Difference between pinch and approach temperature in a boiler / hrsg

Pinch point: It is the difference in temperature of flue gas at evaporator outlet and water temperature at evaporator. The lesser the pinch point, more shall be evaporator surface area (as higher surface area would be required to absorb more heat from flue gas). The decision of pinch point should ideally be made keeping in mind overall costs and boiler efficiency.

Approach temperature: It is the difference in temperature between economiser outlet temperature and saturation temperature. Approach temperature should not be kept very less due to the possibility of steaming.