapor-flow capacity characteristics, centrifugal compressors dominate the 200 ton and larger chiller market, where they are the least costly and most efficient cooling compressor design.
Centrifugal compressors - performance
Packaged water cooled centrifugal compressors are available in sizes ranging from 85 tons to over 5,000 tons. Larger sizes, typically those 1,200 to 1,500 tons and larger are shipped in sub-assemblies. Smaller sizes are shipped as a factory-assembled package. While some smaller air-cooled centrifugal models are manufactured, they are largely exported to the Middle East and other arid areas where water is simply not available for HVAC condensing use, even in cooling towers.
The centrifugal compressors mentioned here will be using HCFC-123, HCFC-22 and HFC-134a. This usually calls for semi-hermetic designs, with single or multi-stage impellers. Two manufacturers (Carrier and McQuay) offer semi-hermetic gear driven models. Trane offers multi-stage direct drive semi-hermetic units. York offers an integrated open-drive geared design.
Chillers 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.
Centrifugal compressors - power requirements
Power requirements for centrifugal chillers are the lowest of all chiller types currently available, and efficiencies have been improving even further over the years as a result of improved impeller designs, better unit configurations, enhanced heat transfer surfaces, and the increased utility emphasis on reducing energy requirements.
At ARI standard rating conditions centrifugal chiller's performance at full design capacity ranges from 0.53 kW per ton or lower to 0.68 kW per ton. This performance includes the semi-hermetic refrigerant cooled or open type compressor motors.
Open drive chiller power requirements are sometimes rated in shaft brake horsepower (bhp). To convert from bhp to electric input in kW, the efficiency of the motor must be considered (which is usually between 90 and 95 percent for centrifugal machines). For example, a rating of 1,000 bhp at 93 % motor efficiency would translate to 802 kilowatt input.
(1,000bhp x 0.746 kW/bhp) = 80.2 kW input
93% Motor efficiency
Centrifugal compressors - first cost
Centrifugals chillers 200 tons and larger cost less to install than reciprocating chillers (available up to the 175 to 200 ton range) and the same or slightly less than screw chillers in most all sizes. Centrifugals offer the advantages of high efficiency, infinitely variable capacity control (down to about 10 percent of full load), they're lighter (which reduces floor loadings) and they take up much less space for a given tonnage.
First cost of centrifugal chiller packages generally start higher than recips under 200 tons, and then cost less in the larger sizes. More definitive costs are shown in the Compare segment.