Chiller Systems

These systems provide chilled water for use in air conditioning or other cooling applications. While some models of chillers are more energy efficient than others, all generate a valuable commodity: waste heat. When skilled operating personnel will not be on duty during system operation, operations are planned to use absorption chiller as a peak shaving unit.

Centrifugal Compressors

Highly efficient systems, centrifugal compressors use one or more rotating impellers to compress refrigerant.

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.

Advantages vs. Disadvantages

Advantages

  • Factory packaged for ease of proper installation
  • Most efficient chilling package – low kW per ton (centrifugals at 0.50 and less available, with screws at somewhat higher kW per ton)
  • Several major reliable suppliers, each with a service network of trained technicians
  • Available from 100 to 10,000 tons capacity in a single centrifugal chiller, with screws available in a lower tonnage range – check suppliers)
  • Lowest weight, height and footprint of any alternative
  • Use environmentally acceptable refrigerants
  • Have excellent and step-less part load characteristics
  • Are available in some sizes in dual compressor models for even more efficient part load operation
  • Chillers are relatively easy to operate with their modern controls and designs

 

Disadvantages

  • More costly in smaller sizes than other types of chiller packages
  • Centrifugal chillers are available only in water-cooled models
  • Screw compressor chillers are somewhat noisier than other designs
  • Screw compressor chillers are somewhat less efficient than centrifugals

Applications

Centrifugal and screw packaged chillers are typically applied in single and multiple units to provide chilled water for air conditioning large buildings and complexes, and in district cooling systems. They are also used in heat pump form for both heating and cooling. They are also used to chill brine in industrial processes.

Best Applications

Centrifugal chillers are usually the best selection among alternatives when used to provide chilled water for cooling large buildings with automated operation

Efficiency

Centrifugal chillers are the most efficient chilling packages available. Recent design improvements result in a low kW per ton (centrifugals are available at 0.50 and less kW/ton, with screws at somewhat higher kW per ton). Higher kW per ton can also be supplied at lower first costs; these may or may not be prudent depending on the present and anticipated cost of power (both demand and energy).

 

Contact us for a detailed list of manufacturers for this equipment.

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.

 

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.

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