Design and Analysis of CO2 Transcritical Cycle Two-stage Rolling Rotor Compressor in Tianjin University Tian Hua, Ma Yitai, Li Minxia, ​​Wang Wei (School of Mechanical Engineering, Tianjin University, Tianjin 300072), therefore, the two-stage rolling rotor compressor for core components The self-developed design was carried out, and the inhalation, compression and exhaust processes and structural characteristics in the working chamber of the two-stage compressor were analyzed. The C2 transcritical cycle two-stage rolling rotor compressor under certain working conditions was designed; the structural parameters according to the design The movement and force analysis were carried out, and as a guide, structural specialization was carried out in severely rubbed parts, such as adding a sealing column at the end of the sliding plate to reduce friction and leakage and improve compressor efficiency.
Fund Project: National Natural Science Foundation of China (50676064); National High Technology Research and Development Program (863 Program) funded project (2007AA05Z262).
In recent years, the international community has taken many measures to deal with global warming and the destruction of the ozone layer. To further protect the ozone layer, the 19th Conference of the Parties to the Montreal Protocol, held on September 17, 2007, agreed to accelerate the phase-out of the production and consumption of hydrochlorofluorocarbons (HCFCs). In addressing global warming, the EU's EC842/2006 fluorine gas regulations and the 2006/40/EC Directive impose clear limits on working fluids with GWP values ​​greater than 150. To this end, CO2 has received extensive attention due to its environmental friendliness, ideal thermodynamic properties, non-toxicity, incombustibility and low cost.
°C), the cycle is usually run under transcritical conditions. Since the operating pressure of the system is much higher than that of the conventional refrigerant, and the pressure difference is large (about 6 MPa), the throttling loss is serious and the system performance is relatively low. The use of two-stage circulation to reduce exhaust gas temperature (ie, reduce equivalent condensation temperature) and reduce compressor power consumption are the primary means of improving system performance. Although the CO2 compressor has a small pressure ratio (about 3), the high pressure can reach 10 MPa, the low pressure is 3.5 MPa, and the pressure difference is very large. Therefore, the traditional CO2 single-stage compressor has very serious leakage, and the imbalance force and friction caused by the pressure difference are also very serious. At the same time, the single-stage compressor has a high exhaust temperature (about 120C), and the high-low temperature heat transfer loss is also compared. serious. These have led to the inefficiency of CO2 single-stage compressors (60% to 70%). At present, the CO2 two-stage compressor developed by large foreign companies shows that the isentropic efficiency can be increased to more than 80%. The domestic Shanghai Hitachi Electric Co., Ltd. also developed a two-stage C2 compressor prototype (fixed speed and AC frequency conversion) for heat pump water heaters, but no substantive results were published.
To this end, the Institute of Thermal Energy of Tianjin University has carried out research on the C2 transcritical two-stage cycle. The author designed the CO2 two-stage rolling rotor compressor to provide a theoretical basis for the independent development of the C2 transcritical cycle two-stage rolling rotor compressor.
The 1C2 transcritical two-stage cycle is shown as a typical C2 transcritical two-stage cycle principle and ts diagram. This cycle is a two-stage compression intermediate fully cooled type. The circulation process is as follows: the saturated gaseous C21 from the evaporator is compressed to the intermediate pressure state 2 through the low-pressure stage of the two-stage compressor; then it is cooled by the intermediate cooling device to the saturated liquid 3, and then enters the high-pressure stage compression of the two-stage compressor. Supercritical state 4 to exhaust pressure; high pressure high temperature supercritical C2 discharged from the compressor is cooled by a gas cooler to 5. In the intercooler, the fluid coming out of the gas cooler is throttled through a section 1' After the flow is cooled, another high-pressure fluid is cooled, and the cooled fluid enters the throttle to throttle and cool down, and then enters the evaporator to absorb heat, and simultaneously cools the high-temperature gas discharged from the low-pressure stage to the inlet state of the high-pressure stage.
The equivalent condensation temperature is a common parameter for comparing system efficiency in a variable temperature cooling system. The system efficiency increases with the decrease of the equivalent condensation temperature. The definition is as follows: In (b), the C2 transcritical single-stage circulation cooling process is 2-5. The equivalent condensation temperature is shown as line A; the two-stage circulation cooling process is 2-3 and 4-7, and the equivalent condensing temperature is significantly reduced as indicated by line B. According to calculation, when the evaporation temperature is °C, the superheat degree is 5 °C, the gas cooler outlet temperature is 34 °C, and the high pressure exhaust pressure is 10 MPa, the COP of the two-stage compression is 2.85, which is more than single-stage compression (2.398). 18% higher. C2 transcritical two-stage cycle principle and t-sC2 two-stage rolling rotor compressor cavity. Its work includes the following four stages.
2.1C2 two-stage rolling rotor compressor working principle The suction chamber pressure on the left side of the rotor is reduced, and the suction starts; the compression on the right side shows the working cavity of the C2 two-stage rolling rotor compressor to form a closed space, and the compression process begins. When the pressure in the compression chamber is sufficient to overcome the exhaust valve resistance, the exhaust process begins; the suction chamber still performs the suction process. The angle at which the rolling rotor rotates at this time is called the exhaust angle, see (b).
When the edge angle of the exhaust orifice is turned, the exhaust port communicates with the suction chamber, the exhaust valve closes, and the exhaust process ends, see (c).
At the end of the inhalation process, the compression chamber forms a closed volume and begins the next cycle, see (d).
2.2 Structural design features The structure of the CO2 two-stage rolling rotor compressor designed and developed by the author. It has the following four characteristics.
In order to reduce differential pressure deformation and leakage, a suitable intermediate pressure is selected to reduce the differential pressure at the inlet and outlet of the compression process, and the pressure ratio of the two-stage compression unit is also kept as relatively consistent as possible, while the first and second stages are simultaneously made. The mass flow is equal.
The two compression units are held by a single drive shaft to maintain a 180° phase difference and are driven by the motor at the top of the spindle. Since the pressure difference and the force of the two-stage compression itself are relatively small, and the arrangement structure of the 180° phase difference is also advantageous for the force equalization, the resistance torque of the compressor shaft changes smoothly.
The low-pressure compression unit is divided into two channels: one enters the high-pressure compression chamber; the other enters the housing to ensure that the pressure of the housing is low, and then enters the high-pressure compression chamber. This design not only ensures that the pressure in the compressor casing is intermediate pressure, but also reduces the pressure strength requirements of the casing, thereby reducing the size and facilitating lubrication of the shaft and other components.
Since the axis between the two eccentric wheels is a stress concentrated portion, the shape of the shaft can be tentatively improved, and the slide plate can be modified to significantly reduce the deformation.
The force analysis of the 3CO2 two-stage rolling rotor compressor gives the force acting on the sliding plate: the contact force Fn and Ft with the rolling rotor; the contact force Fri, Fr2 and Frti, Frt2 with the sliding plate groove; Elasticity Fk; inertial force Flv of the skateboard; force caused by gas or lubricant pressure around the skateboard.
As shown, it is assumed that the pressure in the gap between the slider and the slider slot acts linearly around the slider. The differential pressure force Fc received at both ends of the sliding plate is the pressure difference force Fh of the portion of the sliding plate that protrudes into the cylinder. The formula for the speed and acceleration of the sliding plate can be obtained by the first and second derivatives of the equation (4), that is, other rolling rotor compressors. The calculation formula of the force is the design parameters and dimensions of the table 1C2 transcritical cycle two-stage rolling rotor compressor. The suction pressure / MPa suction temperature / 'C cylinder radius / mm low pressure level 3.5550 high pressure level 7.03437 item exhaust pressure / MPa row Gas Temperature / C Rotor Outer Radius / mm Low Pressure Stage 76545 High Pressure Stage 106233 4 Results and Analysis Table 1 shows the design parameters and main structural dimensions of the C2 transcritical cycle two-stage rolling rotor compressor. The compressor input power is 3kW, and the nominal cooling capacity is 8kW. The relationship between the frictional force of the c2 transcritical cycle two-stage rolling rotor compressor slide and the sliding groove is shown. As the angle of rotation increases, the friction between the slide and the slide groove increases first and then decreases, and both reach a maximum around 180°. This is because when the corner reaches 180°, there is no horizontal component of the positive pressure of the rolling rotor against the sliding plate, resulting in the maximum positive pressure of the sliding plate and the sliding groove. The maximum friction of the low-pressure stage slide plate and the slide groove on the suction chamber side is 129N, and the maximum friction force on the compression chamber side is 23.7N; the maximum friction force of the high-pressure stage slide plate and the slide groove on the suction chamber side is 65N, in compression The maximum frictional force on the cavity side is 15N. In order to reduce the friction loss between the sliding plate and the sliding plate groove, it is conceivable to add a needle roller between the sliding plate and the sliding plate groove to generate rolling friction instead of sliding friction and reduce friction.
The relationship between the frictional force of the slide and the rolling rotor of the CO2 transcritical cycle two-stage rolling rotor compressor with the rotation angle is given. It can be seen that as the angle of rotation increases, the friction of the sliding plate and the rolling rotor first decreases and then increases. This is because, as the angle of rotation increases, the angle between the positive pressure and the vertical direction of the sliding plate and the rolling rotor becomes smaller and then becomes larger, resulting in the same variation of the positive pressure of the two contacts. When the exhaust starts, the differential pressure F at both ends of the slide and the differential pressure Fh that the slide extends into the cylinder are kept to a maximum, which causes the positive pressure of the slide and the rotor to be smaller than before the exhaust, thus appearing The friction between the two is not equal before and after the exhaust. The friction of the low-pressure slide and the rolling rotor can reach a maximum of 72N; the maximum friction of the high-pressure slide and the rolling rotor can reach 42N. In order to reduce this friction, a sealing column with a concave circular surface can be installed at the end of the sliding plate. So that the concave circular surface can coincide with the outer circumference of the rolling rotor. In this way, on the one hand, the sealing column swings left and right with the rotation of the rolling rotor, so that the contact surface is in a better lubrication state, thereby achieving the purpose of reducing friction; at the same time, because the sealing column has a small arc coincides with the rolling rotor, strengthening The sealing effect.
5 Conclusion Under certain working conditions, the structural design of the two-stage compressor is carried out, and the appropriate intermediate pressure is selected. The two compression units maintain a 180° phase difference with a single drive shaft, so that the force is balanced and the rotational torque is stable.
The back pressure uses an intermediate pressure so that the compressor casing is relatively pressure-bearing. The tentative design of the shaft shape and the slide plate is designed to significantly reduce the deformation.
Through force analysis, it is found that the friction between the sliding plate and the sliding plate groove and the end of the sliding plate and the rolling rotor is serious, and specific design improvement measures are proposed.
Symbol Description: Inner,. The pressure of a suction chamber and the compression chamber, Pa; rv - the radius of the arc of the end of the slider, m; H - cylinder height, m; e - compressor eccentricity, m; 0 - rolling rotor rotation angle, rad; K ―spring elastic coefficient, N/m; mv—the mass of the skateboard, kg; F—spring force on the back of the skateboard, N; Fiv—the inertial force of the skateboard, N; F; i—the slide and the slide groove are positive on the compression chamber side Pressure, N; Fr2 - the positive pressure of the sliding plate and the sliding groove on the side of the suction chamber, N; F - the positive pressure of the sliding plate and the rolling rotor, N; Frti - the friction between the sliding plate and the sliding groove on the side of the suction chamber, N; Frt2—the friction between the sliding plate and the sliding groove on the side of the compression chamber, N; Ft—the friction between the sliding plate and the rolling rotor, N; fs—the friction coefficient between the sliding plate and the sliding groove; fv—the friction between the sliding plate and the rolling rotor Coefficient; f - radius ratio of rolling rotor to cylinder; x - length of uncompressed or stretched spring, m; a - rotational speed of rolling rotor, rad / min H - dynamic viscosity, Pa's.
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