# Latent heat of vaporization & pressure

I'm making my way through some workbooks at my job (coal-fired power plant) and I've hit a stumbling block. I could just memorize what the books says, but I really want to understand, so here goes...

One question states: "As the pressure exerted on a steam/water mixture increases, (answer: the value for latent heat of vaporization decreases). According to the steam tables, this is so (with a change in temperature as well). I'm just wondering how this could be though. Because according to the book, a decrease in pressure allows for an increase in steam. This I get. So conversely, a decrease in pressure would result in less steam. So, if the water/steam mixture contains LESS steam (because of the pressure increase), wouldn't it require more heat to turn to complete steam (in other words, "a higher value for latent heat of vaporization"). I would think that the value for latent heat would decrease when there is MORE steam, i.e., less pressure. In that situation, it is much more close to total vaporization.

In the whole discussion, temperature is not mentioned as a factor. Could this be why I'm confused? Because the steam tables don't allow for a change in pressure without a change in temperature. Plus, diagrams in the book make it appear that temperature should be considered. For example, one diagram shows that at 1 psia, and 100 F, and about 100 BTU/LB, water is saturated, and the transition zone begins, and its length represents latent heat of vaporization. As the pressure increases to 14.7 psia, the transition zone begins LATER at about 200 BTU/LB and 200F. The transition period of both pressures ends at ROUGHLY the same place, so the fact that the transition period of the higher psia begins later helps explain the decreased latent heat of vaporization. But, again, that is with an increase in temperature as well. If I can assume that the original question intended the same increase, then it might make a little more sense.

I guess my question boils down to "will latent heat decrease with a pressure increase AND a non-changing temperature?"

Thanks for any help!