A cascade refrigeration system and a method for controlling exhaust temperature during rapid cooling

Abstract

The present invention provides a cascade refrigeration system and a method for controlling exhaust temperature during rapid cooling thereof. The cascade refrigeration system includes a low-temperature subsystem and a high-temperature subsystem, wherein the low-temperature subsystem and the high-temperature subsystem are connected via a condenser evaporator. The low-temperature subsystem includes a low-temperature compressor, a low-temperature heat recovery device, and a low-temperature heat recovery device bypass valve. The outlet of the low-temperature compressor is connected to the high-pressure side inlet of the low-temperature heat recovery device, and the high-pressure side outlet of the low-temperature heat recovery device is connected to the low-temperature side inlet of the condenser evaporator. The high-pressure side of the low-temperature heat recovery device is connected in parallel to the low-temperature heat recovery device bypass valve, and the opening adjustment range of the low-temperature heat recovery device bypass valve is 0-100%. The cascade refrigeration system of the present invention is provided with a low-temperature heat recovery device bypass valve. The low-temperature compressor superheat can be adjusted by adjusting the refrigerant flow rate of the heat recovery device, thereby avoiding the problem of systemic exhaust temperature exceeding the limit.

Description

Technical Field

[0001] The present invention belongs to the field of refrigeration and cryogenic technology, and in particular relates to a cascade refrigeration system and an exhaust temperature control method during a rapid cooling process thereof. Background Art

[0002] With the significant improvement in the quality of life of the people, the demand for cold and hot energy in various industries has become more extensive. In this context, refrigeration technology has also ushered in a new round of development climax. In the fields of medicine, food, chemical industry, etc., due to the development of technology and the specific needs of the industry background, low-temperature refrigeration technology, as an important branch of the refrigeration industry, has been rapidly developed in recent years. From a conventional perspective, low-temperature refrigerators with a target temperature of -60℃ to -120℃ are usually implemented using cascade refrigeration technology. The low-temperature subsystem of the cascade refrigeration system continuously absorbs heat from the low-temperature box in the target temperature range, transfers it to the high-temperature subsystem through the cascade heat exchanger, and finally releases it to the environment through the high-temperature condenser, achieving the effect of balancing the cooling capacity and heat leakage in the low-temperature box, thereby maintaining stable low-temperature conditions.

[0003] Due to limitations in compressor materials and lubricants, the low-temperature refrigerant (-60°C to -120°C) at the outlet of the low-temperature evaporator in cascade refrigeration systems requires extensive heat recovery before entering the compressor to ensure the safety and stability of moving parts. In this context, low-temperature cascade refrigeration systems often employ an additional heat recovery unit to directly exchange heat between the low-temperature compressor's intake and exhaust pipes. This reduces the temperature of the low-temperature refrigerant entering the condenser evaporator, maintaining a stable temperature within the condenser evaporator. Furthermore, it significantly increases the actual intake temperature of the low-temperature compressor, ensuring the maintenance of the low-temperature compressor oil temperature. However, given that the temperature difference between the heat source and heat sink in low-temperature refrigerators is typically around 100K, even with a two-stage cascade refrigeration system to achieve a low target temperature, the compression ratio of each stage in the cascade refrigeration system typically exceeds 10. This results in extremely high exhaust temperatures in the low-temperature subsystem under conditions of high heat recovery and high pressure ratio, a problem that often constitutes a major technical bottleneck for cascade refrigeration systems in the low-temperature refrigerator industry. At the same time, even though the intermediate temperature of the cascade refrigeration system is generally around -30°C, when the ambient temperature is too high, the high-temperature subsystem also faces the problem of huge pressure ratio. The exhaust temperature of the high-temperature compressor also has the risk of exceeding the limit, and it often triggers high-temperature alarms and other faults. Summary of the Invention

[0004] The purpose of the present invention is to provide a cascade refrigeration system and a method for controlling the exhaust temperature during the rapid cooling process thereof, so as to provide a solution to the problems such as lubricating oil carbonization, compressor failure, and cascade refrigeration system failure caused by the exhaust temperature exceeding the limit of the high-temperature compressor and the low-temperature compressor, which usually occurs during the rapid cooling process of the cascade refrigeration system or even during normal operation time.

[0005] The present invention is achieved through the following technical solutions:

[0006] A cascade refrigeration system comprises a low-temperature subsystem and a high-temperature subsystem, wherein the low-temperature subsystem and the high-temperature subsystem are connected via a condenser evaporator;

[0007] The low-temperature subsystem includes a low-temperature compressor, a low-temperature heat recovery device and a low-temperature heat recovery device bypass valve. The outlet of the low-temperature compressor is connected to the high-pressure side inlet of the low-temperature heat recovery device, and the high-pressure side outlet of the low-temperature heat recovery device is connected to the low-temperature side inlet of the condenser evaporator; the high-pressure side of the low-temperature heat recovery device is connected in parallel with the low-temperature heat recovery device bypass valve, and the opening adjustment range of the low-temperature heat recovery device bypass valve is 0 to 100%.

[0008] Preferably, the low-temperature subsystem also includes a low-temperature capillary and a low-temperature evaporator, the low-temperature side outlet of the condenser evaporator is connected to the inlet of the low-temperature capillary, the outlet of the low-temperature capillary is connected to the inlet of the low-temperature evaporator, the outlet of the low-temperature evaporator is connected to the low-pressure side inlet of the low-temperature heat recovery device, and the low-pressure side outlet of the low-temperature heat recovery device is connected to the inlet of the low-temperature compressor.

[0009] Preferably, the high-temperature subsystem includes a high-temperature compressor and a high-temperature condenser, the outlet of the high-temperature compressor is connected to the inlet of the high-temperature condenser, the high-temperature condenser is equipped with a condensing fan, the outlet of the high-temperature condenser is connected to the inlet of the high-temperature capillary tube, the outlet of the high-temperature capillary tube is connected to the high-temperature side inlet of the condenser evaporator, and the high-temperature side outlet of the condenser evaporator is connected to the inlet of the high-temperature compressor.

[0010] Preferably, the refrigerant of the high-temperature subsystem is a mixture of R404A and isopentane, and the refrigerant of the low-temperature subsystem is a mixture of R508A and isopentane.

[0011] Furthermore, the refrigerant charge ratio of isopentane in the refrigerant of the high-temperature subsystem is as shown in the following formula (5), and the refrigerant charge ratio of isopentane in the refrigerant of the low-temperature subsystem is as shown in the following formula (6):

[0012]

[0013]

[0014] Where, π high is the refrigerant charge ratio of the high-temperature subsystem mixed with isopentane, k1 is the empirical coefficient, P sat,R404A is the saturated vapor pressure of R404A, P sat,R508A is the saturated vapor pressure of R508A, P sat,R601a is the saturated vapor pressure of isopentane, π low is the refrigerant charge ratio of the low-temperature subsystem mixed with isopentane, k2 is the empirical coefficient, V high is the internal volume of the high temperature condenser, V low is the internal volume of the low-temperature evaporator, d is the inner diameter of the low-temperature capillary and the high-temperature capillary, l high is the length of high temperature capillary, l low is the low temperature grade capillary length.

[0015] A method for controlling exhaust gas temperature during rapid cooling of a cascade refrigeration system, based on the cascade refrigeration system;

[0016] During the operation of the cascade refrigeration system, at the beginning of the cooling process and the rapid cooling process, the bypass valve of the low-temperature heat recovery device is partially opened, so that part of the exhaust gas of the low-temperature compressor directly enters the condenser evaporator, and the other part of the exhaust gas enters the low-temperature heat recovery device; at the end of the cooling process, the bypass valve of the low-temperature heat recovery device is closed, so that all the exhaust gas of the low-temperature compressor enters the low-temperature heat recovery device.

[0017] Preferably, in the cooling start stage and the rapid cooling stage, the opening of the low-temperature stage heat recovery device bypass valve is calculated according to formula (1):

[0018]

[0019] Where α is the opening of the bypass valve of the low-temperature heat recovery device, n is the empirical coefficient, and P evap,low is the outlet pressure of the low temperature stage evaporator, P cond,low is the outlet pressure of the low temperature side of the condenser evaporator, T out,low is the exhaust temperature of the low temperature compressor, T o is the ambient temperature.

[0020] Preferably, the rapid cooling stage includes at most one pause protection stage, and during the pause protection stage, the entire cascade refrigeration system is in a static state.

[0021] Furthermore, when x on Exceeds the standard set threshold x for entering the pause protection phase on,set When x off Exceeds the judgment threshold x for exiting the pause protection phase off,set When the cascade refrigeration system exits the pause protection stage; wherein the xon According to formula (3), the x off Calculate according to formula (4);

[0022]

[0023]

[0024] Where m is the empirical coefficient, T out,low is the exhaust temperature of the low temperature compressor, T out,high is the exhaust temperature of the high-temperature compressor, P evap,high is the high temperature side inlet pressure of the condenser evaporator 3, P cond,low is the low temperature side inlet pressure of condenser evaporator 3, P cond,high is the outlet pressure of the high temperature stage condenser 7, P evap,low is the outlet pressure of the low-temperature stage evaporator 5, T car,tar is the target temperature inside the box; B is the empirical coefficient, and T0 is the ambient temperature.

[0025] Preferably, the method for judging the cooling start stage, the rapid cooling stage and the cooling end stage is as follows: the cooling factor is calculated according to formula (2); when the cooling factor is less than 1, the cascade refrigeration system is in the cooling start stage; when the cooling factor is greater than 1 and less than 5, the cascade refrigeration system is in the rapid cooling stage; when the cooling factor is greater than 5, the cascade refrigeration system is in the cooling end stage;

[0026]

[0027] In the formula, σ is the cooling factor, A is the empirical coefficient, is the temperature gradient of the box, T car is the box temperature, ΔT overlap is the cascade heat transfer temperature difference, T evap,low is the outlet temperature of the low-temperature stage evaporator 5, T o is the ambient temperature.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] The cascade refrigeration system of the present invention is equipped with a low-temperature heat recovery bypass valve, which allows for infinitely adjustable flow pipe opening from 0 to 100%. The low-temperature heat recovery unit and the bypass valve are connected in parallel. This parallel connection allows for infinitely adjustable refrigerant flow through the low-temperature heat recovery unit. This adjustable refrigerant flow in the heat recovery unit allows for adjustment of the superheat of the low-temperature compressor, preventing systemic exhaust temperature overshoots.

[0030] Furthermore, considering the application requirements of low-temperature refrigerators, the refrigeration system needs to quickly provide a target low-temperature range of -60°C to -100°C from a normal temperature state. Therefore, a cascade refrigeration system is formed by combining refrigerants R404A and R508A, with the R404A cycle as the high-temperature subsystem and the R508A cycle as the low-temperature subsystem. This is in line with the optimal working range of the above two refrigerants, and both refrigerants can maintain appropriate operating pressures within their respective stable operating temperature ranges, with evaporation pressures of 1 to 2 bar and condensing pressures of 10 to 15 bar. However, the miscibility of refrigerants R404A and R508A with lubricating oils decreases under their respective low-temperature operating conditions. The poor oil return of the high and low-temperature subsystems has led to the problem of the exhaust temperature exceeding the subsystem limit. However, in the present invention, considering the excellent low-temperature miscibility of isopentane (R601a) and lubricating oil, isopentane (R601a) is mixed into the high-temperature and low-temperature subsystems of the cascade system as an additive to ensure that the oil return of the compressor remains unobstructed under extremely low-temperature conditions, thereby avoiding the problem of systemic exhaust temperature exceeding the limit.

[0031] The present invention also provides a method for controlling the exhaust temperature during the rapid cooling process of a cascade refrigeration system. In common cascade refrigeration systems, an additional low-temperature heat recovery device is used in the low-temperature subsystem to realize the heat exchange process between the intake and exhaust gases of the low-temperature compressor, so as to cause a large amount of overheating of the low-temperature compressor intake air to ensure that the low-temperature intake air temperature is high enough so as not to cause failure of the low-temperature compressor and the lubricating oil. However, considering the ultra-high compression ratio of the low-temperature subsystem, a large degree of overheating will definitely cause serious exhaust temperature exceeding the limit problem. After research, it was found that the highest exhaust temperature during the rapid cooling process of the cascade refrigeration system often appears in the middle of the cooling process, while the exhaust temperature at the beginning and near the end is relatively stable. Therefore, the present invention innovatively proposes to use an adjustable heat recovery device to adjust the superheat of the low-temperature compressor: in the initial stage of cooling and the rapid cooling stage in the middle, the exhaust temperature is relatively high, but because the evaporation temperature of the low-temperature subsystem is still relatively high at this time, the low-temperature compressor suction temperature can also be guaranteed without too much superheat, so the low-temperature heat recovery device bypass valve is partially opened, so that a part of the compressor exhaust does not pass through the low-temperature heat recovery device and directly enters the condenser evaporator, thereby reducing the superheat and the low-temperature compressor exhaust temperature; and in the final stage of cooling, that is, the end of cooling, the evaporation temperature of the low-temperature subsystem is very low, but the system exhaust temperature is also relatively stable. At this time, the low-temperature heat recovery device bypass valve can be closed, so that a large amount of low-temperature compressor exhaust can be passed into the low-temperature heat recovery device, greatly improving the low-temperature compressor suction temperature.

[0032] Furthermore, even with the protection of the above mitigation method, the cascade system may still experience an exhaust temperature exceeding the limit during the entire cooling process. The present invention also proposes a method for protecting the exhaust temperature of the high- and low-temperature compressors through a pause protection mechanism. Since the target temperature in the intermediate box decreases rapidly during the rapid cooling process, and it takes a certain amount of time for the high- and low-temperature stages to establish a stable pressure difference and form the corresponding exhaust temperature, the pause protection mechanism proposed in the present invention can be effective: that is, during the rapid cooling process, through certain parameter measurement and calculation, when the calculated value exceeds the set threshold for entering the pause protection stage, the cascade refrigeration system will temporarily shut down and wait for the exhaust temperature to gradually recover to a normal temperature state; and when the calculated value of the real-time measurement and calculation reaches the judgment threshold for exiting the pause protection stage again, the cascade refrigeration system will restart again. Through reasonable parameter settings, the cascade refrigeration system can achieve a short shutdown of the system when the exhaust temperature is seriously exceeded and the system adjustment range cannot alleviate the severe working condition, and then restart the system to continue cooling when the exhaust temperature drops, thereby achieving a protective effect without affecting the ultimate cooling purpose. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 Schematic diagram of the low-temperature refrigerator cascade system. DETAILED DESCRIPTION

[0034] In order to further understand the present invention, the present invention is described below in conjunction with embodiments. These descriptions are only for further explaining the features and advantages of the present invention and are not intended to limit the claims of the present invention.

[0035] For a two-stage cascade refrigeration system with a temperature difference of 100°C or even higher between the heat source and the heat sink, the compression ratio of each subsystem is usually above 10. Considering that the introduction of the low-temperature heat recovery device causes a huge suction superheat, and the high-temperature refrigerant (R404A) and the low-temperature refrigerant (R508A) have poor lubricating oil miscibility under their respective low-temperature operating conditions, the exhaust temperature exceeding the limit of the cascade system has always restricted the service life and stability of low-temperature refrigerators.

[0036] At the same time, the results of a large number of experimental studies have found that in the rapid cooling process after the cascade refrigeration system is started, the highest exhaust temperatures of the high-temperature subsystem and the low-temperature subsystem both occur in the middle of the cooling process, that is: when the system is just started, since the temperature in the target box is still relatively high, the cooling process has just begun, and the pressure difference between the subsystems has not been fully established. At this time, the pressure ratio is small and the exhaust temperature is not high; as the cooling process gradually develops and stabilizes, the pressure difference in the subsystems gradually increases, and the exhaust temperature also gradually increases, and the system reaches a rapid cooling stage; in the rapid cooling stage, the cascade system compressor has sufficient capacity and can still maintain a large cooling capacity, that is, the evaporation temperature of the low-temperature subsystem is low, and the temperature difference between it and the target box temperature is large, and at the same time, the low-temperature condensation temperature The temperature difference between the low temperature stage and the high temperature stage evaporation temperature is also large, so as to ensure that a large amount of heat can be stably transferred from the box to the environment; in the rapid cooling process, due to the low evaporation temperature and high condensation temperature of the low temperature stage subsystem, the exhaust temperature over-limit problem is easy to occur; in the rapid cooling process, due to the low evaporation temperature of the high temperature stage subsystem, the exhaust temperature over-limit problem is also easy to occur; when the cooling enters the final stage, the temperature in the box is already low, and it is difficult to absorb heat from the box at this time, so the cooling capacity of the system is very small, the evaporation temperature of the low temperature stage subsystem is very close to the box temperature, and the low temperature stage condensation temperature is also very close to the high temperature stage evaporation temperature. At this time, the pressure ratio of the two subsystems is relatively stable, the exhaust temperature is appropriate, and the over-limit problem rarely occurs. Therefore, the present invention provides an exhaust temperature control method during the rapid cooling process of a cascade refrigeration system, and provides a solution to the problems of lubricating oil carbonization, compressor failure, and cascade system failure caused by the exhaust temperature over-limit of the high temperature stage compressor and the low temperature stage compressor that usually occur during the rapid cooling process of the cascade refrigeration system or even during the normal operating time.

[0037] The two-stage cascade refrigeration system targeted by the present invention belongs to the conventional system configuration, comprising a low-temperature stage subsystem and a high-temperature stage subsystem, the two subsystems being connected via a condenser evaporator 3. In the low-temperature stage subsystem, the outlet of the low-temperature stage compressor 1 is connected to the high-pressure side inlet of the low-temperature stage heat recovery device 2, the high-pressure side outlet of the low-temperature stage heat recovery device 2 is connected to the low-temperature side inlet of the condenser evaporator 3, and then sequentially connected to the low-temperature stage capillary tube 4 and the low-temperature stage evaporator 5, the outlet of the low-temperature stage evaporator 5 is connected to the low-pressure side inlet of the low-temperature stage heat recovery device 2, and its outlet is finally connected to the inlet of the low-temperature stage compressor 1, ensuring that the circuit is unobstructed and the low-temperature refrigerant can complete the circulation inside the low-temperature stage. In the high-temperature stage subsystem, the outlet of the high-temperature stage compressor 6 is connected to the inlet of the high-temperature stage condenser 7, and the high-temperature stage condenser 7 is equipped with a condensing fan 8 to ensure that the high-temperature refrigerant can undergo sufficient forced heat exchange inside it. The outlet of the high-temperature stage condenser 7 is sequentially connected to the high-temperature stage capillary tube 9 and the high-temperature side inlet of the condenser evaporator 3, and finally connected to the inlet of the high-temperature stage compressor 6, ensuring that the circuit is unobstructed and the high-temperature refrigerant can complete the circulation inside the high-temperature stage.

[0038] like Figure 1 As shown, in particular, unlike the conventional system configuration, the present invention is provided with a low-temperature heat recovery bypass valve 10. The low-temperature compressor 1 of the low-temperature subsystem is connected to the low-temperature heat recovery bypass valve 10 through a three-way valve. The bypass valve is a two-way proportional control valve that can achieve stepless adjustment of the flow pipe opening from 0 to 100%. The low-temperature heat recovery device is connected in parallel with the low-temperature heat recovery bypass valve 10. This parallel connection method can achieve the purpose of stepless adjustment of the refrigerant flow through the low-temperature heat recovery device 2. During operation, the opening setting value of the low-temperature heat recovery bypass valve 10 is determined by the evaporation pressure of the low-temperature subsystem, the condensation pressure of the low-temperature subsystem and the exhaust temperature of the low-temperature compressor. The opening setting correlation formula of the low-temperature heat recovery bypass valve 10 is shown in the following formula (1):

[0039]

[0040] Where α is the opening of the bypass valve of the low-temperature heat recovery device, n is the empirical coefficient, P evap,low is the outlet pressure of the low temperature stage evaporator 5, P cond,low is the outlet pressure of the low temperature side of the condenser evaporator 3, T out,low is the exhaust temperature of the low temperature compressor, T o is the ambient temperature.

[0041] When the system is turned on, it can achieve a rapid cooling process of the target area in the box. The temperature of the target area will drop from room temperature to -60℃~-100℃ within a few hours. During the entire rapid cooling process, the exhaust temperature of the high and low temperature subsystems is protected based on a certain control strategy.

[0042] The present invention proposes a cooling factor to characterize the cooling state in the box, that is, to judge whether the system is in the initial cooling stage, the rapid cooling stage, or the final cooling stage. The expression of the cooling factor is shown as follows (2): when the cooling factor is less than 1, the system is in the initial cooling stage; when the cooling factor is greater than 1 and less than 5, the system is in the rapid cooling stage; when the cooling factor is greater than 5, the system is in the final cooling stage.

[0043]

[0044] In the formula, σ is the cooling factor, A is the empirical coefficient, is the temperature gradient of the box (continuously negative during the cooling process), T car is the box temperature, ΔT overlap is the cascade heat transfer temperature difference, T evap,low is the outlet temperature of the low-temperature stage evaporator 5, T o is the ambient temperature.

[0045] In the rapid cooling stage, a compressor pause protection mechanism is included at most once: by measuring and calculating parameters, it is determined whether the system should enter the pause protection stage. The parameter calculation value for entering the pause protection stage is determined by the high-temperature compressor exhaust temperature, the low-temperature compressor exhaust temperature, the target temperature in the box, the outlet pressure of the low-temperature evaporator 5, the low-temperature side inlet pressure of the condenser evaporator 3, the high-temperature side inlet pressure of the condenser evaporator 3, and the outlet pressure of the high-temperature condenser 7, as shown in the following formula (3). When the calculated value x on Above the standard set threshold x on,set The refrigerator enters the pause protection stage:

[0046]

[0047] Where x on The parameter calculation value for judging the entry into the pause protection phase, m is the empirical coefficient, T out,low is the exhaust temperature of the low temperature compressor, T out,high is the exhaust temperature of the high-temperature compressor, P evap,high is the high temperature side inlet pressure of the condenser evaporator 3, P cond,low is the low temperature side inlet pressure of condenser evaporator 3, P cond,high is the outlet pressure of the high temperature stage condenser 7, P evap,low is the outlet pressure of the low-temperature stage evaporator 5, T car,tar is the target temperature inside the box.

[0048] After the system enters the pause protection stage, the high-temperature compressor stops running, the low-temperature compressor stops running, the condensing fan stops running, and the entire system is in a static state until the pause protection stage ends.

[0049] Similarly, the parameter calculation value for judging whether the system exits the pause protection stage is determined by the high-temperature compressor exhaust temperature, the low-temperature compressor exhaust temperature, and the target temperature in the box. The parameter calculation value is shown in the following formula (4). When the calculated value x off Above the standard set threshold x off,set The refrigerator exits the pause protection stage:

[0050]

[0051] Where x off The calculated value of the parameter for judging the exit from the pause protection stage, B is the empirical coefficient, T car,tar is the target temperature in the box, T out,low is the exhaust temperature of the low temperature compressor, T out,high is the exhaust temperature of the high-temperature compressor, and T0 is the ambient temperature.

[0052] In a cascade refrigeration system, the refrigerant used in the high-temperature subsystem is R404A, and the refrigerant used in the low-temperature subsystem is R508A. When charging the refrigerant in both the high-temperature and low-temperature subsystems, a certain proportion of isopentane (R601a) needs to be mixed. The refrigerant charge ratios of isopentane (R601a) in the two subsystems are shown in the following equations (5) and (6):

[0053]

[0054]

[0055] Where, π high is the refrigerant charge ratio of the high-temperature subsystem mixed with isopentane (R601a), k1 is the empirical coefficient, P sat,R404A is the saturated vapor pressure of R404A, P sat,R508A is the saturated vapor pressure of R508A, P sat,R601a is the saturated vapor pressure of R604a, π low is the refrigerant charge ratio of the low-temperature subsystem mixed with isopentane (R601a), k2 is the empirical coefficient, V high is the internal volume of the high temperature condenser 7, V low is the internal volume of the low-temperature evaporator 5, d is the inner diameter of the low-temperature capillary 4 and the high-temperature capillary 9 (the inner diameter of the capillary is generally of uniform specification), l high The length of high temperature capillary is 9, l low Length of low temperature capillary 4.

[0056] The present invention mixes isopentane (R601a) as an additive into the high-temperature subsystem and the low-temperature subsystem of the cascade refrigeration system to ensure that the oil return of the compressor remains unobstructed under extremely low-temperature conditions, thereby avoiding the problem of systemic exhaust temperature exceeding the limit.

[0057] The meanings of the symbols and subscripts used in the present invention are shown in Table 1 below.

[0058] Table 1 Symbols and subscripts used in the present invention

[0059]

[0060]

[0061] Compared with traditional cascade refrigeration systems, the present invention significantly improves the low-temperature oil return performance of the refrigerant in the cascade system under the protection of the control strategy of the present invention. The suction superheat of the low-temperature stage is optimized and controlled in real time during the rapid cooling process. The introduction of a pause protection mechanism also avoids the problem of exhaust temperature exceeding the limit that cannot be alleviated under extreme working conditions. Through the above method, the present invention realizes real-time exhaust temperature protection for the cascade refrigeration system, avoiding the problems such as lubricating oil carbonization, compressor failure, and cascade system failure caused by exhaust temperature exceeding the limit that may occur during the rapid cooling process and stable operation.

[0062] The above content is only for explaining the technical idea of the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A cascade refrigeration system, characterized in that: It includes a low-temperature subsystem and a high-temperature subsystem, wherein the low-temperature subsystem and the high-temperature subsystem are connected via a condenser evaporator (3); The low-temperature subsystem comprises a low-temperature compressor (1), a low-temperature heat recovery device (2) and a low-temperature heat recovery device bypass valve (10); the outlet of the low-temperature compressor (1) is connected to the high-pressure side inlet of the low-temperature heat recovery device (2); the high-pressure side outlet of the low-temperature heat recovery device (2) is connected to the low-temperature side inlet of the condenser evaporator (3); the high-pressure side of the low-temperature heat recovery device is connected in parallel to the low-temperature heat recovery device bypass valve (10); the opening adjustment range of the low-temperature heat recovery device bypass valve (10) is 0-100%; The low-temperature subsystem further includes a low-temperature capillary tube (4) and a low-temperature evaporator (5), the low-temperature side outlet of the condenser evaporator (3) is connected to the inlet of the low-temperature capillary tube (4), the outlet of the low-temperature capillary tube (4) is connected to the inlet of the low-temperature evaporator (5), the outlet of the low-temperature evaporator (5) is connected to the low-pressure side inlet of the low-temperature heat recovery device (2), and the low-pressure side outlet of the low-temperature heat recovery device (2) is connected to the inlet of the low-temperature compressor (1).

2. The cascade refrigeration system according to claim 1, characterized in that: The high-temperature subsystem includes a high-temperature compressor (6) and a high-temperature condenser (7), the outlet of the high-temperature compressor (6) is connected to the inlet of the high-temperature condenser (7), the high-temperature condenser (7) is equipped with a condensing fan (8), the outlet of the high-temperature condenser (7) is connected to the inlet of the high-temperature capillary tube (9), the outlet of the high-temperature capillary tube (9) is connected to the high-temperature side inlet of the condenser evaporator (3), and the high-temperature side outlet of the condenser evaporator (3) is connected to the inlet of the high-temperature compressor (6).

3. The cascade refrigeration system according to claim 1, characterized in that: The refrigerant of the high-temperature subsystem is a mixture of R404A and isopentane, and the refrigerant of the low-temperature subsystem is a mixture of R508A and isopentane.

4. The cascade refrigeration system according to claim 3, wherein: The refrigerant charge ratio of isopentane in the refrigerant of the high-temperature subsystem is shown in the following formula (5), and the refrigerant charge ratio of isopentane in the refrigerant of the low-temperature subsystem is shown in the following formula (6): （5） （6） Where, The refrigerant charge ratio of the high temperature subsystem mixed with isopentane is occupied. is the empirical coefficient, is the saturated vapor pressure of R404A, is the saturated vapor pressure of R508A, is the saturated vapor pressure of isopentane, The refrigerant charge ratio of the low temperature subsystem mixed with isopentane is occupied. is the empirical coefficient, is the internal volume of the high-temperature condenser (7), is the internal volume of the low-temperature evaporator (5), is the inner diameter of the low-temperature capillary (4) and the high-temperature capillary (9), is the length of the high temperature capillary (9), is the length of the low temperature capillary (4).

5. A method for controlling exhaust gas temperature during rapid cooling of a cascade refrigeration system, characterized in that: The cascade refrigeration system is the cascade refrigeration system according to any one of claims 1 to 4; During the operation of the cascade refrigeration system, at the beginning of the cooling process and the rapid cooling process, the bypass valve of the low-temperature heat recovery device is partially opened, so that part of the exhaust gas of the low-temperature compressor directly enters the condenser evaporator, and the other part of the exhaust gas enters the low-temperature heat recovery device; At the end of the temperature drop, the bypass valve of the low-temperature stage heat recovery device is closed, so that all the exhaust gas of the low-temperature stage compressor enters the low-temperature stage heat recovery device.

6. The method for controlling exhaust gas temperature during rapid cooling of a cascade refrigeration system according to claim 5, wherein: At the beginning of cooling and the rapid cooling stage, the opening of the bypass valve of the low-temperature heat recovery device is calculated according to formula (1): （1） Where, is the opening of the bypass valve of the low-temperature heat recovery device, is the empirical coefficient, is the outlet pressure of the low temperature stage evaporator (5), is the outlet pressure of the low temperature side of the condenser evaporator (3), is the exhaust temperature of the low temperature compressor, is the ambient temperature.

7. The method for controlling exhaust gas temperature during rapid cooling of a cascade refrigeration system according to claim 5, characterized in that: The rapid cooling stage includes at most one pause protection stage, during which the entire cascade refrigeration system is in a static state.

8. The method for controlling exhaust gas temperature during rapid cooling of a cascade refrigeration system according to claim 7, wherein: when Exceeds the standard threshold for entering the pause protection phase When the cascade refrigeration system enters the pause protection stage, Exceeding the judgment threshold for exiting the pause protection phase When the cascade refrigeration system exits the pause protection stage; wherein, According to formula (3), the Calculate according to formula (4); （3） （4） Where, is the empirical coefficient, is the exhaust temperature of the low temperature compressor, is the exhaust temperature of the high-temperature compressor, is the high temperature side inlet pressure of the condenser evaporator (3), is the low temperature side inlet pressure of the condenser evaporator (3), is the outlet pressure of the high temperature stage condenser (7), is the outlet pressure of the low temperature stage evaporator (5), is the target temperature in the box; B is the empirical coefficient, is the ambient temperature.

9. The method for controlling exhaust gas temperature during rapid cooling of a cascade refrigeration system according to claim 5, wherein: The judgment method of the cooling start stage, rapid cooling stage and cooling end stage is as follows: the cooling factor is calculated according to formula (2). When the cooling factor is less than 1, the cascade refrigeration system is in the cooling start stage; when the cooling factor is greater than 1 and less than 5, the cascade refrigeration system is in the rapid cooling stage; when the cooling factor is greater than 5, the cascade refrigeration system is in the cooling end stage. （2） Where, is the cooling factor, is the empirical coefficient, is the temperature gradient of the box, is the box temperature, is the cascade heat exchange temperature difference, is the outlet temperature of the low-temperature stage evaporator (5), is the ambient temperature.

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