Compressed Air Homework
All problems refer to a single plant visit. For this plant, the cost of electricity is $0.10 /kWh including demand. Compressed air to the plant is supplied by a 75-hp air-cooled rotory-screw compressor with a 93% efficiency motor that generates 4 scfm/hp. The compressor runs in load/unload mode and draws 60% of full load power when unloaded. The plant and compressor run 5,000 hr/yr.
1)A production line uses 10 copper tubes with nominal diameters of 1/8-inch and inside diameters of 0.125 inch to blow moisture off a product. Air-saver nozzles that consume 10 scfm of compressed air are found to perform just as well as the open tubes. Calculate the annual cost savings from installing air-saver nozzles on the open tubes. Assume the compressed air pressure is 100 psig at the tubes and ambient pressure is 15 psia.
2)Six 1/16-inch air leaks are identified in the compressed air system. Calculate the annual cost savings from fixing these leaks. Assume the compressed air pressure is 100 psig at the leaks and ambient pressure is 15 psia.
3)The average current draw of the three-phase compressor operating at 480 V is 88 A per leg when loaded and 53 A per leg when unloaded. The power factor remains constant at 84%. The compressor control panel reports a total runtime of 10,832 hours and loaded runtime of 4,126 hours. The temperature outside air temperature is 35 F and the temperature in the compressor room is 55 F. The average annual outdoor air temperature in Dayton is 50 F. Assuming that the temperature difference between the compressor room and outside air stays constant, calculate the annual cost savings from compressing outside air instead of inside air.
4)The plant is heated by 80% efficient natural-gas unit heaters. Natural gas costs $7 /mmBtu. Heat rejected from the compressor is currently trapped in the compressor room and does not contribute to plant heating. Assuming that 75% of the power input to the compressor could be utilized for space heating for 2,500 hours per year, calculate the annual cost savings from installing a duct that would direct the warm air from the compressor into the plant.
5)The compressor loads when the air pressure reaches 110 psig and unloads when the pressure reaches 120 psig. The highest pressure required in the plant is 90 psig. Calculate the annual cost savings from reducing the load/unload pressures by 10 psig. Assume ambient pressure is 15 psia.
6)In addition to the 75-hp compressor, the company also owns a 50-hp reciprocating compressor that meets variable loads by turning on and off. Could the 50-hp reciprocating compressor carry the load? If so, calculate the annual cost savings from operating the 50-hp recip instead of the 75-hp rotary-screw. Assume the efficiency of the 50-hp motor is 93%.
7)(Required for Graduate Students Only) Download and install AirSim. Develop a baseline model for the following compressor:
Rated power = 75-hp
Motor efficiency = 93%
Specific capacity = 4 scfm/hp
Power factor = 84%
Storage = 500 gal
Control = Load/unload with 30 sec blowdown
Autoshutoff enabled with 5 min delay
Pressure load/unload set points = 110-120 psig
Fraction full load power when unloaded = 60%
Fraction rate power when loaded = 100%
Plant air demand = 100 cfm
Simulation period = 5 minutes
a)Determine baseline compressor power (kW)
b)Determine power savings (kW) if plant air demand is reduced by 20 cfm
c)Determine power savings (kW) if storage is increased to 800 gal
d)Determine power savings (kW) if pressure setpoints are each reduced by 10 psi
e)Determine the sum of power savings (kW) from b, c, and d.
f)Determine power savings if b, c and d were implemented simultaneously.
Compressed Air Homework1