What is a scroll compressor? Everything You Need to Know

Figure 1: HVAC refrigeration cycle.

A scroll compressor (also called spiral compressor, scroll pump and scroll vacuum pump) is a device for compressing air or refrigerant. So there are scroll air compressors and scroll refrigeration compressor (refrigeration scroll compressor). Among various compressors, the scroll compressor has developed rapidly in the refrigeration industry because of its high efficiency, energy saving, micro-vibration and low noise, and has been widely used in household air conditioners, automobile air conditioners and other refrigeration and air conditioning fields and other special fields (such as scroll turbochargers), etc.

This article introduces some basic knowledge about scroll refrigeration compressors, including the composition and structural characteristics, working principle, advantages and disadvantages and applications, and the performance comparison between scroll compressors and other compressors.

Here is scroll compressor diagram, scroll compressor gif and scroll compressor animation for you to better understand the basics about scroll compressor.

1. Scroll Compressor Components and Structure

Figure 2: Refrigeration scroll compressor structure diagram.

Scroll refrigeration compressors are mainly composed of fixed scrolls, orbiting scrolls, motor rotors, motor stators, eccentric shafts, main bearings, lubricating parts and casings. Like the fully hermetic reciprocating and rolling piston compressors, the refrigeration scroll compressor also uses the lubrication method of "pumping oil" through the eccentric hole, but its internal structure is different from them. The refrigeration scroll compressor motor is placed at the lower part of the fully hermetic steel shell, and the compressor is located at the top.

Figure 3: Fixed and orbiting scrolls.

2. Scroll Compressor Working Principle

Figure 4: Scroll compressor gif.

The scroll refrigeration compressor is a compressor with compressible volumes composed of a fixed scroll and an orbiting scroll. Its working principle is to use the relative revolution motion of the fixed and orbiting scrolls to form a continuous change in the closed volumes, thus achieving the purpose of compressing the gas.

During the working process of suction, compression and exhaust, the fixed scroll is fixed on the frame, the orbiting scroll driven by the eccentric shaft and restricted by the anti-rotation mechanism, rotates in a plane with a small radius around the center of the base circle of the fixed scroll, so a series of crescent-shaped cylindrical working volumes form between the fixed and orbiting scrolls and the end plate.

The gas is sucked into the periphery of the fixed scroll through the air filter element. With the rotation of the eccentric shaft, the gas is gradually compressed in several crescent-shaped compression chambers formed by the engagement of the fixed and orbiting scrolls and then continuously discharged from the axial hole of the central part of the fixed scroll.

Figure 5: Pressure change in crescent-shaped spaces of scroll compressor.

3. Advantages and Disadvantages of Scroll Compressors

3.1 Advantages

Advantages of scroll refrigeration compressors:
①low noise;
②high efficiency within the range of cooling capacity it adapts to;
③unique design making it an energy-saving compressor;
④smooth running, little vibration, low noise, and known as an "ultra-quiet compressor";
⑤small torque change, high balance, easy operation and easy automation;
⑥small size, light weight, few parts (few wearing parts), high reliability;
⑦the main moving part of the scroll compressor is the scroll, and there is only engagement and has no wear, so it has a longer life, known as a maintenance-free compressor.

3.2 Disadvantages

Disadvantages of scroll refrigeration compressor :
①high sealing requirements and complicated sealing structure.
②the surface of its moving parts is mostly in the shape of curved surfaces. The processing and inspection of these curved surfaces are relatively complicated, and some require special equipment, so the manufacturing cost is relatively high.
③between the moving parts and the fixed parts, a certain movement clearance is often maintained to achieve sealing, and the gas passing through the clearance will inevitably cause leakage, which limits the rotary compressor to achieve a large compression ratio. Therefore, most rotary compressors are mostly used in air-conditioning conditions.

Figure 6: Evaporation and condensation process diagram.

Figure 7: Digital scroll compressor.

4.2 Vibration

Since the suction, compression, and exhaust processes of the scroll compressor are carried out continuously at the same time, and the pressure rises slowly, the torque variation range is small and the vibration is small. At any frequency, the vibration and noise of scroll compressors are lower than those of piston compressors and scroll compressors.

For the rolling piston compressor, only the sliding vane has a small reciprocating inertial force, and the rotating inertial force can be completely balanced, so it can have high speed, small vibration and stable operation.

Piston compressors vibrate greatly during operation.

The screw compressor has no reciprocating mass inertia force, and has good dynamic balance performance.

4.3 Volumetric Efficiency

Figure 8: Scroll compressor animation.

The suction, compression, and exhaust of the scroll compressor are continuously performed in one direction. It sucks in gas directly, the harmful overheating of the drawn gas is small, and there is no expansion process of the gas in the clearance volume, so the volumetric efficiency is high.

The rolling piston compressor has no suction valve plate, small clearance volume, high volumetric efficiency, and small resistance in the suction process (because of low suction speed and no suction valve), so its volumetric efficiency is about 20% higher than that of the same capacity piston compressor.

Due to the influence of factors such as clearance volume, suction and exhaust pressure loss, heat exchange between gas and cylinder wall, and leakage, the actual volumetric efficiency of the piston compressor is always less than its theoretical one.

For screw refrigeration compressors, when the working conditions are different, the volumetric efficiency is different, which is roughly 0.7 to 0.92. The lower value is taken when the working condition is small volumetric efficiency and high-pressure ratio, and the upper value is taken when the working condition is high volumetric efficiency and low-pressure ratio. The volumetric efficiency of the screw type is higher than that of the piston type and other types of rotary compressors, and the change is flat.

4.4 Gas Volume Adjustment

The gas volume regulating slide valve is a structural element used to adjust the gas volume in screw refrigeration compressors. Although there are many ways to adjust the gas delivery volume of the screw refrigeration compressor, the adjustment method of using the slide valve has been widely used.

Both the scroll compressor and the rolling piston compressor can use the variable speed adjustment method to change their gas delivery, and both type compressors are suitable for occasions where the speed changes within a wide range.

The simplest way to adjust the gas delivery of the piston compressor is to run the compressor intermittently. When the refrigeration system reaches the set minimum temperature, the compressor stops; when the system temperature is higher than the set maximum temperature, the compressor starts.
This energy adjustment method is only suitable for small refrigeration machines with a power of about 10KW. For a compressor with a large capacity, the frequent start and stop of the machine will not only cause a large energy loss, but also affect the life of the machine, the voltage stability in the power supply circuit, and the normal operation of other devices.

Figure 9: Scroll compressor animation.

4.5 Conclusion

The above shows that scroll compressors are applicable to a wider speed range than piston compressors and rolling piston compressors. It is promising to use scroll compressors to adjust the gas delivery volume by frequency conversion in air conditioners or heat pumps.

In addition, in order to reduce production costs, increase economic benefits, and facilitate equipment management, in refrigeration and air-conditioning projects within the range of medium cooling capacity, it is an inevitable trend that high-efficiency and high-reliability screw compressors will eliminate the existing piston compressors.

5. Application and Development Trend of Scroll Compressor

As a product widely used in refrigeration fields such as household air conditioners and automobile air conditioners at this stage, refrigeration scroll compressors have significantly better performance parameters than the previous two generations of compressors, and the performance of the whole machine is constantly improving, and the future applications will become more and more extensive.

At present, there are mainly two technical categories of scroll refrigeration compressors on the market, one is the high-pressure chamber scroll compressor represented by Japan's HITACHI, and the other is the low-pressure chamber scroll compressor represented by the American COPELAND.
When selecting an air-conditioning refrigeration system, it is necessary to deeply consider the pros and cons of the high-pressure chamber and low-pressure chamber structure of the compressor, clarify the target effect of the refrigeration system, and select refrigeration scroll compressors with different structures in a more targeted manner.
Looking at the current research status of scroll refrigeration compressors, the following development trends are shown:
(1The processing cost of the scroll compressor, especially the scroll disk, is gradually reduced, and researchers are seeking scroll profile with better performance, and simplified processing method.

(2) The power range of the scroll compressor continues to increase, and the application field is further expanded, especially the research on its frequency conversion characteristics and application in air-conditioning heat pumps.

(3) There are more and more researches on oil-free scroll compressor technology, which further promotes the energy saving and environmental protection of compressor technology.

(4) New refrigerants put forward new requirements for the structural design of scroll compressors.

(5) The requirements for sealing and lubrication performance are getting higher and higher to further reduce gas leakage and mechanical friction loss, thereby improving the working efficiency and reliability of the scroll compressor.

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