Thermodynamics

392 | ThermodynamicsCompressor: 125 hp = 93.21 kWOperating hours: 6000 h/yrUnit cost of electricity: $0.085/kWhMotor efficiency: 0.90Annual energy usage: 621,417 kWhAnnual electricity cost: $52,820/yrFIGURE 7–72The cost of electricity to operate acompressor for one year can exceedthe purchase price of the compressor.The hissing of air leaks can sometimes be heard even in high-noise manufacturingfacilities. Pressure drops at end-use points in the order of 40 percentof the compressor-discharged pressure are not uncommon. Yet acommon response to such a problem is the installation of a larger compressorinstead of checking the system and finding out what the problem is. Thelatter corrective action is usually taken only after the larger compressor alsofails to eliminate the problem. The energy wasted in compressed-air systemsbecause of poor installation and maintenance can account for up to 50 percentof the energy consumed by the compressor, and about half of thisamount can be saved by simple measures.The cost of electricity to operate a compressor for one year can exceed thepurchase price of the compressor. This is especially the case for larger compressorsoperating two or three shifts. For example, operating a 125-hp compressorpowered by a 90-percent efficient electric motor at full load for 6000hours a year at $0.085/kWh will cost $52,820 a year in electricity cost, whichgreatly exceeds the purchase and installation cost of a typical unit (Fig. 7–72).Below we describe some procedures to reduce the cost of compressed airin industrial facilities and quantify the energy and cost savings associatedwith them. Once the compressor power wasted is determined, the annualenergy (usually electricity) and cost savings can be determined fromandEnergy savings 1Power saved2 1Operating hours2>h motorCost savings 1Energy savings21Unit cost of energy2(7–87)(7–88)where h motor is the efficiency of the motor driving the compressor and theunit cost of energy is usually expressed in dollars per kilowatt hour (1 kWh 3600 kJ).CompressedairJointAir leakFIGURE 7–73Air leaks commonly occur at jointsand connections.1 Repairing Air Leaks on Compressed-Air LinesAir leaks are the greatest single cause of energy loss in manufacturing facilitiesassociated with compressed-air systems. It takes energy to compress theair, and thus the loss of compressed air is a loss of energy for the facility. Acompressor must work harder and longer to make up for the lost air andmust use more energy in the process. Several studies at plants have revealedthat up to 40 percent of the compressed air is lost through leaks. Eliminatingthe air leaks totally is impractical, and a leakage rate of 10 percent is consideredacceptable.Air leaks, in general, occur at the joints, flange connections, elbows,reducing bushes, sudden expansions, valve systems, filters, hoses, check valves,relief valves, extensions, and the equipment connected to the compressed-airlines (Fig. 7–73). Expansion and contraction as a result of thermal cyclingand vibration are common causes of loosening at the joints, and thus airleaks. Therefore, it is a good practice to check the joints for tightness and totighten them periodically. Air leaks also commonly occur at the points of enduse or where the compressed-air lines are connected to the equipment thatoperates on compressed air. Because of the frequent opening and closing ofthe compressed-air lines at these points, the gaskets wear out quickly, andthey need to be replaced periodically.