Reply To: Porta's Derivation of Lempor Theory

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Martin Johnson

    Yes Chris, all those “losses” need to be subracted off the exhaust steam mass flow.

    And then there is the combustion gas flow, for which Porta does give some suggested “Design Allowances”. There is also the following to consider:
    1) The air to coal ratio increases significantly at lower grate loadings, thus increasing the combustion gas to steam ratio. Question – does the flat out condition actually represent the worst design point?
    2) Similarly, the resistance through ashpan, grate, firebed, tubes, cinder catchers (if fitted) etc. varies broadly as the square of the gas flow. However, the firebed thickness also tends to vary with grate loading so that changes more as a linear relation(???).
    3) A significant proportion of the coal goes up the chimney unburnt – especially at high grate loadings. Moving that mass of unburnt coal eats up momentum from the steam blast. We are effectively pumping a two phase fluid / solid mix with a density over that of combustion gas. Should that represent the design point?
    4) Then the system needs to be controllable – so there must be allowance for a pressure drop across ashpan dampers – even at flat out condition.

    As to the real importance of those factors (and others, no doubt), I don’t know until I start doing some sums.