can you rank projects simply by inspecting the cash flows?

That's a start. After you discount back the cash flows and get the proper NPV for a project, you might want to do a feasibility analysis, for lack of a better term. How accurate were your projections for future flows? Possible things that could go wrong?

Of course, you also have to make sure you have the initial and future outlay necessary to start up and continue the project in order to get the cash flows, especially in a cyclic industry.

As long as you take in the present value of all future cash flows into account. You can't just look at spend this much get this much. If must be considered how long you will be paying for supplies, man power, etc. and how long you will be reaping rewards.

Theres a financial calculator for that kind of stuff.