Heat exchanger for anaerobic digestion

  1. I have a problem whereby sludge of quantity 1000 kg/day is required to fill an anaerobic digester tank. The sludge enters at 20deg and is heated to 35deg. With refernece to this link http://books.google.co.uk/books?id=DwMWvTki7h8C&pg=PA197&dq=heat+required+to+heat+sludge+anaerobic+digestion
    it can be seen that if I have a quantity of 1000kg/day then the formula shown can be used to calculate the heat energy required.

    My problem is understanding the difference between this value of heat energy and the value Q for heat transfer in a heat exchanger. As Q = m*cp*(T2-T1) if I have a set flow velocity through the exchanger pipe of 1 m/sec and a pipe diameter my mass flow rate m works out greater than the 1000kg/day and thus my value of Q heat required is greater.

    MAIN QUESTION >>>>
    Does this mean that the mass flow rate through the exchanger dictates how much how much heat energy I require and not the amount suggested by the equation in the book link which does not take into account pipe dimensions and flow velocities??



    Thanks for the help
     
  2. jcsd
  3. In this equation you have an inlet and exit temperature (T1 and T2). If you changed the pipe dimensions and velocities you would likely get a different exit temperature. If you were designing this system you would not know the exit temperature and your your equation for T2 would include pipe dimensions and velocities.
     
  4. Redbelly98

    Redbelly98 12,039
    Staff Emeritus
    Science Advisor
    Homework Helper

    Both the mass flow rate AND the equation dictate the required heat energy.

    The equation gives the heat required in terms of the sludge mass. The mass flow rate tells you what the mass of the sludge (per day) is.
     
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