Evaporator pinch is the difference between the flue gas temperature leaving the evaporator and the stauration temperatute corresponding to the drum working pressure.
Typically values are 8 to 10 degrees C, although I have seen values as low as 7 degrees C. In very simple terms, the lower the pinch point the higher the steam generation but the larger the required surface area of the evaporator and hence cost.
The relationship between pinh point and required area is not linear since it depends on the Log Mean Temperature Difference.
