Page 195 - Handbook of Energy Engineering Calculations
P. 195

of 25° API fuel for a 900-kWh output; mechanical efficiency, 82.0 percent;
               generator  efficiency,  92.0  percent.  Compute  engine  fuel  rate,  engine-
               generator fuel rate, indicated thermal efficiency, overall thermal efficiency,
               brake thermal efficiency.


               Calculation Procedure:


               1. Compute the engine fuel rate

               The fuel rate of an engine driving a generator is the weight of fuel, lb, used to
               generate 1 kWh at the generator input shaft. Since this engine burns 1.5 bbl
               (238.5  L)  of  fuel  for  900  kW  at  the  generator  terminals,  the  total  fuel

               consumption  is  (1.5  bbl)(42  gal/bbl)  =  63  gal  (238.5  L),  at  a  generator
               efficiency of 92.0 percent.
                  To determine the weight of this oil, compute its specific gravity s from s =
               141.5/(131.5  +  °API),  where°API  =  API  gravity  of  the  fuel.  Hence,  s  =
               141.5(131.5 + 25) = 0.904. Since 1 gal (3.8 L) of water weighs 8.33 lb (3.8

               kg) at 60°F (15.6°C), 1 gal (3.8 L) of this oil weighs (0.904)(8.33) = 7.529 lb
               (3.39 kg). The total weight of fuel used when burning 63 gal is (63 gal)(7.529
               lb/gal) = 474.5 lb (213.5 kg).

                  The generator is 92 percent efficient. Hence, the engine actually delivers
               enough power to generate 900/0.92 = 977 kWh at the generator terminals.
               Thus, the engine fuel rate = 474.5-lb fuel/977 kWh = 0.485 lb/kWh (0.218
               kg/kWh).


               2. Compute the engine-generator fuel rate

               The engine-generator fuel rate takes these two units into consideration and is
               the  weight  of  fuel  required  to  generate  1  kWh  at  the  generator  terminals.
               Using the fuel-consumption data from step 1 and the given output of 900 kW,
               we  see  that  engine-generator  fuel  rate  =  474.5-lb  fuel/900  kWh  output  =

               0.527 lb/kWh (0.237 kg/kWh).


               3. Compute the indicated thermal efficiency
               Indicated thermal efficiency is the thermal efficiency based on the indicated
               horsepower  of  the  engine.  This  is  the  horsepower  developed  in  the  engine
               cylinder. The engine fuel rate, computed in step 1, is the fuel consumed to

               produce  the  brake  or  shaft  horsepower  output,  after  friction  losses  are
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