Saturated water vapor, i.e., steam, is commonly used as a heat source in heat-exchanger  applications. Why saturated vapor? Why saturated water vapor? In a plant of any reasonable size, several varieties of saturated steam are commonly available; for example, saturated steam at 4.5, 9, 17, and 33 bar. But the higher the pressure the lower the useful energy content (why?), and the greater the unit cost. Why then is higher-pressure steam used?