Primary Digester Heating Requirements

For the purposes of this example, a digestion temperature of 95°F and a plant location in the northern U.S. where the lowest (two-week mean) daily temperature is 10°F are used. All digester sidewall areas are earth-covered, with 75% of the areas covered by dry earth and the remaining 25%, as well as the floor area, in groundwater. The mean winter ground temperature is 25°F and the groundwater temperature is 40°F. Equation 7.45(2) calculates the heat losses for both primary digesters as follows:

1. Fixed cover (concrete): Q = (^)(r2)(2)(U)(T2 — T1)

2. Dry-earth-covered sidewall area:

Q = 0.75(3.1416[48][15][2][0.18][95 — 25]) = 42,751 Btu/hr

3. Wet sidewall area:

Q = 0.25(3.1416[48][15][2][0.25][95 — 40]) = 15,551 Btu/hr

4. Floor area:

Q = 3.1416(24)2(2)(0.12)(95 — 40) = 23,886 Btu/hr

5. Total primary digester area heat loss:

153,813 42,751 15,551 +23,886

Alternatively, the approximate digester heat loss in the northern United States can be computed as 2600 Btu/ hr/1000 ft3 of digester volume as follows:

This estimate, however, must be modified for the prevailing conditions, actual winter temperatures, or the presence of groundwater, e.g., at the plant site. 6. Sludge heating requirement using Equation 7.45(3): (The sludge temperature as it enters the primary digester is 50°F for this example.)

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