At present, screw, centrifugal and reciprocating are the three most common types of compressors, and they have significant differences in terms of operating principle, structural characteristics, performance parameters and application range. This article will comprehensively introduce the basic situation of these three types of compressors, and conduct a detailed comparative analysis for the reference and selection of practitioners.
1. Screw compressor
1. Working principle
A screw compressor consists primarily of a pair of meshing male and female screw rotors. When working, the motor drives the male screw to rotate at high speed, and the female screw is driven to rotate in the opposite direction through the synchronous gear. The gas enters the cavity between the screws from the suction port, and continuously moves towards the exhaust port with the rotation of the rotor, while the volume of the cavity gradually decreases, the gas is compressed, and finally discharged in a high-pressure state.
2. Structural characteristics
(1) Screw rotor: asymmetrical profile design, mostly made of ductile iron, the surface is phosphated or nitrided, with good sealing and wear resistance.
(2) Spool valve: By controlling the axial displacement of the spool valve, stepless capacity adjustment can be realized within a certain range, and the suction and exhaust phase angle can be optimized.
(3) Bearings: cylindrical roller bearings are mostly used, with elastic gaskets, which can effectively carry radial and axial loads.
(4) Oil separator: The use of high-efficiency oil-gas separation elements to separate more than 99% of the liquid refrigerated oil from the exhaust.
(5) Synchronous gear: ensure that the male and female rotors rotate in the opposite direction at the same speed and transmit torque.
3. Performance characteristics of screw compressor
Merit:
(1) Compact structure, small size and light weight. The single unit capacity can reach more than 1000kW, which is widely used in large-scale industrial and commercial fields.
(2) High speed, usually 3000~30000rpm, large refrigeration capacity. With frequency conversion speed regulation, it realizes a wide range of stepless energy adjustment.
(3) Strong overpressure capacity, exhaust pressure up to 2.5MPa, suitable for heat pumps, ground source heat pumps and other high-pressure working conditions.
(4) Low vibration, low noise, stable and reliable operation.
(5) The requirement for suction superheat is not high, generally 5~15°C, and the system is well matched.
Shortage:
(1) The processing technology requirements are high, and the manufacturing cost is high.
(2) It needs continuous oil supply and lubrication, and the oil circuit system is complex.
(3) There is a leakage gap in the screw rotor, and the volumetric efficiency is slightly lower than that of the reciprocating type.
(4) It is inconvenient to maintain and the cost of replacing parts is high.
4. Screw compressors are widely used in the following fields:
(1) Central air conditioning: centralized air conditioning system for large public buildings such as office buildings, shopping malls, and hotels.
(2) Industrial refrigeration: process cooling and large-scale cold storage in chemical, metallurgy, printing and other industries.
(3) Refrigeration and refrigeration: display cabinets and quick-freezing tunnels in supermarkets and food factories.
(4) Heat pump hot water: domestic hot water production in hotels, swimming pools and other places.
(5) Rail transit air conditioning: air conditioning and platform environmental control of high-speed rail and subway vehicles.
2. Centrifugal compressor
1. Working principle
The core component of a centrifugal compressor is an impeller that rotates at high speed. The gas is introduced into the impeller through the inlet guide vane, and the flow is accelerated along the flow path under the action of centrifugal force, and the kinetic energy is significantly increased. After the gas leaves the impeller, the kinetic energy is converted into pressure energy through the diffuser, so that the pressure is boosted. Multi-stage series connection can achieve a higher pressure ratio.
2. The main structure of centrifugal compressor is as follows:
(1) Impeller: Three-dimensional space curved surface profile, usually cast into an integral closed impeller, mostly made of aluminum alloy. The large unit is equipped with 2~4 parallel impellers.
(2) Adjustable air intake guide vane: adjust the guide vane angle through hydraulic cylinder or stepper motor to achieve 0~100% continuous capacity adjustment.
(3) Bearing: tilt pad thrust bearing + sleeve type radial bearing is mostly used, and the electromagnetic suspension bearing is used to achieve oil-free lubrication.
(4) Sealing system: non-contact labyrinth seal, carbon brush seal or mechanical seal is used to prevent gas leakage.
(5) Anti-surge system: including adjustable surge hole, suction circuit, etc., to suppress surge and expand the stable operation range.
3. Performance characteristics
Key benefits of centrifugal compressors include:
(1) Large air volume, single machine capacity up to 5000kW or more, suitable for large-scale industrial users.
(2) The volume flow rate adjustment range is wide, usually 10%~105%, which meets the requirements of various working conditions.
(3) Compact structure, light weight, small footprint.
(4) High mechanical efficiency, stable operation and low noise.
(5) No oil lubrication, clean exhaust, no oil separation.
However, centrifugal compressors also have the following limitations:
(1) The speed is extremely high, and the bearing and balance requirements are demanding.
(2) The pressure ratio is usually not more than 5, and the pressurization capacity is not as good as that of the reciprocating type.
(3) Surge is prone to occur under small loads, and complex anti-asthma measures are required.
(4) Suitable for freon system, not suitable for ammonia system, etc.
4. Centrifugal compressors are mainly used in large-scale commercial and industrial fields
(1) Central air-conditioning system for high-rise buildings.
(2) Process cooling in petroleum, chemical and other industries.
(3) Large-scale cold storage and food processing plants.
(4) Cutting-edge scientific research fields such as nuclear power and military industry.
(5) Water-cooled ice storage air-conditioning system.
3. Reciprocating compressor
1. Working principle
Reciprocating compressors mainly work by the reciprocating motion of the piston in the cylinder. The piston is driven by the crankshaft connecting rod mechanism, and when it moves downward, the suction valve is opened, and a low pressure is formed in the cylinder to inhale low-pressure vapor; When the piston goes up, the suction valve closes and the gas is compressed to continue.
2. Structural characteristics
The main structures of a reciprocating compressor include:
(1) Cylinder: Usually made of cast iron, the inner wall is sprayed with cemented carbide or inlaid bushing, which has excellent wear resistance and sealing.
(2) Piston: with compression ring and oil ring, mostly made of aluminum alloy, the surface is anodized or sprayed.
(3) Crankshaft connecting rod: made of ductile iron or forged steel, quenched and ground to ensure strength and rigidity.
(4) Suction and exhaust valves: mostly reed valves or butterfly valves, which are automatically opened and closed by springs.
(5) Lubrication system: equipped with oil pump and oil channel to provide sufficient lubrication and cooling for moving parts.
3. Performance characteristics of reciprocating compressors
(1) The compression ratio is high, and the single-stage pressure ratio can reach more than 10, which is suitable for a wide range of refrigerants, including freon, ammonia, etc.
(2) High volume efficiency, generally above 85%, large refrigeration capacity.
(3) There are various adjustment methods, and the capacity can be adjusted through speed control, cylinder intermittent operation, etc.
(4) Simple structure, low manufacturing cost and easy maintenance.
(5) High reliability, long service life, small units can be operated without maintenance.
However, reciprocating compressors also have some drawbacks:
(1) There are many moving parts, large mechanical loss, and high energy consumption.
(2) There is a lot of internal leakage, and the volumetric efficiency gradually decreases with the use time.
(3) The vibration and noise are large, and vibration and noise reduction measures need to be taken.
(4) The inspiratory superheat requirement is high, and the system matching is not good.
4. Main applications
Reciprocating compressors are mainly used in the following applications:
(1) Household refrigeration: refrigerators, freezers, small air conditioners, etc.
(2) Commercial refrigeration: cold storage, display cabinets, ice machines, etc.
(3) Transportation refrigeration: refrigerated trucks, containers, etc.
(4) Process refrigeration: small cooling systems such as plastic machinery and laser equipment.
(5) Non-standard refrigeration: aviation, medical, laboratory and other special environments.
Fourth, the comparison of the three compressors
(1) Capacity range:
The centrifuge capacity is the largest, up to more than 5000kW; The screw machine is second, at about 1000kW; The piston machine has a relatively small capacity, but it is the most flexible.
(2) Compression ratio:
The piston pressure ratio is the highest, which can reach more than 10; The screw machine is second, around 5; The centrifuge pressure ratio is the lowest, with a single stage not exceeding 3.
(3) Volumetric efficiency:
The piston machine is the highest, above 85%; The screw machine is second, at about 80%; Centrifuges are the lowest, usually below 70%.
(4) Capacity adjustment:
The centrifuge adopts adjustable air inlet guide vanes, which has the widest adjustment range; The screw machine is adjusted by the slide valve, and the range is narrow; The piston machine needs to be controlled by variable speed, and the adjustment is the most inconvenient.
(5) Refrigerant adaptability:
The piston machine has the widest range of applications, including freon, ammonia, etc.; Screw machines are mainly used for freon; Centrifuges are only suitable for Freon.
(6) Reliability:
The screw machine is the highest, with low vibration and long life; Centrifuges are secondary; The piston machine is the worst and has a high failure rate.
(7) Noise:
The centrifuge is the lowest, and the rotor dynamic balance is good; The screw machine is slightly higher, and the gear meshing is noisy; The piston machine is the highest, and the airflow pulsates significantly.
(8) Energy consumption:
Centrifuge is the lowest and has high mechanical efficiency; The screw machine is second; The piston machine is the highest, and the reciprocating loss is large.