Multi-stage cascade fresh air grading treatment system

Abstract

The present invention provides a multi-stage cascade fresh air grading treatment system, which includes an air-conditioning unit and an air flow channel: The air-conditioning unit includes N air-conditioning units, where N is an integer greater than or equal to 2; Each stage of the air-conditioning unit includes a compressor and a flow channel air-cooled heat exchanger connected in sequence through pipelines. A three-medium heat exchanger is provided between adjacent air-conditioning units. The three-medium heat exchanger includes an air channel, a first medium channel, and a second medium channel, and a heat exchange fan is provided at the air channel; In the first-stage air-conditioning unit, an outdoor air-cooled heat exchanger is further included, and the compressor, the flow channel air-cooled heat exchanger, and the outdoor air-cooled heat exchanger are connected in sequence through pipelines; The air flow channel has an air inlet and an air outlet, and the number of stages of the air flow channel corresponds to the number of stages of the air-conditioning unit. The flow channel air-cooled heat exchanger in each stage of the air-conditioning unit is located in the corresponding stage of the air flow channel. A flow channel fan is provided in the air flow channel, and the flow channel fan is used to drive external air into the air flow channel and perform heat exchange with the flow channel air-cooled heat exchanger.

Description

Technical Field

[0001] The present invention relates to the technical field of air conditioners, and particularly to a multi-stage cascade fresh air grading treatment system. Background Art

[0002] Among the building operation energy consumptions, the energy consumption for fresh air treatment accounts for a relatively high proportion; and when the outdoor is in an extremely low temperature environment, it is difficult to treat the outdoor fresh air to the set value due to the performance limitation of the unit. Therefore, ensuring the treatment demand of fresh air and reducing the annual energy consumption for fresh air treatment are of great significance for achieving the "dual carbon" goal.

[0003] In the related art, the commonly used method of exhaust air heat recovery is to pre-boost the temperature of fresh air, so that the unit can meet the demand for fresh air treatment. However, in actual projects, most buildings do not have a centralized exhaust air system, and the fresh air sent in penetrates to the outside through doors and windows, and the heat of the penetrated air cannot be recovered for preheating the fresh air. Therefore, a solution for directly treating fresh air alone is greatly needed in actual projects.

[0004] In addition, in the prior art, traditional fresh air units use one kind of cold and heat source to treat fresh air, which requires a high grade of cold and heat source and high energy consumption; to solve this problem, the related system grades the fresh air and cascades the heat pump units, which can achieve air grading treatment under a large temperature difference and improve the system performance; but when the fresh air condition changes, especially during part-load operation, the performance of this system is poor. And when the air passes through multiple heat exchangers, due to the high air resistance, there is a large distribution energy consumption.

[0005] Therefore, there is an urgent need for a multi-stage cascade fresh air grading treatment system that can solve the above problems. Summary of the Invention

[0006] The present invention provides a multi-stage cascade fresh air grading treatment system, which uses multiple direct expansion air conditioning units to grade the fresh air. The compression ratio of each stage of direct expansion matches the required compression ratio, and the required number of stages also matches the fresh air state, so as to reduce the compressor power consumption and fan power consumption during the fresh air treatment process, ensure the air treatment demand under a large temperature difference, avoid the risk of freezing during winter fresh air treatment, and improve the energy efficiency of the fresh air treatment system during the whole-year operation.

[0007] The present invention provides a multi-stage cascade fresh air grading treatment system, including an air conditioning unit and an air flow channel:

[0008] The air conditioning unit includes N air conditioning units, where N is an integer greater than or equal to 2;

[0009] Each stage of the air-conditioning unit includes a compressor and a flow-channel air-cooled heat exchanger connected in sequence through pipelines. A three-medium heat exchanger is provided between adjacent air-conditioning units. The three-medium heat exchanger includes an air channel, a first-medium channel, and a second-medium channel. A heat exchange fan is provided in the air channel. In each stage of the air-conditioning unit, a first expansion valve and a second expansion valve are respectively provided in connection with the flow-channel air-cooled heat exchanger and the first-medium channel.

[0010] In every two adjacent air-conditioning units, the first-medium channel corresponding to the three-medium heat exchanger is in parallel connection with the flow-channel air-cooled heat exchanger in the previous air-conditioning unit, and the second-medium channel corresponding to the three-medium heat exchanger is connected in sequence through pipelines with the compressor and the flow-channel air-cooled heat exchanger in the subsequent air-conditioning unit.

[0011] In the first-stage air-conditioning unit, an outdoor air-cooled heat exchanger is further included. The compressor, the flow-channel air-cooled heat exchanger, the first expansion valve, and the outdoor air-cooled heat exchanger are connected in sequence through pipelines.

[0012] The air flow channel has an air inlet and an air outlet. The number of stages of the air flow channel corresponds to the number of stages of the air-conditioning unit. The flow-channel air-cooled heat exchangers in each stage of the air-conditioning unit are respectively located in the corresponding stage of the air flow channel. A flow-channel fan is provided in the air flow channel, and the flow-channel fan is used to drive external air into the air flow channel and perform heat exchange with the flow-channel air-cooled heat exchanger.

[0013] According to the multi-stage cascade fresh air grading treatment system provided by the present invention, in the air flow channel, a bypass device is correspondingly provided for each flow-channel air-cooled heat exchanger.

[0014] According to the multi-stage cascade fresh air grading treatment system provided by the present invention, the first-stage air-conditioning unit further includes a four-way valve for changing the flow direction of the coolant.

[0015] According to the multi-stage cascade fresh air grading treatment system provided by the present invention, a four-way valve for changing the flow direction of the coolant is included in each stage of the air-conditioning unit.

[0016] According to the multi-stage cascade fresh air grading treatment system provided by the present invention, in the Nth-stage air-conditioning unit, a heat exchange circuit and one three-medium heat exchanger are further included. A heat exchange device is provided on the heat exchange circuit. The first-medium channel of the three-medium heat exchanger is in parallel connection with the flow-channel air-cooled heat exchanger. The second-medium channel of the three-medium heat exchanger is connected with the heat exchange device through the heat exchange circuit, and a second expansion valve is provided in connection with the first heat exchange channel of the three-medium heat exchanger.

[0017] According to the multi-stage cascade fresh air grading treatment system provided by the present invention, a valve is provided on the heat exchange circuit.

[0018] According to the multi-stage cascade fresh air grading treatment system provided by the present invention, the heat exchange device includes one or a combination of a fan coil unit, a radiant panel, or a domestic hot water collection device.

[0019] According to the multi-stage cascade fresh air grading treatment system provided by the present invention, it further includes an air treatment unit, and the air treatment unit includes a heating device and a flow channel air-cooled heat exchanger connected in sequence through a pipeline, and the flow channel air-cooled heat exchanger is arranged in the (N + 1)-th stage of the air flow channel.

[0020] A multi-stage cascade fresh air grading treatment system provided by the present invention uses multiple direct expansion air conditioning units to grade-treat fresh air. The compression ratio of each stage of direct expansion matches the required compression ratio, and the required number of stages also matches the fresh air state, thereby reducing the power consumption of the compressor and the fan during the fresh air treatment process. While ensuring the air treatment requirements under a large temperature difference, it avoids the risk of freezing during winter fresh air treatment and improves the energy efficiency of the fresh air treatment system during annual operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0022] Figure 1 It is a schematic diagram of Embodiment 1 of the multi-stage cascade fresh air grading treatment system provided by the present invention;

[0023] Figure 2 It is a schematic diagram of the system when N = 2 in the multi-stage cascade fresh air grading treatment system of Embodiment 1;

[0024] Figure 3 It is a schematic diagram of one of the operation modes of the multi-stage cascade fresh air grading treatment system of Embodiment 1;

[0025] Figure 4 It is a schematic diagram of the second operation mode of the multi-stage cascade fresh air grading treatment system of Embodiment 1;

[0026] Figure 5 It is a schematic diagram of the third operation mode of the multi-stage cascade fresh air grading treatment system of Embodiment 1;

[0027] Figure 6 It is a schematic diagram of the fourth operation mode of the multi-stage cascade fresh air grading treatment system of Embodiment 1;

[0028] Figure 7It is a schematic diagram of the fifth operation mode of the multi-stage cascade fresh air grading treatment system in Embodiment 1;

[0029] Figure 8 It is a schematic diagram of the sixth operation mode of the multi-stage cascade fresh air grading treatment system in Embodiment 1;

[0030] Figure 9 It is a schematic diagram of the seventh operation mode of the multi-stage cascade fresh air grading treatment system in Embodiment 1;

[0031] Figure 10 It is a schematic diagram of Embodiment 2 of the multi-stage cascade fresh air grading treatment system provided by the present invention;

[0032] Figure 11 It is a schematic diagram of Embodiment 2 of the multi-stage cascade fresh air grading treatment system provided by the present invention.

[0033] Reference numerals:

[0034] 1. Compressor; 2. Flow channel air-cooled heat exchanger; 3. Three-medium heat exchanger; 301. Heat exchange fan; 302. First medium channel; 303. Second medium channel; 304. Air channel; 4. Outdoor air-cooled heat exchanger; 5. Air flow channel; 502. Air inlet; 502. Air outlet; 6. Flow channel fan; 7. Bypass device; 8. Four-way valve; 9. First expansion valve; 10. Second expansion valve; 11. Heat exchange device; 12. Heat exchange circuit; 13. Valve; 14. Heating device. Detailed implementation manners

[0035] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the present invention will be clearly and completely described below with reference to the accompanying drawings in the present invention. Apparently, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present invention without making creative efforts shall fall within the protection scope of the present invention.

[0036] The following is combined with Figures 1-11 to describe the multi-stage cascade fresh air grading treatment system of the present invention.

[0037] Embodiment 1

[0038] As Figure 1 shown, it is a schematic diagram of Embodiment 1 of the multi-stage cascade fresh air grading treatment system provided by the present invention. When N = 2, the multi-stage cascade fresh air grading treatment system is as Figure 2 shown. The multi-stage cascade fresh air grading treatment system in this embodiment includes an air-conditioning unit and an air flow channel 5:

[0039] The air-conditioning unit includes N air-conditioning units, where N is an integer greater than or equal to 2;

[0040] Each stage of the air-conditioning unit includes a compressor 1 and a flow-channel air-cooled heat exchanger 2 connected in sequence through pipelines. A three-medium heat exchanger 3 is provided between adjacent air-conditioning units. The three-medium heat exchanger 3 includes an air channel 304, a first-medium channel 302, and a second-medium channel 303. The first-medium channel 302 and the second-medium channel 303 are located in the air channel 304 of the three-medium heat exchanger 3. A heat exchange fan 301 is provided in the air channel 304. A first expansion valve 9 and a second expansion valve 10 are respectively connected to the flow-channel air-cooled heat exchanger 2 and the first-medium channel.

[0041] Among every two adjacent air-conditioning units, the first-medium channel 302 of the corresponding three-medium heat exchanger 3 is in parallel with the flow-channel air-cooled heat exchanger 2 in the previous air-conditioning unit, and the second-medium channel 303 of the corresponding three-medium heat exchanger 3 is connected in sequence through pipelines with the compressor 1 and the flow-channel air-cooled heat exchanger 2 in the subsequent air-conditioning unit.

[0042] In the first-stage air-conditioning unit, an outdoor air-cooled heat exchanger 4 is further included. The compressor 1, the flow-channel air-cooled heat exchanger 2, and the outdoor air-cooled heat exchanger 4 are connected in sequence through pipelines.

[0043] The air flow channel 5 has an air inlet 501 and an air outlet 502. The number of stages of the air flow channel 5 corresponds to the number of stages of the air-conditioning unit. The flow-channel air-cooled heat exchanger 2 in each stage of the air-conditioning unit is located in the corresponding stage of the air flow channel 5. A flow-channel fan 6 is provided in the air flow channel 5. The flow-channel fan 6 is used to drive the outside air into the air flow channel 5 and perform heat exchange with the flow-channel air-cooled heat exchanger 2.

[0044] As Figure 1 shown, in this embodiment, in the air flow channel 5, a bypass device 7 is correspondingly provided for each flow-channel air-cooled heat exchanger 2. The bypass device 7 is specifically a bypass valve. The outdoor air enters the air flow channel 5 through the air inlet 501. The flow-channel air-cooled heat exchangers 2 provided in each stage in the air flow channel 5 perform cascade heat exchange on the air, which can ensure the air treatment requirements under large temperature differences. After the bypass device 7 is provided, when the air passes through the unoperated air-conditioning unit, the corresponding bypass device 7 in the air-conditioning unit can be opened to allow the air to pass through, which can reduce the transmission and distribution energy consumption caused by large air resistance during the operation of some air-conditioning units in the system and improve the performance of the system during the operation of some air-conditioning units.

[0045] As Figure 1 shown, in this embodiment, the first-stage air-conditioning unit further includes a four-way valve 8 for changing the flow direction of the refrigerant. By providing the four-way valve 8, the system can be switched between the winter heating mode and the summer cooling mode. In winter, the outdoor fresh air can be heated stage by stage to meet the fresh air heating requirements under large temperature differences and improve the performance of fresh air heating during the heating season. In summer, only the first-stage refrigeration operation can meet the fresh air cooling treatment requirements of civil buildings.

[0046] Of course, in some embodiments, each stage of the air-conditioning unit includes a four-way valve 8 for changing the flow direction of the refrigerant carrier. When the system operates in winter, the outdoor fresh air can be heated stage by stage to meet the fresh air heating requirements under large temperature differences and improve the fresh air heating performance during the heating season. At the same time, in summer, the air can be processed in stages, which can not only meet the fresh air treatment requirements in some special cases but also further improve the annual operating energy efficiency of the system.

[0047] The operation modes of the multi-stage cascade fresh air staged treatment system of this embodiment will be specifically described below.

[0048] Mode 1 - Operation mode at extremely low outdoor temperature in winter

[0049] As Figure 3 shown, it is a schematic diagram of the multi-stage cascade fresh air staged treatment system of this embodiment when operating in the operation mode at extremely low outdoor temperature in winter. In this mode, the four-way valves 8 are all adjusted to the heating working condition, N air-conditioning units are turned on, the heat exchange fans 301 in each three-medium heat exchanger 3 stop operating, the first medium channel 302 and the second medium channel 303 of the three-medium heat exchanger 1 exchange heat, and the bypass devices 7 corresponding to the N flow channel air-cooled heat exchangers 2 of the N air-conditioning units are in the closed working condition. This mode can achieve the reliable operation of the system at extremely low outdoor temperatures in winter.

[0050] Mode 2 - Operation mode at low outdoor temperature in winter

[0051] As Figure 4 shown, it is a schematic diagram of the multi-stage cascade fresh air staged treatment system of this embodiment when operating in the operation mode at low outdoor temperature in winter. In this mode, the four-way valves 8 are all adjusted to the heating working condition, m (1 < m < N) air-conditioning units are turned on, the heat exchange fans 301 of the three-medium heat exchangers 3 in the first m air-conditioning units stop operating, the first medium channel 302 and the second medium channel 303 of the three-medium heat exchanger 3 exchange heat, and the bypass devices 7 corresponding to the m flow channel air-cooled heat exchangers 2 of the first m air-conditioning units are in the closed working condition, and the bypass devices 7 corresponding to the N - m flow channel air-cooled heat exchangers 2 of the latter N - m air-conditioning units are in the open working condition. This mode can achieve the efficient operation of the system at low outdoor temperatures in winter.

[0052] Mode 3 - Operation mode when the outdoor temperature is relatively high in winter

[0053] As Figure 5As shown in the figure, it is a schematic diagram of the operation mode of the multi-stage cascade fresh air grading treatment system of this embodiment when the outdoor temperature is relatively high in winter. In this mode, the four-way valves 8 are all adjusted to the heating condition, and m (1 < m < N) air-conditioning units are turned on. The heat exchange fans 301 of the three-medium heat exchangers 1 of each of the first m air-conditioning units are turned on, and the second expansion valve 10 corresponding to the first medium channel 302 of the three-medium heat exchanger 3 is closed, so as to realize heat exchange between the air channel 304 and the second medium channel 303. Moreover, the bypass devices 7 corresponding to the m flow channel air-cooled heat exchangers 2 of the first m air-conditioning units are in the closed condition, and the bypass devices 7 corresponding to the N - m flow channel air-cooled heat exchangers 2 of the latter N - m air-conditioning units are in the open condition. This mode can achieve the efficient operation of the system when the outdoor temperature is relatively high in winter.

[0054] Mode Four - Operation Mode When the Outdoor Temperature is High in Winter

[0055] As Figure 6 shown in the figure, it is a schematic diagram of the operation mode of the multi-stage cascade fresh air grading treatment system of this embodiment when the outdoor temperature is high in winter. In this mode, the four-way valves 8 are all adjusted to the heating condition, only the first-stage air-conditioning unit is turned on, the heat exchange fan 301 of the three-medium heat exchanger 3 of the first-stage air-conditioning unit is closed, the second expansion valve 10 corresponding to the first medium channel 302 of the three-medium heat exchanger 3 is closed, and the bypass device 7 corresponding to the flow channel air-cooled heat exchanger 2 of the first-stage air-conditioning unit is in the closed condition, while the bypass devices 7 corresponding to the N - 1 flow channel air-cooled heat exchangers 2 of the latter N - 1 air-conditioning units are in the open condition. This mode can achieve the efficient operation of the system when the outdoor temperature is high in winter.

[0056] Mode Five - Operation Mode in Extremely High Outdoor Temperature in Summer

[0057] As Figure 7 shown in the figure, it is a schematic diagram of the operation mode of the multi-stage cascade fresh air grading treatment system of this embodiment when the outdoor temperature is extremely high in summer. In this mode, the four-way valves 8 are all adjusted to the cooling condition, N air-conditioning units are turned on, the heat exchange fans 301 of the three-medium heat exchangers 3 of each stage of air-conditioning units are turned on, the second expansion valve 10 corresponding to the first medium channel 302 of the three-medium heat exchanger 3 is closed, so as to realize heat exchange between the air channel 304 and the second medium channel 303, and the bypass devices 7 corresponding to the N flow channel air-cooled heat exchangers 2 of the N air-conditioning units are in the closed condition. This mode can achieve the efficient operation of the system when the outdoor temperature is extremely high in summer.

[0058] Mode Six - Operation Mode When the Outdoor Temperature is Relatively High in Summer

[0059] As Figure 8As shown in the figure, it is a schematic diagram of the multi-stage cascade fresh air grading treatment system of this embodiment when operating in the summer outdoor high temperature operation mode. In this mode, the four-way valve 8 is adjusted to the refrigeration condition, and m (1 < m < N) air conditioning units are turned on. The heat exchange fans 301 of the three-medium heat exchangers 3 of each of the first m air conditioning units are turned on, and the second expansion valve 10 corresponding to the first medium channel 302 of the three-medium heat exchanger 3 is closed, realizing heat exchange between the air channel 304 and the second medium channel 303. And the bypass devices 7 corresponding to the m flow channel air-cooled heat exchangers 2 of the first m air conditioning units are in the closed condition, and the bypass devices 7 corresponding to the N - m flow channel air-cooled heat exchangers 2 of the latter N - m air conditioning units are in the open condition. This mode can achieve the efficient operation of the system when the outdoor temperature is relatively high in summer.

[0060] Mode Seven - Operation Mode in Summer with Relatively Low Outdoor Temperature

[0061] As Figure 9 shown in the figure, it is a schematic diagram of the multi-stage cascade fresh air grading treatment system of this embodiment when operating in the summer outdoor low temperature operation mode. In this mode, the four-way valve 8 is adjusted to the refrigeration condition, the heat exchange fan 301 of the three-medium heat exchanger 3 of the first-stage air conditioning unit is turned off, the second expansion valve 10 corresponding to the first medium channel 302 of the three-medium heat exchanger 3 is closed, and the bypass device 7 corresponding to the first flow channel air-cooled heat exchanger 2 of the first air conditioning unit is in the closed condition, and the bypass devices 7 corresponding to the N - 1 flow channel air-cooled heat exchangers 2 of the latter N - 1 air conditioning units are in the open condition. This mode can achieve the efficient operation of the system when the outdoor temperature is relatively low in summer.

[0062] Embodiment 2

[0063] As Figure 10 shown in the figure, it is a schematic diagram of Embodiment 2 of the multi-stage cascade fresh air grading treatment system provided by the present invention. Compared with Embodiment 1, in the Nth-stage air conditioning unit of this embodiment of the multi-stage cascade fresh air grading treatment system, there is also a heat exchange circuit 12 and a three-medium heat exchanger 3. A heat exchange device 11 is provided on the heat exchange circuit 12. The first medium channel 302 of the three-medium heat exchanger 3 is connected in parallel with the flow channel air-cooled heat exchanger 2. The second medium channel 303 of the three-medium heat exchanger 3 is connected to the heat exchange device 11 through the heat exchange circuit 12. A second expansion valve 10 is provided on the first heat exchange channel connection of the three-medium heat exchanger 3. This system can directly produce hot / cold water while grading the air.

[0064] As Figure 3 shown in the figure, in this embodiment, a valve 13 is provided on the heat exchange circuit 12. By setting the valve 13, the on / off and flow rate adjustment of the coolant located in the heat exchange circuit 12 can be controlled.

[0065] In this embodiment, the heat exchange device 11 is one of a fan coil unit, a radiant panel, or a domestic hot water collection device. Of course, in some embodiments, the heat exchange device 11 may also be a combination of two or more of the above three devices.

[0066] Embodiment 3

[0067] As Figure 4 shown, it is a schematic diagram of Embodiment 3 of the multi-stage cascade fresh air grading treatment system provided by the present invention. Compared with Embodiment 1, the multi-stage cascade fresh air grading treatment system of this embodiment further includes an air treatment unit, and the air treatment unit includes a heating device 14 and a flow channel air-cooled heat exchanger 2 that are sequentially connected through a pipeline. The flow channel air-cooled heat exchanger 2 is arranged in the (N + 1)-th stage of the air flow channel 5. This system can effectively simplify the system form and reduce costs while meeting the fresh air treatment requirements of the system.

[0068] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A multi-stage cascade fresh air grading treatment system, characterized in that, Comprising an air-conditioning unit and an air flow path: The air-conditioning unit comprises N levels of air-conditioning units, where N is an integer greater than or equal to 2; Each level of the air-conditioning unit comprises a compressor and a flow path air-cooled heat exchanger connected in sequence through pipelines. A three-medium heat exchanger is provided between adjacent air-conditioning units. The three-medium heat exchanger comprises an air channel, a first medium channel, and a second medium channel. A heat exchange fan is provided at the air channel. In each level of the air-conditioning unit, a first expansion valve and a second expansion valve are respectively provided at the flow path air-cooled heat exchanger and the first medium channel; In every two adjacent air-conditioning units, the first medium channel corresponding to the three-medium heat exchanger is in parallel with the flow path air-cooled heat exchanger in the previous air-conditioning unit, and the second medium channel corresponding to the three-medium heat exchanger is connected in sequence through pipelines with the compressor and the flow path air-cooled heat exchanger in the subsequent air-conditioning unit; In the first-level air-conditioning unit, an outdoor air-cooled heat exchanger is further included, and the compressor, the flow path air-cooled heat exchanger, the first expansion valve, and the outdoor air-cooled heat exchanger are connected in sequence through pipelines; The air flow path has an air inlet and an air outlet. The number of levels of the air flow path corresponds to the number of levels of the air-conditioning units. The flow path air-cooled heat exchangers in each level of the air-conditioning unit are respectively located in the corresponding levels of the air flow path. A flow path fan is provided in the air flow path, and the flow path fan is used to drive external air into the air flow path and perform heat exchange with the flow path air-cooled heat exchanger.

2. The multi-stage cascade fresh air grading treatment system according to claim 1, wherein In the air flow path, a bypass device is respectively provided for each flow path air-cooled heat exchanger.

3. The multi-stage cascade fresh air grading treatment system according to claim 1, wherein The first-level air-conditioning unit further includes a four-way valve for changing the flow direction of the coolant.

4. The multi-stage cascade fresh air grading treatment system according to claim 1, characterized in that, In each level of the air-conditioning unit, a four-way valve for changing the flow direction of the coolant is included.

5. The multi-stage cascade fresh air grading treatment system according to any one of claims 1-4, characterized in that, In the Nth-level air-conditioning unit, a heat exchange circuit and one of the three-medium heat exchangers are further included. A heat exchange device is provided on the heat exchange circuit. The first medium channel of the three-medium heat exchanger is in parallel with the flow path air-cooled heat exchanger. The second medium channel of the three-medium heat exchanger is connected to the heat exchange device through the heat exchange circuit. A second expansion valve is provided at the first heat exchange channel of the three-medium heat exchanger.

6. The multi-stage cascade fresh air grading treatment system according to claim 5, wherein, A valve is provided on the heat exchange circuit.

7. The multi-stage cascade fresh air grading treatment system according to claim 5, characterized in that, The heat exchange device includes one or a combination of a fan coil unit, a radiant panel, or a domestic hot water collection device.

8. The multi-stage cascade fresh air grading treatment system according to any one of claims 1-4, characterized in that, An air treatment unit is further included. The air treatment unit includes a heating device and a flow path air-cooled heat exchanger connected in sequence through pipelines. The flow path air-cooled heat exchanger is arranged in the (N + 1)th level of the air flow path.

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