0068

7. Total primary digester heating requirement:

Tank

Sludge

Total

236,001 +556,213 792,214 Btu/hr

For digester heating, a gas-fired heat exchanger can use the digester gas as fuel. The digester gas is assumed to have a heating value of 650 Btu per ft3. Therefore, the excess heat can be calculated as:

76,936,600 Btu/day 3,205,692 Btu/hour

3,205,692 - 792,214 = 2,413,478 Btu/hr excess heat is available for sludge incineration, building heat, and so forth.

2. Heat exchanger length: In this example, an external heat exchanger with 4-in sludge tubes jacketed by 6-in water tubes is used. Water enters the exchanger at 145°F and exits at 100°F. The sludge enters at 80°F and exits at 95°F. An overall heat transfer coefficient of 140 can be assumed. Equation 7.45(4) calculates the required heat exchanger length is calculated as follows:

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