1. Due to leakage and other faults, the high pressure heater is out of service, the boiler feed water temperature will be significantly reduced. At this time, the working medium of unit mass inside the boiler will have more heat absorption capacity. In order to maintain a certain amount of evaporation D, the fuel amount B shall be increased, in order to make the ratio B/D of fuel quantity and evaporation increase.

2. Due to the increased ratio B/D, the fume temperature and low rate at outlet of hearth will rise and the convective heat transfer will rise too, and the outlet steam temperature of superheater and reheater with convective characteristics will increase. In order to maintain steam temperature stable, it is required to increase the attemperation water to take other auxiliary thermal control measures. When the steam temperature cannot be controlled or because of the increased amount of coal other problems occur, it is possibly to force to reduce the boiler output. 

4. The high-pressure preheater is out of service, the water supply temperature at boiler inlet will be reduced, thus increasing the heat -transfer temperature difference in the economizer, so that the flue gas temperature at economizer outlet drops. As for the dropping extent of the exhaust fumetemperature and the hot air temperature at air preheater outlet, they will depend on the size of the air preheater heating surface. 

5. The high-pressure preheater is out of service, the water supply temperature at boiler inlet will be reduced, thus increasing the heat-transfer temperature difference in the economizer, so that the flue gas temperature at economizer outlet drops. As for the dropping extent of the exhaust fume temperature and the hot air temperature at air preheater outlet, they will depend on the size of the air preheater heating surface. 5. After the high- pressure heater is out of service, even if the boiler thermal efficiency is improved, it cannot offset the cyclic thermal efficiency dropping of the unit, as a result, the efficiency of the whole plant is reduced.