Application of quasi-isothermal compression technology in rotor compressor

In an ideal refrigeration cycle, the compression process is generally considered to be an isentropic process. According to the engineering thermodynamics knowledge, the power consumed by the isentropic compression is the largest, the power consumed by isothermal compression is the smallest, and the compression is variable (<1 isothermal compression technology. 1.1 Thermodynamics theory, direct to the gaseous refrigerant in the refrigeration compressor Isothermal compression will cause the refrigerant state to enter the liquid phase, that is, the phenomenon of "wet compression" occurs, which is not allowed, and the actual physical process cannot be realized. Only by reducing the temperature of the compressor and its components The temperature of the refrigerant in the compression chamber is reduced to a degree, thereby obtaining an effect similar to the isothermal compression process.

Essentially, the cooling treatment of the high-temperature components of the compressor utilizes thermal management means to achieve optimal utilization of energy. Cooling the refrigerant compression process can be carried out in the following aspects: 1) removing the heat carried by the refrigerant during the compression process, reducing the compression work and improving the efficiency of the whole machine;) removing the heat in the exhaust gas and reducing Heat transfer to the inhalation; 3) Cooling the motor or housing to increase motor efficiency while reducing heat transfer to the suction.

1.2 Isentropic-isothermal compression and isentropic compression process power consumption comparison According to the usual inhalation state, the refrigerant in the compressor is isothermally compressed, it is very likely that droplets will start to appear before the exhaust pressure is reached, which is not feasible. If a compression method can be found, the temperature of the working medium after compression is not changed much, and no droplets appear during the compression process. At this time, the power consumed by the compression is smaller than the isentropic compression. Theoretically, an isentropic, re-isothermal compression path can be used, that is, the refrigerant is firstly compressed from the inspiratory state to the intermediate state, and then compressed from the intermediate state to the final state.

The following quantitative analysis of the isentropic isothermal compression process (the compression point of the compression process is assumed to be the condensation temperature of the refrigeration cycle) and the power consumption of the isentropic compression process, the theoretical verification of the technical significance of isothermal compression.

Table 1 Standard operating conditions Parameter operating conditions Parameters Intake and exhaust evaporation pressure Nine / Pressure Ai / Temperature / Condensation temperature Intake temperature Pre-valve temperature values ​​As shown, the isentropic compression process can be expressed as 1 - V - 2 on the graph The compression work is equivalent to the area enclosed by the area 1 - 2 - 5 - 6 - 1; the first isentropic and isothermal compression process can be expressed as 1 - 1 ' - 2 ' on the graph, and the compression work is equivalent to 1 - 2 The area enclosed by '―5―6―1 area, compared with the isentropic compression process, saves the area surrounded by the V-2-2'-V area.

For the system using R22 and R410A refrigerant, calculate the power consumption of the above two compression paths under the conditions of Table 1 (the following is the R22 system calculation process): the isentropic-isothermal compression process dynamometer and the pressure-焓 map isentropic The total power consumption of the isothermal compression process is equal to the sum of the power consumption of the 1 -1 isentropic process and the power consumption of the 1 2 'isothermal process. By determining the 1 and 1 state parameters, the energy consumption (k/kg) of the 1~1 isentropic compression process is: /, -hi-439.2-428. 6-10.6. The compression process of the temple temperature process, ie 1 The area enclosed by the 2'-5-7-1 area is calculated using the difference method. It can be determined that: 19.1 MPa, and 0.915 MPa, and the difference is 10 aliquots, the pressure step is 0.123 MPa. The curve of the 1 2' process is an isotherm, which can be obtained from the pressure and temperature. Each step corresponds to the specific volume of the point, and then the integral, you can get the area of ​​〗 1, 〖3, 〗 10, as shown. The calculation result is as follows 12. = 9.34k/kg. Available, the isentropic - the total consumed work of the isothermal compression process (k / kg): u1 = 10.6 + 2.77 + 9.34 - 32.71. Visible, if the refrigeration compressor compression process (Partial stage) maintained in the isothermal process, to save compressor power consumption. Under the standard operating conditions of Table 2, the power consumption of different compression processes is compared with the isentropic refrigerant - the isothermal compression process isentropic compression elimination rate / 2 quasi-isothermal compression cooling In the operation of the rolling rotor compressor, the crankshaft speed is very fast, and the heat generated by the compressed gas is difficult to eliminate in time, and the compression process is close to the adiabatic process. To quickly remove heat and achieve isothermalization of the actual compression process, it is necessary to adopt a corresponding cooling scheme.

The following is an analysis of the typical structure of the rolling rotor compressor. According to the refrigerant gas compression process in the compressor, the application of quasi-isothermal compression technology is carried out from two key steps: heat removal in the cylinder and cooling of the motor components outside the cylinder.

2.1 Cylinder cooling is based on the isothermal compression theory. Cooling and cooling of the compression carrier-cylinder of the compressor refrigerant gas is one of the effective ways to improve performance and efficiency; exhaust gas temperature reduction, exhaust density increase, exhaust valve, muffler The refrigerant gas flow rate and pressure loss in the exhaust pipe are also reduced accordingly. However, the difficulty in implementing cylinder cooling is that due to the extremely fast compression speed, the compression process can be used for heat exchange for a very short period of time. At the same time, excessive heat is removed, which may cause the refrigerant to condense during compression. As a result, the valve plate and the friction pair are subjected to excessive pressure and fail.

According to the technical implementation, cylinder cooling can be divided into direct cooling and indirect cooling. Direct cooling is the exchange of heat in the cylinder chamber with direct contact of the cryogenic fluid with the refrigerant; indirect cooling is performed by cooling the cylinder wall surface and cooling the refrigerant in the cylinder by heat conduction inside and outside the cylinder wall.

Direct cooling A common type of direct cooling scheme is to inject refrigeration oil directly into the suction stream. The high turbulent flow of the refrigerant during the suction and compression process forces the refrigerating machine oil to be dispersed to have a sufficient area to exchange heat with the refrigerant, effectively absorbing the heat generated during the compression process. Since the specific heat capacity of the refrigerating machine oil is much higher than the specific heat capacity of the refrigerant gas, there is no significant temperature rise after the heat is absorbed. On the other hand, although the heat absorption during compression helps to reduce the compression work of the refrigerant, the refrigeration oil inevitably consumes part of the work during the compression process. The specific quantitative data on the compression and power consumption of incompressible fluids such as refrigerating machine oil needs further theoretical and experimental research. The theoretical evaluation can assume that the refrigerating machine oil density is constant. The compression work of the incompressible fluid can be expressed by the following formula: In addition, it is recommended to conduct an in-depth and detailed CFD study on the cylinder suction and exhaust passage to confirm the pressure loss of the refrigerant flow caused by the injection of the refrigeration oil; The exhaust valve piece is reinforced.

After the throttling and cooling, the liquid refrigerant enters the drainage part of the compressor, participates in the refrigerant compression process in the cylinder, and needs to consume part of the power of the compressor, but through the control of the flow and the pipe diameter of the drainage throttling device, the refrigerant is drained. The compression work is much less energy-saving than the near isothermal compression process of the refrigerant. In short, by controlling the parameters of the drainage refrigerant, the energy consumption of the compressor can be effectively reduced, the compression process of the refrigerant can be similar to the isothermal compression process, and the refrigerant discharge temperature can be lowered.

When the motor component is cooled at room temperature, the resistance change rate of copper is 0.333%/°C; and the ohmic loss is the product of the resistance and the square of the current. Decreasing the winding temperature of the motor will effectively reduce the motor resistance, which is beneficial to the improvement of motor efficiency. . The method of cooling the motor is to fully immerse the motor in the refrigeration oil and pump the oil to an external oil cooling cycle to cool down. The current rolling rotor compressor is a high back pressure structure, and the motor is located above the pump body. The use of the refrigerating machine oil to achieve motor cooling is an important subject that the designer deliberately explores.

3 Conclusions Exploring the energy-saving significance of the staged isothermal compression in the working process of the refrigeration compressor, and based on the quasi-isothermal compression theory, the rolling rotor compressor is used as the object to discuss the cooling scheme of the cylinder and motor of the compressor: cylinder cooling . In-cylinder cooling scheme, due to the rapid compression process, the effect of directly cooling the cylinder is not obvious. The refrigerating machine oil or low-temperature refrigerant is injected into the suction airflow, and the refrigerant can be sufficiently cooled by direct heat exchange; the cylinder external cooling scheme is in the cylinder block. A drain tank is opened, and an external cryogenic liquid refrigerant directly introduced into the refrigeration system is used as a cold source to perform cylinder cooling.

Motor cooling: Oil cooling is a key technical solution. Due to the limited structure of the rotor compressor, its feasibility is subject to further experimental and theoretical research.

In short, through the heat management of the functional sections such as the cylinder section and the motor section of the rotor compressor, the gas compression process in the compressor is approached to the isothermal path, effectively reducing the input power consumption and controlling the exhaust temperature of the refrigerant. Quasi-isothermal compression technology has important guiding significance for the structural design and optimization of rotor compressors.

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