7223 Reciprocating Engine Systems

Reciprocating engines include a variety of internally fired, piston driven engines. Their sizes range from 10 bhp to 50,000 bhp. According to Kovacik (1982), the largest unit supplied by a U.S. manufacturer is rated at 13,500 bhp. In larger plants, several units are used to accommodate part load and to provide redundancy and better availability.

In these engines, combustion heat rejected through the jacket water, lube oil and exhaust gases, can be recovered through heat exchangers to generate hot water and/or steam. Fig. 7.13 shows an internal combustion engine cogeneration system.

Exhaust gases have also been used directly. Reciprocating engines are classified by:

— the thermodynamic cycle: Diesel or Otto cycle.

— the rotation speed: high-speed (1200-1800 rpm) medium-speed (500-900 rpm) low speed (450 rpm or less)

— the aspiration type: naturally aspirated or tur-bocharged

— the operating cycle: two-cycle or four-cycle

— the fuel burned: fuel-oil fired, natural-gas fired.

Reciprocating engines are widely used to move vehicles, generators and a variety of shaft loads. Larger engines are associated to lower speeds, increased torque, and heavier duties. The total heat utilization of CHP systems based on gas-fired or fuel-oil fired engines approach 60-75%. Figure 7.12 shows the CHP balance vs. load of a diesel engine.

Example 5. Estimate the amount of 180 F water that can be produced by recovering heat; first from the jacket water and then from the exhaust of a 1200 kW diesel generator. In the average, the engine runs at a 75% load and the inlet water temperature is 70°F. The effectiveness of the jacket water heat exchanger is 90% and exhaust heat exchanger is 80%.

From Figure 7.12, at 75% load, the exhaust heat and the jacket water heat are 22% and 33%, respectively. Thus, the flow rate of water heated from 70 to 180°F

(1200 kW x 75%) [(90% x 22%) + (80% x 33%)] (3412 Btu/kW) (1 lb.°F/Btu)(1 lb/8.33 gal) (1 hr/60 min)/(180 - 70°F) 25.8 gal per minute.

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