If we look at Table 3.4 For performance of a boiler at, say, 100% load, we see that the gas temperature leaving the evaporator is 739°F and leaving the economizer it is 327°F. Now if the economizer is removed from service, can we assume that the exit gas temperature will still be 739°F? The answer is No, for the following reasons:

1. The boiler efficiency drops significantly, by at least (739 — 327)/40 = 10.3%. Hence the efficiency will be at best 83.66 — 10.3 = 73.36% HHV

2. The boiler fuel input increases by this ratio. The new heat input is (118.71/0.7336) = 161.8 MM Btu/h versus (118.71/0.8366) = 141.9 MM Btu/h. Hence the flue gas flow, which is proportional to heat input, will be higher by 161.8/141.9 =1.14 or 14%, or about

1.14x 125,246 =142,800 lb/h.

3. The furnace heat input and heat release rate will also be higher due to the lower efficiency and hence higher furnace exit gas temperature. The combination of higher gas flow and higher gas inlet temperature to the convection bank will increase the exit gas temperature from the evaporator from 739°F to a slightly higher value. Therefore another iteration will have to be performed to arrive at the exit gas temperature based on the revised efficiency and fuel input. The exit gas temperature could be close to 770-780°F.

4. Because of the larger flue gas flow and higher operating temperature in the evaporator bank, the gas pressure drop will also be higher; it could be as much as in the earlier case or even more. Hence, the assumption that removing the economizer will reduce the total gas pressure drop is incorrect. One has to do the performance calculations before arriving at any conclusion.