Originally posted by Thomas:
As has been said TANSTAAFL.
Based on his last reply, I think
Rob S. gets that. He just isn't fully thinking through the logistics of what he is proposing. And he has us telling him that he can't extract energy out of these items without putting more in. I see where he is confused.
Take the material falling off the conveyor example he gave in his last post. You let this material drop 10 feet from the conveyor to the top of the pile below. The potential energy the material swapped for kinetic energy on the way down is going to get completely converted to heat when it hits the pile below. It is now ambient heat and is very hard to recover. This falling material was a byproduct of the process. It will happen no matter what anybody does. So
Rob S. is saying why not recover that energy. Well, that sounds good but how do you do it? To get the most out of it you need to move your energy recovery device with the material height, since you want to extract the energy until the new material is at pile height. This takes energy itself.
The same can be said for fluid streams, especially exhaust air. Exhaust airflow is generally just blown into the surrounding atmosphere. Fluid friction turns all the air velocity into heat in the atmosphere. Again, that energy is just lost. So why not recover some of that energy? Again, because you would influence the process you are trying to exhaust by restricting the exhaust flow.
So our saying you need to add more energy to extract that energy is not unconditionally correct. You need to add more energy
only if you want to maintain the previous process conditions. Since maintaining process conditions is what most plants try to do we tend to implicitly consider this as a precondition to anything we do.
Rob S. wasn't holding himself to that precondition.
Keith
