Economizers are used as heat recovery equipment in packaged boilers instead of air heaters because of NOx concerns as discussed in Chapter 4. They are also less expensive and have lower gas pressure drops across them. Economizers for gas firing typically use serrated fins at four to five fins per inch. For distillate fuel, about 4 fins/in, solid fins are preferred. For heavy oil, bare tubes or a maximum of 2-3 fins/in. are used, depending upon the dirtiness of the flue gas and the ash content of the fuel.
Economizers are generally of vertical gas flow and counterflow configuration with horizontal tubes as shown in Fig. 3.23. The water-side velocity ranges from 3 to 7 ft/s. Small packaged boilers, below 40,000 lb/h capacity, use circular economizers that can be fitted into the stack. Another variation is the horizontal gas flow configuration with vertical headers and horizontal tubes.
Generally, steaming in the economizer is not a concern, as discussed earlier. Feedwater temperatures of 230-320°F are common, depending on acid dew point concerns. The feedwater is sometimes preheated in a steam-water exchanger if the deaerator delivers a lower feedwater temperature than that desired to avoid acid corrosion in the case of oil-fired boilers.