Contemporary Engineering Economics 6th Edition by Chan Park

تمرين 58

Birmingham Steel, Inc. is considering whether to buy conventional 25-HP, 230-V, 60-Hz, 1800-rpm induction motors in its plant or modem premium efficiency (PE) motors with the same specifications. Both types of motor have a power output of 18.650 kW per motor (25 HP × 0.746 kW/HP). Conventional motors have a published efficiency of 89.5%, while the PE motors are 93% efficient. The initial cost of the conventional motors is $ 14,000, whereas the initial cost of the proposed PE motors is $ 16,600. The motors are operated 12 hours per day, 5 days per week, 52 weeks per year, with a local utility cost of $0.08 per kilowatt-hour (kWh). The motors are to be operated at a 75% load, and the lifecycle of both the conventional and PE motors is 20 years with no appreciable salvage value.
(a) At an interest rate of 12%, what are the savings per kWh achieved by switching from the conventional motors to the PE motors
(b) At what operating hours are the two motors equally economical