I am very fascinated by steam engines. When I took my thermodynamics course a few years back, I remember learning that the steam is recirculated after it does work. I am confused on how this happens. After the steam is used to do work, it is expelled at a lower pressure than it started with. If this steam were to be pumped back into the boiler via a one way valve, shouldn't it need to be at a higher pressure to be able to open the valve and move back into the boiler? How can low pressure steam be pumped into a boiler of higher pressure? Is there some external source that provides the extra energy to pump back into the boiler?