In an air separation plant, the cooling capacity includes two parts: the cooling capacity of the expansion mechanism and the cooling capacity of the throttling effect. After the expansion air from the medium pressure air separation unit enters the lower tower for liquefaction, it also flows into the upper tower through a liquid throttle, while the expansion air from the low pressure air separation unit no longer passes through the throttle valve. So, is it only the gas (or liquid) passing through the throttle valve that produces the throttling effect cooling capacity? This is not actually the case.
Throttling effect refrigeration capacity is due to pressure reduction, volume expansion, and increased potential energy of molecular interaction, resulting in a decrease in kinetic energy of molecular motion, resulting in a decrease in gas temperature, making it have a certain ability to absorb heat. For the entire air separation plant, the air pressure when entering the plant is high, and the pressure decreases when leaving the air separation plant. In theory, the temperature can be reheated to the temperature when entering the plant. At this point, the enthalpy value of the low-pressure gas is greater than the enthalpy value at the inlet, and the enthalpy difference between it and the inlet gas is the throttling effect cooling capacity, regardless of whether this pressure drop occurs in the throttle valve.
When a gas expands in an expander, the pressure decreases, which also increases the molecular potential energy, creating a portion of the throttling effect that corresponds to the cooling capacity. This part of the cooling capacity is calculated based on the total enthalpy difference between the low-pressure gas exiting the device and the pressure gas entering the device, indicating the total throttling effect cooling capacity of the device.
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