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.
Reciprocating & Scroll Packaged Chillers
Providing improved part load efficiencies and employing environmentally acceptable refrigerants, reciprocating & scroll packaged chillers are packaged for easy, lower-cost installation. They’re an excellent option for smaller and medium-sized buildings.
Reciprocating, and to a lesser extent scroll, packaged chillers are the heart of most central systems for medium sized buildings. They consist of one or more reciprocating or scroll compressor-motor assembly, air- or water-cooled condenser (and condenser air fan(s)), insulated liquid cooler, expansion device, interconnecting refrigerant piping, oil and refrigerant charge, control panel and wiring, auxiliaries, and the compressor motor-starter. They are made in both full- and semi-hermetic and open types. They are manufactured, factory assembled and tested, charged, and shipped in one assembly up to about 2,00 tons capacity. Installation consists of piping supply and return chilled water piping, (and cooling water piping where applicable,) power wiring and interconnection of external controls, evacuation and charging when necessary, check-out and startup. Units are also available without any condenser for field piping to a separate remote air-cooled or evaporative condenser.
Advantages vs. Disadvantages
- Factory packaged for ease of proper installation
- Many reliable suppliers, most with a service network of trained technicians or through factory approved service providers
- Reciprocating models available from many manufacturers in capacities up to 200 tons and larger in incremental steps (i.e. 20, 25, 30, etc)
- Scroll chillers are available from a lesser number of manufacturers and more limited models (20 to 60 tons – check suppliers)
- Scroll chillers have improved part load efficiencies
- Use environmentally acceptable refrigerants
- Various condensing options available, including air-cooled and for low ambient operation
- Chillers are relatively easy to operate with their modern controls and designs
- Reciprocating models: Part load capacity is stepped – cylinder unloading, compressor on/off of multiple compressor units
- Central plant systems tend to be more costly than unitary systems
Reciprocating and scroll packaged chillers are typically applied in single and multiple units to provide chilled water for air conditioning small to medium sized buildings using central system designs.
High quality installations requiring central chilled/hot water systems to provide temperature and humidity control of multiple spaces.
Many models will fit through a 30 inch door for ease of installation at minimum cost of structural modifications.
Reciprocating compressor packages have been the mainstay of this segment, and many advances and improvements have been made over the years. They are in wide-spread use and trained service technicians are available almost everywhere.
Scroll compressor packages are the latest advancement in positive displacement compressors and indications are they may offer better reliability and improved efficiency. The network of trained service technicians is in the process of development and may not be available in all locations.
Older water cooled chillers are in the 0.82 to 1.0 kW per ton range, while newer high efficiency models range from 0.78 to 0.85 kW per ton at ARI conditions.
Most cooling systems in use today rely on reciprocating piston-type compressors. Reciprocating compressors are manufactured in three types:
- Hermetic – compressor-motor assembly contained in a welded steel case, typically used in household refrigerators, residential air conditioners, smaller commercial air conditioning and refrigeration units.
- Semi-hermetic – compressor-motor assembly contained in a casting with no penetration by a rotating shaft and with gasketed cover plates for access to key parts such as valves and connecting rods.
- Open – compressor only with shaft seal and external shaft for coupling connection to belt – or direct-drive using as electric motor or natural gas engine. These are largely used for ammonia refrigeration applications as hermetic designs cannot be used with ammonia refrigerant, and for engine-driven units.
As the piston nears the bottom of its stroke within the cylinder, the intake valve opens and the refrigerant vapor enters. As the piston rises, the increased pressure closes the intake valve. Then as the piston nears the top of its stroke, the exhaust valve opens permitting the vapor at the higher pressure to exit. Reciprocating compressor capacity is a function of the bore and stroke of the piston-cylinder configuration as well as the speed of the machine, and the clearance tolerances. Compressor capacity is also related to the compression ratio.
The mechanical design is rugged and reliable but has one significant limitation. Reciprocating compressors are designed to handle vapors, not liquids. When liquid enters the cylinder on the intake stroke, it tends to damage the valves on the compression stroke and possibly the compressor itself. This is why chillers incorporate liquid-to-suction heat exchangers, which assure some level of vapor superheat at the compressor suction. Capacity is controlled by multiple staging of smaller compressors or in large multiple cylinder reciprocating compressors by unloading banks of cylinders on the compressor. This tends to make the machine most efficient at full load. Therefore, for maximum efficiency recips should generally be operated at full load. This is the reason small compressors are cycled on and off in most residential and small commercial applications.