Page 252 - Boiler plant and distribution system optimization manual

Waste Heat Recovery 237

Figure 12.10—Enthalpy of combustion products during condensation, as a function of temperature
and excess air for natural gas.

Teflon coated heat exchangers are capable of cleaning solvent vapors, so storage of chloride
raising water temperature to 200 to 250°F. containing material near the boiler combustion
Another less common heat exchanger uses air inlets could cause problems.
stainless steel tubes. Stainless steel heat exchang- Metals have a very wide range of corrosion
ers were installed in 25 hospitals with gas fired resistance. As sulfuric and sulfurous acid is the
(very low sulfur fuel) boilers, and many of these most likely attack to be encountered, material
heat exchangers have operated trouble free for selection must take this into account. Stainless
more than 8 years. This type 304 stainless steel steel, for example does not stand up well under
has been proven to be a durable material in the this attack. Carbon steel or “open hearth” steel is
stack gas environment for natural gas boilers. The most commonly used for economizer construc-
presence of chlorides can cause metal failure due tion, giving long and reliable service. It has a
to stress corrosion however. A common source is corrosion rate superior to many more expensive