A semi-cascade stepped evaporation refrigeration system

Abstract

The present application relates to a kind of semi-stacked step evaporation refrigeration systems, system is made of refrigerant circuit and cold carrier flow path;Refrigerant circuit includes the high-temperature stage refrigerant circuit, medium-temperature stage refrigerant circuit and low-temperature stage refrigerant circuit connected in order heat exchange.The cold carrier flows through each stage refrigerant-cold carrier heat exchanger in turn, and each stage refrigerant cascade cooling is dropped in turn by the evaporation temperature from high to low.Compared with prior art, the present application has the following advantages: the cascade heat exchange of cold carrier and refrigerant is conducive to improving temperature field uniformity, and reducing heat exchange loss;System can form four kinds of operating modes by the switch of compressor and valve, to adapt to different working conditions;System can start high-temperature stage refrigerant circuit in start-up stage, and low-temperature stage refrigerant circuit is started after high-temperature stage evaporation temperature drops to a certain extent, can avoid low-temperature stage compressor to start up failure due to excessive load, and the start-up stage of system becomes more safe and convenient.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of vapor compression refrigeration, in particular to a semi-cascade step evaporation refrigeration system. BACKGROUND

[0002] At present, cascade refrigeration systems (including general cascade, self-cascade refrigeration systems, etc.) are an important way to obtain-20℃ to-150℃ low temperature, and have a relatively wide application in industrial production, low temperature test and biomedical field, such as gas liquefaction, low temperature purification, plasma preservation, etc. However, whether it is a general cascade system or a self-cascade system, there are still some aspects to be improved.

[0003] On the one hand, in the cascade system, when the temperature drop of the heat carrier is large, the heat exchange process between the heat carrier and the refrigerant is prone to have large heat exchange loss. The general cascade refrigeration system (such as the cascade refrigeration system proposed in patent CN 216769837U) often uses pure refrigerant or azeotropic refrigerant, and the evaporation temperature of the low-temperature stage refrigerant is relatively constant and very low, while the temperature of the heat carrier gradually decreases from a higher value to a lower value. The temperature difference between the two is large, and the temperature matching is not good, which is not conducive to the performance of the system. For the self-cascade refrigeration system, although the temperature glide of the non-azeotropic refrigerant can be used to some extent to match the cooling process of the heat carrier, the temperature glide range is also relatively limited. If the temperature glide of the refrigerant is too large, the evaporation pressure of the system will be too low, and the efficiency of the compressor will decrease. In order to solve this problem, patent CN 108895694A proposes an improved self-cascade refrigeration cycle system and a control method thereof, which reduces the pressure ratio of the system by adding a regenerator and a throttling valve.

[0004] On the other hand, when the operating conditions change greatly, the adjustment ability of the cascade system is weak. For the general cascade refrigeration system, since the refrigerant circuits of each stage are connected through the condenser evaporator, all the refrigerant circuits can only run simultaneously in the stable state. Therefore, when the operating conditions change greatly, the system is difficult to meet the requirements by changing the compression stages, and the compressor pressure ratio may be too large or too small. For the self-cascade system, since the system utilizes the component separation characteristics of the mixed refrigerant to realize multi-stage cascade through a single compressor, once the evaporation temperature of the system is greatly reduced, the suction pressure of the compressor will rapidly decrease, the pressure ratio will increase, and the efficiency will decrease, thereby causing the operating efficiency and refrigeration capacity of the system to significantly decrease. SUMMARY

[0005] The present application aims to overcome the defects of the prior art and provides a semi-cascade step evaporation refrigeration system, which is composed of a refrigerant circuit and a coolant flow path. Compared with the prior art, the present application has the following advantages: first, the step cooling of the coolant is conducive to reducing heat exchange loss; second, the system can form four operating modes through the opening and closing of the compressor and the valve to adapt to different working conditions; third, the system can start the high-temperature refrigerant circuit first in the starting stage, and the high-temperature refrigerant and the coolant are exchanged, and then the low-temperature refrigerant circuit is started when the high-temperature evaporation temperature drops to a certain extent. This can avoid the failure of the low-temperature compressor due to excessive load, and the starting stage of the system becomes safer and more convenient.

[0006] The object of the present application can be achieved by the following technical solutions:

[0007] The present application first provides a semi-cascade step evaporation refrigeration system (A system), which includes a refrigerant circuit and a coolant flow path.

[0008] The refrigerant circuit includes a high-temperature refrigerant circuit, a medium-temperature refrigerant circuit, and a low-temperature refrigerant circuit.

[0009] The first refrigerant passage of the medium-temperature condenser evaporator is connected with the first refrigerant passage of the low-temperature condenser evaporator.

[0010] The coolant flow path includes a coolant passage of the high-temperature refrigerant-coolant heat exchanger, a coolant passage of the medium-temperature refrigerant-coolant heat exchanger, and a coolant passage of the low-temperature refrigerant-coolant heat exchanger connected in sequence.

[0011] The high-temperature refrigerant circuit includes a refrigerant passage of the high-temperature refrigerant-coolant heat exchanger, a first refrigerant passage of the medium-temperature condenser evaporator, a first refrigerant passage of the low-temperature condenser evaporator, a high-temperature compressor, a refrigerant passage of the high-temperature condenser, and a high-temperature throttling valve connected in sequence.

[0012] The medium-temperature refrigerant circuit includes a refrigerant passage of the medium-temperature refrigerant-coolant heat exchanger, a medium-temperature compressor, a second refrigerant passage of the medium-temperature condenser evaporator, and a medium-temperature throttling valve connected in sequence.

[0013] The low-temperature stage refrigerant circuit comprises a refrigerant passage of a low-temperature stage refrigerant-carrying-refrigerant heat exchanger, a low-temperature stage compressor, a second refrigerant passage of a low-temperature stage condensing evaporator and a low-temperature stage throttling valve connected in sequence.

[0014] The high-temperature stage refrigerant circulation process of the semi-cascade stage evaporation refrigeration system is as follows: high-temperature high-pressure refrigerant gas discharged from the high-temperature stage compressor enters the refrigerant passage of the high-temperature stage condenser, releases heat to the environment and is condensed into high-temperature high-pressure refrigerant liquid; the refrigerant liquid enters the high-temperature stage throttling valve and is throttled into low-temperature low-pressure two-phase flow; the low-temperature low-pressure two-phase flow refrigerant enters the refrigerant passage of the high-temperature stage refrigerant-carrying-refrigerant heat exchanger, absorbs heat from the flowing carrying refrigerant and evaporates; the refrigerant continues to flow into the first refrigerant passages of the medium-temperature stage condensing evaporator and the low-temperature stage condensing evaporator, absorbs heat from the flowing medium-temperature stage refrigerant and low-temperature stage refrigerant in turn, and finally evaporates into low-temperature low-pressure refrigerant gas; the low-temperature low-pressure refrigerant gas flows into the high-temperature stage compressor, forming the high-temperature stage refrigerant circulation.

[0015] Further, the high-temperature stage refrigerant circuit of the semi-cascade stage evaporation refrigeration system uses pure refrigerant, and the saturation pressure of the refrigerant corresponds to the temperature. That is, under ideal constant-pressure evaporation conditions, the evaporation temperatures of the high-temperature stage refrigerant in the high-temperature stage refrigerant-carrying-refrigerant heat exchanger, the medium-temperature stage condensing evaporator and the low-temperature stage condensing evaporator are equal. Considering the flow resistance in the actual system, the saturation pressure of the refrigerant slightly decreases during the heat exchange process, so the evaporation temperatures of the high-temperature stage refrigerant in the three heat exchangers also slightly decrease in turn, which helps to reduce the pressure ratio of the medium-temperature stage and low-temperature stage refrigerant circuits, thereby improving the efficiency of the medium-temperature stage and low-temperature stage compressors.

[0016] The medium-temperature stage refrigerant circulation process of the semi-cascade stage evaporation refrigeration system is as follows: high-temperature high-pressure refrigerant gas discharged from the medium-temperature stage compressor enters the second refrigerant passage of the medium-temperature stage condensing evaporator, releases heat to the flowing high-temperature stage refrigerant, and is condensed into high-temperature high-pressure refrigerant liquid; the refrigerant liquid enters the medium-temperature stage throttling valve and is throttled into low-temperature low-pressure two-phase flow; the low-temperature low-pressure two-phase flow refrigerant enters the refrigerant passage of the medium-temperature stage refrigerant-carrying-refrigerant heat exchanger, absorbs heat from the flowing carrying refrigerant, and evaporates into low-temperature low-pressure refrigerant gas; the low-temperature low-pressure refrigerant gas flows into the medium-temperature stage compressor, forming the medium-temperature stage refrigerant circulation.

[0017] The low-temperature stage refrigerant circulation process of the semi-cascade stage evaporation refrigeration system is as follows: high-temperature and high-pressure refrigerant gas discharged from the low-temperature stage compressor enters the second refrigerant passage of the low-temperature stage condenser evaporator, releases heat to the high-temperature stage refrigerant flowing therethrough, and is condensed into high-temperature and high-pressure refrigerant liquid; the refrigerant liquid enters the low-temperature stage throttling valve, is throttled into low-temperature and low-pressure two-phase flow; the low-temperature and low-pressure two-phase flow refrigerant enters the refrigerant passage of the low-temperature stage refrigerant-carrying coolant heat exchanger, absorbs heat from the carrying coolant flowing therethrough, and is evaporated into low-temperature and low-pressure refrigerant gas; the low-temperature and low-pressure refrigerant gas continues to flow into the low-temperature stage compressor, thereby constituting the low-temperature stage refrigerant circulation.

[0018] The carrying coolant cooling process of the semi-cascade stage evaporation refrigeration system is as follows: the carrying coolant sequentially flows through the carrying coolant passages of the high-temperature stage refrigerant-carrying coolant heat exchanger, the medium-temperature stage refrigerant-carrying coolant heat exchanger and the low-temperature stage refrigerant-carrying coolant heat exchanger, releases heat to the refrigerants of the respective stages, and is greatly cooled through the stages to achieve a target temperature.

[0019] The starting sequence of the semi-cascade stage evaporation refrigeration system is to sequentially start the high-temperature stage compressor, the medium-temperature stage compressor and the low-temperature stage compressor.

[0020] The semi-cascade stage evaporation refrigeration system switches between four modes according to changes in working conditions:

[0021] 1) When the ambient temperature is higher than the ambient temperature threshold value, the inlet temperature of the carrying coolant is higher than the inlet temperature threshold value, and the temperature drop is greater than the first temperature drop threshold value, the refrigerant circuits of the system are all operated, the carrying coolant is sequentially cooled by the high-temperature stage, the medium-temperature stage and the low-temperature stage refrigerant-carrying coolant heat exchangers, and thereby the target temperature is reached;

[0022] 2) When the ambient temperature is higher than the ambient temperature threshold value, the inlet temperature of the carrying coolant is higher than the inlet temperature threshold value, and the temperature drop is lower than the first temperature drop threshold value and greater than the second temperature drop threshold value, the high-temperature stage and the medium-temperature stage refrigerant circuits are operated, and the carrying coolant is cooled by the high-temperature stage and the medium-temperature stage refrigerant-carrying coolant heat exchangers to the target temperature;

[0023] 3) When the ambient temperature is higher than the ambient temperature threshold value, the inlet temperature of the carrying coolant is lower than the inlet temperature threshold value, the high-temperature stage and the medium-temperature stage refrigerant circuits are operated, the carrying coolant does not flow through the high-temperature stage refrigerant-carrying coolant heat exchanger, and is only cooled by the medium-temperature stage refrigerant-carrying coolant heat exchanger to the target temperature, so that the high-temperature stage refrigerant circuit is operated in a low-load state to save energy;

[0024] 4) When the ambient temperature is lower than the ambient temperature threshold value, and the temperature drop of the carrying coolant is less than the second temperature drop threshold value, only the high-temperature stage refrigerant circuit is operated, and the carrying coolant is cooled by the high-temperature stage refrigerant-carrying coolant heat exchanger to the target temperature.

[0025] The application also provides another semi-cascade cascade evaporation refrigeration system (B system), wherein the high-temperature stage refrigerant circuit comprises, in sequence, a refrigerant passage of a high-temperature stage refrigerant-carrying refrigerant heat exchanger, a high-temperature stage compressor, a refrigerant passage of a high-temperature stage condenser, a high-temperature stage throttling valve, a first refrigerant passage of a low-temperature stage condensing evaporator and a first refrigerant passage of a medium-temperature stage condensing evaporator.

[0026] The medium-temperature stage refrigerant circuit comprises, in sequence, a refrigerant passage of a medium-temperature stage refrigerant-carrying refrigerant heat exchanger, a medium-temperature stage compressor, a second refrigerant passage of a medium-temperature stage condensing evaporator and a medium-temperature stage throttling valve.

[0027] The low-temperature stage refrigerant circuit comprises, in sequence, a refrigerant passage of a low-temperature stage refrigerant-carrying refrigerant heat exchanger, a low-temperature stage compressor, a second refrigerant passage of a low-temperature stage condensing evaporator and a low-temperature stage throttling valve.

[0028] The main difference between the B system and the A system is that the high-temperature stage refrigerant circuit of the B system uses a non-azeotropic refrigerant. Such a refrigerant has a temperature glide during the evaporation phase change, that is, the high-temperature stage refrigerant first reaches the bubble point temperature and then rises to the dew point temperature. Therefore, under the ideal constant-pressure evaporation condition, the evaporation temperature of the high-temperature stage refrigerant in the low-temperature stage condensing evaporator, the medium-temperature stage condensing evaporator and the high-temperature stage refrigerant-carrying refrigerant heat exchanger of the B system is successively increased. Although the flow resistance of the high-temperature stage refrigerant causes a slight drop in pressure in the actual system, it does not change the glide trend of the evaporation temperature. The evaporation temperature glide of the high-temperature stage refrigerant brings two benefits to the semi-cascade cascade evaporation refrigeration B system: first, the lower high-temperature stage evaporation temperature in the low-temperature stage and medium-temperature stage condensing evaporators is conducive to reducing the pressure ratio of the low-temperature stage and medium-temperature stage refrigerant circuits, thereby improving the efficiency of the compressors of the two circuits. Second, the carrying refrigerant first exchanges heat with the high-temperature stage refrigerant-carrying refrigerant heat exchanger having a higher evaporation temperature, which is conducive to further reducing the heat exchange loss.

[0029] The application also provides another semi-cascade cascade evaporation refrigeration system (C system), wherein the high-temperature stage refrigerant circuit comprises, in sequence, a refrigerant passage of a high-temperature stage refrigerant-carrying refrigerant heat exchanger, a high-temperature stage compressor, a refrigerant passage of a high-temperature stage condenser, a third high-temperature stage throttling valve.

[0030] The high-temperature stage refrigerant circuit further comprises a second high-temperature stage throttling valve, one end of the second high-temperature stage throttling valve is connected with the refrigerant passage of the high-temperature stage condenser, and the other end is connected with one end of the first refrigerant passage of the medium-temperature stage condensing evaporator, and the other end of the first refrigerant passage of the medium-temperature stage condensing evaporator is connected with the high-temperature stage compressor.

[0031] The high-temperature stage refrigerant circuit further comprises a first high-temperature stage throttling valve, one end of which is connected with the refrigerant passage of the high-temperature stage condenser, and the other end of which is connected with one end of the first refrigerant passage of the low-temperature stage condensing evaporator, and the other end of the first refrigerant passage of the low-temperature stage condensing evaporator is connected with the high-temperature stage compressor.

[0032] The medium-temperature stage refrigerant circuit comprises, in sequence, a refrigerant passage of a medium-temperature stage refrigerant-carrying refrigerant heat exchanger, a medium-temperature stage compressor, a second refrigerant passage of a medium-temperature stage condensing evaporator, and a medium-temperature stage throttling valve.

[0033] The low-temperature stage refrigerant circuit comprises, in sequence, a refrigerant passage of a low-temperature stage refrigerant-carrying refrigerant heat exchanger, a low-temperature stage compressor, a second refrigerant passage of a low-temperature stage condensing evaporator, and a low-temperature stage throttling valve.

[0034] The main difference between the C system and the A system lies in the different connection modes of the high-temperature stage refrigerant evaporation side heat exchanger, and each connection mode has its own advantages.

[0035] In the A system, the refrigerant passage of the high-temperature stage refrigerant-carrying refrigerant heat exchanger, the first refrigerant passage of the medium-temperature stage condensing evaporator, and the first refrigerant passage of the low-temperature stage condensing evaporator are connected in series. The high-temperature and high-pressure refrigerant liquid first enters the refrigerant passage of the high-temperature stage refrigerant-carrying refrigerant heat exchanger after throttling, absorbs heat from the flowing carrying refrigerant to evaporate, and then flows into the first refrigerant passages of the medium-temperature stage and low-temperature stage condensing evaporators in sequence to absorb heat from the flowing medium-temperature stage and low-temperature stage refrigerants to evaporate. Due to the existence of flow resistance, the evaporation temperature in the two condensing evaporators is successively reduced compared with the high-temperature stage refrigerant-carrying refrigerant heat exchanger, which is beneficial to reducing the pressure ratio of the medium-temperature stage and low-temperature stage refrigerant circuits, thereby improving the efficiency of the compressors of the two circuits.

[0036] In the C system, the refrigerant passage of the high-temperature refrigerant-carrying coolant heat exchanger, the first refrigerant passage of the medium-temperature condensing evaporator, and the first refrigerant passage of the low-temperature condensing evaporator are connected in parallel. The high-temperature refrigerant is throttled, absorbs heat and evaporates in the three parallel flow paths, and then converges at the suction port of the high-temperature compressor. The first flow path of the high-temperature refrigerant enters the first refrigerant passage of the low-temperature condensing evaporator through the first high-temperature throttling valve, absorbs heat from the low-temperature refrigerant, and evaporates. The second flow path enters the first refrigerant passage of the medium-temperature condensing evaporator through the second high-temperature throttling valve, absorbs heat from the medium-temperature refrigerant, and evaporates. The third flow path enters the refrigerant passage of the high-temperature refrigerant-carrying coolant heat exchanger through the third high-temperature throttling valve, absorbs heat from the carrying coolant, and evaporates. In the three flow paths, the evaporation pressures of the high-temperature refrigerant are equal. The advantage of this connection mode is that when the high-temperature refrigerant circuit does not need to exchange heat with the carrying coolant, only the third high-temperature throttling valve needs to be closed, and the three parallel refrigerant flow paths do not interfere with each other, facilitating control.

[0037] Compared with the prior art, the application has the advantages and beneficial effects that:

[0038] 1. The temperature matching of the refrigerant and the carrying coolant in the system is good. In the high-temperature, medium-temperature and low-temperature refrigerant-carrying coolant heat exchangers of the system, the evaporation temperatures of the refrigerants (pure refrigerant, azeotropic refrigerant or non-azeotropic refrigerant) are distributed from high to low. The carrying coolant flows through the refrigerant-carrying coolant heat exchangers in turn and is cooled by the refrigerants at different temperatures in stages, which can reduce the heat exchange loss caused by uneven heat exchange temperature difference and improve the system performance.

[0039] 2. The system can be switched between four operating modes through the opening and closing of the compressor and the valve, so as to adapt to different ambient temperatures, inlet temperatures of the carrying coolant and temperature drops of the carrying coolant, and the system has good adjustment ability.

[0040] 3. The control of the system during the starting stage is flexible. When the system starts, the high-temperature refrigerant circuit can be opened first, the heat exchange between the high-temperature refrigerant and the carrying coolant is performed, and then the medium-temperature and low-temperature refrigerant circuits are opened in turn when the evaporation temperature of the high-temperature refrigerant decreases to a certain extent. In this way, the low-temperature compressor can be prevented from failing to start due to excessive load, and the starting process of the system is safe and convenient.

[0041] 4. The structure of the system can be flexibly configured according to actual needs. The high-temperature refrigerant circuit of the system is connected in heat exchange mode with the medium-temperature and low-temperature refrigerant circuits through the condensing evaporators. The more the number of condensing evaporators is, the more refrigerant circuits can be connected, and correspondingly, the lower the evaporation temperature that the system can achieve is. Therefore, in actual application, the system structure can be configured according to the lowest evaporation temperature, and the system has strong flexibility. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 A schematic diagram of the principle of embodiment 1 of the present application.

[0043] Figure 2 A schematic diagram of the principle of embodiment 2 of the present application.

[0044] Figure 3 A schematic diagram of the principle of embodiment 3 of the present application.

[0045] Figures 1-3 Medium: 1-1, high-temperature stage refrigerant-carrying refrigerant heat exchanger, 1-2, medium-temperature stage condenser-evaporator, 1-3, low-temperature stage condenser-evaporator, 1-4, high-temperature stage compressor, 1-5, high-temperature stage condenser, 1-6, first high-temperature stage throttling valve, 1-7, second high-temperature stage throttling valve, 1-8, third high-temperature stage throttling valve, 2-1, medium-temperature stage refrigerant-carrying refrigerant heat exchanger, 2-2, medium-temperature stage compressor, 2-3, medium-temperature stage throttling valve, 3-1, low-temperature stage refrigerant-carrying refrigerant heat exchanger, 3-2, low-temperature stage compressor, 3-3, low-temperature stage throttling valve. DETAILED DESCRIPTION

[0046] The present application will be described in detail below in conjunction with the accompanying drawings and specific embodiments. In the technical solution, if the structure / module name, control mode, algorithm, process, or component ratio and other features are not explicitly described, they are regarded as common technical features disclosed in the prior art.

[0047] Among them, the drawings are only used for illustrative explanation, and the representation is only a schematic diagram, not a physical diagram, and cannot be understood as a limitation on the patent; in order to better illustrate the embodiments of the present application, some components of the drawings may be omitted, enlarged or reduced, and do not represent the size of the actual product; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0048] Embodiment 1

[0049] The present embodiment provides a semi-cascade cascade evaporative refrigeration system (see Figure 1 ), comprising a refrigerant circuit and a carrying refrigerant flow path.

[0050] The refrigerant circuit comprises a high-temperature stage, a medium-temperature stage and a low-temperature stage refrigerant circuit, the high-temperature stage and the medium-temperature stage refrigerant circuit are connected to each other in a heat exchangeable manner by a medium-temperature stage condenser-evaporator 1-2, and the high-temperature stage and the low-temperature stage refrigerant circuit are connected to each other in a heat exchangeable manner by a low-temperature stage condenser-evaporator 1-3.

[0051] The first refrigerant passage of the medium-temperature stage condenser-evaporator 1-2 is connected to the first refrigerant passage of the low-temperature stage condenser-evaporator 1-3.

[0052] The coolant flow path includes sequentially connected coolant channels of the high-temperature level refrigerant-coolant heat exchanger 1-1, the coolant channels of the medium-temperature level refrigerant-coolant heat exchanger 2-1, and the coolant channels of the low-temperature level refrigerant-coolant heat exchanger 3-1.

[0053] The high-temperature level refrigerant circuit includes sequentially connected refrigerant channels of the high-temperature level refrigerant-coolant heat exchanger 1-1, the first refrigerant channels of the medium-temperature level condenser-evaporator 1-2, the first refrigerant channels of the low-temperature level condenser-evaporator 1-3, the high-temperature level compressor 1-4, the refrigerant channels of the high-temperature level condenser 1-5, and the high-temperature level throttling valve 1-6.

[0054] The medium-temperature level refrigerant circuit includes sequentially connected refrigerant channels of the medium-temperature level refrigerant-coolant heat exchanger 2-1, the medium-temperature level compressor 2-2, the second refrigerant channels of the medium-temperature level condenser-evaporator 1-2, and the medium-temperature level throttling valve 2-3.

[0055] The low-temperature level refrigerant circuit includes sequentially connected refrigerant channels of the low-temperature level refrigerant-coolant heat exchanger 3-1, the low-temperature level compressor 3-2, the second refrigerant channels of the low-temperature level condenser-evaporator 1-3, and the low-temperature level throttling valve 3-3.

[0056] The high-temperature level refrigerant circulation process of the system is as follows: ① The high-temperature high-pressure refrigerant gas discharged from the high-temperature level compressor 1-4 enters the refrigerant channels of the high-temperature level condenser 1-5, releases heat to the environment, and condenses into high-temperature high-pressure refrigerant liquid; ② The refrigerant liquid enters the high-temperature level throttling valve 1-6, and throttles into low-temperature low-pressure two-phase flow; ③ The low-temperature low-pressure two-phase flow refrigerant enters the refrigerant channels of the high-temperature level refrigerant-coolant heat exchanger 1-1, absorbs heat from the flowing coolant to evaporate, and then enters the first refrigerant channels of the medium-temperature level condenser-evaporator 1-2 and the low-temperature level condenser-evaporator 1-3 in turn, absorbs heat from the flowing medium-temperature level and low-temperature level refrigerants, and finally evaporates into low-temperature low-pressure refrigerant gas; ④ The low-temperature low-pressure refrigerant gas flows into the high-temperature level compressor 1-4, forming the high-temperature level refrigerant circulation.

[0057] The high-temperature level refrigerant circuit of the system uses pure refrigerant, and the saturation pressure of the refrigerant corresponds to the temperature. That is, under ideal constant-pressure evaporation conditions, the evaporation temperatures of the high-temperature level refrigerant in the high-temperature level refrigerant-coolant heat exchanger 1-1, the medium-temperature level condenser-evaporator 1-2, and the low-temperature level condenser-evaporator 1-3 are equal. Considering the flow resistance in the actual system, the saturation pressure of the refrigerant slightly decreases during the heat exchange process, so the evaporation temperatures of the high-temperature level refrigerant in the three heat exchangers 1-1, 1-2, and 1-3 also slightly decrease in turn, which helps to reduce the pressure ratio of the medium-temperature level and low-temperature level refrigerant circuits, thereby improving the efficiency of the medium-temperature level and low-temperature level compressors.

[0058] The medium-temperature stage refrigerant circulation process of the system is: ① the high-temperature and high-pressure refrigerant gas discharged from the medium-temperature stage compressor 2-2 enters the second refrigerant passage of the medium-temperature stage condenser-evaporator 1-2, releases heat to the high-temperature stage refrigerant flowing therethrough, and is condensed into high-temperature and high-pressure refrigerant liquid; ② the refrigerant liquid enters the medium-temperature stage throttling valve 2-3, is throttled into low-temperature and low-pressure two-phase flow; ③ the low-temperature and low-pressure two-phase flow refrigerant enters the refrigerant passage of the medium-temperature stage refrigerant-carrying coolant heat exchanger 2-1, absorbs heat from the carrying coolant flowing therethrough, and is evaporated into low-temperature and low-pressure refrigerant gas; ④ the low-temperature and low-pressure refrigerant gas flows into the medium-temperature stage compressor 2-2, and forms the medium-temperature stage refrigerant circulation.

[0059] The low-temperature stage refrigerant circulation process of the system is: ① the high-temperature and high-pressure refrigerant gas discharged from the low-temperature stage compressor 3-2 enters the second refrigerant passage of the low-temperature stage condenser-evaporator 1-3, releases heat to the high-temperature stage refrigerant flowing therethrough, and is condensed into high-temperature and high-pressure refrigerant liquid; ② the refrigerant liquid enters the low-temperature stage throttling valve 3-3, is throttled into low-temperature and low-pressure two-phase flow; ③ the low-temperature and low-pressure two-phase flow refrigerant enters the refrigerant passage of the low-temperature stage refrigerant-carrying coolant heat exchanger 3-1, absorbs heat from the carrying coolant flowing therethrough, and is evaporated into low-temperature and low-pressure refrigerant gas; ④ the low-temperature and low-pressure refrigerant gas continues to flow into the low-temperature stage compressor 3-2, and forms the low-temperature stage refrigerant circulation.

[0060] The carrying coolant cooling process of the system is: the carrying coolant flows through the carrying coolant passage of the high-temperature stage refrigerant-carrying coolant heat exchanger 1-1, the carrying coolant passage of the medium-temperature stage refrigerant-carrying coolant heat exchanger 2-1, and the carrying coolant passage of the low-temperature stage refrigerant-carrying coolant heat exchanger 3-1 in sequence, releases heat to the refrigerants of each stage, and realizes a large temperature drop through cascade cooling, so as to reach the target temperature.

[0061] The starting sequence of the system is to start the high-temperature stage compressor 1-4, the medium-temperature stage compressor 2-2, and the low-temperature stage compressor 3-2 in sequence.

[0062] The system can switch between the four modes according to the changes of the working conditions:

[0063] 1) When the ambient temperature is higher than the ambient temperature threshold value, the inlet temperature of the carrying coolant is higher than the inlet temperature threshold value, and the temperature drop is greater than the first temperature drop threshold value, the refrigerant circuits of the system are all operated, the carrying coolant is cascade cooled by the high-temperature stage refrigerant-carrying coolant heat exchanger 1-1, the medium-temperature stage refrigerant-carrying coolant heat exchanger 2-1, and the low-temperature stage refrigerant-carrying coolant heat exchanger 3-1 in sequence, so as to reach the target temperature;

[0064] 2) When the ambient temperature is higher than the ambient temperature threshold, the inlet temperature of the secondary refrigerant is higher than the inlet temperature threshold, and the temperature drop is lower than the first temperature drop threshold and greater than the second temperature drop threshold, the high-temperature stage and the medium-temperature stage refrigerant circuits are operated, and the secondary refrigerant is stepwise cooled to the target temperature by the high-temperature stage refrigerant-secondary refrigerant heat exchanger 1-1 and the medium-temperature stage refrigerant-secondary refrigerant heat exchanger 2-1;

[0065] 3) When the ambient temperature is higher than the ambient temperature threshold, the inlet temperature of the secondary refrigerant is lower than the inlet temperature threshold, the high-temperature stage and the medium-temperature stage refrigerant circuits are operated, but the secondary refrigerant does not flow through the high-temperature stage refrigerant-secondary refrigerant heat exchanger 1-1, and is only cooled to the target temperature by the medium-temperature stage refrigerant-secondary refrigerant heat exchanger 2-1, so that the high-temperature stage refrigerant circuit is operated in a low-load state to save energy.

[0066] 4) When the ambient temperature is lower than the ambient temperature threshold, and the temperature drop of the secondary refrigerant is less than the second temperature drop threshold, only the high-temperature stage refrigerant circuit is operated, and the secondary refrigerant is cooled to the target temperature by the high-temperature stage refrigerant-secondary refrigerant heat exchanger 1-1.

[0067] Embodiment 2

[0068] This embodiment provides a semi-cascade step evaporation refrigeration system (see Figure 2 ), which comprises a refrigerant circuit and a secondary refrigerant flow path.

[0069] The main difference between this embodiment and Embodiment 1 is that the high-temperature stage refrigerant circuit of this embodiment uses a non-azeotropic refrigerant. Such a refrigerant has a temperature glide during the evaporation phase change, that is, the high-temperature stage refrigerant first reaches the bubble point temperature and then rises to the dew point temperature. Therefore, under ideal constant-pressure evaporation conditions, the evaporation temperature of the high-temperature stage refrigerant in the low-temperature stage condenser-evaporator 1-3, the medium-temperature stage condenser-evaporator 1-2, and the high-temperature stage refrigerant-secondary refrigerant heat exchanger 1-1 of this embodiment increases in turn. Although the flow resistance of the high-temperature stage refrigerant will cause the pressure to drop in the actual system, it does not change the glide trend of the evaporation temperature. The evaporation temperature glide of the high-temperature stage refrigerant has two benefits for the system performance: first, the lower evaporation temperature in the low-temperature stage condenser-evaporator 1-3 and the medium-temperature stage condenser-evaporator 1-2 is conducive to reducing the pressure ratio of the low-temperature stage and the medium-temperature stage refrigerant circuits, thereby improving the efficiency of the compressors of the two circuits. Second, the secondary refrigerant first exchanges heat with the high-temperature stage refrigerant-secondary refrigerant heat exchanger 1-1 having a higher evaporation temperature, which is conducive to further reducing the heat exchange loss.

[0070] Embodiment 3

[0071] This embodiment provides a semi-cascade step evaporation refrigeration system (see Figure 3), including a refrigerant circuit and a secondary refrigerant circuit. The main difference between this embodiment and embodiment 1 is the connection mode of the high-temperature refrigerant- secondary refrigerant heat exchanger, and each connection mode has its own advantages:

[0072] In embodiment 1, the refrigerant passage of the high-temperature refrigerant- secondary refrigerant heat exchanger 1-1, the first refrigerant passage of the medium-temperature condenser-evaporator 1-2, and the first refrigerant passage of the low-temperature condenser-evaporator 1-3 are connected in series. The high-temperature and high-pressure refrigerant liquid first enters the refrigerant passage of the high-temperature refrigerant- secondary refrigerant heat exchanger 1-1, absorbs heat from the flowing secondary refrigerant to evaporate, and then flows into the first refrigerant passages of the medium-temperature condenser-evaporator 1-2 and the low-temperature condenser-evaporator 1-3 in turn, absorbing heat from the flowing medium-temperature and low-temperature refrigerants to evaporate. Due to the existence of flow resistance, the evaporation temperature in the condenser- evaporator 1-2 and 1-3 is lower than that in the high-temperature refrigerant- secondary refrigerant heat exchanger 1-1, which is conducive to reducing the pressure ratio of the medium-temperature and low-temperature refrigerant circuits, thereby improving the efficiency of the compressors of the two circuits.

[0073] In embodiment 3, the refrigerant passage of the high-temperature refrigerant- secondary refrigerant heat exchanger 1-1, the first refrigerant passage of the medium-temperature condenser-evaporator 1-2, and the first refrigerant passage of the low-temperature condenser-evaporator 1-3 are connected in parallel. The high-temperature and high-pressure refrigerant liquid is throttled and absorbs heat to evaporate in the three flow paths respectively, and then converges at the suction port of the high-temperature compressor 1-4. The first flow path of the high-temperature refrigerant enters the first refrigerant passage of the low-temperature condenser-evaporator 1-3 through the first high-temperature throttling valve 1-6, absorbs heat from the low-temperature refrigerant to evaporate. The second flow path enters the first refrigerant passage of the medium-temperature condenser-evaporator 1-2 through the second high-temperature throttling valve 1-7, absorbs heat from the medium-temperature refrigerant to evaporate. The third flow path enters the refrigerant passage of the high-temperature refrigerant- secondary refrigerant heat exchanger 1-1 through the third high-temperature throttling valve 1-8, absorbs heat from the secondary refrigerant to evaporate. In the three flow paths, the evaporation pressures of the high-temperature refrigerant are equal. The advantage of this connection mode is that when the high-temperature refrigerant circuit does not need to exchange heat with the secondary refrigerant, only the third high-temperature throttling valve 1-8 needs to be closed, and the three parallel refrigerant flow paths do not interfere with each other, which is convenient for control.

[0074] The above description of the embodiments is to facilitate those of ordinary skill in the art to understand and use the present application. Those skilled in the art can easily make various modifications to these embodiments, and apply the general principles described herein to other embodiments without having to go through creative labor. Therefore, the present application is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art without departing from the scope of the present application should be within the scope of protection of the present application.

Claims

1. A semi-cascade stepped evaporation refrigeration system characterized by, The refrigerant circuit and the secondary refrigerant circuit are connected in series. The high-temperature stage and the medium-temperature stage refrigerant circuits are connected in heat exchange mode through a medium-temperature stage condenser-evaporator (1-2), and the high-temperature stage and the low-temperature stage refrigerant circuits are connected in heat exchange mode through a low-temperature stage condenser-evaporator (1-3). The first refrigerant passage of the medium-temperature stage condenser-evaporator (1-2) is connected with the first refrigerant passage of the low-temperature stage condenser-evaporator (1-3). The secondary refrigerant circuit comprises the secondary refrigerant passages of the high-temperature stage refrigerant-secondary refrigerant heat exchanger (1-1), the medium-temperature stage refrigerant-secondary refrigerant heat exchanger (2-1) and the low-temperature stage refrigerant-secondary refrigerant heat exchanger (3-1) connected in series. The starting sequence of the semi-cascade evaporative refrigeration system is to start the high-temperature stage compressor, the medium-temperature stage compressor and the low-temperature stage compressor in sequence. The semi-cascade evaporative refrigeration system switches among the four modes according to the change of working conditions: 1) When the ambient temperature is higher than the ambient temperature threshold value, the inlet temperature of the secondary refrigerant is higher than the inlet temperature threshold value, and the temperature drop is greater than the first temperature drop threshold value, the refrigerant circuit of the system is operated in full load, and the secondary refrigerant is sequentially cooled by the high-temperature stage refrigerant-secondary refrigerant heat exchanger (1-1), the medium-temperature stage refrigerant-secondary refrigerant heat exchanger (2-1) and the low-temperature stage refrigerant-secondary refrigerant heat exchanger (3-1) to reach the target temperature; 2) When the ambient temperature is higher than the ambient temperature threshold value, the inlet temperature of the secondary refrigerant is higher than the inlet temperature threshold value, and the temperature drop is lower than the first temperature drop threshold value and greater than the second temperature drop threshold value, the high-temperature stage and the medium-temperature stage refrigerant circuits are operated, and the secondary refrigerant is cooled by the high-temperature stage refrigerant-secondary refrigerant heat exchanger (1-1) and the medium-temperature stage refrigerant-secondary refrigerant heat exchanger (2-1) to the target temperature; 3) When the ambient temperature is higher than the ambient temperature threshold value, the inlet temperature of the secondary refrigerant is lower than the inlet temperature threshold value, the high-temperature stage and the medium-temperature stage refrigerant circuits are operated, the secondary refrigerant does not flow through the high-temperature stage refrigerant-secondary refrigerant heat exchanger (1-1), and is only cooled by the medium-temperature stage refrigerant-secondary refrigerant heat exchanger (2-1) to the target temperature, so that the high-temperature stage refrigerant circuit is operated in a low-load state to save energy; 4) When the ambient temperature is lower than the ambient temperature threshold value, and the temperature drop of the secondary refrigerant is less than the second temperature drop threshold value, only the high-temperature stage refrigerant circuit is operated, and the secondary refrigerant is cooled by the high-temperature stage refrigerant-secondary refrigerant heat exchanger (1-1) to the target temperature.

2. A semi-cascade stepped evaporation refrigeration system according to claim 1, characterized in that, The high-temperature stage refrigerant circuit comprises the refrigerant passages of the high-temperature stage refrigerant-secondary refrigerant heat exchanger (1-1), the first refrigerant passage of the medium-temperature stage condenser-evaporator (1-2), the first refrigerant passage of the low-temperature stage condenser-evaporator (1-3), the high-temperature stage compressor (1-4), the refrigerant passage of the high-temperature stage condenser (1-5) and the high-temperature stage throttling valve (1-6).

3. A semi-cascade stepped evaporative refrigeration system according to claim 2, wherein, The medium-temperature stage refrigerant circuit comprises a refrigerant passage of a medium-temperature stage refrigerant-carrying coolant heat exchanger (2-1), a medium-temperature stage compressor (2-2), a second refrigerant passage of a medium-temperature stage condenser-evaporator (1-2), and a medium-temperature stage throttling valve (2-3) connected in sequence. The low-temperature stage refrigerant circuit comprises a refrigerant passage of a low-temperature stage refrigerant-carrying coolant heat exchanger (3-1), a low-temperature stage compressor (3-2), a second refrigerant passage of a low-temperature stage condenser-evaporator (1-3), and a low-temperature stage throttling valve (3-3) connected in sequence.

4. A semi-cascade cascade stepped evaporative refrigeration system as claimed in claim 2 wherein, The high-temperature stage refrigerant circuit comprises a refrigerant passage of a high-temperature stage refrigerant-carrying coolant heat exchanger (1-1), a high-temperature stage compressor (1-4), a refrigerant passage of a high-temperature stage condenser (1-5), a high-temperature stage throttling valve (1-6), a first refrigerant passage of a low-temperature stage condenser-evaporator (1-3), and a first refrigerant passage of a medium-temperature stage condenser-evaporator (1-2).

5. A semi-cascade stepped evaporative refrigeration system according to claim 4, wherein, The medium-temperature stage refrigerant circuit comprises a refrigerant passage of a medium-temperature stage refrigerant-carrying coolant heat exchanger (2-1), a medium-temperature stage compressor (2-2), a second refrigerant passage of a medium-temperature stage condenser-evaporator (1-2), and a medium-temperature stage throttling valve (2-3) connected in sequence. The low-temperature stage refrigerant circuit comprises a refrigerant passage of a low-temperature stage refrigerant-carrying coolant heat exchanger (3-1), a low-temperature stage compressor (3-2), a second refrigerant passage of a low-temperature stage condenser-evaporator (1-3), and a low-temperature stage throttling valve (3-3) connected in sequence.

6. A semi-cascade stepped evaporative refrigeration system as claimed in claim 1 wherein, The high-temperature stage refrigerant circuit comprises a refrigerant passage of a high-temperature stage refrigerant-carrying coolant heat exchanger (1-1), a high-temperature stage compressor (1-4), a refrigerant passage of a high-temperature stage condenser (1-5), and a third high-temperature stage throttling valve (1-8) connected in sequence. The high-temperature stage refrigerant circuit further comprises a second high-temperature stage throttling valve (1-7), one end of the second high-temperature stage throttling valve (1-7) being connected with the refrigerant passage of the high-temperature stage condenser (1-5), and the other end being connected with one end of the first refrigerant passage of the medium-temperature stage condenser-evaporator (1-2), the other end of the first refrigerant passage of the medium-temperature stage condenser-evaporator (1-2) being connected with the high-temperature stage compressor (1-4); The high-temperature stage refrigerant circuit further comprises a first high-temperature stage throttling valve (1-6), one end of the first high-temperature stage throttling valve (1-6) being connected with the refrigerant passage of the high-temperature stage condenser (1-5), and the other end being connected with one end of the first refrigerant passage of the low-temperature stage condenser-evaporator (1-3), the other end of the first refrigerant passage of the low-temperature stage condenser-evaporator (1-3) being connected with the high-temperature stage compressor (1-4).

7. A semi-cascade stepped evaporative refrigeration system according to claim 6, wherein, The medium-temperature stage refrigerant circuit comprises a refrigerant passage of a medium-temperature stage refrigerant-carrying coolant heat exchanger (2-1), a medium-temperature stage compressor (2-2), a second refrigerant passage of a medium-temperature stage condenser-evaporator (1-2), and a medium-temperature stage throttling valve (2-3) connected in sequence. The low-temperature stage refrigerant circuit comprises, in sequence, a refrigerant passage of the low-temperature stage refrigerant-carrying-refrigerant heat exchanger (3-1), a low-temperature stage compressor (3-2), a second refrigerant passage of the low-temperature stage condenser-evaporator (1-3), and a low-temperature stage throttling valve (3-3). The low-temperature stage refrigerant circuit comprises, in sequence, a refrigerant passage of the low-temperature stage refrigerant-carrying-refrigerant heat exchanger (3-1), a low-temperature stage compressor (3-2), a second refrigerant passage of the low-temperature stage condenser-evaporator (1-3), and a low-temperature stage throttling valve (3-

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