Air Heaters

Air Heaters

Air heaters are used in steam generators when low BTU fuels are fired or in a few waste heat boilers for preheating combustion air. Incineration plants and reformer furnaces also use preheated air. Decades ago, they were used in boilers firing oil, gas, and solid fuels. However, with NO x emission limitations throughout the world, as discussed in Chapter 1, they are used only if the fuel combustion process warrants it. If the gaseous fuel has a low heating value or if the solid fuel has significant amount of moisture, then hot air is required for drying the fuel and also to ensure combustion with a stable flame. A gas to gas heater, which is similar to the tubular air heater (Figure 4.41), is also used in incineration plants.

Air heaters can be of tubular type or regenerative or heat pipes (Figure 4.41). In the tubu- lar air heater, air or flue gas could flow inside the tubes. If the flue gas contains dust or ash particles, it is preferable to make the flue gas flow inside the tubes so that cleaning is easier. The air then takes a multipass route outside the tubes.

One of the concerns with air heaters is low-temperature corrosion at the cold end. The tube wall temperature at the cold end falls below the acid dew point temperature of the flue gas if the inlet air temperature is low or during part loads when both the air and flue gas flows are reduced. Steam is often used to preheat air to mitigate this concern and raise the air inlet temperature, which in turn raises the flue gas exit temperature and thus the average tube wall temperature. Some boiler suppliers use corten steel tubes to minimize corrosion concerns.

There are two types of regenerative air heaters, one in which the heater matrix rotates (Ljungstrom) and the other in which the connecting air and flue gas duct rotate (Rothemuelle). The energy from the hot flue gas is transferred to the slowly rotating matrix (1–5 rpm) made of enamel or alloy steel material, which absorbs the heat and transfers the energy to the cold air as it rotates. The elements are contained in baskets, which makes

Waste Heat Boilers 251

Seal sector

Gas out

Gas sector

Air sector

(a) Tubular and (b) regenerative air heaters. cleaning or replacement easier. The air- and gas-side pressure drops are high in both types

of air heaters, adding to the fan power consumption. Due to the low air- and gas-side heat transfer coefficients and low overall U and the low LMTD, the surface area required is large. However, in the case of Ljungstrom air heater, a lot of surface area can be packed into each basket and hence can be made compact while the tubular air heater (where finned tubes are not effective as discussed in Appendix E) will be huge often as large as the steam generator itself.

One of the problems with regenerative-type air heaters is the leakage (from 5% to 10%) from air to flue gas side (air is often at higher pressure), and this affects the performance even with good design of seals.

In case of low-temperature corrosion, the cold-end baskets can be made of alloy or corten material, while in the case of tubular type, the entire tube has to be replaced.