Thermodynamics

398 | ThermodynamicsOutsideairWallAir intake ductAir filterCompressorFIGURE 7–78The power consumption of acompressor can be reduced by takingin air from the outside.Air inlet85.6 kPanormally drawn from inside the building or the shelter. However, in manylocations the air temperature in the building is higher than the outside airtemperature, because of space heaters in the winter and the heat given up bya large number of mechanical and electrical equipment as well as the furnacesyear round. The temperature rise in the shelter is also due to the heatdissipation from the compressor and its motor. The outside air is generallycooler and thus denser than the air in the compressor room even on hotsummer days. Therefore, it is advisable to install an intake duct to the compressorinlet so that the air is supplied directly from the outside of the buildinginstead of the inside, as shown in Fig. 7–78. This will reduce the energyconsumption of the compressor since it takes less energy to compress aspecified amount of cool air than the same amount of warm air. Compressingthe warm air in a building in winter also wastes the energy used to heatthe air.4 Reducing the Air Pressure SettingAnother source of energy waste in compressed-air systems is compressingthe air to a higher pressure than required by the air-driven equipment since ittakes more energy to compress air to a higher pressure. In such cases considerableenergy savings can be realized by determining the minimum requiredpressure and then reducing the air pressure control setting on the compressoraccordingly. This can be done on both screw-type and reciprocating compressorsby simply adjusting the pressure setting to match the needs.The amount of energy it takes to compress a unit mass of air is determinedfrom Eq. 7–89. We note from that relation that the higher the pressure P 2 atthe compressor exit, the larger the work required for compression. Reducingthe exit pressure of the compressor to P 2,reduced will reduce the power inputrequirements of the compressor by a factor off reduction w comp,current w comp,reducedw comp,current 1 1P 2,reduced>P 1 2 1n12>n 11P 2 >P 1 2 1n12>n 1(7–96)A power reduction (or savings) factor of f reduction 0.08, for example, indicatesthat the power consumption of the compressor is reduced by 8 percentas a result of reducing the pressure setting.Some applications require slightly compressed air. In such cases, the needcan be met by a blower instead of a compressor. Considerable energy can besaved in this manner since a blower requires a small fraction of the powerneeded by a compressor for a specified mass flow rate.MotorAirCompressorFIGURE 7–79Schematic for Example 7–23.800 kPa900 kPaCompressedairEXAMPLE 7–23Reducing the Pressure Setting to Reduce CostThe compressed-air requirements of a plant located at 1400-m elevation isbeing met by a 75-hp compressor that takes in air at the local atmosphericpressure of 85.6 kPa and the average temperature of 15°C and compresses itto 900 kPa gauge (Fig. 7–79). The plant is currently paying $12,000 a yearin electricity costs to run the compressor. An investigation of the compressedairsystem and the equipment using the compressed air reveals thatcompressing the air to 800 kPa is sufficient for this plant. Determine how