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What is the function of the clearance of the piston expander in the DEAR air separation unit?
2022-01-05

When the resistance of each group is different, the gas will produce bias flow, resulting in uneven gas flow distribution. The group with small resistance has a large amount of backflow, which reduces the temperature difference at the cold end and expands the temperature difference at the hot end, which is conducive to self removal, but the cold loss at the hot end will increase; the group with large resistance has a small amount of backflow, which expands the temperature difference at the cold end and makes the self removal incomplete, which will increase the freezing amount of water and carbon dioxide.When the refrigeration capacity needs to be increased in operation, it is difficult to increase the unit refrigeration capacity only by increasing the temperature in front of the expander without increasing the expansion capacity. When liquid products are produced, the liquid will move the cooling capacity Q0 directly out of the device, and the device needs to increase the same amount of cooling capacity to maintain the balance of cooling capacity.
According to the flow rate of chilled water and the temperature of inlet and outlet, the cooling capacity of the refrigerator can be determined. Generally, the temperature of chilled water entering evaporator is 16 ~ 18 ℃, and the outlet temperature is 5 ~ 7 ℃. In this temperature range, the commonly used refrigerants are R11, R12, etc.It should be noted that the refrigeration capacity of the expander is not only related to the expansion capacity, but also related to the inlet and outlet parameters of the expander. The total cooling capacity of two expanders may not be as large as that of one expander at full load. For example, if an expander with an expansion capacity of 2700m3 / h operates under the following conditions: P1 = 0.55mpa, T1 = 123k; P2 = 0.125mpa, T1 = 85K, the unit cooling capacity is △ H = h1-h2 = 8270 (kJ / KMOL) - 7264 (kJ / km01) = 1006kj / KMOL.The obvious symbol of freezing of heat exchanger in small air separation column is that the temperature difference at hot end increases, the pressure difference before and after heat exchanger increases, and the temperature in front of high pressure air throttle valve increases. This is because the small tubes in the heat exchanger are blocked by grease, water and carbon dioxide, and the heat exchange performance is reduced.
This part of the cold loss is not considered in the design, to make up for this part of the cold loss will damage the normal operation of the device, or even unable to maintain production, forced to shut down. Therefore, leakage is the enemy of the DEAR air separation unit. In the process of installation and pressure test, it must be strictly controlled. The more serious the leakage is, the more serious it becomes and finally it becomes uncontrollable.After cold blowing, the temperature decreases and a flat head peak appears. The longer the curve distance of the flat head peak is, the more water enters into the molecular sieve. In order to prevent the water inflow accident of Molecular Sieve Purifier, attention should be paid to the operation: the air cooling tower should be operated according to the operating procedures, first the gas is injected, and then the water is injected after the pressure rises and stabilizes; the gas volume should not be increased or reduced suddenly;
The backflow cryogenic gas is composed of several gases which have been separated into product oxygen, product nitrogen and waste nitrogen. When calculating the heat exchange incomplete loss, the total heat exchange incomplete loss should be calculated separately and then added up. Because of the largest amount of polluted nitrogen, the temperature difference at the hot end has the greatest influence on the incomplete heat exchange loss.Moreover, the cooling capacity of throttling effect accounts for a large proportion of the total cooling capacity. For example, when the working pressure is 3.0MPa, the refrigeration capacity of throttling effect per cubic meter of process air is 8.4kj/m3, and the refrigeration capacity of expander per cubic meter of expanded air is 37.7kj/m3.Changing the valve hanger into a rigid support can prevent the displacement of the valve and the deformation of the valve stem when the pipeline shrinks; in the installation and leak detection, the small pipe and the valve stem should not be used as scaffolds; the temperature of the DEAR air separation tower changes greatly, from normal temperature to - 190 ℃. The change of temperature will cause the expansion or contraction of pipeline, so the compensation of cold and heat should be considered in the design of pipeline. In actual installation, pipe elbows or expansion joints should be added to compensate for the parts that are not well considered in design.