The storage capacity of the gas holder should provide for a detention time of 0.25 to 0.50 days. The normal gas production is 0.9 to 1.0 ft3 per capita of population per day when mixed primary and activated sludges are digested. Gas production is also estimated as 15 ft3 per lb VSS destroyed. The Btu value of digester gases is generally 640 to 703 Btu/ft3.
The following equation gives the digester heat loss:
Q = heat loss, Btu/hr U = Overall coefficient of heat transfer,
Btu/hr/ft2/°F (see Table 7.45.1). A = Area (ft2) normal to direction of heat flow T2 = Digestion temperature, °F
T1 = Average external temperature for coldest two-week period, °F
Alternatively, the heat loss is 1°F per day for the entire digester contents, or 2600 Btu/hr/1000 ft3 of tank contents in the northern United States. For the southern United States, half of this rate can be assumed.
The following equation gives the sludge heat requirement:
Heat required to raise sludge to the digestion temperature, Btu per day W = Sludge added to the digester daily, lb C = Mean specific heat of sludge, «1.0, Btu/lbm-°F T2 = Digestion temperature, °F
T1 = Sludge temperature, coldest two-week period, °F
In the absence of other information, the sludge temperature can be assumed to be 50°F in the southern United States, 45°F in the central parts of the United States, and 40°F in the northern United States.
The following equation gives the length of the external, jacketed, sludge pipe, heat exchanger:
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