Double-machine double-stage screw refrigeration compressor unit special for ground freezing

Abstract

The application provides a double-machine double-stage screw refrigeration compressor unit special for ground freezing, which comprises a low-pressure stage compressor, a low-pressure stage variable frequency motor, a plate-and-shell heat exchanger, a high-pressure stage compressor, a high-pressure liquid accumulator, an evaporative condenser, a high-pressure stage power frequency motor, an oil separator, a hot siphon oil cooler, an oil pump, an oil fine filter, a brine pump, a brine pool, a hot siphon evaporator, a throttle valve and a control console. The double-machine double-stage screw refrigeration compressor unit special for ground freezing combines the variable frequency technology and the unique features of the double-machine double-stage screw compressor unit operation, can realize flexible adjustment of the volume ratio of the high-pressure stage compressor and the low-pressure stage compressor at different temperature points, reaches the highest efficiency, improves the refrigerating capacity at the medium temperature stage, reduces the use cost, improves the motor efficiency and the refrigerating capacity at the low temperature stage, shortens the freezing time while saving energy, and is convenient for dismounting, installing and transporting.

Description

Technical Field

[0001] This invention relates to the technical field of refrigeration compressor units, and more particularly to a dual-machine, two-stage screw refrigeration compressor unit specifically designed for ground freezing. Background Technology

[0002] Artificial freezing is a cooling technology that achieves ground freezing through a refrigerant heat transfer process. After the salt solution is cooled by a refrigeration unit, it is used as a refrigerant and circulated in a freezing pipe buried in the rock and soil. The low-temperature salt solution exchanges heat with the rock and soil, absorbing heat from the rock and soil, increasing its own sensible heat, and causing most of the water in the surrounding rock and soil to freeze. This effectively enhances the strength and stability of the rock and soil, reduces deformation, and isolates groundwater. Under the protection of the frozen wall, well casing is excavated and constructed until the frozen section of the well casing is completed.

[0003] The refrigeration compressor selected in this cooling technology is a dual-machine, two-stage screw refrigeration compressor unit. The refrigeration cycle of this unit is an improved two-stage compression, one-stage throttling, and intermediate incomplete cooling cycle. It adopts two-stage subcooling and has the characteristics of compact structure, low cost, small footprint, simple and convenient operation, simple system, high refrigeration efficiency, and wide coverage of low-temperature system refrigeration capacity.

[0004] Artificial ground freezing can be divided into three stages: high, medium, and low temperature. In the high-temperature stage, single-stage compression is generally used. The evaporation temperature in this stage is 10℃ to -10℃, and the brine temperature drops from room temperature (25℃) to -5℃, taking about 10 to 20 days. In the medium-temperature stage, the evaporation temperature is -10℃ to -30℃, and the brine temperature is -5℃ to -25℃, also taking about 10 to 20 days. In the low-temperature stage, the evaporation temperature is -30℃ to -45℃, and the brine temperature is -25℃ to -40℃, accounting for about 90% of the total time. Depending on the well depth, it takes more than a year from excavation to reaching the predetermined depth. Throughout the process, the refrigeration unit runs continuously, and its operating costs account for more than 60% of the total project cost, with electricity costs reaching tens of millions. Therefore, energy saving of the refrigeration unit is extremely important and can save huge costs for the project.

[0005] Existing conventional two-stage compressor units face the following problems:

[0006] (i) During the entire freezing process of the strata, the evaporation temperature and load are constantly changing. When the evaporation temperature is lower, the higher the volume ratio of the two stages, the more energy-efficient the unit is. However, the volume ratio of the high and low pressure stage compressors of conventional dual-machine dual-stage units is fixed and cannot be changed with different operating conditions, resulting in lower energy efficiency.

[0007] (ii) During the freezing and intermediate temperature stage, a higher cooling capacity is required for rapid cooling because it takes longer to break through this temperature point, but once it is broken, the cooling rate accelerates. At this time, the economizer needs to be activated to provide a higher cooling capacity. However, due to the limited motor capacity during the intermediate temperature stage, the low-pressure compressor can only operate at partial load through the energy slide valve. At partial load, there is a leakage channel between the energy slide valve and the compressor body, preventing the economizer from activating. Furthermore, the low-pressure compressor operates at partial load for a relatively long time during freezing, but the motor still runs at full speed, resulting in energy waste.

[0008] (iii) During the freezing and low temperature stage, as the pressure ratio of the low-pressure stage compressor continues to increase, the discharge volume decreases significantly, and the shaft power also decreases. The shaft power required for the compressor to operate at full load is less than 50% of the motor capacity. Moreover, this stage accounts for 90% of the total operating time. The motor operates at low load for a long time with low efficiency and poor power factor, which may result in fines from the power supply department.

[0009] (iv) The equipment for artificial ground freezing needs to be moved to another mine to rebuild the system after the completion of one project. Conventional dual-unit dual-stage units do not have an integrated starter cabinet, which means that the power cable must be reconnected every time the refrigeration system is rebuilt. In addition, the components in the unit cannot be disassembled individually, making the installation inflexible and the unit not easy to move. Summary of the Invention

[0010] This invention addresses the aforementioned shortcomings by providing a dual-stage, two-component screw refrigeration compressor unit specifically designed for ground freezing. This unit combines variable frequency technology with the unique characteristics of dual-stage, two-component screw compressor operation, enabling flexible adjustment of the volume ratio of the high and low pressure compressor stages at different temperature points to achieve maximum efficiency. Furthermore, it increases the cooling capacity in the intermediate temperature stage, reduces operating costs, and improves motor efficiency and cooling capacity in the low temperature stage. This results in energy savings while shortening freezing time and facilitates disassembly, installation, and transportation.

[0011] To achieve the above objectives, the present invention provides a dual-stage screw refrigeration compressor unit specifically for ground freezing, comprising a low-pressure stage compressor, a low-pressure stage variable frequency motor, a plate heat exchanger, a high-pressure stage compressor, a high-pressure liquid receiver, an evaporative condenser, a high-pressure stage industrial frequency motor, an oil separator, a thermosiphon oil cooler, an oil pump, an oil filter, a brine pump, a brine tank, a thermosiphon evaporator, a throttling valve, and a control console;

[0012] The low-pressure stage compressor is driven by a low-pressure stage variable frequency motor, and the high-pressure stage compressor is driven by a high-pressure stage industrial frequency motor. The discharge end of the low-pressure stage compressor is connected to the suction end of the high-pressure stage compressor. The high-pressure stage compressor is connected to the inlet of the oil separator. The discharge end of the oil separator is connected to the inlet of the evaporative condenser. The refrigerant output end of the evaporative condenser is connected to the inlet of the high-pressure liquid receiver. The refrigerant in the high-pressure liquid receiver is connected to the input end of the thermosiphon evaporator in sequence through a plate-and-shell heat exchanger and a throttling valve. The output end of the thermosiphon evaporator is connected to the suction port of the low-pressure stage compressor.

[0013] The lubricating oil outlet of the oil separator is connected to the oil inlet of the thermosiphon oil cooler, and the oil outlet of the thermosiphon oil cooler is connected in sequence to the oil injection ports of the low-pressure stage compressor and the high-pressure stage compressor through an oil pump and an oil filter.

[0014] The inlet of the brine pump is connected to the refrigerant outlet of the thermosiphon evaporator, and the outlet of the brine pump passes through the brine pool and the ground in sequence, and is connected to the refrigerant inlet of the thermosiphon evaporator.

[0015] Furthermore, the control console is a control device that integrates a starter cabinet and a frequency converter, and can start high-voltage power frequency motors and start low-voltage frequency variable frequency motors via a touch screen;

[0016] When operating at variable frequency, the high-pressure stage motor is started first, and the high-pressure stage compressor is driven by the high-pressure stage coupling. The load is gradually and automatically increased to 100% full load through the energy regulating slide valve. After the low-pressure stage compressor starts, the low-pressure stage variable frequency motor is started in variable frequency mode, and the low-pressure stage compressor is driven by the low-pressure stage coupling. Then the liquid supply valve of the plate heat exchanger is opened to achieve secondary subcooling of the refrigerant from the high-pressure liquid receiver.

[0017] Furthermore, in the refrigeration cycle, the low-pressure refrigerant vapor generated by the thermosiphon evaporator is drawn into and compressed by the low-pressure stage compressor and then enters the high-pressure stage compressor for secondary compression. After being compressed, the high-pressure refrigerant gas flows into the evaporative condenser after being de-oiled by the oil separator, where it is condensed into liquid by the cooling water. The condensed high-pressure refrigerant then flows through the plate heat exchanger for secondary subcooling. After being throttled and depressurized by the expansion valve, the refrigerant enters the thermosiphon evaporator to absorb heat and evaporate. The resulting low-pressure vapor returns to the low-pressure stage compressor, completing the refrigeration cycle.

[0018] Furthermore, the lubricating oil separated by the oil separator enters the unit's lubrication oil circuit, is transported by the oil pump, flows through the thermosiphon oil cooler for cooling, and then flows back to the low-pressure stage compressor and the high-pressure stage compressor for lubrication after passing through the oil filter. The oil filter adopts a dual parallel structure, in which when one oil filter is working, the other is in standby mode, used to replace the filter element without stopping the machine.

[0019] Furthermore, on the refrigerant side, the brine exchanges heat with the refrigerant in the evaporator and then enters the brine pool. It is then pumped by the brine pump into the mine excavation opening and the pipeline next to the stratum to absorb heat from the soil, and then returns to the evaporator to release heat and complete the cycle.

[0020] Furthermore, the plate heat exchanger integrates an economizer and an intercooler, with the heat exchange cores of the economizer and the intercooler integrated within the same housing; the plate heat exchanger is equipped with a liquid supply valve, which opens immediately after the low-pressure stage compressor starts, allowing the refrigerant from the high-pressure liquid receiver to be subcooled first through the plate heat exchanger before entering the throttling valve for secondary expansion.

[0021] Furthermore, the operating frequency of the low-pressure stage variable frequency motor is automatically adjusted in segments according to the changes in the evaporation temperature of the thermosiphon evaporator during unit operation; when the evaporation temperature drops, the frequency of the low-pressure stage variable frequency motor is first reduced to 40-45Hz to adapt to the partial load state, and then the frequency is gradually increased to no more than 60Hz.

[0022] Furthermore, the low-pressure stage compressor and the low-pressure stage variable frequency motor are mounted on top of the thermosiphon oil cooler, and the high-pressure stage compressor and the high-pressure stage power frequency motor are mounted on a common base; the plate-shell heat exchanger is arranged in the form of an integrated module on the base located behind the low-pressure stage compressor; the control console is located behind the high-pressure stage power frequency motor and connected to the power cable.

[0023] Furthermore, the low-pressure stage variable frequency motor is a 20-60Hz variable frequency motor; the high-pressure stage power frequency motor is a motor with a rated frequency of 50Hz, used to drive the high-pressure stage compressor to operate at full load under high-temperature conditions.

[0024] Furthermore, both the low-pressure stage compressor and the high-pressure stage compressor are open-type screw compressors.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] Firstly, the dual-stage screw refrigeration compressor unit for ground freezing of this invention provides higher cooling capacity under medium-temperature conditions. The cooling capacity of the dual-stage refrigeration mainly comes from the low-pressure stage compressor. When the low-pressure stage compressor operates under low-temperature conditions, its actual shaft power is relatively small. Considering that the low-pressure stage compressor starts when the evaporation temperature is close to -10°C, the load is large and the starting torque is large, the power of the motor is more than two levels higher than the actual shaft power when operating at low temperatures. The high-pressure stage compressor basically operates under a fixed high-temperature condition, and the power of the motor is more reasonable. Under high evaporation temperature conditions, the shaft power required for the compressor to operate at full load exceeds the rated power of the low-pressure stage motor. By reducing the frequency, the compressor can operate under partial load. Traditional dual-stage compressor units regulate cooling capacity by changing the compressor's working volume using a slide valve. Under partial load, the motor operates at a constant speed, unable to adjust according to load changes. This reduces the compressor's effective working volume and the number of effective sealing teeth, leading to increased inter-rotor leakage. If the suction and discharge pressure difference is large, internal and external leakage increases rapidly, drastically reducing compressor efficiency. Increased mechanical friction and internal leakage also impact compressor lifespan. Furthermore, slide valve regulation prevents the economizer from activating before full compressor load because superheated gas exchanging heat with the high-pressure liquid in the economizer leaks through the channel between the slide valve and the compressor body to the compressor suction port, reducing refrigerant flow in the low-pressure stage. Inverter frequency reduction perfectly solves these problems, allowing the economizer to activate immediately upon low-pressure stage compressor startup for rapid cooling. Inverter regulation is also more energy-efficient than slide valve regulation, saving 35% to 10% energy when the load is between 50% and 90%. Therefore, the longer the partial load operation, the greater the energy savings. Due to the complexity and special nature of the application scenarios of ground freezing, the heat load of soil is much greater than that of air. While cooling down, the soil and the water vapor in the soil are also heating up. The load is large and the cooling time is long. The low-pressure compressor runs at partial load for a long time. Therefore, frequency reduction energy-saving technology is more suitable for ground freezing.

[0027] Secondly, the dual-stage, two-component screw refrigeration compressor unit for ground freezing of this invention can improve the cooling capacity and COP under low-temperature conditions. After the low-pressure stage compressor starts, as the evaporation temperature decreases, the volumetric efficiency of the low-pressure stage compressor decreases, the suction volume also decreases, and the cooling capacity decreases. At this time, the rated power of the low-pressure stage motor is much higher than the shaft power required for the compressor to operate at full load. Long-term light-load operation of the motor will lead to a decrease in power factor and motor efficiency, making the motor matching very unreasonable. However, considering that the low-pressure stage compressor operates under high-temperature conditions, it is necessary to configure it with a motor with a higher rated power. This problem can be solved by using a frequency converter to increase the frequency, simultaneously increasing the speed and suction / discharge volume of the low-pressure stage compressor, thereby improving the cooling capacity, motor efficiency, and motor power factor to achieve energy saving. Since the demand for cooling capacity is relatively small at this stage, improving the unit's COP becomes the focus. At this time, the COP can be improved by changing the ratio of the two-stage compression displacement through frequency conversion speed regulation. Tests show that, at a constant condensing temperature, the lower the evaporation temperature and the larger the ratio of the two-stage compression displacement, the higher the unit's COP.

[0028] Thirdly, the investment cost recovery period of the dual-machine, two-stage screw refrigeration compressor unit for ground freezing of the present invention is short. The dual-machine, two-stage screw refrigeration compressor unit for ground freezing has two motors. Since the high-pressure stage compressor operates stably under high temperature conditions and basically at full load, the high-pressure stage motor does not need to use frequency conversion technology. Only the low-pressure stage motor uses a 20~60Hz frequency conversion motor and is equipped with a frequency converter.

[0029] Fourth, the dual-machine, two-stage screw refrigeration compressor unit for ground freezing of the present invention is installed on a common base along with the oil circuit and other circulation pipelines. This forms a dual-machine, two-stage screw refrigeration compressor unit for ground freezing. The highly integrated design integrates the motor start-up and control on a single skid, requiring only a single-wire connection to complete the electrical installation. This improves deployment and transportation efficiency by more than 50%. Its structure is particularly suitable for space-constrained scenarios such as mines and subway tunnels, facilitating the rapid advancement of projects. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of a dual-machine, two-stage screw refrigeration compressor unit specifically designed for ground freezing, according to an embodiment of the present invention.

[0031] Figure 2 A flowchart illustrating the operation of a dual-machine, two-stage screw refrigeration compressor unit specifically designed for ground freezing in an embodiment of the present invention at a ground freezing site;

[0032] In the diagram, the components are: 1. Low-pressure stage compressor; 2. Low-pressure stage variable frequency motor; 3. Plate heat exchanger; 4. High-pressure stage compressor; 5. High-pressure liquid receiver; 6. Evaporative condenser; 7. High-pressure stage industrial frequency motor; 8. Oil separator; 9. Thermosiphon oil cooler; 10. Oil filter; 11. Brine pump; 12. Formation; 13. Brine tank; 14. Thermosiphon evaporator; 15. Throttling valve; 16. Control console; 17. Detailed Implementation

[0033] The following examples illustrate the implementation of the present invention in detail, but they do not constitute a limitation on the invention and are merely illustrative. Furthermore, the advantages of the present invention will become clearer and easier to understand by explaining them.

[0034] like Figure 1 and Figure 2 As shown, the dual-stage, two-component screw refrigeration compressor unit for ground freezing of the present invention includes a low-pressure stage compressor 1, a low-pressure stage variable frequency motor 2, a plate heat exchanger 3, a high-pressure stage compressor 4, a high-pressure liquid receiver 5, an evaporative condenser 6, a high-pressure stage industrial frequency motor 7, an oil separator 8, a thermosiphon oil cooler 9, an oil pump 10, an oil filter 11, a brine pump 12, a brine tank 14, a thermosiphon evaporator 15, a throttle valve 16, and a control console 17 integrating a starter cabinet and a frequency converter. The low-pressure stage variable frequency motor 2 is a 20-60Hz variable frequency motor; the high-pressure stage industrial frequency motor 7 is a motor with a rated frequency of 50Hz, used to drive the high-pressure stage compressor 4 to operate at full load under high-temperature conditions. Both the low-pressure stage compressor 1 and the high-pressure stage compressor 4 are open-type screw compressors, and the low-pressure stage compressor adopts a non-energy sliding valve mechanism or is a compressor that is always in a full-load state.

[0035] The low-pressure stage compressor 1 is driven by the low-pressure stage variable frequency motor 2, and the high-pressure stage compressor 4 is driven by the high-pressure stage fixed frequency motor 7. The discharge end of the low-pressure stage compressor 1 is connected to the suction end of the high-pressure stage compressor 4. The high-pressure stage compressor 4 is connected to the inlet of the oil separator 8. The discharge end of the oil separator 8 is connected to the inlet of the evaporative condenser 6. The refrigerant output end of the evaporative condenser 6 is connected to the inlet of the high-pressure liquid receiver 5. The refrigerant in the high-pressure liquid receiver 5 is connected to the input end of the thermosiphon evaporator 15 through the plate heat exchanger 3 and the expansion valve 16 in sequence. The output end of the thermosiphon evaporator 15 is connected to the suction port of the low-pressure stage compressor 1; the lubricating oil outlet of the oil separator 8 is connected to the oil inlet of the thermosiphon oil cooler 9, and the oil outlet of the thermosiphon oil cooler 9 is connected to the oil injection ports of the low-pressure stage compressor 1 and the high-pressure stage compressor 4 in sequence through the oil pump 10 and the oil filter 11; the inlet of the brine pump 12 is connected to the refrigerant outlet of the thermosiphon evaporator 15, and the outlet of the brine pump 12 passes through the brine pool 14, the mine excavation opening and the formation 13 in sequence, and is connected to the refrigerant inlet of the thermosiphon evaporator 15.

[0036] The control console 17 is a control device that integrates a starter cabinet and a frequency converter. It can start the high-voltage industrial frequency motor 7 and start the low-voltage variable frequency motor 2 via a touch screen. The control console 17, which integrates the starter cabinet and frequency converter, is integrated with the low-voltage variable frequency motor 2 and the high-voltage industrial frequency motor 7 on a common base. Electrical installation can be completed with only a single wire connection, which improves deployment and transportation efficiency by more than 50%.

[0037] When the variable frequency is running, the high-pressure stage industrial frequency motor 7 is started first, and the high-pressure stage compressor 4 is driven by the high-pressure stage coupling. The load is gradually and automatically increased to 100% full load through the energy regulating slide valve. After the starting conditions of the low-pressure stage compressor 1 are met, the low-pressure stage variable frequency motor 2 is started by the variable frequency, and the low-pressure stage compressor 1 is driven by the low-pressure stage coupling. Then the liquid supply valve of the plate heat exchanger 3 is opened to achieve secondary subcooling of the refrigerant from the high-pressure liquid receiver 5.

[0038] In the refrigeration cycle, the low-pressure refrigerant vapor generated by the thermosiphon evaporator 15 is drawn into and compressed by the low-pressure stage compressor 1 and then enters the high-pressure stage compressor 4 for secondary compression. After the compressed high-pressure refrigerant gas is de-oiled by the oil separator 8, it flows into the evaporative condenser 6 and is condensed into liquid by the cooling water. The condensed high-pressure refrigerant flows through the plate heat exchanger 3 for secondary subcooling. After the refrigerant is throttled and depressurized by the expansion valve 16, it enters the thermosiphon evaporator 15 to absorb heat and evaporate. The resulting low-pressure vapor returns to the low-pressure stage compressor 1, completing the refrigeration cycle.

[0039] The lubricating oil separated by oil separator 8 enters the unit's lubrication oil circuit, is pumped by oil pump 10, flows through thermosiphon oil cooler 9 for cooling, and then flows back to low-pressure compressor 1 and high-pressure compressor 4 for lubrication after passing through oil filter 11. Oil filter 11 adopts a dual parallel structure, where one oil filter is working while the other is in standby mode, used for replacing the filter element without stopping the machine. There are two oil filters 11 in total, installed in parallel, one in operation and one on standby, and they are led to the edge of the skid for easier and faster replacement of the oil filter element.

[0040] On the refrigerant side, the brine exchanges heat with the refrigerant in the evaporator 15 and then enters the brine pool 14. It is then pumped by the brine pump 12 into the pipes next to the mine excavation opening and the stratum 13 to absorb heat from the soil, and then returns to the evaporator 15 to release heat and complete the cycle.

[0041] The plate heat exchanger 3 integrates the economizer and the intercooler, with the heat exchange cores of the economizer and the intercooler integrated in the same shell. The plate heat exchanger 3 is equipped with a liquid supply valve, which opens immediately after the low-pressure stage compressor 1 starts, so that the refrigerant from the high-pressure liquid receiver 5 is first subcooled through the plate heat exchanger 3, and then enters the throttle valve 16 for secondary expansion.

[0042] The operating frequency of the low-pressure stage variable frequency motor 2 is automatically adjusted in segments according to the evaporation temperature change of the thermosiphon evaporator 15 during unit operation; when the evaporation temperature drops, the frequency of the low-pressure stage variable frequency motor is first reduced to 40-45Hz to adapt to the partial load state, and then the frequency is gradually increased to no more than 60Hz.

[0043] The low-pressure stage compressor 1 and the low-pressure stage variable frequency motor 2 are mounted on top of the thermosiphon oil cooler 9, and the high-pressure stage compressor 4 and the high-pressure stage industrial frequency motor 7 are mounted on a common base; the plate heat exchanger 3 is set in the form of an integrated module on the base located behind the low-pressure stage compressor 1; the control console 17 is set behind the high-pressure stage industrial frequency motor 7 and connected to the power cable.

[0044] The variable frequency operation scheme of this invention is as follows: Connect the 50Hz power supply to the low-pressure control cabinet, and start the high-pressure stage power frequency motor 7 (rated frequency 50Hz, rated speed 3000r / min) using a star-delta starter via buttons on the control cabinet touch screen. The high-pressure stage compressor 4 is driven by the high-pressure stage coupling. The load is gradually and automatically increased to 100% full load through the energy regulating slide valve. After the low-pressure stage compressor starts at -10℃ and the evaporation temperature meets the starting conditions, the low-pressure stage variable frequency motor 2 is started using a variable frequency method. The low-pressure stage compressor is driven by the low-pressure stage coupling. Then, the intercooler liquid supply valve is opened, followed by the economizer liquid supply valve to allow the ammonia liquid from the high-pressure liquid storage tank to be subcooled a second time.

[0045] At this point, the evaporation temperature is -10℃. Since the low-pressure stage motor was selected for operation at an evaporation temperature of -30℃, the low-pressure stage compressor cannot operate at full load due to motor capacity limitations. Therefore, the low-pressure stage inverter motor 2 is increased to 35 Hz, and the low-pressure stage compressor 1 operates at 2100 r / min. As the evaporation temperature decreases, the frequency is gradually increased to 50 Hz, and the low-pressure stage compressor 1 reaches 3000 r / min. When the dual-machine, two-stage screw refrigeration compressor unit reaches the low-temperature stage, the cooling capacity and shaft power also continuously decrease, the load decreases, and the required cooling capacity is not as large as before. The frequency of the low-pressure stage inverter motor 2 is first reduced to 45 Hz, and the compressor speed is 2700 r / min. Then, the frequency is gradually increased to 50~60 Hz, and the speed reaches 3000 r / min~3600 r / min. The aforementioned gradual frequency increase process continues until the evaporation temperature drops to the low-temperature operating conditions for normal user use. The maximum frequency is controlled below 60 Hz. By controlling the displacement of the low-pressure stage compressor, the low- and high-pressure stage compressors are kept in optimal condition under different operating conditions, which greatly improves the unit's coefficient of performance and motor efficiency under low-temperature conditions.

[0046] Example:

[0047] This embodiment of the dual-stage screw refrigeration compressor unit for ground freezing mainly consists of a low-pressure stage compressor 1, a low-pressure stage variable frequency motor 2, a high-pressure stage compressor 4, a high-pressure stage fixed frequency motor 7, a vertical oil separator 8, a thermosiphon oil cooler 9, an oil pump 10, an oil filter 11, a plate heat exchanger 3, and a control console 17 integrating a starter cabinet and a frequency converter, all connected by pipes and valves. The low-pressure stage compressor 1 and the low-pressure stage variable frequency motor 2 are mounted on the thermosiphon oil cooler 9, while the high-pressure stage compressor and the high-pressure stage motor are mounted on a common base. The plate heat exchanger 3, which integrates an intercooler and an economizer, is placed on the common base behind the low-pressure stage compressor. The control console 17, which integrates the starter cabinet and the frequency converter, is located close to the rear of the motor for easy connection of the power cable.

[0048] The main path of the refrigeration cycle in the ground freezing is as follows: the low-pressure refrigerant vapor from the return port of the thermosiphon evaporator 15 is compressed through the suction port of the low-pressure stage compressor 1 and enters the suction port of the high-pressure stage compressor 4. The gas after secondary compression enters the oil separator 8 for separation and flows to the evaporative condenser 6. After being condensed into refrigerant liquid by cooling water, it flows to the plate heat exchanger 3 for secondary subcooling. The subcooled high-pressure refrigerant liquid is throttled by the expansion valve 16 and then absorbs heat from the heat transfer fluid liquid in the evaporator to boil and evaporate. The resulting refrigerant vapor is then drawn into the low-pressure stage screw refrigeration compressor 1 to complete the refrigeration cycle.

[0049] Meanwhile, the lubricating oil separated in the oil separator enters the unit's oil circuit system. The oil circuit system is equipped with an oil filter 11, which ensures the cleanliness of the lubricating oil through fine filtration. The oil filter 11 adopts a dual oil filter parallel structure, with one oil filter on and one on standby during operation, so that the oil filter element can be replaced without stopping the machine.

[0050] On the refrigerant side, the brine exchanges heat with the refrigerant in the evaporator 15 and then enters the brine pool 14. It is then pumped by the brine pump 12 into the PVC pipe next to the mine excavation opening and the stratum 13 to absorb the heat from the soil. Finally, it returns to the evaporator 15 to release the heat and complete the cycle.

[0051] In the above embodiments, a W-DSAHLG25ⅢT250 / 20ⅢD200 dual-stage screw refrigeration compressor unit for ground freezing and a W-SAHLG25ⅢT250 / 20ⅢD200 conventional dual-stage screw refrigeration compressor unit were selected for comparative testing. To date, 300 sets of this model of dual-stage unit have been used in key coal mine freezing projects in China, representing the gold standard configuration among dual-stage units, making the comparison more convincing. The design operating temperature is -45℃ evaporation temperature and +35℃, with ammonia R717 as the refrigerant. The cooling process and operating data under low-temperature conditions are shown in Table 1. Since the operating parameters of the two units are the same during the cooling process from 5℃ to -10℃, and this only accounts for 5% of the total freezing time, the operating data recorded in the appendix ranges from -15℃ to -45℃.

[0052] Table 1

[0053]

[0054] The experimental data comparison in Table 1 shows that:

[0055] I. The intermediate temperature range from -15 to -30℃ is the main cooling point, requiring a larger cooling capacity to overcome this point. Once this point is overcome, the cooling rate will accelerate. Compared to conventional dual-unit screw refrigeration compressor units, the cooling capacity of the ground freezing dual-unit screw refrigeration compressor unit is increased by approximately 6%, and the COP is increased by approximately 1.5%. Since the energy-saving effect of inverter frequency reduction regulation compared to slide valve frequency reduction regulation cannot be precisely calculated (the internal leakage of the slide valve cannot be calculated), practical experience shows that when energy is below 100%, slide valve regulation is definitely more energy-consuming. The specific reasons have been described in detail above.

[0056] Second, in the low-temperature stage, the evaporation temperature is -30℃ to -45℃. This stage requires less cooling capacity than the medium-temperature stage, but the operating time is longer. Compared to conventional dual-unit screw refrigeration compressor units, the ground-freezing dual-unit screw refrigeration compressor unit increases cooling capacity by approximately 8%, COP by approximately 1%, and motor efficiency by 3%.

[0057] Throughout the cooling phase, the cooling capacity increased by an average of 7%, the COP increased by an average of 1.25%, and the motor efficiency increased by an average of 2.5%, maintaining high-efficiency operation. Since the energy-saving effect of inverter frequency reduction regulation compared to slide valve frequency reduction regulation cannot be precisely calculated, and the internal leakage of the slide valve cannot be calculated, practical experience shows that when energy is below 100%, slide valve regulation is definitely more power-consuming, conservatively estimated to save up to 10% of electricity.

[0058] The above are merely specific embodiments of the present invention. It should be noted that any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the protection scope of the present invention. Any other aspects not described in detail are prior art.

Claims

1. A dual-stage, two-component screw refrigeration compressor unit specifically designed for ground freezing, characterized in that: It includes a low-pressure stage compressor (1), a low-pressure stage variable frequency motor (2), a plate heat exchanger (3), a high-pressure stage compressor (4), a high-pressure liquid receiver (5), an evaporative condenser (6), a high-pressure stage industrial frequency motor (7), an oil separator (8), a thermosiphon oil cooler (9), an oil pump (10), an oil filter (11), a brine pump (12), a brine tank (14), a thermosiphon evaporator (15), a throttle valve (16), and a control console (17). Both the low-pressure stage compressor (1) and the high-pressure stage compressor (4) are open screw compressors, and the low-pressure stage compressor (1) adopts an energy-free slide valve mechanism or is a compressor that is always in full load. The low-pressure stage compressor (1) is driven by the low-pressure stage variable frequency motor (2), and the high-pressure stage compressor (4) is driven by the high-pressure stage industrial frequency motor (7). The exhaust end of the low-pressure stage compressor (1) is connected to the suction end of the high-pressure stage compressor (4). The high-pressure stage compressor (4) is connected to the inlet of the oil separator (8). The exhaust end of the oil separator (8) is connected to the inlet of the evaporative condenser (6). The refrigerant output end of the evaporative condenser (6) is connected to the inlet of the high-pressure liquid receiver (5). The refrigerant in the high-pressure liquid receiver (5) is connected to the input end of the thermosiphon evaporator (15) in sequence through the plate heat exchanger (3) and the throttle valve (16). The output end of the thermosiphon evaporator (15) is connected to the suction port of the low-pressure stage compressor (1). The lubricating oil outlet of the oil separator (8) is connected to the oil inlet of the thermosiphon oil cooler (9), and the oil outlet of the thermosiphon oil cooler (9) is connected to the oil injection ports of the low-pressure stage compressor (1) and the high-pressure stage compressor (4) in sequence through the oil pump (10) and the oil filter (11). The inlet of the brine pump (12) is connected to the refrigerant outlet of the thermosiphon evaporator (15). The outlet of the brine pump (12) passes through the brine pool (14) and the stratum (13) in sequence, and is connected to the refrigerant inlet of the thermosiphon evaporator (15). The control console (17) is a control device that integrates a starter cabinet and a frequency converter. It can start the high-pressure stage power frequency motor (7) and start the low-pressure stage variable frequency motor (2) via the touch screen. When the frequency converter is running, the high-pressure stage power frequency motor (7) is started first, and the high-pressure stage compressor (4) is driven by the high-pressure stage coupling. The load is gradually increased to 100% full load through the energy regulating slide valve. After the low-pressure stage compressor (1) is started, the low-pressure stage variable frequency motor (2) is started by the frequency converter. The low-pressure stage compressor (1) is driven by the low-pressure stage coupling. Then the liquid supply valve of the plate heat exchanger (3) is opened to achieve secondary subcooling of the refrigerant from the high-pressure liquid receiver (5). The plate heat exchanger (3) is integrated with an economizer and an intercooler, and the heat exchange cores of the economizer and the intercooler are integrated in the same shell; the plate heat exchanger (3) is provided with a liquid supply valve, which is opened immediately after the low-pressure stage compressor (1) is started, so that the refrigerant from the high-pressure liquid receiver (5) is first subcooled through the plate heat exchanger (3) and then enters the throttle valve (16) for secondary expansion; The operating frequency of the low-pressure stage variable frequency motor (2) is automatically adjusted in segments according to the evaporation temperature change of the thermosiphon evaporator (15) during the operation of the unit. When the evaporation temperature drops, the frequency of the low-pressure stage variable frequency motor is first reduced to 40-45Hz to adapt to the partial load state, and then the frequency is gradually increased to no more than 60Hz.

2. The dual-machine, two-stage screw refrigeration compressor unit for ground freezing as described in claim 1, characterized in that: In the refrigeration cycle, the low-pressure refrigerant vapor generated by the thermosiphon evaporator (15) is drawn into and compressed by the low-pressure stage compressor (1) and then enters the high-pressure stage compressor (4) for secondary compression. After the high-pressure refrigerant gas is compressed, it flows into the evaporative condenser (6) after the oil is removed by the oil separator (8), and is condensed into liquid under the action of cooling water. The condensed high-pressure refrigerant flows through the plate heat exchanger (3) for secondary subcooling. After the refrigerant is throttled and depressurized by the throttling valve (16), it enters the thermosiphon evaporator (15) to absorb heat and evaporate. The generated low-pressure vapor returns to the low-pressure stage compressor (1) to complete the refrigeration cycle.

3. The dual-machine, two-stage screw refrigeration compressor unit for ground freezing as described in claim 1 or 2, characterized in that: The lubricating oil separated by the oil separator (8) enters the unit's lubrication oil circuit, is transported by the oil pump (10), flows through the thermosiphon oil cooler (9) for cooling, and then flows back to the low-pressure stage compressor (1) and the high-pressure stage compressor (4) for lubrication after passing through the oil filter (11). The oil filter (11) adopts a dual parallel structure, in which when one oil filter is working, the other is in standby mode, used to replace the filter element without stopping the machine.

4. The dual-machine, two-stage screw refrigeration compressor unit for ground freezing as described in claim 1 or 2, characterized in that: On the refrigerant side, the brine exchanges heat with the refrigerant in the thermosiphon evaporator (15) and then enters the brine pool (14). It is then pumped by the brine pump (12) into the pipes next to the mine excavation opening and the stratum (13) to absorb the heat from the soil. Finally, it returns to the thermosiphon evaporator (15) to release heat and complete the cycle.

5. The dual-machine, two-stage screw refrigeration compressor unit for ground freezing as described in claim 1 or 2, characterized in that: The low-pressure stage compressor (1) and the low-pressure stage variable frequency motor (2) are mounted on top of the thermosiphon oil cooler (9), and the high-pressure stage compressor (4) and the high-pressure stage power frequency motor (7) are mounted on a common base; the plate heat exchanger (3) is set in the form of an integrated module on the base located behind the low-pressure stage compressor (1); the control console (17) is set behind the high-pressure stage power frequency motor (7) and connected to the power cable.

6. The dual-machine, two-stage screw refrigeration compressor unit for ground freezing as described in claim 1 or 2, characterized in that: The low-pressure stage variable frequency motor (2) is a 20-60Hz variable frequency motor; the high-pressure stage power frequency motor (7) is a motor with a rated frequency of 50Hz, used to drive the high-pressure stage compressor (4) to operate at full load under high temperature conditions.

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