Process chillers are refrigeration systems that maintain tight temperature control for industrial processes and applications. Operators depend on process chillers to remove significant quantities of heat during constant operation in demanding environments. Common applications include plastic injection molding, food processing, pharmaceuticals and chemical manufacturing processes.

Ammonia (R-717) is the refrigerant most often used for industrial refrigeration, cold storage warehouses, food processing plants, ice rinks, wineries and breweries because it is efficient, economical and environmentally friendly. With rising environmental awareness and energy efficiency needs among industrial processors, pharmaceutical manufacturers and food producers, satisfying process cooling requirements with an environmentally friendly refrigerant such as ammonia has become a growing trend.

Traditional ammonia plants often require refrigerant charges above 10,000 pounds of ammonia, and systems are usually one of a kind, assembled in the field. But low charge ammonia refrigeration systems are starting to gain favor for large-capacity process cooling applications in the 100 to 1,000 hp range. Refrigerant charges can be as low as one pound per horsepower. These process chillers, packaged with single-screw compressors, are a durable process solution.

Single-Screw Compressor Packages

Ammonia is an efficient natural refrigerant that provides maximum coefficient of performance (COP). In low charge packaged systems, the ammonia is contained within a small area where only trained operators are able to service the system. The system design isolates the ammonia refrigerant on a packaged skid that does not need to be installed inside the plant. Only a secondary fluid is sent to the working spaces. These skid-mounted packages can be portable as well to be used on different processes.

Factory-engineered and assembled packaged refrigeration systems have many advantages over field-built units, including:
  • Lower on-site assembly costs.
  • Quicker installation and commissioning.
  • Refrigerant is confined to the packaged refrigeration system and not to the owner’s plant operation spaces.
  • Space saving compact design.
  • Can be designed for mobility.
  • Can be designed with enclosures to allow remote, stand-alone refrigeration plants.
  • Factory wired and tested.
  • Factory warranty.
The heart of the refrigeration system is the compressor. Single-screw compressors are well suited for larger horsepower applications because of their high capacity, high efficiency, low maintenance, high reliability and operating flexibility. Single-screw compressors benefit from balanced forces around the main rotor. Balanced axial and radial forces offset one another so that, effectively, the only net force on the main rotor of the single-screw compressor is gravity. The low bearing loads result in long compressor life and high reliability. At least one single-screw manufacturer is able to offer a 15-year bearing warranty. Long bearing life reduces maintenance costs and downtime events for operators.

The symmetrical design of single-screw compressors allows parallel capacity and compression ratio slides. This design advantage over twin-screw compressors allows single-screw compressors to independently control capacity and compression ratio, allowing the system to operate at optimal efficiency at full load and part-load conditions. In contrast, twin screws are designed with only one slide; therefore, they are unable to simultaneously optimize capacity and compression ratio. Partial load efficiency on single-screw systems can be further improved by application of variable-speed drives in large compression ratio applications, but this is unnecessary for compression ratios below 6.0. Performance of the single screw can be optimized in high compression ratio applications with large dual economizer ports. These allow much greater flow rates into the machine for improved COP efficiency and can be used to allow the compressor to pull in intermediate temperature loads from other sources.

Process Heating with Heat Pumps

The heat removed by industrial process cooling systems usually is discarded to the atmosphere. However, recent developments in single-screw compressors now enable industrial heat pumps to capture rejected heat and put it to use for beneficial process heating purposes.

Capturing the rejected heat of refrigeration systems is not a new concept, but ammonia heat pumps have not been workable until recently because of compressor limitations. Reciprocating and twin-screw compressors have inherent design limitations that restrict their ability to deliver high water temperatures, and their efficiency limitations result in poor economic payback in these heat recovery applications. Industrial heat pumps designed with high-pressure single screw compressors can efficiently deliver high quality hot water from 140 to 190°F (60 to 88°C).

Single-screw systems are already being used by major food processing companies in the United States, Europe and Latin America. A few case histories can demonstrate these real world applications.

Pharmaceutical Processing. One of the world’s largest pharmaceutical companies has an initiative to use only natural refrigerants like R-717 ammonia. For its 800 gal/min water chiller, the 700 hp system takes advantage of the flexibility of the single-screw compressor and the VFD motor control. This combination improves economizer performance and partial load performance, saving energy and money. The system uses a low ammonia charge - about 1 lb per ton of refrigeration - meeting the requirement to reduce the amount of ammonia used on site.

Chocolate Processing. In a typical chocolate processing plant, refrigeration is needed to cool and store chocolate, and heat is needed to separate the chocolate from the shaping molds. The largest food company in the world wanted to find a way to capture waste heat from its process cooling operations to replace the need for gas-fired equipment in its process heating operations.

The solution was a dual-purpose ammonia heat pump system that delivers chilled glycol at 32°F (0°C) and hot water at 140°F (60°C) using waste heat. It uses single-screw compressors. With the new system, heat can be taken from the 32°F process glycol and lifted to 140°F in one stage for heating. Since commissioning in May 2010, the system is heating around 14,000 gal of water each day to 140°F. As a result of the new system, hot water is delivered more efficiently than from the previous coal-fired steam generator. The ammonia heat pump solution has cut process utility costs at the site by more than $394,000 per year. In addition, the reduction of gas combustion has reduced CO2< emissions by more than 1.1 million pounds per year.

Cheese Processing. This 300 hp package is used in a glycol cooling system that isolates the ammonia refrigerant from the rest of the plant. This method reduces the amount of ammonia used and improves safety. The package is constructed with a plate-and-frame chiller. The entire package is smaller than a shell-and-tube system, and the compact size simplifies installation and cuts the setup’s footprint.

Industrial process cooling applications are demanding and require chillers built to industrial standards. Low charge ammonia refrigeration systems provide a durable, safe solution for many applications. Compact factory packages lower costs and simplify installation. Single-screw compressors with independent capacity and compression ratio slides and VFD motor control have good partial load performance and economizer operation. In addition, the high pressure capability of single screws enables industrial heat pumps to capture rejected heat and put it to use for beneficial process heating purposes.