Scroll compressors are single-shaft machines of the positive displacement principle. As is known, machines of this operating principle are reversible, i.e. can operate practically without changing the design, both as compressors and as motors (expanders or expanders.
The idea of such a machine has been known for more than a hundred years, but it was only possible to realize it and bring it to industrial production and widespread use in the 80s of the twentieth century. The reason is the same as during the development of screw compressors, there was not sufficiently accurate equipment to produce such a part shape as a spiral.
Currently in refrigeration technology scroll compressors are used in domestic and transport air conditioners, heat pumps, refrigeration machines low and medium power up to 50 kW. But calculations show that the refrigerating power of scroll compressors can be increased to 100 kW or more as their design and manufacturing technology are improved.
28 Classification of scroll compressors
Scroll compressors are classified as follows: oil-flooded; with injection of drip liquid (for example refrigerant); dry compression.
And, naturally, one- and two-stage with different arrangements of stages in relation to the engine.
Depending on the type of gas, power and other conditions: sealed, sealless, stuffed.
According to the type of spirals used: with involute spirals, with Archimedes spirals, with piecewise-circular ones, etc.
It is important to divide scroll compressors into vertical and horizontal. In the latter, shaft 1 is located horizontally (see Figure 65). In horizontally located scroll compressors, for example in a transport air conditioner with a parallel shaft arrangement and the longitudinal axis of the vehicle, it is more difficult to ensure reliable operation of the compressor lubrication system.
29 Advantages and disadvantages of scroll compressors
The main advantages of scroll compressors are:
1.High energy efficiency; their effective efficiency reaches 80-86%;
2.High reliability and durability, determined by the durability of the bearings;
3.Good balance; slight change in torque on the compressor shaft; low gas speeds in the car - all this ensures the car moves with low level noise.
4. Speed - the number of revolutions of the compressor shaft is from 1000 to 13000, and this range is expanding.
5. No dead volume, low leakage rate, and, therefore, higher indicator efficiency; the gas sucked in by the compressor does not come into contact with the hot walls of the compressor parts;
6. The processes of suction, compression and discharge are “stretched” along the angle of rotation of the shaft and therefore, even at high shaft frequencies, gas velocities are low.
7. Lack of valves on the suction, and often on the discharge;
8. A scroll compressor, like a screw compressor, can operate in a cycle with “recharging”;
9. The scroll compressor, like all compressors of the positive displacement principle, can operate on any refrigerant, on any gas, and even with droplet liquid injection.
Compared to piston compressors of the same power, a scroll compressor has the following advantages:
The disadvantages of scroll compressors include the following:
1. Spiral machines require new spiral parts for mechanical engineering, for the manufacture of which milling machines with CNC.
2. A complex system of forces acts on the moving spiral: axial, centrifugal, tangential, requiring competent calculation and balancing, and, consequently, balancing of the rotor.
3. If there is no discharge valve, then the theoretical indicator diagram of a scroll compressor will be the same in appearance as that of a screw compressor, with possible undercompression and overcompression of the gas, i.e. with additional losses.
Scroll compressor- a device for compressing gas (air or refrigerant) by reducing its volume in chambers formed by the surfaces of the spirals.
Scroll compressors are used in air conditioning, refrigeration, heating, automobiles, cryogenic and refrigeration systems, and as vacuum pumps.
Design and principle of operation of a scroll compressor
There are several standard designs scroll compressors.
The most common option is use of two spiral elements, installed with eccentricity. One of these elements is movable, the other is not.
Compressor design with one moving scroll
A scroll compressor is shown in the figure.
The sealed housing houses an electric motor that rotates the shaft. A fixed spiral is installed in the upper part of the body. A movable spiral is installed on the shaft, which can move along guides making a complex movement relative to the fixed spiral.
As a result of movement between the spirals, chambers (pockets) are formed, the volume of which decreases with further movement, and as a result, the gas located in these pockets is compressed.
The operating principle of such a compressor is shown in the video:
Also found compressors with two moving scrolls performing rotational motion relative to different axes. As a result of the rotation of the spiral elements, chambers are also formed, the volume of which decreases during rotation.
To a greater extent, the compressor differs from the options presented above, in which the rigid element is made in the form Archimedean spiral affects flexible elastic tube. The principle of operation of such a compressor is similar to a peristaltic pump. Such scroll compressors usually filled with liquid lubricant to reduce wear flexible tube and heat removal. Such compressors are often called hose.
Dynamic valves
In scroll compressors, a suction valve is not needed, because the movable spiral itself cuts off the working chamber from the suction channel. A dynamic valve may be installed in the discharge line of a scroll compressor, which prevents backflow and, as a result, rotation of the scroll under the action when the engine is turned off. It should be taken into account that the dynamic valve creates additional resistance in the discharge line.
Dynamic valves are installed in the discharge line of Copeland medium and low temperature refrigeration compressors.
Advantages of scroll compressors
Scroll compressor runs smoother, and more reliable than most other volumetric machines. Unlike pistons, the moving scroll can be perfectly balanced, minimizing vibration.
The absence of dead volume in scroll compressors results in increased volumetric efficiency.
Scroll compressors usually have less pulsation than single-piston machines, but more pulsation than multi-piston machines.
Scroll compressors have fewer moving parts, compared to piston engines, which, theoretically, ensures their greater reliability.
Scroll compressors are generally very compact and do not require spring suspension due to their smooth operation.
Disadvantages of scroll compressors
Scroll compressors are sensitive to contamination of the pumped gas, because... small particles may settle on the surface of the spiral, which will not ensure sufficient tightness of the working chamber.
A scroll compressor must rotate in one direction only.
Variable Scroll Compressors
For a long time, scroll compressors were produced without the ability to adjust performance. If it was necessary to reduce the flow, frequency control of the drive electric motor was used, or part of the gas was bypassed from the discharge line to the suction line.
Currently adjustable scroll compressors are manufactured by Emerson. In these compressors, the distance between the axes of rotation of the scrolls can be changed; if necessary, this distance can be chosen such that no chambers will form between the spiral elements, which means the compressor supply will be wound 0. By alternating two different operating states (idle and working stroke) using an electronic control, the required performance can be achieved.
Rice. 2. 26. Performer scroll compressor (Danfoss). 1 – movable spiral; 2 – fixed spiral; 3 - terminal box; 4 – electric motor protection; 5 – sight glass; 6 – suction; 7 – oil pump; 8 - electric motor; 9 – injection; 10 – reverse rotation protection; 11 – check valve.
The electric motor is located in the lower part of the compressor; the shaft, with the help of an eccentric, ensures the ellipsoidal movement of a movable spiral inserted into a stationary spiral installed in the upper part of the compressor. The sucked gas enters the compressor through the suction pipe, flows around the electric motor casing and enters it through the holes in the lower part of the casing (Fig. 2.26). The oil contained in the refrigerant vapor is separated from it as a result of rotation of the refrigerant-oil mixture under the influence of centrifugal forces and flows to the bottom of the compressor crankcase. The steam passes through the electric motor, ensuring complete cooling of the compressor in all operating modes. After passing through the electric motor, the steam enters the compressor scroll elements, which are located at the top of the compressor above the electric motor. The working cycle is completed in three revolutions of the shaft: the first revolution is suction, the second revolution is compression, the third revolution is discharge. There is a check valve immediately above the outlet of the fixed volute. It protects the compressor from backflow of gas after it is turned off. After passing the check valve, the gas leaves the compressor through the discharge pipe.
The efficiency of scroll compressors is largely determined by the magnitude of internal radial and axial gas leaks during the compression process. Radial leaks occur between the touching side surfaces of the spirals, axial leaks - between the upper end of one spiral and the base plate of the other (Fig. 2.24). Leaks lead to an increase in compressor power consumption, reducing its cooling capacity and operating efficiency.
The main difference between this compressor and other scroll compressors is the principle of compaction of the scroll elements. A common way to ensure radial sealing is to create tight contact by pressing the movable spiral onto the stationary one under the influence of centrifugal force. However, freshly manufactured compressors only create an effective, uniform seal after a “break-in” period during which the necessary contact is established between the surfaces. Touching the side surfaces of the spirals is prerequisite for such compressors.
Danfoss uses the so-called “controlled orbiting principle” in Performer brand compressors, which implies the movement of the scrolls along a fixed path without contact between the moving and stationary scrolls under any operating conditions of the compressor.
Performer controlled rotation compressors must have ultra-precise scroll profiles to ensure a guaranteed seal. The side surfaces of such spirals never come into contact with each other, and a thin film of oil, sealing the gap, ensures lubrication of the spirals without friction and wear on their surface.
When creating an axial seal, some compressor manufacturers press a moving scroll against a stationary one to seal using compressed gas pressure.
On Performer compressors, dynamic contact between the top end of the moving scroll and the fixed scroll base plate is maintained by a floating seal (Fig. 2.27).
Rice. 2.27. Performer Rotation Controlled Scroll Floating Seal:
1 - base plate; 2 - the gap between the end and the base plate; 3 - floating seal; 4 - spiral; 5 - oil film preventing seal gas leakage; 6 - gas high pressure
This sealing element is located in a groove cut into the upper end of the movable spiral (Fig. 2.27). The pressurized gas presses against the floating seal from below and forces it against the scroll support plate, creating dynamic contact as the compressor operates. The pressing forces are very small, which in combination with small area contact reduces friction and increases compressor efficiency.
Characteristic feature These compressors are designed to run idle, even when the pressure in the system is unbalanced. This happens due to the installation check valve on the discharge line, which closes when it stops. Under these conditions, only the gas compressed in the compressor to the valve installation location returns to the crankcase, passing through the spirals. This equalizes the internal pressure. When the compressor stops, the two spirals open both vertically and horizontally. When restarted, the compressor does not experience a load, since the pressure increases gradually. The scroll compressor is equipped with a safety valve that opens when the pressure exceeds 28 bar and bypasses the refrigerant from the discharge cavity to the suction cavity.
The oil in scroll compressors serves only to lubricate the bearings and floating seal ring. Lubrication of the spirals is not required due to the low rotation speed and friction force at each contact point. The oil content in the coolant-oil mixture is sufficient to provide the necessary lubrication, which is why the oil is not exposed to high temperatures which can lead to deterioration of oil performance over time. Another positive feature is a high ability to resist oil carryover during startup.
Questions for self-control in Chapter 2.
What is the difference between direct-flow and indirect-flow compressors? 2. What is the design difference of the compressor? simple action from a double acting compressor? 3. What water hammer protection device is included in the compressor? 4. What is the difference between a piston seal ring and an oil seal ring? 5. How is the compressor seal lubricated? 6. What is the purpose of a safety valve in a compressor? 7. How does the oil carried away by the refrigerant vapor return to the compressor crankcase? 8. Why does a compressor running on ammonia have greater cooling capacity than when running on R22? 9. How can you change the cooling capacity of a refrigeration compressor? 10. How does compression occur in a screw compressor? 11. Why do energy losses occur in a screw compressor when the pressure at the end of compression does not coincide with the discharge pressure? 12. Why does the refrigeration capacity of a screw compressor change when the spool moves? 13. What advantages and disadvantages does a screw compressor have compared to a piston compressor? 14. What are the advantages of scroll compressors? 15. Seals for scroll compressors. 16. Operating principle of scroll compressors. 17. What is the “pinched” volume in screw compressors?
Readings for Chapter 2.
1.Baranenko A.V., Bukharin N.N., Pekarev V.I., Timofeevsky L.S. Refrigerating machines - St. Petersburg: Politekhnika, 2006.-944 p.
2. Quick selection of automatic regulators, compressors and compressor-condensing units. Catalog. Danfoss. 2009.-234s
3. Ladin N.V., Abdulmanov Kh.A., Lalaev G.G. Marine refrigeration units. Textbook. Moscow, Transport, 1993.-246 p.
4. Shvetsov G. M., Ladin N. V. Marine refrigeration units: A textbook for
universities - M.: Transport, 1986. - 232 p.
Scroll compressors have proven themselves to be reliable, energy-efficient and easy-to-use units for producing compressed air. In such compressors, air is compressed by two spirals - one of them is stationary, and the second rotates at high speeds and moves at the same time. The movement of the movable spiral reduces the volume of the chamber containing air - and due to this, the density of the gas increases.
Application
Spiral installations allow you to obtain an output stream of the highest degree cleaning: during the compression process, the air does not come into contact with oil or other lubricants and, accordingly, does not mix with it. Therefore, scroll compressors are used in those industries where particularly stringent requirements are imposed on air quality (medical and dental clinics, food and pharmaceutical production, manufacturing of high-precision electronics, etc.). And the use of such equipment together with dehumidifiers and additional filtering devices ultimately allows you to obtain the best air flow in terms of quality.
Advantages of scroll compressors
The main element of any is the compressor. It serves to ensure the movement of refrigerant in the system and create a pressure difference.
Relatively recently, scroll-type compressors began to be used in refrigeration technology. They mainly work as part of air conditioning systems, heat pumps, medium and high temperature refrigeration units.
The working element of a scroll compressor is a spiral. The operating principle of a refrigeration scroll compressor is based on the coordinated rotation of one spiral relative to another.
Operating principle of a scroll refrigeration compressor.
In a scroll compressor, refrigerant vapor is compressed between two spirals.
One spiral is stationary, the second rotates around it. Moreover, this movement has a complex trajectory. The electric motor, located in one sealed compressor housing, does the work - it rotates the shaft, at the end of which there is an eccentrically installed spiral. While rotating, the movable spiral rolls along the walls of the fixed spiral, sliding along the oily film. The contact points of the spirals gradually move from the edge to the center, and they are located on each turn of the working element. Capturing the sucked-in refrigerant vapors in the area of a larger volume of compressed gas, the spirals gradually compress them as they approach working area towards the center, as its volume decreases. Accordingly, in the center of the spirals the maximum gas pressure is achieved, which then enters the condenser through the compressor discharge line. In a scroll compressor, during operation, vapor compression occurs continuously, since there is more than one point of contact between the spirals and several working compression zones are formed. The electric motors of hermetic scroll compressors are cooled by suction refrigerant vapors.
Let's look at the design of a spiral refrigeration compressor using the example of a product . The design of compressors from other manufacturers is similar. The main components of a scroll compressor are shown in Figure 2.
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Figure 2. Design of a scroll refrigeration compressor.
Due to its design, the number of mutually rubbing parts in a scroll compressor is significantly less than what theoretically indicates its reliability.
Also, the advantages of the design include the absence of dead harmful space in the compression zone, which increases operating efficiency.
Due to the fact that several working zones are formed simultaneously during the gas compression process, refrigerant vapors are pumped more evenly than in piston compressors and with smaller working volumes, which reduces the load on the electric motor.
To improve work efficiency, great attention in scroll compressors, attention is paid to sealing the side and end surfaces of the contacts of the spirals to reduce gas flows between adjacent compression zones.
Scroll compressors were originally designed and found their greatest application in the field of high and medium temperature refrigeration systems - air conditioning, chillers, heat pumps. But also in low temperature refrigeration units they are also used thanks to the technology of injecting a small amount of refrigerant into the center of the spirals during operation.
Regulating the performance of scroll compressors is possible using frequency converters, changing the speed of rotation of the shaft. In addition, the manufacturer of scroll compressors Copeland , has developed a technology for adjusting performance by changing the distance between the spirals during rotation. This technology allows the scroll compressor to operate idle, without forming any working compression zones at all.
Today, spiral refrigeration compressors are produced and supplied to Russia and, accordingly, to Chelyabinsk by such world-famous companies as, Danfoss Performer , .