Fig. 3-18 explains the increased rate of energy utilization in the waste heat boiler: the temperature/heat diagrams are for two examples with live steam pressures of 15 and 60 bar (203 and 855 psig) respectively. At the lower live steam pressure, there more thermal energy available for evaporation and super- heating, since the evaporation temperature is correspondingly lower. The pinch point of the evaporator is the same in both cases, and the surface area of the heat exchanger is therefore similar in size. As a result, the stack temperature at 15 bars is about 40°C (72°F) lower than at 60 bars, which means that the waste heat energy is being better utilized.