Research on CO_2 Condensation Vacuum Insulation Scheme of Cryogenic Transfer System

CO2 condensation is an effective measure to obtain and improve the vacuum. The specific method is to fill the vacuum interlayer with atmospheric CO2. Under low temperature operation, the CO2 gas condenses into a frost-like solid, so that a vacuum is formed in the interlayer space or The original vacuum is higher. As early as the 1960s, the US National Bureau of Standards conducted an experimental study of condensing vacuum. It is believed that CO2SO2 and NO are three more suitable gases; CO2 is finally selected as the condensing vacuum gas, because CO2 is easy to obtain a fairly pure state, and the price is low. Non-toxic, asphyxiation is lower than SO2 and NO. The main purpose of adopting CO2 condensation vacuum technology is to reduce the degree of vacuum sealing vacuum or to completely replace the commonly used evacuation process, in order to obtain and maintain long-term operation under low temperature conditions. The degree of vacuum. For long-term storage of vacuum insulation pipes for short-term use, in order to keep the vacuum degree in the adiabatic space of the pipes more efficient, the vacuum jacket is filled with CO2 gas that is easily condensed at a low temperature, and is normally at a normal temperature when it is not in operation, causing a vacuum. The pressure in the space is in a low vacuum state or maintained at atmospheric pressure, reducing the pressure difference between inside and outside, that is, reducing or avoiding the amount of gas entering the vacuum space due to leakage. When the cryogenic liquid is filled, CO2 gas condenses into a solid state on a cold surface with very low temperature due to the cooling effect of the cryogenic medium entering the conduit. The vacuum chamber quickly forms a vacuum and the vacuum insulation is used. The liquid hydrogen filling line used CO2 condensation vacuum insulation and was very satisfactory. At present, some domestic companies have also adopted the CO2 condensation vacuum process to produce liquid nitrogen cryogenic containers or cryogenic liquid transfer tubes. However, the purity of CO2 in the structure is currently very high (>99.99%). It is difficult to maintain high purity of CO2 in the long term. , and can not reduce the welding and leak detection requirements of the vacuum sandwich weld We have used a variety of insulation materials for the experimental study of the vacuum insulation layer CO2 at different filling pressures in the use of low purity industrial CO2 vacuum insulation layer Under conditions of low vacuum, different insulation materials, and different CO2 charging pressures, the degree of vacuum in the insulation layer and the effect of the insulation effect of the insulation layer 2CO2 purity on the vacuum degree of the insulation layer are generally 99.9% pure for industrial CO2. The use of CO2 to condense the vacuum needs to purify the vacuum. The effect of purity of the CO2 on the vacuum degree of the vacuum insulation layer is calculated in Table 1 in Table 1 without considering the entrainment and adsorption of impurity gases during the CO2 condensation.

3 Experimental apparatus and principle The schematic diagram of the whole experimental apparatus see the inner diameter of the inner tube of the vacuum insulation layer is 110mm, the thickness is 3mm, the outer diameter of the outer tube is 164mm, the thickness of the vacuum sandwich is 20mm, the outer tube is covered with 10 layers of radiation protection layer and 10 layers of insulating layers are intertwined. The thickness of the covered insulation layer is lmm. The radiation protection layer is aluminum foil. The insulation layer is aluminum foil or vegetable fiber paper vacuum insulation layer without using getters, and the surface has not hit the L data acquisition system. Using HengHe DC-1 Data Acquisition System Table 1 CO2 Condensation Vacuum Adiabatic Theory Calculate Vacuum Degree C2 Filling Pressure C2 Purity % Theory Calculate Vacuum Degree Pa For the thermal conductivity of the thermal insulation layer, use the calorimetric principle. At the steady state, the known amount of liquid nitrogen vaporization of the latent heat of vaporization is measured, and the total amount of heat transferred from the test specimen radially into the test section through the heat insulating layer is obtained. According to the size of the heat flux, the apparent thermal conductivity of the vacuum multilayer insulation layer is calculated: where: r is the latent heat of vaporization of liquid nitrogen under standard conditions (/kg); d, To is the saturated gaseous nitrogen under standard conditions Density (kg/m3) and temperature (K); PT is the pressure (Pa) temperature (K) of the evaporative liquid nitrogen measured under the experimental conditions; dud2 is the inner and outer diameter (m) of the insulating layer; / is the thermal insulation layer Length of the test section (m); TuT2 is the temperature (K) of the inner and outer surfaces of the insulating layer; the amount of gas evaporated by the liquid nitrogen per unit time (m3/s) The entire experimental device was evaporated from the experimental test line and the liquid nitrogen container. Rate measuring system components The upper end of the experimental line is connected directly to the bottom of the vessel via a flange. There are two orifices on the flange, one upward, connected to the gaseous nitrogen outlet pipe; the other down, followed by a thin tube straight The main function of the bottom tube of the pipe is to introduce LN2 into the pipe and prevent evaporation of LN2 from the pipe into the container. During the experiment, the LN2 in the pipe continuously evaporates. The LN2 in the container continuously flows into the pipe to maintain the LN2 in the pipe. Liquid-filled state prevents fluid in the pipeline The continuous decrease in the surface caused the pipeline test status to be inconsistent with the actual status.

4 Test process According to the purpose of the test and the actual conditions of the entire test device, the contents of the test arrangement are as follows: (1) vacuum insulation layer test of different insulation layer materials; (2) research of vacuum insulation layer under different CO2 filling pressure After the final filling of CO2, the pressure in the vacuum sandwich is 0.2 MPa and the CO2 filling process of 5 Pa vacuum sandwich is as follows: (1) Heat the vacuum line and heat the pipe to 120~140 °C before vacuum The metal deflation and the gas attached to the pipeline and the insulating layer are released, then the pre-vacuum is started; (2) The pre-vacuum is used, and the vacuum pump is used by the mechanical pump of the vacuum unit, and the maximum vacuum degree of each vacuum is only For 5Pa, the vacuuming time is 3 hours; (3) C2 for industrial filling (purity 99.9%), the charging pressure is 0.1 MPa, and the time lasts for half an hour; (4) The vacuum layer is again evacuated. Repeat this three times, and then seal the vacuum insulation layer and finally seal it. According to the experimental requirements, fill the CO2 gas with different pressure.

Before filling the experimental tank with liquid nitrogen, the pipeline shall be replaced. After replacing the residual gas in the pipeline and replacing it, open the container and the pipeline vent valve, open the valve of the Dewar and fill the container and pipeline with liquid nitrogen. When the vessel and the pipeline are cold and the internal liquid level is stable, the pipeline vent valve is closed, and the evaporation rate of liquid nitrogen in the pipeline and the temperature of each point outside the vacuum insulation layer are measured.

The measured wall temperature and evaporation rate of the insulated pipe are shown in Table 2. The insulating layer 1 of the heat insulating pipe is made of aluminum foil plus vegetable fiber paper, 10 layers in total, and the thickness is 10mm. The heat-insulating pipe No. 2 is directly made of aluminum foil. The total is 10 layers. The thickness of 8mm No. 1 tube was tested under different CO2 charging pressures, and No. 2 tube was only subjected to the experimental pressure of 10Pa. Table 2 CO2 condensation Vacuum insulation layer Wall temperature of the outer tube and the liquid nitrogen evaporation rate in the tube Insulation Vacuum sandwich The pressure (Pa)T, T2T3T4T5m3/s11c520.118.67.8-5, which is charged into the temperature line C2 at each measuring point, is calculated in Table 3 for the thermal conductivity of the insulating layer and the measured vacuum degree in the vacuum chamber. The vacuum insulation layer of different materials, apparent No. 1 pipe has good thermal insulation performance. Main note: “-” in the table indicates untested parameters. The measured pressure is the pressure gauge pressure if the radiation protection layer and the isolation layer are loosely wound. Plant fiber paper has better adsorption performance. Aluminum foil used in No. 2 pipe is a two-layered aluminum foil. When CO 2 is adsorbed, the effect is not obvious. The temperature of the outer wall of the pipe is significantly lower than that of the No. 1 pipe. Fill in the calculated thermal conductivity into the obtained Pressure of the vacuum line C2 PaW/m.Pa11050. 0036-100.0022210-2210-9

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