Most of the high capacity boilers firing coal operate with an efficiency of around 86% on the Higher Heat Value basis. Loss of around 14% can be attributed to various losses of which the dry gas loss is about 35% of the total. When both economizers and air pre-heaters are not provided the boiler efficiency drops to around 66% from 86%. When air pre-heater is not provided the boiler efficiency will be around 74 % only. Thus we can conserve about 20% extra fuel when we provide both economizers and air pre-heaters in boilers.
The feed water from the high pressure heaters enters the economizer and picks up heat from the flue gases after the low temperature superheater. Many types of economizer are designed for picking up heat from the flue gas. These can be classified as an inline or staggered arrangement based on the type of tube arrangement. The staggered arrangement is compact and occupies less volume for the same amount of heat transfer when compared to the inline arrangement. Economizers are also designed with plain tube and fined tubes. The fins can be longitudinal or spiral. All these types are suitable for clean fuels like gas, oil, and low ash coals. For high ash coals, only the plain tube inline arrangement is used. This is mainly to reduce ash erosion and thus reduce erosion failures. These economizers pick up about 50 to 55 degrees centigrade in a large capacity boiler, which will reduce the flue gas temperature by about 150 to 170 degree centigrade. The boiler designers always keep the economizer water outlet temperature to about 25 to 35 degrees below the drum saturation temperature. This is done to mainly avoid steaming in the economizer. A steaming economizer generally is less reliable. As a rule of thumb, for every one degree pick up of economizer water temperature, there will be a drop of about 3 to 3.5 degrees.
Air pre heaters
Air pre-heaters are provided in boilers to preheat the combustion air. There are two main types: recuperative and regenerative air heaters.
Tubular or recuperative air pre-heaters are provided in boilers of medium and small range of steam generation. This type of air pre-heater becomes very large in size if they have to be used in very high capacity boilers like 600 tons/hr of steam production and above. In these cases regenerative air pre-heaters are used. The arrangement of all these air pre-heaters differ with the design and, in large, the way they are combined for very high capacity boilers. Regenerative air per-heaters are compact and can have a stationary or rotating hood. A combination of tubular and regenerative type of air pre-heaters is used in very high capacity boilers. The tubular being used for primary air heating and the regenerative used for the secondary air heating. In case the boiler designers do not want to go for a combination of tubular and regenerative air pre-heater, then they have a choice of tri-sector regenerative air heater. Normally the ambient air is heated to about 300 to 350 degree centigrade. This results in a flue gas temperature drop of around 230 to 250 degree centigrade. So for each degree pick up in air temperature, roughly 0.8 degree drop in flue gas temperature is achieved.
Steam coil air pre-heaters are another type. These are used only during start up of the boiler to prevent low temperature corrosion. This air heater does not contribute to improving the efficiency of boilers, but are provided to improve availability. It is seen that during start up the chances of low temperature corrosion is high, and hence the need to provide the steam coil air heaters is evident.
Both economizer and air pre-heaters are called heat recovery systems in a boiler. Were it not for these heat recovery systems, present day boilers would be operating at much lower efficiency levels.