Return to Economizer Information Main Page One method for economizer heat recovery on firetube boilers has been with cylindrical, or coiled type economizers. There are many serious concerns regarding the proper selection and operation of coiled type economizers. Any economizer selected should be of a rectangular configuration.
The first concern is in reference to the actuated flue gas bypass arrangment required on a coiled type unit. The problem is that the damper arrangment does not provide a physical seperation between the gas bypass channel and the internal tube bundle. If the flue gas is not physically separated from the coil, some gas will inevitably pass over the tubes. This could actually lead to accelerated corrosion as the bulk of the hot flue gas will bypass the coil leaving relatively low temperature tubes unprotected. In a low fire or cycling operation, flue gas could begin to condense on the tube wall which would lead to a premature failure of the economizer. These units are also not repairable in place. these coiled units tend to be inexpensive up front, however since they cannot be repaired in place without significant time and expense, they are considered to be disposable units.
The aforementioned bypass arrangment is proposed to be a benefit. In reality the only reason that the damper is there is to act as a flue gas plug to force gas over and around the tube bundle. If not for this damper the gas would bypass the tube bundle and no energy recovery would be recognized. It has been inferred that bypassing flue gas has an advantage of corrosion control. One point must be established, an economizer is installed to recovery energy. Bypassing energy defeats the purpose.
The correct answer for an economizer installation is to select a unit with a rectangular tube configuration. These tubes must also be individually and fully removable in place to facilitate easy maintanance. No welding or torching, nor should any A.S.M.E. inspection or hydrotest be required for the replacement of tube elements. The materials of construction should also be capable of withstanding possible corrosive environments. As a minimum, all of the internal tube supports and casing in contact with the flue gas should be manufactured out of stainless steel. The heat exchanger bundle should also have the option of being manufactured out of stainless or other alloy tubes suitable for the application.
Economizer condensation and also tube failures are a reality of any economizer installation. An economizer should also be designed with internal provisions for collecting and draining away this water before it has a chance to enter the boiler and cause serious damage. The entire stack opening must be covered as a minimum to protect against water damage inside of the boiler.
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