A three-stage auto-cascade refrigeration system and its control method

Abstract

The present invention discloses a three-stage auto-cascade refrigeration system, comprising a variable-frequency compressor, an evaporator, a condenser, multiple evaporative condensers, a gas-liquid separator, and an electronic expansion valve. By constructing a novel system configuration with two-stage intermediate air supply, the system requires only a portion of the refrigerant to be throttled to an intermediate pressure during throttling, and then returned to the compressor through the compressor's intermediate air supply port. This significantly reduces throttling losses and effectively improves system performance.

Description

Technical Field

[0001] The present invention belongs to the field of low-temperature refrigeration equipment, and in particular relates to a three-stage auto-cascade refrigeration system and a control method thereof. Background Art

[0002] By leveraging the component separation and temperature glide characteristics of non-azeotropic mixtures, auto-cascade refrigeration systems require only a single compressor to achieve multi-stage self-cascading within the system. This system boasts numerous advantages, including simple structure, compact size, and low cost. It has found widespread application and development in low-temperature refrigeration fields such as medicine, electronics, gas liquefaction, and freeze-drying. To achieve even lower temperatures, auto-cascade systems have gradually evolved from two-stage to three-stage, four-stage, and other multi-stage systems by increasing the number of internal cascade stages. Three-stage auto-cascade systems, by achieving three stages of internal cascade, can achieve temperatures below -120°C, gaining widespread application and development in the ultra-low temperature refrigeration field.

[0003] However, the current three-stage auto-cascade refrigeration system has a relatively high working pressure and suffers from a large amount of irreversible losses during the throttling process, resulting in poor refrigeration performance. Summary of the Invention

[0004] In order to address the deficiencies of the prior art, the present invention aims to provide a three-stage auto-cascade refrigeration system. By implementing two-stage intermediate air supply, the system can greatly reduce the throttling loss of the system and significantly improve the refrigeration performance of the system.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] A three-stage self-cascade refrigeration system includes a variable frequency compressor, an evaporator and a condenser, wherein the outlet of the variable frequency compressor is connected to the inlet of the condenser, the outlet of the condenser is connected to the hot fluid inlet of the first evaporative condenser, the hot fluid outlet of the first evaporative condenser is connected to the hot fluid inlet of the second evaporative condenser, the hot fluid outlet of the second evaporative condenser is connected to the inlet of the first gas-liquid separator, the liquid outlet of the first gas-liquid separator is connected to the cold fluid inlet of the third evaporative condenser through a first electronic expansion valve, the cold fluid outlet of the third evaporative condenser is connected to the second cold fluid inlet of the first evaporative condenser; the gas outlet of the first gas-liquid separator is connected to the hot fluid inlet of the third evaporative condenser, the hot fluid outlet of the third evaporative condenser is connected to the second gas-liquid separator. The inlet of the separator, the liquid outlet of the second gas-liquid separator is connected to the cold fluid inlet of the fourth evaporative condenser through the second electronic expansion valve, the cold fluid outlet of the fourth evaporative condenser is connected to the first cold fluid inlet of the first evaporative condenser, the gas outlet of the second gas-liquid separator is connected to the hot fluid inlet of the fourth evaporative condenser, the hot fluid outlet of the fourth evaporative condenser is connected to the inlet of the evaporator through the third electronic expansion valve, the outlet of the evaporator is connected to the cold fluid inlet of the second evaporative condenser, the cold fluid outlet of the second evaporative condenser is connected to the intake port of the variable frequency compressor, the first cold fluid outlet of the first evaporative condenser is connected to the first air supply port of the variable frequency compressor, and the second cold fluid outlet of the first evaporative condenser is connected to the second air supply port of the variable frequency compressor.

[0007] Preferably, the first evaporative condenser has three fluid flow channels, including a first cold fluid flow channel connecting the first cold fluid inlet and the first cold fluid outlet, a second cold fluid flow channel connecting the second cold fluid inlet and the second cold fluid outlet, and a hot fluid flow channel connecting the hot fluid inlet and the hot fluid outlet.

[0008] Preferably, the variable frequency compressor is a two-stage air supply compressor, the first air supply port is located between the first stage compression and the second stage compression, and the second air supply port is located between the second stage compression and the third stage compression.

[0009] Preferably, the second evaporative condenser, the third evaporative condenser, and the fourth evaporative condenser each include a cold fluid circulation channel and a hot fluid circulation channel.

[0010] According to the control method of the three-stage auto-cascade refrigeration system, the following steps are specifically included:

[0011] 1) According to the refrigerant temperature at the condenser outlet T c,out and air temperature T a Adjust the adjustable speed fan of the condenser; if T c,out minus T a The difference ΔT c Less than the preset lower limit Δ T c,down , then reduce the fan speed; if Δ T c Greater than the preset upper limit Δ T c,up , then increase the fan speed;

[0012] 2) According to the refrigerant temperature at the evaporator outlet T e,out and refrigeration space T cb Adjust the speed of the variable frequency compressor; if T cb minus T e,out The difference Δ T e Less than the preset lower limit Δ T e,down , then increase the speed of the variable frequency compressor; if Δ T e Greater than the preset upper limit Δ T e,up , then reduce the speed of the variable frequency compressor;

[0013] 3) Adjust the opening of the third electronic expansion valve according to the refrigerant superheat at the suction port of the variable frequency compressor; if the refrigerant superheat at the suction port of the variable frequency compressor is less than the preset lower limit value Δ T sh1,down , then reduce the opening of the third electronic expansion valve; if the refrigerant superheat at the suction port of the variable frequency compressor is greater than the preset upper limit value Δ T sh1,up , then increase the opening of the third electronic expansion valve;

[0014] 4) Adjust the opening of the second electronic expansion valve according to the refrigerant superheat at the first air supply port of the variable frequency compressor; if the refrigerant superheat at the first air supply port of the variable frequency compressor is less than the preset lower limit value Δ T sh2,down , then reduce the opening of the second electronic expansion valve; if the refrigerant superheat at the first air supply port of the variable frequency compressor is greater than the preset upper limit value Δ T sh2,up , then increase the opening of the second electronic expansion valve;

[0015] 5) Adjust the opening of the first electronic expansion valve according to the refrigerant superheat at the second air supply port of the variable frequency compressor; if the refrigerant superheat at the second air supply port of the variable frequency compressor is less than the preset lower limit value Δ T sh3,down, then reduce the opening of the first electronic expansion valve; if the refrigerant superheat at the second air supply port of the variable frequency compressor is greater than the preset upper limit value Δ T sh3,up , the opening of the first electronic expansion valve is increased.

[0016] Among them, the upper limit value Δ in the above steps T c,up , Δ T e,up , Δ T sh1,up , Δ T sh2,up , Δ T sh3,up It is a preset value, ranging from 5 ºC to 10 ºC.

[0017] Lower limit Δ T c,down , Δ T e,down , Δ T sh1,down , Δ T sh2,down , Δ T sh3,down It is a preset value, ranging from 0°C to 5°C.

[0018] The beneficial effects of the present invention are:

[0019] 1. The present invention constructs a new system configuration with two-stage intermediate air supply. When throttling, part of the refrigerant in the three-stage auto-cascade refrigeration system only needs to be throttled to the intermediate pressure and then returned to the compressor from the intermediate air supply port of the compressor. There is no need to throttle all the refrigerant to the evaporation pressure and then return to the compressor from the compressor suction port, thereby greatly reducing the system's throttling loss and effectively improving the system's working performance.

[0020] 2. Because the refrigerant temperature at the gas outlet of the first gas-liquid separator is higher than that at the gas outlet of the second gas-liquid separator, to minimize refrigerant throttling losses, the low-temperature refrigerant entering the fourth evaporative condenser is throttled to a lower first intermediate pressure and returned to the first air supply port of the compressor. The low-temperature refrigerant entering the third evaporative condenser is throttled to a higher second intermediate pressure and returned to the second air supply port of the compressor. By achieving two different intermediate pressures, refrigerant throttling losses are minimized, maximizing system performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a system structure diagram of the present invention. DETAILED DESCRIPTION

[0022] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only used to explain the present invention and are not intended to limit the scope of application of the present invention.

[0023] like Figure 1 As shown, the present invention proposes a three-stage auto-cascade refrigeration system, comprising a variable frequency compressor 101, a condenser 102, an evaporator 112, four evaporative condensers, two gas-liquid separators, and three electronic expansion valves. The first evaporative condenser 103 has three fluid flow channels: a first cold fluid flow channel connecting a first cold fluid inlet and a first cold fluid outlet, a second cold fluid flow channel connecting a second cold fluid inlet and a second cold fluid outlet, and a hot fluid flow channel connecting a hot fluid inlet and a hot fluid outlet.

[0024] The second evaporative condenser 104 , the third evaporative condenser 107 , and the fourth evaporative condenser 110 have two fluid circulation channels, namely a cold fluid circulation channel and a hot fluid circulation channel.

[0025] The variable frequency compressor 101 is a two-stage air supply compressor having an air intake port, a first air supply port, a second air supply port and an outlet. The first air supply port is located between the first stage compression and the second stage compression, and the second air supply port is located between the second stage compression and the third stage compression.

[0026] The structure of the three-stage auto-cascade refrigeration system is specifically as follows: the outlet of the variable frequency compressor 101 is connected to the inlet of the condenser 102, the outlet of the condenser 102 is connected to the hot fluid inlet of the first evaporative condenser 103, the hot fluid outlet of the first evaporative condenser 103 is connected to the hot fluid inlet of the second evaporative condenser 104, the hot fluid outlet of the second evaporative condenser 104 is connected to the inlet of the first gas-liquid separator 105, the liquid outlet of the first gas-liquid separator 105 is connected to the cold fluid inlet of the third evaporative condenser 107 through the first electronic expansion valve 106, and the cold fluid outlet of the third evaporative condenser 107 is connected to the second cold fluid inlet of the first evaporative condenser 103; the gas outlet of the first gas-liquid separator 105 is connected to the hot fluid inlet of the third evaporative condenser 107, the hot fluid outlet of the third evaporative condenser 107 is connected to the inlet of the second gas-liquid separator 108, and the gas outlet of the first gas-liquid separator 105 is connected to the hot fluid inlet of the third evaporative condenser 107. The liquid outlet of the second gas-liquid separator 108 is connected to the cold fluid inlet of the fourth evaporative condenser 110 through the second electronic expansion valve 109, the cold fluid outlet of the fourth evaporative condenser 110 is connected to the first cold fluid inlet of the first evaporative condenser 103, the gas outlet of the second gas-liquid separator 108 is connected to the hot fluid inlet of the fourth evaporative condenser 110, the hot fluid outlet of the fourth evaporative condenser 110 is connected to the inlet of the evaporator 112 through the third electronic expansion valve 111, the outlet of the evaporator 112 is connected to the cold fluid inlet of the second evaporative condenser 104, the cold fluid outlet of the second evaporative condenser 104 is connected to the intake port of the variable frequency compressor 101, the first cold fluid outlet of the first evaporative condenser 103 is connected to the first air supply port of the variable frequency compressor 101, and the second cold fluid outlet of the first evaporative condenser 103 is connected to the second air supply port of the variable frequency compressor 101.

[0027] Accordingly, the present invention also proposes a control method based on the above three-stage auto-cascade refrigeration system, which specifically includes the following steps:

[0028] 1) According to the refrigerant temperature at the outlet of condenser 102 T c,out and air temperature T a Adjust the adjustable speed fan of the condenser 102; if T c,out minus T a The difference Δ T c Less than the preset lower limit Δ T c,down , then reduce the fan speed; if Δ T c Greater than the preset upper limit Δ T c,up , then increase the fan speed.

[0029] 2) According to the refrigerant temperature at the outlet of evaporator 112T e,out and refrigeration space T cb Adjust the speed of the variable frequency compressor 101; if T cb minus T e,out The difference Δ T e Less than the preset lower limit Δ T e,down , then increase the speed of the variable frequency compressor 101; if Δ T e Greater than the preset upper limit Δ T e,up , the speed of the variable frequency compressor 101 is reduced.

[0030] 3) The opening of the third electronic expansion valve 111 is adjusted according to the refrigerant superheat at the suction port of the variable frequency compressor 101; if the refrigerant superheat at the suction port of the variable frequency compressor 101 is less than the preset lower limit value Δ T sh1,down , then reduce the opening of the third electronic expansion valve 111; if the refrigerant superheat at the suction port of the variable frequency compressor 101 is greater than the preset upper limit value Δ T sh1,up , the opening of the third electronic expansion valve 111 is increased.

[0031] 4) The opening of the second electronic expansion valve 109 is adjusted according to the refrigerant superheat at the first air supply port of the variable frequency compressor 101; if the refrigerant superheat at the first air supply port of the variable frequency compressor 101 is less than the preset lower limit value Δ T sh2,down , then reduce the opening of the second electronic expansion valve 109; if the refrigerant superheat at the first air supply port of the variable frequency compressor 101 is greater than the preset upper limit value Δ T sh2,up , the opening of the second electronic expansion valve 109 is increased.

[0032] 5) Adjust the opening of the first electronic expansion valve 106 according to the refrigerant superheat at the second air supply port of the variable frequency compressor 101; if the refrigerant superheat at the second air supply port of the variable frequency compressor 101 is less than the preset lower limit value Δ T sh3,down , then reduce the opening of the first electronic expansion valve 106; if the refrigerant superheat at the second air supply port of the variable frequency compressor 101 is greater than the preset upper limit value Δ T sh3,up , the opening of the first electronic expansion valve 106 is increased.

[0033] The upper limit value Δ of the above parameters T c,up , ΔT e,up , Δ T sh1,up , Δ T sh2,up , Δ T sh3,up It is a preset value, ranging from 5 ºC to 10 ºC.

[0034] The lower limit of the above parameters Δ T c,down , Δ T e,down , Δ T sh1,down , Δ T sh2,down , Δ T sh3,down It is a preset value, ranging from 0 ºC to 5 ºC.

[0035] Because existing auto-cascade refrigeration systems lack an intermediate air supply function, all refrigerants must be throttled to the evaporation pressure, resulting in large system throttling losses and poor performance. In the present invention, however, since the refrigerant temperature at the gas outlet of the first gas-liquid separator 105 is higher than the refrigerant temperature at the gas outlet of the second gas-liquid separator 108, in order to minimize the throttling losses of the refrigerant, the low-temperature refrigerant entering the fourth evaporative condenser 110 is throttled to a lower first intermediate pressure and returned to the first air supply port of the variable frequency compressor 101. The low-temperature refrigerant entering the third evaporative condenser 107 is throttled to a higher second intermediate pressure and returned to the second air supply port of the variable frequency compressor 101. By achieving two different intermediate pressures, the throttling losses of the refrigerant are minimized, achieving the greatest possible improvement in system performance.

[0036] Compared with the prior art, the present invention constructs a new system configuration with two-stage intermediate air supply. In this three-stage self-cascade low-temperature refrigeration system, part of the refrigerant throttling only needs to be throttled to the intermediate pressure and then returned to the compressor from the intermediate air supply port of the compressor. There is no need to throttle all the refrigerant to the evaporation pressure and then return to the compressor from the compressor suction port, thereby greatly reducing the throttling loss of the system and significantly improving the working performance of the system.

[0037] Obviously, the embodiments described above are only some of the embodiments of the present application, rather than all of the embodiments. The preferred embodiments of the present application are given in the accompanying drawings, but they do not limit the patent scope of the present application. The present application can be implemented in many different forms. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present application more thorough and comprehensive. Although the present application has been described in detail with reference to the aforementioned embodiments, for those skilled in the art, it is still possible to modify the technical solutions described in the aforementioned specific embodiments, or to make equivalent replacements for some of the technical features therein. Any equivalent structure made using the contents of the present application specification and the accompanying drawings, directly or indirectly used in other related technical fields, is also within the scope of patent protection of the present application.

Claims

1. A three-stage auto-cascade refrigeration system, comprising a variable frequency compressor, an evaporator and a condenser, characterized in that: The outlet of the variable frequency compressor is connected to the inlet of the condenser, the outlet of the condenser is connected to the hot fluid inlet of the first evaporative condenser, the hot fluid outlet of the first evaporative condenser is connected to the hot fluid inlet of the second evaporative condenser, the hot fluid outlet of the second evaporative condenser is connected to the inlet of the first gas-liquid separator, the liquid outlet of the first gas-liquid separator is connected to the cold fluid inlet of the third evaporative condenser through the first electronic expansion valve, the cold fluid outlet of the third evaporative condenser is connected to the second cold fluid inlet of the first evaporative condenser; the gas outlet of the first gas-liquid separator is connected to the hot fluid inlet of the third evaporative condenser, the hot fluid outlet of the third evaporative condenser is connected to the inlet of the second gas-liquid separator, the second gas-liquid separator The liquid outlet is connected to the cold fluid inlet of the fourth evaporative condenser through the second electronic expansion valve, the cold fluid outlet of the fourth evaporative condenser is connected to the first cold fluid inlet of the first evaporative condenser, the gas outlet of the second gas-liquid separator is connected to the hot fluid inlet of the fourth evaporative condenser, the hot fluid outlet of the fourth evaporative condenser is connected to the inlet of the evaporator through the third electronic expansion valve, the outlet of the evaporator is connected to the cold fluid inlet of the second evaporative condenser, the cold fluid outlet of the second evaporative condenser is connected to the intake port of the variable frequency compressor, the first cold fluid outlet of the first evaporative condenser is connected to the first air supply port of the variable frequency compressor, and the second cold fluid outlet of the first evaporative condenser is connected to the second air supply port of the variable frequency compressor.

2. The three-stage auto-cascade refrigeration system according to claim 1, characterized in that: The first evaporative condenser has three fluid flow channels, including a first cold fluid flow channel connecting the first cold fluid inlet and the first cold fluid outlet, a second cold fluid flow channel connecting the second cold fluid inlet and the second cold fluid outlet, and a hot fluid flow channel connecting the hot fluid inlet and the hot fluid outlet.

3. The three-stage auto-cascade refrigeration system according to claim 2, characterized in that: The variable frequency compressor is a two-stage air supply compressor, wherein the first air supply port is located between the first stage compression and the second stage compression, and the second air supply port is located between the second stage compression and the third stage compression.

4. The three-stage auto-cascade refrigeration system according to claim 3, characterized in that: The second evaporative condenser, the third evaporative condenser, and the fourth evaporative condenser each include a cold fluid circulation channel and a hot fluid circulation channel.

5. The control method of the three-stage auto-cascade refrigeration system according to claim 4, characterized in that: The specific steps include: According to the refrigerant temperature at the condenser outlet T c,out and air temperature T a Adjust the adjustable speed fan of the condenser; if T c,out minus T a The difference Δ T c Less than the preset lower limit Δ T c,down , then reduce the fan speed; if Δ T c Greater than the preset upper limit Δ T c,up , then increase the fan speed; According to the refrigerant temperature at the evaporator outlet T e,out and refrigeration space T cb Adjust the speed of the variable frequency compressor; if T cb minus T e,out The difference Δ T e Less than the preset lower limit Δ T e,down , then increase the speed of the variable frequency compressor; if Δ T e Greater than the preset upper limit Δ T e,up , then reduce the speed of the variable frequency compressor; The opening of the third electronic expansion valve is adjusted according to the refrigerant superheat at the suction port of the variable frequency compressor; if the refrigerant superheat at the suction port of the variable frequency compressor is less than the preset lower limit value Δ T sh1,down , then reduce the opening of the third electronic expansion valve; if the refrigerant superheat at the suction port of the variable frequency compressor is greater than the preset upper limit value Δ T sh1,up , then increase the opening of the third electronic expansion valve; The opening of the second electronic expansion valve is adjusted according to the refrigerant superheat at the first air supply port of the variable frequency compressor; if the refrigerant superheat at the first air supply port of the variable frequency compressor is less than the preset lower limit value Δ T sh2,down , then reduce the opening of the second electronic expansion valve; if the refrigerant superheat at the first air supply port of the variable frequency compressor is greater than the preset upper limit value Δ T sh2,up , then increase the opening of the second electronic expansion valve; The opening of the first electronic expansion valve is adjusted according to the refrigerant superheat at the second air supply port of the variable frequency compressor; if the refrigerant superheat at the second air supply port of the variable frequency compressor is less than the preset lower limit value Δ T sh3,down , then reduce the opening of the first electronic expansion valve; if the refrigerant superheat at the second air supply port of the variable frequency compressor is greater than the preset upper limit value Δ T sh3,up , the opening of the first electronic expansion valve is increased.

6. The control method of the three-stage auto-cascade refrigeration system according to claim 5, characterized in that: The upper limit value Δ in the above steps T c,up , Δ T e,up , Δ T sh1,up , Δ T sh2,up , Δ T sh3,up It is a preset value, ranging from 5 ºC to 10 ºC.

7. The control method of the three-stage auto-cascade refrigeration system according to claim 5, characterized in that: The lower limit value Δ in the above steps T c,down , Δ T e,down , Δ T sh1,down , Δ T sh2,down , Δ T sh3,down It is a preset value, ranging from 0 ºC to 5 ºC.

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