Centrifugal & Screw

General

Centrifugal, and to a lesser extent screw, packaged chillers are the heart of most central systems for large buildings. They consist of the centrifugal or screw compressor-motor assembly, water-cooled condenser, insulated liquid cooler, expansion device, interconnecting refrigerant piping, lube system, oil and refrigerant charge, control panel and wiring, auxiliaries, and in some cases the compressor motor-starter. They are made in both hermetic and open types, and with single- and multi-stage centrifugal designs. They are manufactured, factory assembled and tested, charged, and shipped in one assembly up to about 2,000 tons capacity; in large sizes they are factory disassemble in major pieces for shipment and installation. Installation consists of piping supply and return chilled water or brine piping, and cooling water piping, power wiring and interconnection of external controls, evacuation and charging when necessary, check-out and startup.

NOTE: Screw compressors are also sometimes referred to as ahelical rotary compressors.

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.

Applications to Avoid

Centrifugal chillers are relatively Aconstant head machines and they are not particularly suited to applications with widely varying differences in leaving chilled and condenser water temperatures. Screw chillers should be considered. Screw chillers tend to be noisy and installation near spaces where this could be a problem should be avoided.

For additional information see also:

Centrifugal Compressors
Screw Compressors

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).

Links to Related Topics

Reciprocating & Scroll
Free Cooling
Absorption
Natural Gas - Engine Driven