Heat Exchanger for Anaerobic Digestion Process

cabellos2

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=D...obic+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