Not sure where the "low pressure" designation came from, but 20 bar would not normally be considered low pressure.
If they are both water tube boilers, the design and operation could be relatively similar, but there are variations on water tube boiler design also. The saturation temperature is higher obviously in the 41 bar boiler. Another possibilty is that the 41 bar boiler has a superheater tube bank which adds complexity to the design.
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