Questions about the increase or decrease in expansion volume
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I would like to ask for your advice! In my plant, the air separation process uses full-low pressure expansion air to feed into the upper tower. I don’t quite understand why, when the opening of the expander nozzle is increased from full open to 75 degrees, the rotational speed increases (from around 23,800 to around 24,100), and the bearing temperature rises slightly. The pressure after the booster increases from 0.67 Mpa to 0.710 Mpa, while the temperature before the expander rises from -119 to -113. The control valves for intermediate and bottom extraction have not been adjusted at all. Please explain the reasons behind these phenomena. Thank you!
Some analysis for your reference: When the expander nozzle is fully open, the amount of expansion is excessive, which causes the temperature at the inlet of the expander to drop. This leads to a decrease in the specific enthalpy drop, and as a result, although the amount of expansion is large, less work is done, resulting in a lower rotational speed. When the nozzle is reduced, the situation is reversed: the expansion amount decreases -> the temperature at the inlet of the expander rises -> the specific enthalpy drop increases -> the total work output increases -> the rotational speed rises. Generally, it is said that high temperature and high enthalpy drop, as well as an excessively low inlet temperature of the air separation expander, are often detrimental to cold production.
The poster is performing a reduction operation on the expander, right? Generally, it’s necessary to first open some of the booster pump return valves to prevent surging in the booster pump, and then reduce the size of the expander nozzles. Based on the poster’s description, it seems that only the expansion turbine nozzle was reduced in size, which led to an increase in speed. This is actually easy to explain: when the opening of the expansion turbine nozzle is reduced from full open to 75 degrees, it’s as if the outlet valve of the compressor is being closed, resulting in less gas passing through the nozzle. However, the pressure behind the compressor increases from 0.67 Mpa to 0.710 Mpa; the internal energy of the gas increases. Since the pressure at the inlet of the expansion turbine rises, the unit enthalpy drop increases, and thus the work done by the turbine increases. As a result, both the bearing temperature and the speed increase accordingly. When driving, we often encounter a situation where as the inlet temperature of the expander decreases, its rotational speed also drops; this is due to the change in specific enthalpy drop.