Expander operation
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I am a student who is new to air separation. I saw in the operation manual that "the expander should avoid operating at low speeds under normal circumstances. ”But I think it can run at a low speed when the cooling capacity balance is met. Could you please give me a reason to run at a low speed?
How low is the low speed, 10k? 5k？ 20k？ There is also a surge boundary to consider.
Expanders generally have a lower speed limit. Below the lower limit, there is a risk of surge. Your question is not clear.
Generally, there is an adjustment range in machine design. When the machine is running at normal speed, it has the highest efficiency. It is uneconomical to reduce the rotational speed to reduce the cooling capacity, and the effect is not obvious.
Some expanders have a critical speed, which usually must be run higher than the critical speed, otherwise the machine will be damaged.
I think there are several reasons for low speed interlocking. Low speed interlocking is generally caused by a sudden decrease in flow. If the shaft is running at low speed, the vibration will increase, and it is easy to surge, and the graphite sealing sleeve seal will also be worn. Since the inlet pressure of the low-speed expander is low, the unit cooling capacity is reduced. In order to maintain the cooling capacity balance, the expansion amount will be increased, which will increase the power consumption. In addition, the amount of positive flow expansion air to remove the plate exchanger at low speed will be reduced, which will cause a large temperature difference at the hot end of the plate exchanger, which may freeze the pipes, etc.
Thank you, everyone can give me some insights. I think it should be due to increased vibration at low speeds, fearing damage to the machine.