The Source of Steam Velocity

Let s assume I wish to generate 1 lb of saturated 400-psig steam in a boiler. I ll be starting with a pound of water at 60°F and atmospheric pressure. I ll do this in three steps and express the energy required for each step in Btu. 

Any boiler plant operator trying to optimize steam generation efficiency will worry most about the firebox and convective section economizer, and not the boiler feed water pump. About two-thirds of the energy required to generate steam is devoted to latent heat and one-third to sensible heat. The energy needed to increase the pressure of the water is very small in comparison. 

A hydroelectric power station generating electricity from a waterfall requires the flow from an entire river. A coal-fired power station generating electricity from steam requires a small water rate that could flow through a 12-inch pipe. The hydroelectric station is only using the potential energy of the water that has been converted to velocity. The thermal power station is using the heat content of the steam that has been converted to velocity. 

I have two ways of expanding steam. The bad way and the good way. In both ways I am not changing the energy content of the steam. I am not extracting any work from the steam there is no friction and no heat is added or lost. This is called an adiabatic expansion. 

This sort of expansion is bad because it accomplishes nothing. The velocity of the steam has not increased at all. It s called an isoen-thalpic expansion because the heat content of the steam has not been reduced. None of the heat has been converted to velocity. And since it is the velocity— not the heat—of the steam that does work, nothing has been accomplished. 

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However, the source of steam velocity does not originate with the pressure of the steam, but with the heat content of the steam. Hence, the original text on thermodynamics was called Heat, a Mode of Motion. The term thermodynamics was introduced as an afterthought because it sounds more important. 

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