In the field of refrigeration, HVAC and industrial pneumatics, scroll compressors are gradually replacing traditional small and medium-sized reciprocating compressors with high efficiency and low noise.
However, engineers are often puzzled by the deep connection between their structure and performance in the selection and system design. In this article, we will dismantle the scroll compressor from the structure, principle, advantages and disadvantages and performance factors, to provide reference for refrigeration project design and equipment selection.
What is a Scroll Compressor?
Scroll compressor is a volumetric compressor, the core component is dynamic and static scroll disk, through the relative rotation to form a closed volume change to achieve gas compression. Its core features are no suction and discharge valves, low vibration, can be oil-free or lubricated compression, gas purity and operational stability advantages.
Its applications cover air conditioning, refrigeration, automotive superchargers, vacuum pumps and other fields, especially suitable for medical (high gas purity), commercial buildings (low noise), cold chain storage (stable output) and other scenarios.
From the machining accuracy of scroll body profiles to the selection of housing types, from the design of back pressure chambers to the optimization of sealing mechanisms, every structural detail has a direct impact on the compression efficiency, energy consumption level and operating life. Understanding this correlation is a prerequisite for good equipment selection and system optimization.
Structural Design of Scroll Compressors
Basic Components and Their Functions
- Cooling fan: The core function is to dissipate the compression heat generated during the compressor’s working process in a timely manner, to avoid overheating of the equipment leading to a decline in operating efficiency, and at the same time to prevent high temperatures on the internal components to cause damage to the compressor to protect the long-term stability of the operation of the compressor;
- Safety temperature sensor: belongs to the core components of the safety protection category, real-time uninterrupted monitoring of the compressor internal and exhaust side of the temperature changes, when the detected temperature exceeds the safety threshold, can trigger the shutdown and other protection mechanisms to prevent equipment failure or damage due to overload overheating;
- Suction chamber: as the initial buffer area for the gas to enter the compressor, it provides temporary storage space for the inhaled gas, which can stabilize the inlet flow and reduce the impact of inlet fluctuations on the subsequent compression process, and at the same time guide the gas to enter the compression chamber evenly;
- Suction port: is the exclusive channel for the outside gas to enter the compressor, its caliber design is directly related to the inlet resistance, need to match the compressor displacement requirements, to ensure that the gas is smoothly imported into the suction chamber, reduce the inlet pressure loss;
- Exhaust port: responsible for the compressed high-pressure gas out of the compressor, its structural design needs to be adapted to the demand for exhaust pressure, and at the same time to reduce the exhaust resistance, to avoid high-pressure gas reflux, to ensure the stability of the compressed gas transported to the after-treatment system;
- Static scroll disk: it is a fixed core compression part, its scroll teeth and the scroll teeth of the dynamic scroll disk engage with each other, forming the basic structure of the compression chamber, and realizing the gradual compression of the gas through the relative movement with the dynamic scroll disk;
- Dynamic scroll disk: driven by the motor through the crankshaft, under the constraint of the anti-rotation mechanism to do a small radius eccentric rotation movement, through the meshing movement with the static scroll disk, so that the compression chamber volume continues to change, to complete the gas inhalation, compression and pushing process.
Core Structure Features
The dynamic and static scroll disks are designed with double function equations and are 180° opposite to each other; the dynamic scroll disk is driven by a small eccentric crank shaft, and under the constraint of the anti-rotation mechanism, it makes a small radius plane movement, which forms a crescent-shaped cylindrical working volume with the end plate.
In order to balance the axial gas force borne by the dynamic scroll disk, the equipment is specially set up with a back-pressure chamber structure: the gas is introduced into the intermediate pressure chamber through the back-pressure holes, so that the back-pressure chamber is in the middle of suction and exhaust pressures, and the axial force and torque are balanced by the force of the gas in the chamber.
Shell Design Comparison: High-pressure Chamber vs.Low-pressure Chamber
High-pressure chamber scroll compressor (represented by Hitachi): the motor is located in the exhaust side, the shell for the exhaust pressure; advantage is a large exhaust cushion, vibration is small, less suction preheating, high volumetric efficiency, lubrication and reliable; disadvantages are poor suction muffling, weak resistance to liquid strike, the shell of the airtightness and strength of the requirements of the high.
Low-pressure chamber scroll compressor (represented by Copeland): the motor is located in the return side, the shell for the return pressure; advantage is the suction cushion is large, the motor working conditions are mild (low temperature and low pressure), anti-liquid strike impurity resistance is strong; disadvantage is the suction preheating obvious, the volumetric efficiency is affected, high and low pressures coexist in the shell, sealing is difficult.
How Do Scroll Compressors Work?
Core Working Mechanism
Scroll compressor is based on the closed compression principle: motor-driven spindle rotation, through the cross-slider coupling will be converted into rotary motion of the dynamic scroll disk eccentric circular motion.
Dynamic scroll disk rotation, and the static scroll disk surrounded by the gas gap to the center of the move, the volume is reduced, so that the gas temperature and pressure rise, to achieve compression.
As the two scroll disks only engage with each other without mechanical friction, the oil-free scroll compressor does not need to add lubricating oil at the gas end for lubrication and cooling, and is able to produce pure oil-free compressed air.
At the same time, the scroll structure provides radial and axial stability, effectively avoiding gas leakage during the compression process and guaranteeing high compression efficiency.
Step-by-step Operation Sycle
- Suction stage: After the scroll compressor is started, the motor drives the moving scroll disk to make a small radius eccentric rotation. With the dynamic and static scroll disk mesh position changes, the two constitute the crescent-shaped working volume gradually expanded.
So that the suction chamber to form a negative pressure; in the internal and external pressure difference, the outside world to be compressed gas through the suction port into the gas end of the compression chamber, complete the suction process.
- Compression stage: the dynamic scroll disk continues to carry out eccentric rotation, and the compression chamber formed with the static scroll disk is driven by the meshing motion, and gradually moves to the center of the vortex along the vortex line trajectory.
In this process, the compression cavity volume is constantly reduced, the gas molecules in the cavity density increases, collision intensification, and promote the gas pressure and temperature rise synchronously, to achieve gas compression and pressurization.
- Continuous cycle: the rotational movement of the dynamic vortex disk is continuous, so that the suction and compression process forms a continuous cycle.
The external gas is continuously sucked into the newly formed compression chamber, while the compressed gas is continuously transported to the center for pressurization, so that the gas pressure in the system accumulates smoothly, effectively avoiding pressure fluctuations and ensuring the stability of the output gas.
- Exhaust phase: When the compressed gas is delivered to the center of the scroll and the pressure reaches the preset exhaust threshold, the exhaust port opens automatically.
The high-pressure gas is discharged from the compressor through the exhaust port, and then enters the aftercooler to cool down, and then is purified by drying, filtration and other post-processing equipments, which ultimately meets the requirements of different scenarios in terms of gas purity, temperature and pressure, and realizes a stable supply.
This continuous cycle of operation allows the scroll compressor to efficiently produce oil-free compressed air, which is particularly suitable for scenarios requiring high gas purity.
Pros of Scroll Compressor
Sealing and Leakage Control
Relying on the unique double function equation line meshing design, the adjacent compression chamber forms a gentle pressure gradient and low pressure difference, which weakens the driving force of high-pressure gas leakage from the root and reduces the amount of leakage; avoiding repetitive compression and ineffective energy consumption, it guarantees the stable compression efficiency in all conditions and ensures that the gas compression effective utilization is at a high level.
Operational Stability
the scroll disk meshing structure realizes continuous synchronization of suction, compression and discharge processes, and seamless connection of crescent-shaped compression chambers.
The gas pressure rises gently without any sudden rise or fall, and the torque fluctuation is only 50% (much lower than that of the traditional reciprocating type), which reduces vibration significantly and suppresses noise, and it is suitable for noise-sensitive scenarios, such as commercial buildings and medical places.
Reliability and Maintenance
the design without suction and exhaust valves not only improves the compression efficiency and reliability of the equipment, but also avoids the maintenance problems caused by valve failures; with the flexible structural design, strong resistance to impurities and liquid strikes.
When the pressure in the compression chamber is too high, the dynamic and static scroll disks can be automatically detached from the end surface of the disk to realize the pressure relief, thus reducing the risk of malfunctioning.
Optimization of Energy Efficiency
the inner chamber of the casing is an exhaust chamber, which can reduce the loss of suction preheating and improve the volumetric efficiency,relying on the characteristic of compressed gas moving from outside to inside.
It supports liquid injection cooling and intermediate air replenishment, which can realize the economizer operation mode and further improve the energy efficiency level of the whole machine;
Structure-related limitations and manufacturing challenges
- Manufacturing precision and cost: The processing precision of the scroll body profile is demanding, with the flatness of the end plate and the perpendicularity between the end plate and the side wall of the scroll body controlled at the micron level; the need to rely on special precision machining equipment and precise centering assembly technology directly pushes up the manufacturing cost;
- Limitation of application scope: constrained by the power specifications, the current scroll compressor is mainly adapted to 1~15kW air conditioners, which is difficult to cover the application scenarios of higher power or special working conditions;
- Sealing and structural design limitations: demanding sealing performance, sealing mechanism design is complex; and subject to the constraints of the design of the valve structure, the compression chamber is prone to over-compression or under-compression phenomenon, which affects the stability of operation;
Scroll Compressors vs Dual-rotor Compressors
Adaptability of Technical Issues
When the pressure difference increases, the double rotor structure can realize the optimal adaptation of the sealing gap, which has a significant advantage; while the scroll structure has more sealing parts, the effect of suppressing leakage loss is poor.
In the face of increased shaft load, the double rotor structure can be effectively dealt with by reducing the cylinder height; although the scroll structure can reduce the shaft load by reducing the height of the scroll teeth, but it will be accompanied by the problem of increased axial load, and the adaptability is relatively unfavorable.
R410A Suitability and Energy-saving Properties
R410A is a high-pressure environmentally friendly refrigerant, which requires high adaptability, sealing and pressure resistance of the compressor. In terms of adaptability, the double rotor structure has significant advantages, with better stability and reliability.
In terms of energy saving, the double rotor can be stable and efficient under both low and high compression ratios, whereas the scroll structure is limited by the adapted range of the profile, and only saves energy under fixed compression ratios, with a narrow range of applicability, making it difficult to adapt to fluctuating operating conditions.
Operating Performance Range
In practice, the scroll compressor by structural limitations, low-frequency low-load conditions due to increased clearance, increased leakage led to efficiency degradation, high-frequency high-load conditions due to excessive stress, insufficient heat dissipation performance saturation.
Double-rotor compressor by virtue of the symmetrical structure and precise gap control, high and low-frequency conditions can protect the efficiency and stability of the ability to cover a wide range of scenarios more suitable for fluctuating operating conditions.
Key Factors Affecting the Performance of Scroll Compressors
Mechanical Friction Loss
When the compressor is working, the relative sliding of the movable disk, anti-rotation mechanism and keyway, crankshaft and main and auxiliary bearings will generate friction loss. Inadequate machining accuracy of the moving disk and excessive impurities will cause abnormal friction.
Poor precision of the cross key/keyway and contact friction of the anti-rotation mechanism of the vertical model will increase power consumption; the crankshaft and bearing eccentricity error will also generate friction losses.
Fluid Resistance and Lubrication Influence
Fluid obstruction at the back of the rotating disk and rotation of the balance block with the crankshaft will generate flow resistance loss; friction between the gas and the pipe wall in the suction and exhaust process will also generate resistance; suction resistance reduces the volume of exhaust gas, and exhaust resistance increases power consumption.
Lubrication of moving parts is critical: poor lubrication increases wear and tear, good lubrication can absorb heat and reduce friction. Refrigeration compressor, the lubricating oil into the friction surface after cooling, heat absorption, and then discharged with the high-pressure gas, forming a stable lubrication cycle.
Gas Leakage (Internal and External)
Internal leakage of high-pressure gas to the low-pressure cavity leakage (such as adjacent compression chamber, compression chamber and back pressure between the cavity), resulting in repeated compression, increasing power consumption; external leakage of high-pressure gas into the suction chamber expansion, reducing the suction volume, increasing consumption of cooling volume.
Internal leakage of the main channel for the adjacent working chamber engagement gap, the working chamber and the back pressure chamber connecting gap and the safety valve hole sealing gap, although the gap is small, but easy to form a leakage flow, affecting the compression efficiency.
External leakage is mainly caused by the fixed disk suction hole O-ring sealing failure or defective (eg, aging, deformation, abrasion, assembly deviation), destroying the integrity of the seal, so that the high-pressure gas infiltration into the suction chamber.
Suction Preheating
Suction gas heating will occur after the specific volume increases, the phenomenon of reduced suction volume, which leads to a decline in refrigeration capacity, power consumption (data show that the suction preheat every 3 ℃, the compressor energy efficiency ratio decreased by 1%).
In addition, the suction tube oil churning loss, gas flow friction loss, as well as the difference between the coefficient of thermal expansion of the dynamic fixed disk materials, and other factors, will also adversely affect the performance of the compressor.
Conclusion
The advantages of scroll compressors in terms of high efficiency, low noise and stability come from scroll meshing, back pressure balance and valve-less design; the limitations and losses are related to machining accuracy, sealing and casing design. Structural design is the core of performance, selection and optimization is the key.
Engineers can grasp the key points of structure and performance to accurately match demand and optimize efficiency. Model line innovation and capacity expansion can broaden the application, it is recommended that practitioners pay attention to the new inverter equipment, combined with simulation to optimize the selection; refrigeration projects from the structural details to maximize the advantages.
