I can understand its utility in static system like pressure cooker, where water is having no velocity but in a boiler, water has enough velocity to circulate so this velocity should also get imparted in the steam ideally.
If you ever had accompanied the process of heating water in a pan (no a pressure cooker because the cap must be covered) you would see that early in the heating process that creates convective currents from the bottom up by temperature difference and and consequent density between the bottom and the surface.
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No use I speak here of Rayleigh-Bénard convection if thou you correctly observe the nature. No need to make great culinary recipes only observing tea leaves in a small saucepan in which the water is being heated you'll have a preview of the pattern of movement that occurs both in a saucepan as a boiler. The difference between the two is that a free surface and shalt have no other but the macroscopic movement will have the same pattern except in a thin layer on the top surface.
Sir,your example for cooking was good.but that was too basic..essentially in a boiler, water gets sufficient velocity from pump outlet,i have asked regarding that.thank you for your reply.