Heat Recovery From Boiler Stack-Gases
Waste heat energy in any stack gas consists of:
1. The sensible heat going out the stack with the flue gases, other than water vapor, and
2. The latent and the sensible heat in the water vapor.
Steam temperatures corresponding to the pressure levels used in most of our boilers are shown in Figure 3200-2.
The temperature of the flue gases leaving the boiler is related to the temperature of the boiling water. For heat to transfer to the steam generating surfaces, the flue gases have to be at a still higher temperature. For typical boilers, this is in the 600°F to 800°F range.
An economizer or an air preheater recover heat from these high temperature flue gases in a steam boiler.
For industrial boilers, a dual installation using both an economizer and an air preheater is rarely economical or installed.
An economizer takes boiler feedwater (BFW), on its way to the boiler at 225°F to 275°F, and lets it absorb much of the heat in the hot flue gases with substantial improvement to overall efficiency.
An air preheater recovers the heat from flue gases and increases the temperature of the combustion air to the boiler. The air preheater improves the boiler efficiency by reducing the stack gas temperature and returning the heat to the combustion air, thereby reducing fuel consumption.
Depending on the cost of fuel, an economizer or air preheater for water-tube boilers are typically not attractive for the following conditions:
• Water-tube boilers operating under 150 PSIG
• Water tube boilers operating below 30,000 to 40,000 pounds/hour of steam production
• Any size boiler that will normally run at reduced capacity
For a preliminary evaluation, the following investments, based on 1983 costs (EDPI) can be used to estimate a payout:
• New Economizer, Convection Section:
$ = 6000 ´ (Q MMBH)0.9
• Combustion Air Preheater:
$ = 53,000 ´ (Q MMBH)0.7