CN1162667C - Throttling Control Mechanism of Transcritical Carbon Dioxide Refrigeration System - Google Patents

Abstract

The throttling control mechanism of the transcritical carbon dioxide refrigeration system mainly includes throttle valves, vapor-liquid separators, solenoid valves, mixers, thermal expansion valves, etc. The opening of the thermal expansion valve is controlled by the superheat of the refrigerant at the outlet of the evaporator. Through the adjustment of the opening, the evaporation pressure can be adjusted and the superheat of the refrigerant can be controlled. The pressure of the high pressure side is controlled by the switch of the solenoid valve. , to control the inlet state of the refrigerant entering the expansion valve. The main advantages of this device are its economy and reliability. These components used are general components, and the economy and reliability of the device itself are relatively good. Using the device can not only effectively control the evaporation pressure, improve the heat transfer efficiency of the evaporator, but also control the pressure on the high pressure side.

Description

Throttling Control Mechanism of Transcritical Carbon Dioxide Refrigeration System

Technical field:

The invention relates to a throttling control mechanism of a refrigeration system, in particular to a throttling control mechanism of a transcritical carbon dioxide refrigeration system, which is used to control the pressure of the high-pressure side and the low-pressure side of the transcritical carbon dioxide refrigeration system, and belongs to the technical field of refrigeration.

Background technique:

At present, human beings are facing more and more serious environmental problems, among which the ozone layer depletion and the greenhouse effect are increasingly concerned by the whole world, and the replacement of refrigerants is imminent. The CFC refrigerants used in the past destroyed the ozone layer. Although the commonly used HFC refrigerants do not destroy the ozone layer, the greenhouse effect is obvious. Although people can work hard to synthesize better refrigerants, due to the inevitable discharge of refrigerants, the final result is that any unnatural substitutes may cause damage to the ecological balance of the earth. Therefore, the final environmentally friendly The refrigerant used should be a non-synthesized natural refrigerant. Carbon dioxide is a natural refrigerant, which is beneficial to the protection of the environment. At the same time, the performance of the transcritical carbon dioxide refrigeration cycle is comparable to that of the traditional Freon refrigeration cycle, so it is widely considered to have a promising future.

The high and low pressure control characteristics of the transcritical carbon dioxide refrigeration system are quite different from our current common compression refrigeration system. Our current commonly used compression refrigeration system adopts subcritical cycle, no matter it is high pressure side or low pressure side, temperature and pressure are closely related. During the operation of the unit, the evaporation pressure and temperature are mainly controlled by adjusting the throttle valve to improve the working efficiency of the system. For the high pressure side, the condensation temperature mainly depends on the temperature and flow of the cooling medium, the condensation pressure corresponds to the condensation temperature one by one, and the pressure on the high pressure side is the condensation pressure, so the pressure on the high pressure side basically depends on the temperature of the cooling medium As with flow, no special control is required in the system. For the transcritical carbon dioxide system, the refrigerant on the high-pressure side is not the condensation process, but the cooling process of the supercritical gas, and its pressure and temperature are two independent variables. So while the temperature is limited by the cooling medium, the pressure is not directly limited. In a transcritical carbon dioxide refrigeration system, the pressure on the high-pressure side can reach 70-150bar, which is 7-10 times that of common refrigeration devices. In addition, the pressure characteristics of the high-pressure side have a great impact on the working efficiency of the system. Therefore, no matter in terms of safety or thermal efficiency, in the transcritical carbon dioxide refrigeration system, like the usual subcritical cycle refrigeration system, there is no device that directly adjusts the high pressure side pressure.

In order to ensure that the pressure on the high-pressure side of the transcritical carbon dioxide refrigeration device is controlled, a common idea is to use a throttle valve to control the pressure on the high-pressure side. When the pressure on the high-pressure side is too high, increase the opening of the throttle valve. When the pressure drops, the opening is reduced. But in this way, the evaporating pressure cannot be directly controlled, which may cause the superheat of the evaporator outlet to be too large, resulting in a decrease in system efficiency.

Due to the existence of the above problems, a solution should be sought to simultaneously control the pressure on the high pressure side and the pressure on the low pressure side. In the prior art, Shengming Liao and Arne Jakobsen in their paper "Shengming Liao, Arne Jakobsen. Optimal heat rejection pressure in transcritical carbon dioxide airconditioning and heat pump systems. Proc. Natural Working Fluids'98, Oslo, 1998: 301-310" , a control idea is proposed, the core of which is: use the throttle valve to control the pressure on the high pressure side, and use the frequency conversion compressor to control the frequency of the compressor by feeling the temperature of the cold space, so as to control the performance of the low pressure side Effect. Although this solution solves the problem of simultaneous control of the high and low pressure sides to a certain extent, it is not applicable to the carbon dioxide refrigeration system using a fixed-capacity compressor. Moreover, the cost of the variable frequency compressor is much higher than that of the constant capacity compressor, so the economy of the aforementioned scheme is relatively poor.

Invention content:

The object of the present invention is to aim at the deficiencies of the prior art, and to design and provide a throttling control mechanism suitable for a transcritical carbon dioxide refrigeration system using a constant-capacity compressor, capable of simultaneously controlling the high and low side pressures.

The throttling control mechanism of the transcritical carbon dioxide refrigeration system proposed by the present invention is composed of a throttle valve, a gas-liquid separator, a solenoid valve, a mixer, and a thermal expansion valve. The inlet of the throttle valve is connected to the outlet of the gas cooler, the inlet of the gas-liquid separator is connected to the outlet of the throttle valve, and there are two outlets of the gas-liquid separator, among which the gas phase outlet is connected to the inlet of the solenoid valve, and the outlet of the solenoid valve is connected to the outlet of the solenoid valve through a pipeline. The inlet of the mixer is connected, the outlet of the liquid phase is directly connected with the inlet of the mixer, the outlet of the mixer is connected with the inlet of the thermal expansion valve, and the outlet of the thermal expansion valve is connected with the inlet of the evaporator. The inlet and outlet of the compressor are respectively connected with the outlet of the evaporator and the inlet of the gas cooler. Solenoid valve adopts normally open type.

The throttling elements in the entire throttling control mechanism include a throttle valve and a thermal expansion valve.

The opening of the thermal expansion valve is controlled by the superheat of the refrigerant at the outlet of the evaporator. When the evaporating pressure is too low and the superheat is too high, the opening of the thermal expansion valve increases, and the flow rate of the refrigerant increases, so the evaporating pressure and evaporating temperature rise, and the degree of superheat decreases; when the evaporating pressure is too high, the expansion valve passes through reducing Small opening to suppress the rise of evaporating pressure.

The control of the pressure on the high pressure side is realized by controlling the state of the refrigerant entering the thermal expansion valve. The high-pressure carbon dioxide from the gas cooler may be a subcooled liquid or a supercritical fluid, but after the pressure reduction of the throttle valve, it will become a two-phase fluid and flow into the gas-liquid separator. In most cases, the two outlets of the gas-liquid separator flow out the liquid and gas to the mixer respectively. For the switch of the electromagnetic valve, two control pressure values are preset, one is the pressure upper limit action value, and the other is the pressure lower limit action value. If the pressure on the high-pressure side of the system exceeds the pressure upper limit action value, the solenoid valve is closed at this time, and only the liquid reaches the inlet of the thermal expansion valve through the mixing chamber. At this time, under the same opening of the thermal expansion valve, the refrigerant flowing through the thermal expansion valve The flow increases, thereby causing a rapid drop in the amount of refrigerant in the gas cooler, causing a rapid drop in pressure on the high side. When the pressure on the high-pressure side drops below the lower limit operating value, the solenoid valve opens. Since the refrigerant entering the inlet of the thermal expansion valve is a two-phase refrigerant, the flow rate through the thermal expansion valve decreases, making the pressure on the high-pressure side rise again.

Through the above-mentioned control method, while ensuring proper pressure on the low-pressure side, the pressure on the high-pressure side is also controlled, avoiding the danger of the system caused by excessive pressure on the high-pressure side.

The main advantages of this control mechanism are its economy and reliability. In this control mechanism, only economical and common components such as throttle valve, thermal expansion valve, solenoid valve, gas-liquid separator, and mixer are used, so the economy and reliability of the device itself are relatively good. Although it is necessary to control the two coupled variables of the pressure on the high pressure side and the degree of superheat of the evaporator at the same time, it is necessary to pay attention to the stability of the control. There is a certain gap between the two control pressure values, and the response of this control loop is much slower than that of the evaporator superheat control loop. Since the time constants of the two control loops are orders of magnitude different, possible control instability problems are avoided.

Description of drawings:

Fig. 1 is a schematic diagram of the throttling control mechanism of the transcritical carbon dioxide refrigeration system of the present invention.

In the figure, 1 is a throttle valve, 2 is a gas-liquid separator, 3 is a solenoid valve, 4 is a mixer, 5 is a thermal expansion valve, 6 is an evaporator, 7 is a compressor, 8 is a gas cooler,

Detailed ways:

The specific implementation of the present invention will be further described below in conjunction with the accompanying drawings.

As shown in FIG. 1 , the throttling control mechanism of the transcritical carbon dioxide refrigeration system of the present invention mainly includes a throttle valve 1 , a gas-liquid separator 2 , a solenoid valve 3 , a mixer 4 , and a thermal expansion valve 5 . In addition to the throttling control mechanism, the transcritical carbon dioxide refrigeration system also includes an evaporator 6, a compressor 7 and a gas cooler 8, etc.

The outlet of compressor 7 is connected to the inlet of gas cooler 8, the outlet of gas cooler 8 is connected to the inlet of throttle valve 1, the outlet of throttle valve 1 is connected to the inlet of gas-liquid separator 2, and gas-liquid separator 2 has two phases of gas phase and liquid phase. The outlet of the gas phase is connected to the inlet of the solenoid valve 3 through a pipeline, the outlet of the solenoid valve 3 is connected to the inlet of the mixer 4 through a pipeline, the liquid phase outlet of the vapor-liquid separator 2 is directly connected to the inlet of the mixer 4, and the outlet of the mixer 4 It is connected with the inlet of thermal expansion valve 5, the outlet of thermal expansion valve 5 is connected with the inlet of evaporator 6, and the outlet of evaporator 6 is connected with the inlet of compressor 7.

Throttle valve 1 is made of ordinary copper throttle valve, gas-liquid separator 2 is made of cast iron, solenoid valve 3 is normally open, mixer 4 is made of cast iron, thermal expansion valve 5 is F-type thermal expansion valve, evaporator 6 And gas cooler 8 adopts aluminum tube belt heat exchanger, and compressor 7 adopts reciprocating piston compressor.

Under normal working conditions, the compressor 8 discharges supercritical carbon dioxide high-pressure gas, which becomes a liquid or a supercritical gas whose temperature is closer to the ambient temperature after passing through the gas cooler 8, and then becomes a two-phase fluid after passing through the throttle valve 1 , through the action of the gas-liquid separator 2, the liquid phase and the gas phase are separated, the liquid phase directly enters the mixer 4, and the gas phase enters the mixing chamber 4 through the solenoid valve 3, and the two-phase refrigerant coming out of the mixing chamber 4 undergoes thermal expansion. The valve 5 flows into the evaporator 6 and then is sucked in by the compressor 7. For the switch of the solenoid valve 3, two control pressure values are preset, one is the pressure upper limit action value, and the other is the pressure lower limit action value. If the pressure on the high pressure side of the system exceeds the pressure upper limit action value, the solenoid valve 3 is closed at this time, and only the liquid reaches the inlet of the thermal expansion valve 5 through the mixing chamber 4. At this time, under the same opening degree of the thermal expansion valve 5, it flows through the thermal expansion valve. The refrigerant flow rate of valve 5 increases, so that the amount of refrigerant in gas cooler 8 drops rapidly, causing the pressure on the high pressure side to drop rapidly. When the pressure on the high pressure side drops below the lower limit operating value, the solenoid valve 3 opens, and since the two-phase refrigerant reaches the inlet of the thermal expansion valve 5 at this time, the flow rate through the thermal expansion valve 5 decreases, making the high pressure side pressure rises again.

The opening degree of the thermal expansion valve 5 is controlled by the degree of superheat of the refrigerant at the outlet of the evaporator 6 . When the evaporating pressure is too low and the superheat is too high, the opening of the thermal expansion valve 5 increases, and the flow rate of the refrigerant increases, so the evaporating pressure and evaporating temperature rise, and the degree of superheat decreases; when the evaporating pressure is too high, the expansion valve 5 By reducing the opening, the increase in evaporation pressure is suppressed.

Claims (2)

1, a kind of critical-cross carbon dioxide refrigeration system throttling controlling organization, mainly comprise choke valve (1), vapour liquid separator (2), magnetic valve (3), blender (4), heating power expansion valve (5), it is characterized in that: choke valve (1) import links to each other with gas cooler (8) outlet, choke valve (1) outlet links to each other with vapour liquid separator (2) import, vapour liquid separator (2) has gas phase and two outlets of liquid phase, wherein gaseous phase outlet links to each other with magnetic valve (3) import by pipeline, magnetic valve (3) outlet is connected with blender (4) import by pipeline, the liquid phase outlet of vapour liquid separator (2) directly links to each other with blender (4) import, blender (4) outlet links to each other with heating power expansion valve (5) import, and heating power expansion valve (5) outlet is then joined with evaporimeter (6) import.

2, as the said critical-cross carbon dioxide refrigeration system of claim 1 throttling controlling organization, its feature is that also magnetic valve (3) adopts open type.