Thermodynamics

Chapter 7 | 399much money will be saved as a result of reducing the pressure of thecompressed air.Solution It is observed that the compressor of a facility compresses the airto much higher pressures than needed. The cost savings associated withpressure reduction of the compressor are to be determined.Assumptions 1 Air is an ideal gas. 2 Compression process is isentropic, andthus n k 1.4.Analysis The fraction of energy saved as a result of reducing the pressuresetting of the compressor isf reduction 1 1P 2,reduced>P 1 2 1n12>n 11P 2 >P 1 2 1n12>n 1 1 1885.6>85.6211.412>1.4 11985.6>85.62 11.412>1.4 1 0.060That is, reducing the pressure setting will reduce the energy consumed by thecompressor by about 6 percent. Then,Cost savings 1Current cost2f reduction 1$12,000>yr2 10.062 $720/yrTherefore, reducing the pressure setting by 100 kPa will result in annualsavings of $720 in this case.There are also other ways to reduce the cost of compressed air in industrialfacilities. An obvious way is turning the compressor off during nonproductionperiods such as lunch hours, nights, and even weekends. A considerableamount of power can be wasted during this stand-by mode. This is especiallythe case for screw-type compressors since they consume up to 85 percent oftheir rated power in this mode. The reciprocating compressors are notimmune from this deficiency, however, since they also must cycle on and offbecause of the air leaks present in the compressed-air lines. The system canbe shut down manually during nonproduction periods to save energy, butinstalling a timer (with manual override) is preferred to do this automaticallysince it is human nature to put things off when the benefits are not obviousor immediate.The compressed air is sometimes cooled considerably below its dewpoint in refrigerated dryers in order to condense and remove a large fractionof the water vapor in the air as well as other noncondensable gases such asoil vapors. The temperature of air rises considerably as it is compressed,sometimes exceeding 250°C at compressor exit when compressed adiabaticallyto just 700 kPa. Therefore, it is desirable to cool air after compressionin order to minimize the amount of power consumed by the refrigeration system,just as it is desirable to let the hot food in a pan cool to the ambienttemperature before putting it into the refrigerator. The cooling can be doneby either ambient air or water, and the heat picked up by the cooling mediumcan be used for space heating, feedwater heating, or process-related heating.Compressors are commonly cooled directly by air or by circulating a liquidsuch as oil or water through them in order to minimize the power consumption.The heat picked up by the oil or water is usually rejected to theCooling liquidfrom compressorDamper(winter mode)AirLiquid-to-airheat exchangerHeatedairOutsideInsidefacilityDamper(summer mode)FIGURE 7–80Waste heat from a compressor can beused to heat a building in winter.