Besides an economiser ( Section 3.2.5), an air preheater (air-air heat exchanger) can also be installed. The air preheater or APH heats the air which flows to the burner. This means flue- gases can be cooled down even more, as the air is often at ambient temperature. A higher air temperature improves combustion, and the general efficiency of the boiler will increase. In general for every decrease of 20 °C in flue-gas temperature, a 1 % increase in efficiency can be achieved. A scheme of a combustion system with an air preheater is shown in Figure 3.2.
A less efficient but simpler way of preheating might be to install the air intake of the burner on the ceiling of the boilerhouse. Generally, the air here is often 10 to 20 °C warmer compared to the outdoor temperature. This might compensate in part for efficiency losses.
Another solution is to draw air for the burner via a double walled exhaust pipe. Flue-gases exit the boiler room via the inner pipe, and air for the burner is drawn via the second layer. This can preheat the air via losses from the flue-gases.
Alternatively, an air-water heat exchanger can be installed.
一個較為簡單但效果沒那麼好的空氣預熱方法，將燃燒器空氣的進氣口設在鍋爐房的上方， 通常那裡空氣會比室外溫度高10~20 °C，這方式也可補償部分效率的損失。
There are, however, also some practical disadvantages related to an APH, which often inhibit installation:
●the APH is a gas-gas heat exchanger, and thus takes up a lot of space. The heat exchange is also not as efficient as a gas-water exchange
●a higher drop pressure of the flue-gases means the ventilator of the burner has to provide higher pressure
●the burner must ensure that the system is fed with preheated air. Heated air uses up more volume. This also poses a bigger problem for flame stability
●there may be higher emissions of NOx due to higher flame temperatures.
The installation of an air preheater is cost effective for a new boiler. The change in air supply or the installation of the APH often is limited due to technical reasons or fire safety. The fitting of an APH in an existing boiler is often too complex and has a limited efficiency.
Air preheaters are gas-gas heat exchangers, whose designs depend on the range of temperatures. Air preheating is not possible for natural draught burners.
Preheated water can be used as boiler feed or in hot water systems (such as district schemes).
In practice, the possible savings from combustion air preheating amount to several per cent of the steam volume generated, as shown in Table 3.4. Therefore, the energy savings even in small boilers can be in the range of several GWh per year. For example, with a 15 MW boiler, savings of roughly 2 GWh/yr, some EUR 30 000/yr and about 400 t CO2/yr can be attained.
Energy Efficiency (2009) 184.108.40.206