Pinch point in Exhaust gas boiler is difference b/w the temperature of of exhaust gas leaving the suction of evaporator and the saturated steam.
If the pinch Point is lower then the efficiency of the exhaust boiler ( E.G.B ) is higher.It is achieved by having a higher heat surface area.
But if surface area is too large causes reduce in velocity of gas. And causes more soot depositions on exhaust gas boiler tube and this is the main reason for soot fire.
Minimum velocity of the exhaust gas is 35 m/sec and the pressure is not more than 150 mm of water gauge.