There are two important components to the stack loss: (1) exit gas temperature and (2) excess air at the last heat trap.

• Exit gas temperature should be as low as possible, consistent with the avoidance of low-temperature gas side corrosion as depicted in Figure 6.36. This works out to a temperature of ~140°C (285°F) for most coal-fired boilers where there is no appre­ciable sulfur in fuel.

The larger the surface, the better the cooling of gases, and the higher the boiler effi­ciency. But there is a sharp rise in costs, as surfaces required to cool the gases are significantly hotter at lower gas temperatures as shown in Figure 1.6. Approxi­mately 40% of total HS of boiler residing in AH captures only —10% of the total heat. The benefit lies in understanding the cut-off point beyond which the cost of adding surface outweighs the gain in efficiency.

• Excess air should be at minimum without compromising on the completeness of combustion as evidenced by only some traces of CO in flue gas. This requires a good layout of ducting to distribute the combustion air uniformly, besides a careful meter­ing of air and fuel. Optimum excess air for each type of firing is listed in Table 2.12.

• Air infiltration into the furnace setting and boiler should be minimized in a bal­anced draft boiler to reduce the weight of gases exiting. Membrane panelings of furnace, welded joints in all flues and ducts, pressure-tight casing, and high-quality leak-proof expansion joints can keep the air infiltration to a minimum. The joints through which air can leak inside, such as the gaps around the stoker and spaces around the suspension rods, should be carefully sealed.

• Gas bypassing. Many times the desired cooling of the flue gas does not take place despite the provision of adequate surfaces based on the consequent loss of effi­ciency. This can be traced to the flue gas bypassing around the tube banks due to excessive gaps. Lack of suitable deflection baffles is the cause that has to be remedied.

• Gas leakages. Gas and air bypass ducts to protect the AH against low-temperature corrosion during startups and low loads are simple and effective, but are difficult to seal in normal operation, unless they are very small and closed with pneumatically operated dampers. Leakage through them can increase the exit gas temperature.