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Once Through and Drum Type Boiler Designs Compared

written by: Dr V T Sathyanathan • edited by: Lamar Stonecypher • updated: 10/20/2010

As carbon emission is a major concern today, more super critical pressure units are bound to be preferred due to the increase in plant cycle efficiency. This will make once through type boilers take over from drum type boilers. Fuel conservation is an important factor for energy security presently.

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    Drum Type and Once-through Boilers

    The major proportion of boilers operating in the world today are drum type boilers. These boilers have certain restrictions during start-up due to the presence of a high thickness component- the drum. The once through design mainly avoids this, along with a few more advantages. The concept of once through boilers dates back to 1824, referenced through patents in the United States. It was in 1923 that the first commercial 4 tons/hr once-through boiler was made by Mark Benson, a Czechoslovakian, and supplied to English Electric Company Ltd at Rugby, England.

    When we try to analyze two types of boiler design which can cater to the same requirement, it is necessary to look at certain specific factors. These key factors generally include:

    1. How the boiler will respond to load changes
    2. How the efficiency will change
    3. How the auxiliary power consumption will vary in each design
    4. The availability of additional systems or equipment
    5. How the control system for each will vary
    6. What are the water chemistry requirements?
    7. Suitability for cyclic and two shift operation
    8. The operation and maintenance aspects of these designs
    9. The cycle time needed for each design, and
    10. The overall economics of each

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    Once-through Boiler Characteristics

    • The once through boiler has high load response characteristics due to the fact that it does not have a drum and has a much lower water inventory.
    • In the once through boiler, many times the load change response is dictated by the firing system and its controls rather than the boiler, per-say.
    • Once through boilers of super-critical pressure boilers have higher efficiency. However in the sub-critical range there is no difference in efficiency when compared to the drum type design.
    • Generally the power consumption is higher by 5 to 8 % for the same capacity boilers of drum type.
    • Once through boilers have a separate start-up loop along with all its controls.
    • As the load demand is met by varying both fuel and feed water flow simultaneously, the controls are more sophisticated and have to be more reliable. More redundancies are built in.
    • The water quality level is much more stringent than drum type boilers. Normally a condensate polishing unit is employed in once through units.
    • In once through boilers the superheater headers are subjected to both fatigue and creep when cyclic or two shift operation is resorted to. Hence these boilers are more preferred for base load operation. However, the load change rate that theses boilers can take is higher due to the absence of the drum.
    • A closer regime of operation is expected in once through boilers.
    • The absence of the drum makes it possible to reduce the overall cycle time for the once through boiler. However, the overall plant cycle time may not vary only marginally.
    • Once through boilers life time cost is expected to be more than the drum type units.
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    Drum Type Boiler Characteristics

    • Cold start-up takes much more time in drum type units as the metal temperature difference across the thickness dictates this.
    • Drum type units do not find application in super-critical pressure power plants.
    • The drum type boiler is more adaptable to cyclic and two shifting operation.
    • The water chemistry is maintained within a band and can accommodate some variations when compared to once through type boilers.
    • The control system is more simplified when compared to the once through type as load variations are done by fuel control and feed water is controlled by drum level.

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