At the heart of each mechanical process refrigeration system is the refrigerant compressor. Every mechanical refrigeration system uses a compressor that draws in low-pressure refrigerant vapor from the refrigerant evaporator and discharges high-pressure, superheated refrigerant vapor to the refrigerant condenser.
Many types of compressors — reciprocating, centrifugal, scroll and screw compressors, to name a few — can be used for industrial refrigeration. Each type is suited for a range of applications and refrigerants and has its own unique benefits and drawbacks.
One compressor type that works effectively for a broad range of industrial refrigeration applications is the oil-flooded screw compressor. The benefits of using an oil-flooded screw compressor for industrial refrigeration are the focus of this article. They include:
- Good unloading capabilities to adjust the compressor capacity in response to process demands.
- Ability to be used with many types of refrigerant.
- A large application envelope.
- A long maintenance interval.
Oil-flooded screw compressors have been used extensively in the industrial refrigeration industry since the late 1960s. Today, multiple manufacturers offer oil-flooded screw compressors with either a single-screw or a twin-screw design. Both single-screw and the twin-screw compressors inject oil into the compressor that mixes with the refrigerant vapor, resulting in the need to separate the oil after the discharge of the compressor.
Unloading Capabilities for Screw Compressors
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Features of many industrial refrigeration applications are: a large turndown, a highly variable batch process, and demand for fine temperature control. An oil-flooded screw compressor is well suited for these types of applications.
Most oil-flooded screw compressors currently on the market have an internal slide valve that is used to load and unload the compressor. Basically, the slide valve shortens the effective compression length of the rotors, thus reducing the capacity of the compressor. In general, manufacturers of oil-flooded screw compressors utilize a slide valve that allows unloading to a nominal 10 percent capacity. In addition, the slide valve allows continual and stepless unloading and loading from this minimum 10 percent capacity to full capacity. This continual unloading from 10 percent to 100 percent allows the screw compressor to effectively match the specific load profile to which a process chiller is subjected. These features make the oil-flooded screw compressor design suitable for demanding applications such as those with large turndown requirements, highly variable batch loads or fine temperature control.
Other compressors do not offer the same abilities. For instance, a reciprocating compressor often is limited to unloading of cylinders, which results in stepped capacity control and also minimum unloading, based on the number of cylinders. For example, a four-cylinder reciprocating compressor typically has load steps of 25 percent, 50 percent, 75 percent and 100 percent capacity. Likewise, centrifugal compressors are also limited on how much they can unload, and care must be taken to avoid surge at lower load conditions.
Refrigerant Versatility with Screw Compressors
Oil-flooded screw compressors are suited for use with numerous natural refrigerants, including ammonia (R717), carbon dioxide (CO2) and hydrocarbons (propane, propylene) as well as HFCs (R134a, R507, R404a, etc.) and other synthetic refrigerants. Without major modifications, an oil-flooded screw compressor can be applied to all of these refrigerants as well as other process gases.
When adapting an oil-flooded screw compressor to alternate refrigerants, several factors must be considered. For instance, most industrial refrigeration applications require design pressures in the range of 300 to 400 psig. However, some refrigerants like carbon dioxide used in a cascade refrigeration system require higher pressures of up to 750 to 900 psig. Oil-flooded screw compressors are available for both the common refrigeration system pressure range of 300 to 400 psig as well as the high-pressure range for use with carbon dioxide and other high-pressure refrigerants.
Another important factor to consider when applying an oil-flooded screw compressor is the oil. Proper care must be taken to ensure compatible oil is selected and that proper viscosity is maintained throughout the full operating range of the compressor. Also, depending on the refrigerant and the oil used, proper elastomers must be selected.
Other modifications required on an oil-flooded screw compressor often are directed by governing codes or customer preference. For example, it is common for industrial refrigeration systems using hydrocarbons to include a compressor with cast-steel housing in lieu of cast or ductile iron and also to opt for a double mechanical-shaft seal in lieu of a single mechanical-shaft seal.
Broad Application Limits of Screw Compressors
Industrial oil-flooded screw compressors offer a much larger application envelope when compared to a reciprocating or centrifugal compressor. Typically, the oil-flooded screw compressor can operate in a system with a pressure ratio (discharge pressure divided by suction pressure) as low as 1.5 to a pressure ratio beyond 20. In contrast, a reciprocating compressor can operate with pressure ratios up to approximately 10. This large application envelope can be achieved in part because of the oil injected into the compressor.
The injected oil is used for radial and thrust bearings, capacity control and the shaft seal. In addition, oil is injected into the rotor housing for both cooling and sealing of the rotor tips. The sealing of the rotor tips allows for improved efficiency at all pressure ratios, but this is especially true at high-pressure ratios because the oil effectively reduces the blowback internal to the compressor. Also, larger pressure ratios generate more heat, and the injected oil is used to transfer the heat of compression away from the compressor.
Most oil-flooded screw compressors are equipped with a side port — often referred to as an economizer port — at an intermediate pressure. The use of the side port ultimately offsets some of the efficiency losses associated with larger pressure ratios. This port allows introduction of refrigerant vapor at an intermediate pressure between suction and discharge, which, in turn, allows for the use of an economizer. The economizer is a vessel or heat exchanger that allows for expansion of the refrigerant at this intermediate side-port pressure, which in turn improves the capacity and efficiency of the compressor.
Screw Compressors Deliver Longer Maintenance Intervals
One of the more significant benefits of an industrial oil-flooded screw compressor, especially when compared to reciprocating compressors, is the time between overhauls. Historically, manufacturers used time-based schedules for overhauls for screw compressors. Often, this interval is in the range of 25,000 to 30,000 run hours. However, with a strong predictive maintenance program that includes vibration and oil analyses performed multiple times a year, time between overhauls can be extended well beyond the historical time-based interval.
In addition to low maintenance costs, with routine maintenance and periodic overhauls, industrial oil-flooded screw compressors have a useful operating life well beyond 20 years.
In conclusion, the key to creating an optimal industrial refrigeration system is to fully understand the application, the load profile and how the system will operate. With this information in hand, the heart of the matter can be addressed — selection of the most advantageous compressor type. Often, the resulting compressor selection will be an oil-flooded screw compressor.
Oil-flooded screw compressors provide benefits when compared to other compressors and can be efficiently and effectively applied in a magnitude of industrial refrigeration applications.