# Economies of Scale, texbook example

I am currently studying international economics and I'm going over economies of scale, I get the concept, but there is an example in my textbook that I don't understand.

Here is a screenshot for context.

I don't get the last example about tanks and pipes, I understand they are also internal scale economies, but I don't know what they mean by surface area or volume, wouldn't their contribution to output be quantity produce like the airplane example? or is it refering to something else?

• Is screenshoting the text better in some way than copying the text and quoting it? The latter seems to take less space and is searchable. Commented Feb 25 at 14:33

The surface area of a pipe with length $$l$$ and radius $$r$$ is $$l2r\pi$$. This is approximately proportional to how much material you need to make the pipe,1 so it is proportional to the cost of the pipe.
If we assume a constant flow speed $$v$$, the amount of liquid flowing through the pipe is proportional to $$vr^2\pi$$. In this formula $$r$$ is squared. Thus if we double the radius of the pipe, costs are doubled, but flowthrough/liquid carrying capacity is quadrupled, making the internal economics to scale increasing.