A self - cascading refrigeration system and control method for a low - temperature reaction kettle

Abstract

The present invention belongs to the technical field of cryogenic refrigeration, and particularly relates to a self-cascade refrigeration system and a control method for a cryogenic reaction kettle. Starting from the perspective of improving the separation effect of the mixed refrigerant components and recovering the expansion work of the refrigerant, the present application proposes a self-cascade refrigeration system and a control method for a cryogenic reaction kettle. On the basis of a conventional self-cascade refrigeration system, on the one hand, by realizing the secondary separation process of the refrigerant components, the proportion of low-boiling components in the mixed refrigerant entering the evaporator is increased, the evaporation pressure is increased, and the working pressure ratio of the system is reduced. On the other hand, by introducing an ejector to recover the expansion work of the refrigerant, the suction pressure of the compressor is increased, and the system performance is enhanced. In addition, the secondary separation of the refrigerant components can also increase the primary fluid flow rate of the ejector and reduce the secondary fluid flow rate, further improving the pressure boost ratio of the ejector, realizing the synergistic effect of the secondary separation of the components and the efficiency increase of the ejector, and achieving a greater degree of system performance improvement.

Description

Technical Field

[0001] The present invention belongs to the technical field of cryogenic refrigeration, and particularly relates to a self-cascade refrigeration system and a control method for a cryogenic reactor. Background Art

[0002] Due to the many advantages of the self-cascade system, such as low cost, simple structure, and low control difficulty, more and more cryogenic reactors have begun to use the self-cascade refrigeration system for cooling. However, the refrigeration energy efficiency of the current self-cascade refrigeration system is relatively low, which severely restricts its application and development. The reasons are as follows. On the one hand, the separation effect of the mixed refrigerant components in the conventional self-cascade refrigeration system is not sufficient, resulting in a relatively low evaporation pressure of the system and a large working pressure ratio of the compressor. On the other hand, due to the relatively large working pressure ratio of the self-cascade refrigeration system, there are a large number of throttling losses when the refrigerant throttles from a high-pressure fluid to a low-pressure fluid. Summary of the Invention

[0003] To solve these problems, this patent application starts from the perspective of improving the separation effect of the mixed refrigerant components and recovering the expansion work of the refrigerant, and proposes a self-cascade refrigeration system and a control method for a cryogenic reactor. On the basis of the conventional self-cascade refrigeration system, on the one hand, by realizing the secondary separation process of the refrigerant components, the proportion of low-boiling components in the mixed refrigerant entering the evaporator is increased, the evaporation pressure is increased, and the working pressure ratio of the system is reduced. On the other hand, by introducing an ejector to recover the expansion work of the refrigerant, the suction pressure of the compressor is increased, and the system performance is enhanced. In addition, the secondary separation of the refrigerant components can also increase the primary fluid flow rate of the ejector and reduce the secondary fluid flow rate, thereby further increasing the pressure boost ratio of the ejector and realizing the synergistic effect of the secondary separation technology of the components and the ejector efficiency enhancement technology, achieving a greater degree of system performance improvement. In addition, since the working temperature of the cryogenic reactor needs to be set according to actual needs, the system operating conditions are complex and variable, and the traditional fixed ejector can only achieve better performance enhancement within a relatively narrow range. Therefore, to ensure that the ejector can achieve stable and efficient performance enhancement under the complex and variable conditions of the cryogenic reactor, the adjustable ejector is selected in this application system. The adjustable ejector can change the nozzle throat area by adjusting the position of the conical needle penetrating into the nozzle throat of the ejector according to the change of the working conditions, so that the nozzle area of the ejector is always in an appropriate range, maintaining the stable and efficient operation of the ejector and the system under variable working conditions, and effectively preventing the occurrence of compressor liquid slugging.

[0004] To achieve the above object, the technical solution adopted by the present invention is:

[0005] A self - cascading refrigeration system for a low - temperature reactor, characterized in that it includes a compressor, an inlet of a condenser connected to the outlet of the compressor, an inlet of a gas - liquid separator connected to the outlet of the condenser, a hot - fluid inlet of a first evaporation condenser connected to the gas - phase outlet of the gas - liquid separator, an inlet of a T - shaped pipe connected to the hot - fluid outlet of the first evaporation condenser, a hot - fluid inlet of a second evaporation condenser connected to the gas - phase outlet of the T - shaped pipe, an outlet of the second evaporation condenser connected to an inlet of a throttling mechanism, an outlet of the throttling mechanism connected to an inlet of an evaporator, an outlet of the evaporator connected to a secondary - fluid inlet of an adjustable ejector, a primary - fluid inlet of the adjustable ejector connected to the liquid - phase outlet of the gas - liquid separator and the liquid - phase outlet of the T - shaped pipe, an outlet of the adjustable ejector connected to a cold - fluid inlet of the second evaporation condenser, a cold - fluid outlet of the second evaporation condenser connected to a cold - fluid inlet of the first evaporation condenser, and a cold - fluid outlet of the first evaporation condenser connected to an inlet of the compressor.

[0006] Furthermore, the compressor is a variable - frequency compressor, the condenser is an air - cooled condenser and the air volume is controlled by an adjustable - speed fan; the throttling mechanism is one of a manual throttle valve, an automatic throttle valve or a capillary tube; and the working medium used in this system is a non - azeotropic mixed refrigerant of binary or more components.

[0007] A control method for a self - cascading refrigeration system for a low - temperature reactor, comprising the following steps:

[0008] Step 1: After the refrigeration function of the reactor is turned on, taking t n seconds as the time interval, cyclically compare the temperature T n inside the reactor and the set temperature T s . If T n ≥T s +ΔT nu , the system is turned on and steps 2 - 6 are executed; otherwise, the system is not turned on.

[0009] Step 2: After the system receives the startup signal, the condenser fan is turned on to the initial speed n0. After t1 seconds, the compressor is turned on to the initial frequency f0. After the compressor is turned on for t2 seconds, the system performs automatic adjustment of working parameters and sequentially executes steps 3 - 6.

[0010] Step 3: Based on the temperature T n inside the reactor and the set temperature T s , the frequency of the compressor is adjusted using the PID algorithm.

[0011] Step 4: After the compressor frequency changes for t3 seconds, if the suction temperature of the compressor is greater than the target temperature T3, increase the throat area of the ejector nozzle; if the suction temperature of the compressor is less than the target temperature T3, decrease the throat area of the ejector nozzle.

[0012] Step 5: After the nozzle throat area has changed for t4 seconds, if the heat exchange temperature difference of the condenser is greater than the target value ΔT1, increase the rotational speed of the condenser fan; if the heat exchange temperature difference of the condenser is less than the target value ΔT1, decrease the rotational speed of the condenser fan;

[0013] Step 6: After the rotational speed of the condenser fan has changed for t5 seconds, return to Step 3 and repeatedly execute Steps 3 - 6.

[0014] The heat exchange temperature difference of the condenser is the difference between the refrigerant temperature at the condenser outlet and the air temperature; the parameters set for the compressor PID controller need to be adjusted according to the operating characteristics of the system.

[0015] Furthermore, during the operation of the system, if a shutdown signal is received, the compressor is immediately turned off, and after t6 seconds, the condenser fan is turned off, and the nozzle throat area of the ejector returns to the default value; the n0, f0, t n 、t1, t2, t3, t4, t5, t6, ΔT nu 、ΔT1, T3 parameter values are confirmed according to the system configuration and operating conditions, and their recommended values are: where t n is 1 second, t1, t2, t3, t4, t5, t6 are 1 - 30 seconds, ΔT nu 、ΔT1 are 1 - 5 °C, and T3 is 0 - 20 °C.

[0016] The present invention has the following beneficial effects:

[0017] 1. The present invention realizes the secondary separation process of the mixed refrigerant components by introducing a T-shaped pipe, thereby effectively improving the separation effect of the mixed refrigerant components, increasing the evaporation pressure, and reducing the system working pressure ratio;

[0018] 2. The present invention effectively increases the suction pressure of the compressor and reduces the compressor working pressure ratio by introducing an adjustable ejector and recycling the expansion valve in the throttling process;

[0019] 3. In the present invention, the secondary separation process of the refrigerant increases the flow rate of the primary fluid of the ejector and reduces the flow rate of the secondary fluid, thereby further increasing the pressure boost ratio of the ejector, realizing the synergistic effect of the component secondary separation technology and the ejector efficiency enhancement technology, and achieving a greater degree of system performance improvement;

[0020] 4. In the present invention, the T-shaped pipe and the ejector have a simple structure, small volume, and low cost. The system can achieve significant performance improvement only by increasing a small amount of cost and mass, and has a low cost-effectiveness ratio;

[0021] 5. By using an adjustable ejector in the present invention to adjust the nozzle throat area of the ejector according to the compressor suction temperature, the system operating parameters can be ensured to be within an appropriate range under different working conditions, and the occurrence of compression liquid hammer can be effectively prevented. Description of the Drawings

[0022] Figure 1 It is the schematic diagram of the device of the system of the present invention.

[0023] Figure 2 It is the schematic diagram of the structure of the adjustable ejector adopted in the system of the present invention. Detailed implementation manners

[0024] The present invention will be further described below in conjunction with the accompanying drawings and embodiments. Embodiment

[0025] A self - cascade refrigeration system for a low - temperature reactor, comprising a variable - frequency compressor 101. The outlet of the compressor is connected to the inlet of a condenser 102. The condenser is an air - cooled condenser equipped with a variable - speed fan. The outlet of the condenser 102 is connected to the inlet of a gas - liquid separator 103. The gas - phase outlet of the gas - liquid separator 103 is connected to the hot - fluid inlet 104a of a first evaporative condenser 104. The hot - fluid outlet 104b of the first evaporative condenser 104 is connected to the inlet of a T - shaped pipe 105. The gas - phase outlet of the T - shaped pipe 105 is connected to the hot - fluid inlet 106c of a second evaporative condenser 106. The hot - fluid outlet 106d of the second evaporative condenser is connected to the inlet of a throttling mechanism 107. The outlet of the throttling mechanism 107 is connected to the inlet of an evaporator 108. The outlet of the evaporator 108 is connected to the secondary - fluid inlet of an adjustable ejector 109. The primary - fluid inlet of the adjustable ejector 109 is connected to the liquid - phase outlet of the gas - liquid separator 103 and the liquid - phase outlet of the T - shaped pipe 105. The outlet of the adjustable ejector 109 is connected to the cold - fluid inlet 106a of the second evaporative condenser. The cold - fluid outlet 106b of the second evaporative condenser is connected to the cold - fluid inlet 104c of the first evaporative condenser. The cold - fluid outlet 104d of the first evaporative condenser is connected to the inlet of the compressor 101.

[0026] In the present invention, both the first evaporative condenser and the second evaporative condenser are provided with two working - fluid flow channels, namely a hot - fluid channel and a cold - fluid channel. The hot - fluid inlet 104a and the hot - fluid outlet 104b of the first evaporative condenser are connected through the hot - fluid channel, and the cold - fluid inlet 104c and the cold - fluid outlet 104d are connected through the cold - fluid channel; the cold - fluid inlet 106a and the cold - fluid outlet 106b of the second evaporative condenser are connected through the cold - fluid channel, and the hot - fluid inlet 106c and the hot - fluid outlet 106d are connected through the hot - fluid channel; the working fluid used in this system is azeotropic non - zeotropic refrigerant of binary or more; the throttling mechanism is one of a manual throttle valve, an automatic throttle valve or a capillary tube. An adjustable ejector is adopted, and the adjustable ejector can adjust the throat area of the ejector nozzle as required.

[0027] In the present invention, the adjustable ejector is composed of an adjustable needle 201 with a conical needle tip, a nozzle 202, a suction chamber 203, a mixing chamber 204 and a diffuser chamber 205. The adjustable ejector includes a primary fluid inlet 2a, a secondary fluid inlet 2b and an outlet 2c. The actual throat area for the primary fluid to flow through the adjustable ejector is the difference between the outlet area of the nozzle 202 and the cross-sectional area of the conical needle tip of the adjustable needle 201 at the throat. By screwing in or out the adjustable needle 201 to change the cross-sectional area of its conical needle tip at the outlet of the nozzle 202, the adjustment of the nozzle throat area of the ejector can be achieved. The working process of the refrigerant in this adjustable ejector is as follows: The high-pressure primary fluid enters the nozzle 201 from the primary fluid inlet 2a, throttles into a low-pressure high-speed fluid at the throat of the nozzle 201 outlet, and entrains the low-pressure secondary fluid to enter the suction chamber 203 of the ejector from the secondary fluid inlet 2b. The primary fluid and the secondary fluid are fully mixed in the suction chamber 203 and the mixing chamber 204 to become a low-pressure medium-speed fluid, and then enter the diffuser 205 to be compressed into a medium-pressure low-speed fluid, and finally flow out of the ejector outlet 2c, thereby realizing the recovery of the expansion work of the primary fluid.

[0028] The working process of the present invention is as follows:

[0029] The high-temperature and high-pressure non-azeotropic mixed refrigerant leaves the compressor outlet, enters the condenser and is partially condensed into a two-phase fluid, and then enters the gas-liquid separator to realize the separation of the high- and low-boiling components of the mixed refrigerant: The gaseous mixed refrigerant rich in low-boiling components leaves the gas-phase outlet of the gas-liquid separator, enters the hot fluid channel of the first evaporative condenser and is cooled into a two-phase fluid, and then enters the T-shaped pipe to realize further separation of the high- and low-boiling components. The gaseous mixed refrigerant rich in a higher proportion of low-boiling components leaves the gas-phase outlet of the T-shaped pipe, is condensed into a subcooled liquid in the hot fluid channel of the second evaporative condenser, and then enters the throttling mechanism to be throttled into a low-pressure two-phase fluid, and then enters the evaporator to evaporate and absorb heat, and finally returns to the secondary fluid inlet of the adjustable ejector; The high-pressure liquid mixed refrigerant in the T-shaped pipe leaves the liquid-phase outlet of the T-shaped pipe and is mixed with the high-pressure liquid mixed refrigerant rich in high-boiling components from the liquid-phase outlet of the gas-liquid separator, and then enters the primary fluid inlet of the adjustable ejector; The high-pressure primary fluid and the low-pressure secondary fluid go through a series of complex mixing processes inside the adjustable ejector to become a medium-pressure two-phase refrigerant fluid, and then evaporate into superheated refrigerant vapor after passing through the hot fluid channels of the first evaporative condenser and the second evaporative condenser, and finally return to the compressor suction port.

[0030] The present invention also proposes a control method based on the above system, and its control process includes the following contents:

[0031] After the system is powered on, the following steps are executed in sequence:

[0032] Step 1: After the refrigeration function of the reactor is turned on, the temperature T in the reactor is cyclically compared at intervals of t n seconds. If T n ≥ the set temperature T s , and T n ≥ T s +ΔT nu , the system is turned on and Steps 2-6 are executed; otherwise, the system is not turned on.

[0033] Step 2: After the system receives the startup signal, the condenser fan is turned on to the initial speed n0. After t1 seconds, the compressor is turned on to the initial frequency f0. After the compressor is turned on for t2 seconds, the system will perform the automatic adjustment process of working parameters and execute Steps 3-6 in sequence.

[0034] Step 3: Based on the temperature T in the reactor n and the set temperature T s , the compressor frequency is adjusted using the PID algorithm. The parameters set by the PID controller need to be tuned according to the operating characteristics of the system.

[0035] Step 4: After the compressor frequency changes for t3 seconds, the nozzle throat area of the adjustable ejector is adjusted based on the compressor suction temperature. If the compressor suction temperature is greater than the target temperature T3, the nozzle throat area of the ejector is increased; if the compressor suction temperature is less than the target temperature T3, the nozzle throat area of the ejector is decreased.

[0036] Step 5: After the nozzle throat area changes for t4 seconds, the fan speed of the condenser is adjusted based on the heat exchange temperature difference of the condenser (the difference between the refrigerant temperature at the outlet of the condenser and the air temperature). If the heat exchange temperature difference of the condenser is greater than the target value ΔT1, the condenser fan speed is increased; if the heat exchange temperature difference of the condenser is less than the target value ΔT1, the condenser fan speed is decreased.

[0037] Step 6: After the condenser fan speed changes for t5 seconds, return to Step 3 and execute Steps 3-6 in a loop.

[0038] Meanwhile, during the operation of the system, if a shutdown signal is received, the compressor is immediately turned off, and the condenser fan is turned off after t6 seconds. The nozzle throat area of the ejector returns to the default value; the values of n0, f0, t n , t1, t2, t3, t4, t5, t6, ΔT nu , ΔT1, T3 and other parameters are determined according to the system configuration and operating conditions. Among them, the recommended value of t n is 1 second, and the recommended values of t1, t2, t3, t4, t5, t6 are 1-30 seconds. The recommended values of ΔT nu , ΔT1 are 1-5 °C, and the recommended value of T3 is 0-20 °C.

[0039] The above description of the embodiments is provided to enable those of ordinary skill in the art to understand and apply the present invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the embodiments herein, and all improvements and modifications made by those skilled in the art without departing from the scope of the present invention as disclosed should be within the protection scope of the present invention.

Claims

1. A control method for a self - cascading refrigeration system of a low - temperature reactor. The refrigeration system includes a compressor, an inlet of a condenser connected to the outlet of the compressor, an inlet of a gas - liquid separator connected to the outlet of the condenser, an inlet of a first evaporative condenser hot fluid connected to the gas - phase outlet of the gas - liquid separator, an inlet of a T - shaped pipe connected to the outlet of the first evaporative condenser hot fluid, an inlet of a second evaporative condenser hot fluid connected to the gas - phase outlet of the T - shaped pipe, an outlet of the second evaporative condenser hot fluid connected to an inlet of a throttling mechanism, an outlet of the throttling mechanism connected to an inlet of an evaporator, an outlet of the evaporator connected to an inlet of an adjustable ejector secondary fluid, an inlet of the adjustable ejector primary fluid connected to the liquid - phase outlet of the gas - liquid separator and the liquid - phase outlet of the T - shaped pipe, an outlet of the adjustable ejector connected to an inlet of a second evaporative condenser cold fluid, an outlet of the second evaporative condenser cold fluid connected to an inlet of the first evaporative condenser cold fluid, and an outlet of the first evaporative condenser cold fluid connected to an inlet of the compressor. It is characterized in that, The control method includes the following steps: Step 1: After the refrigeration function of the reactor is turned on, at intervals of t n seconds, the temperature T n inside the reactor and the set temperature T s are cyclically compared. If T n ≥T s +ΔT nu , the system is turned on and Steps 2 - 6 are executed; otherwise, the system is not turned on. Step 2: After the system receives the power-on signal, the condenser fan is turned on to the initial speed n0. After t1 seconds, the compressor is turned on to the initial frequency f0. After the compressor is turned on for t2 seconds, the system automatically adjusts the working parameters and sequentially executes Steps 3-6; Step 3: Based on the temperature T inside the reactor n and the set temperature T s , the compressor frequency is adjusted using the PID algorithm; Step 4: After the compressor frequency changes for t3 seconds, if the compressor suction temperature is greater than the target temperature T3, increase the throat area of the ejector nozzle; if the compressor suction temperature is less than the target temperature T3, decrease the throat area of the ejector nozzle; Step 5: After the throat area of the nozzle changes for t4 seconds, if the heat transfer temperature difference of the condenser is greater than the target value ΔT1, increase the speed of the condenser fan; if the heat transfer temperature difference of the condenser is less than the target value ΔT1, decrease the speed of the condenser fan; Step 6: After the speed of the condenser fan changes for t5 seconds, return to Step 3 and loop through Steps 3-6.

2. The control method for a self - cascading refrigeration system of a low - temperature reactor according to claim 1, characterized in that: The compressor is a variable-frequency compressor, the condenser is an air-cooled condenser and the air volume is controlled by a variable-speed fan; the throttling mechanism is one of a manual throttle valve, an automatic throttle valve or a capillary tube; the working fluid used in the system is a non-azeotropic mixed refrigerant of two components or more.

3. The control method for a self - cascading refrigeration system of a low - temperature reactor according to claim 1, characterized in that: The heat transfer temperature difference of the condenser is the difference between the refrigerant temperature at the outlet of the condenser and the air temperature, and the parameters set by the compressor PID controller need to be adjusted according to the operating characteristics of the system.

4. The control method for a self - cascading refrigeration system of a low - temperature reactor according to claim 1, characterized in that: During the operation of the system, if a shutdown signal is received, the compressor will be immediately shut down, and the condenser fan will be shut down after t6 seconds. The throat area of the ejector nozzle will return to the default value; the values of n0, f0, t n , t1, t2, t3, t4, t5, t6, ΔT nu , ΔT1, and T3 are determined according to the system configuration and operating conditions. The recommended values are: t n is 1 second, t1, t2, t3, t4, t5, t6 are 1 to 30 seconds, ΔT nu , ΔT1 are 1 to 5 °C, and T3 is 0 to 20 °C.

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