water passageways are cast in a spiral. Heat transfer coefficients for external heat exchangers range from 0.85 to 1.6 kJ/m2-°C-s (150 to 280 Btu/ft2-°F-hr).
Hot water used in a heat exchanger is most commonly generated in a boiler fueled by sludge gas. Provisions for burning an auxiliary fuel such as natural gas, propane, or fuel oil must be included to maintain heating during periods of low digester gas production or high heating demand, such as for digester startup. If a cogeneration system is used, waste heat from sludge gas-powered engines used to generate electricity or directly to drive pumps or blowers is sufficient to meet digester requirements.
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