Piston Compressor Foundation – Structure, Working Principle, Advantages And Disadvantages
Both the manufacturer and the end customer know that in the refrigeration or air-conditioning system, the compressor is the core of the equipment, and constantly absorbs and outputs cold air. If the compressor fails or crashes, the entire cooling system will stop.
Piston compressors are widely used in large and medium-sized cold storages. Here, we will introduce the structure, working principle, advantages and disadvantages and common fault diagnosis of the piston compressor in detail.
- Structure of piston compressor
The following is a detailed description of the key components of the piston compressor.
1.1 Valves
The main engine of compressor includes cylinder and crankcase. It is usually made of high-strength gray cast iron.
1.2 Cylinder surface
Cylinder is the core component of piston compressor. According to the different pressure, displacement and gas characteristics of the compressor, the user must select the cylinder with appropriate structure with other materials.
The basic requirements of the cylinder are strength and stiffness, good cooling, lubrication and wear resistance.
1.3 Crankshaft
The crankshaft converts the rotary motion of the motor into the reciprocating linear motion of the piston through the connecting rod, transmitting all the power of the compressor. It is one of the core components of the compressor motion system.
1.4 Links
The connecting rod is the connecting piece between the crankshaft and the piston. It converts the rotary motion of the crankshaft into the reciprocating motion of the piston, and transfers the power to the piston for working gas.
Connecting rod includes connecting rod body, small end bushing, large end bearing bushing and bolt.
1.5 Total piston
The piston group is the common name of the piston, including the piston pin and piston ring. Driven by the connecting rod, the piston group moves back and forth in a straight line in the cylinder to form a variable working volume with the cylinder to complete the process of suction, compression and exhaust.
1.6 Piston
The piston can be divided into cylinder and disc. The piston material is generally aluminum alloy or cast iron.
1.7 Piston pin
The piston pin is used to connect the small end of the piston with the connecting rod. It works under complex cross loads.
1.8 Piston ring
A cut is a ring with straight, sloping, or overlapping cuts. The angle of the bevel cut is 45 degrees.
The piston ring is composed of air ring and oil ring. The air ring is used as a seal between the piston and the cylinder wall to prevent compressed air from leaking from the gap between the piston and the cylinder wall. The oil ring is used to distribute oil and scrape off the lubricating oil left on the cylinder wall.
1.9 Shaft seal
The shaft seal is used to prevent the leakage of compressed gas along the extended end of the crankshaft, and also to prevent the leakage of external air when the crankcase pressure is lower than atmospheric pressure.
1.10 Crankcase radiator
The crankcase radiator separates the coolant from the crankcase to ensure normal lubrication of the crankcase coolant radiator.
The user must install the crankcase radiator in all environments. When the crankcase temperature is 20 ℃ higher than the ambient temperature, the power supply of the crankcase will be turned off when the compressor starts. When the compressor stops, the heater will turn on.
1.11 Compressor motor protection device
In order to prevent the compressor from overload or overheating, 6 PTC thermal resistors are built in the motor. Generally, L and N represent power supply, 11 and 14 represent control circuit, and 12 represent alarm indicator.
1.12 Hydraulic safety device
The oil pressure safety device monitors the pressure difference between the inlet and outlet of the pumping unit to ensure the lubrication of the compressor. Generally less than 0.7 bar, delay 90 ± 10 seconds. The differential oil pressure of the compressor is 2.5 to 3 kg/cm.
The oil pressure difference of a single compressor is calculated as follows:
◆ P=pumping unit outlet pressure suction pressure.
The oil pressure difference of the two-stage compressor is calculated as follows:
◆ P=pumping unit outlet pressure – intermediate pressure.
- Working principle of piston compressor
The work of piston compressor is completed by continuous change of working volume from cylinder, gas valve, cylinder to resurrection plug.
In the actual work of the piston compressor, if the volume loss and energy loss are not considered(i.e. in the ideal working process), the work completed by the crankshaft of the piston compressor can be divided into four processes: compression, exhaust, expansion and suction.
2.1 Compression process
When the piston is at the lowest position(called inner point or lower point), the cylinder will be filled with low-pressure coolant steam sucked from the evaporation tank, and the suction process is ended.
Driven by the crankshaft and connecting rod, the piston starts to move upward. At this time, the air inlet valve is closed, the working volume of the cylinder is gradually reduced, the gas in the cylinder is compressed, and the temperature and pressure are gradually increased. The process of cylinder gas rising from low pressure suction to exhaust pressure is called compression process.
The compression process is usually considered as an isometric process.
2.2 Exhaust process
When the gas pressure in the cylinder is slightly higher than that in the exhaust chamber, the piston continues to move upward.
After the exhaust valve is opened, the high-pressure steam in the cylinder is discharged from the cylinder at a certain pressure, and the exhaust process is completed until the piston reaches the top position(outer fulcrum or top point).
2.3 Expansion process
When the piston moves in the cylinder, the top of the piston is not completely consistent with the top of the cylinder, but there is a certain space to ensure the safety and reliability of its operation. This space is called the remaining space volume.
Due to the volume of the remaining space, if the piston moves to the top point, the exhaust will be closed, and a small part of the high-pressure gas in the cylinder will not be discharged. When the piston moves in the opposite direction, the suction valve will open only when the high-pressure gas expands to a pressure lower than the suction pressure, and the coolant can enter the cylinder under the low-pressure gas state.
2.4 Suction process
When the piston moves downward, at the volume of the remaining space, the expansion process of high-pressure gas will unite and start the suction process. Low pressure gas is drawn into the cylinder until the piston moves to the bottom.
So far, the compressor has completed the working cycle consisting of compression, exhaust, expansion and suction.
Then the piston moves down again and repeats the above four processes. This is the ideal working process and principle of piston compressor.
2.5 Cooling process of piston compressor
The cooling process is carried out in a closed circulating system consisting of ammonia compressor, cooler, regulating valve and evaporator.
After the pressure of ammonia liquid is reduced through the regulating valve, it enters the evaporation pool to absorb and evaporate the heat of the cooling medium, reduce the temperature of the medium, and achieve the purpose of cooling.
The vaporized ammonia is absorbed and compressed by the compressor, discharged from the cooler to cool the ammonia, and condensed into ammonia liquid.
Next, enter the evaporation pool through the regulating valve, and then evaporate to complete the refrigeration operation.
- Advantages and disadvantages of piston compressor
3.1 Advantages of piston compressor
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Piston compressors can be used in a variety of pressure environments, regardless of the desired pressure flow rate.
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Piston compressor has higher thermal efficiency and lower unit power consumption.
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Strong adaptability. That is to say, the exhaust range of the piston compressor is large and is not affected by the pressure. The compressor capacity meets a wide range of pressure and cooling capacity requirements.
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The material requirements of piston compressor are relatively low, which can be made into ordinary steel and easy to manufacture at low cost.
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The piston compressor technology is mature and has accumulated rich experience in production and application.
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The installation system of piston compressor is relatively simple.
3.2 Disadvantages of piston compressor
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The speed is not high.
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This machine is big and heavy.
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The structure is complex, there are many vulnerable parts, and the maintenance cost is high.
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Discrete gas flow can cause gas pulses.
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There is severe vibration during operation.
Piston compressor has its disadvantages, but compared with other types of compressors, it can be used in various occasions due to its overall advantages.
Piston compressor is the most widely used type and the largest model for mass production in the refrigeration industry.