TM 5-814-3/AFM 88-11, Volume III
(3) Using a circular tank 20 feet deep and 26 feet in diameter, determine the heat requirement. Given:
Heat Transfer Coefficient
Temperature
40EF
Walls = 0.14
(air)
50EF
Floor = 0.12
(ground)
40EF
Roof = 0.16
(air)
Area
Walls = (26)(20) = 1,634 sq ft;
Floor = (13)2 = 531 sq ft;
Roof = (13)2 = 531 sq ft.
Heat Loss
Walls = (0.14)(1,634)(80 - 40) = 9,150 Btu/hr;
Floor = (0.12)(530)(80 - 50) = 1,908 Btu/hr;
Roof = (0.16)(530)(80 - 40) = 3,392 Btu/hr;
Total Heat Loss = 14,450 Btu/hr = 346,800 Btu/day.
(1,200 lb/day)
(80 & 45)EF ' 1,400,000 Btu/day.
Sludge requirement =
.03
Total heat requirement = 346,800 + 1,400,000 Btu/day = 1,746,800 Btu/day.
(4) Determine heat supplied by utilizing sludge gas, assuming a heat value of 600 Btu/cu ft;
Heat from sludge gas = (8,100 cu ft/day)(600 Btu/cu Ft) = 4,860,000 Btu/day.
C-15. Aerobic sludge digestion. (Refer to para 16-6a.)
a. Design requirements and criteria. Determine digester volume and air requirements for the aerobic
digestion of activated sludge after thickening. Assume the following conditions apply:
Sludge quantity = 1,200 lb/day;
Sludge concentration = 3 percent;
Detention time = 20 days;
Air supply requirement = 30 cfm/l,000 cu ft digester volume.
b. Calculations and results.
(1) Determine digester volume:
1,200 lb/day
1 cu ft
' 4,796 gal day '4,796
' 641 cu ft/day.
Sludge volume '
8.34 (0.03)
7.48 gal
Digester volume = 641 cu ft/day 20 day = 12,820 cu ft.
(2) Determine air requirement:
Air required = (12,820 cu ft)(30 cfm/1,000 cu ft) = 384.6 cfm, use 385 cfm.
(This also satisfies mixing requirements.)
C-16. Sludge pumping. (Refer to para 16-2.)
sludge from a settling tank to a thickener. Assume the following conditions apply:
Sludge is pumped at 6 fps;
150 ft of 8" pipe is used;
Thickener is 10 ft above settling tank (elev diff = 10*);
Sludge specific gravity = 1.02;
Sludge moisture content = 95 percent;
Pressure at inlet side of pumps = 3 psi;
Coefficient of friction = 0.01.
C-38
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