An office room is currently illuminated by ten 75-watt incandescent lamps, each having a luminous efficacy...

 

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An office room is currently illuminated by ten 75-watt incandescent lamps, each having a luminous efficacy of 15 lumens/watt. A proposal has been submitted to replace the incan- descent lighting with a sufficient number of fluorescent lamps to provide the same light flux in the room. The fluorescent lamps are 40 watts and have a luminous efficacy of 80 lumens/watt. (a) Determine the number of fluorescent lamps that would provide the same or slightly greater illuminance as the incandescent lights. (b) Compare the daily electric power costs of the two lighting systems if they operated 12 hours per day and electrical power cost is 7 cents per kW-hr. An office room is currently illuminated by ten 75-watt incandescent lamps, each having a luminous efficacy of 15 lumens/watt. A proposal has been submitted to replace the incan- descent lighting with a sufficient number of fluorescent lamps to provide the same light flux in the room. The fluorescent lamps are 40 watts and have a luminous efficacy of 80 lumens/watt. (a) Determine the number of fluorescent lamps that would provide the same or slightly greater illuminance as the incandescent lights. (b) Compare the daily electric power costs of the two lighting systems if they operated 12 hours per day and electrical power cost is 7 cents per kW-hr.