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Engine Drive Systems: Considerations with Engine Applications - Waste Heat Recovery

Energy in the fuel supplied to an engine is released during combustion and is converted to shaft work and heat. The shaft work drives the compressor, while the heat is liberated from the engine through the coolant, exhaust gas and surface radiation. Approximately 70% of the total energy input is converted to heat. Much of this heat can be recovered from the engine exhaust and jacket coolant, while smaller amounts are also available from the lube oil cooler and the turbocharger's intercooler/aftercooler.

Effective recovery of this heat is an important consideration in fully utilizing the fuel-input energy to the engine. The heat can be used to generate steam or hot water, or it can be used directly in certain industrial applications. Recovered heat is typically used for space heating, domestic or process water heating, absorption chilling, or desiccant regeneration.

Heat recovered from the engine jacket accounts for up to 30% of the energy input and is capable of producing 200°F hot water. Some engines, such as those with ebullient cooling systems, can operate with water jacket temperatures up to 265°F. Almost all of the heat transferred to the engine coolant can be recovered, limited only by the efficiency of the heat exchanger, assuming there is a demand for the heat.

The other major source of heat is the engine exhaust. Exhaust temperatures of 850°-1200°F are typical. Only a portion of the exhaust heat can be recovered, since exhaust gas temperatures are generally kept above condensation thresholds. Most heat recovery units are designed for a 300°-350°F exhaust outlet temperature, to avoid the corrosive effects of condensation in the exhaust piping. Exhaust gas heat recovery is typically used to generate hot water up to about 230°F or low-pressure steam (15 psig).

By recovering heat in the jacket water and exhaust, approximately 75-80% of the fuel's energy can be effectively utilized.