Refrigeration compressors provide a refrigerant circulation in the system and, unlike gas compressors, are not used independently out of the refrigeration system. Differences in the working conditions of compressors in the structure of refrigerating machines from the operating conditions of general purpose compressors ones:
Operated over a wide range of changes in the suction and discharge pressures;
It is not infrequent that the refrigerants dissolve oils and the lubrication conditions of compressor deteriorate;
The aspirated steam has a low temperature and often carries drops of liquid;
Sometimes it can be observed a condensation of refrigerant in the cylinder (with intensive cooling);
Often the refrigerants are very fluid and have a high permeability;
Increased requirements are always demanding to the refrigerating compressors: greater reliability, considerable life-time, high efficiency, etc.
As in the air supply, used in refrigeration compressors, positive displacement (reciprocating, screw, plate) and Kinetic Energy (turbochargers, ejectors).
Piston Compressors
Most commonly in the refrigerating machines are used:
A. Enclose compressors with ammonia and freon. These are vertical or angular (V- and W-shaped) devices in case of the monoblock compressor (for strengthening, rigidity and easiness of layout) of a single-action machine. Usually these are the direct-flow compressors, with false cylinder covers (to protect against the water-hammer effect). Cylinders are cooled. The ammonia compressors have a jacket cooling by water. The freon compressors are air cooled, with fins of the cylinders.
Advantages:
high-speed, light and compact units;
irreplaceable for the small production.
Disadvantages:
complex access to the bearings, the lubrication system and other details, located in the block-crankcase.
B. Open-crankcase compressors with the horizontal opposed cylinders arrangement. These are double-action machines with the increased productivity, one- and two-staged, with a mutually opposite movement of the pistons. They are constructed to work on ammonia, propane, propylene. Cooling − water.
Disadvantages: sensitivity to water hammering. Machines of this type are successfully replaced by the screw compressors.
Screw Compressors
Screw compressors are widely used in the refrigeration machines. These are one-or two-rotor machines of volume effect with a constant geometric degree of compression. They are divided into two types: dry and oil-filled. A large amount of oil is injected into the working cavity to seal the gaps, lubrication and cooling into the oiled compressors. The oil injection allows reducing the noise level significantly.
Advantages:
high-speeding, low weight and dimensions (their dimensions in 2-10 times, and masses in 10-100 times less than in the reciprocating compressors of the same capacity), complete steadiness, practically non-pulsating feed, no vibrations;
a wide range of stepless performance control (from 10 to 100%), operation in variable modes with a slight change in efficiency;
absence of a surging mode;
the ability to work in a humid environment, as these machines are not afraid of water-hammer effects;
high reliability and considerable life-time (no valves and friction parts);
easy servicing, automation and remote control.
Disadvantages:
high noise level;
necessity for cooling at medium and high degrees of the increased pressure.
The cooling capacity of screw compressors is regulated by the movement of the slide, which is moved along the rotor’s axis. Moving it in the direction of exhaust side, a working length of the screw decreases and, accordingly, the productivity is. When starting, the compressor is completely unloaded.
Scroll Compressors
Pumping in this type of compressors is made by two spirals − moving one and stationary other. The fixed spiral is fixed inside the compressor casing, and the movable is mounted on the eccentric. Due to a specially selected profile, chambers are formed between these spirals, along which the gas moves. Herewith, their shape is calculated in such a way that they do not slip, which is much reduce their wear level. This type of equipment is most often used to compress the oil-free gases. Due to their constructive features, they have some advantages over the other types of compressor equipment:
reducing the load on the electric motor, especially at the moment of its start;
passing through the cylindrical body, the coolant cools the engine effectively;
ensuring the uniformity rate of gas supply;
vary of a high level of reliability;
having a reduced noise level.
Along with a number of advantages of the scroll compressors, they also have disadvantages that prevent them from being implemented more actively. Among the most serious, there are two:
complex manufacturing technology,
relatively low productivity.
At the same time, it should be noted that an increasing number of spiral compressors are being offered on the market today, with a constant increase in their quality. It is expected that in the near future this type of equipment will begin to displace the other less reliable systems.
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Turbochargers
This, as a rule, is centrifugal compressors with a volumetric capacity of 50-60 m/min or more. They are used for large refrigerating plants and cooling stations. The number of sections is 2-3, steps from 3 to 7, with branch pipes between sections for the interstage refrigeration and staged throttling. To regulate the compressor capacity, input regulating (guiding) apparatus (IRA or IGA) is used. By swirling the flow in the inlet at the impeller, the capacity can be varied from 100% to 50% of the rated value. These are the high-speed machines (≤13000-15000 rpm); therefore, when using an electric drive, reducers are necessary. The advantages and disadvantages of refrigeration turbochargers are the same as those of air superchargers of this type.
Hermetic, Semi-hermetic and Open Compressors
Refrigeration compressors differ in their construction for the hermetic, semi-hermetic and open devices.
Hermetic Refrigeration Compressors
Hermetic compressors are located in the same housing with an electric motor, which is cooled directly by the coolant. Their advantages include:
economy;
compactness;
universality.
Hermetic refrigeration compressors have a relatively small capacity - up to 35 kW.
Semi-Hermetic Refrigeration Compressors
Semi-hermetic compressors are used in the units, where the large-capacity compact equipment is required. This type of equipment can develop the power up to 350 kW. Most often there are piston and screw models of compressors. The working part and the electric motor of this device of this type are in a dismountable casing, which makes the compressor more repairable. It can be easy dismantled, remove the electric motor and replace the necessary parts. The operating mode can be adjusted by closing a part of the suction valves.
Other advantages of this equipment include:
high power;
compactness;
ease of maintenance.
Open Refrigeration Compressors
The main feature of an open compressor is the presence of an external motor. This is a rather cumbersome construction, but has many advantages. Most often, open compressors are used in large industrial plants, where they are most effective. The shaft driving the compressor mechanics goes beyond it, so it requires a special seal. Typically, an oil seal or rotary seal is used. The compressor can rotate at the engine speed or, with the transmission (most often a wedge-belt), its rotation speed can be increased by selecting a sheave of the desired diameter.
Among the advantages of this compressor type it can be noted:
possibility of starting at the idle to reduce the load on the engine;
reliability;
maintainability;
convenient adjustment of the operating mode by means of inverters.
