Thermodynamics

specifications, on the other hand, may drop from 91 percent at full load to 75percent at quarter load. The first motor is obviously better suited for a situationin which a compressor is expected to operate at quarter load during a significantportion of the time. The efficiency at part-load conditions can beimproved greatly by installing variable voltage controllers if it is economicalto do so. Also, oversizing a motor just to be on the safe side and to have someexcess power just in case is a bad practice since this will cause the motor tooperate almost always at part load and thus at a lower efficiency. Besides,oversized motors have a higher initial cost. However, oversized motors wastelittle energy as long as they operate at loads above 50% of design.Using a Smaller Motor at High CapacityWe tend to purchase larger equipment than needed for reasons like having asafety margin or anticipated future expansion, and compressors are no exception.The uncertainties in plant operation are partially responsible for optingfor a larger compressor, since it is preferred to have an oversized compressorthan an undersized one. Sometimes compressors that have several times therequired capacity are purchased with the perception that the extra capacitymay be needed some day. The result is a compressor that runs intermittentlyat full load, or one that runs continuously at part load.A compressor that operates at part load also causes the motor to operateless efficiently since the efficiency of an electric motor decreases as the pointof operation shifts down from its rated power, as explained above. The resultis a motor that consumes more electricity per unit power delivered, andthus a more expensive operation. The operating costs can be reduced byswitching to a smaller motor that runs at rated power and thus at a higherefficiency.3 Using Outside Air for Compressor IntakeWe have pointed out earlier that the power consumed by a compressor isproportional to the specific volume, which is proportional to the absolutetemperature of the gas at a given pressure. It is also clear from Eq. 7–89that the compressor work is directly proportional to the inlet temperature ofair. Therefore, the lower the inlet temperature of the air, the smaller thecompressor work. Then the power reduction factor, which is the fraction ofcompressor power reduced as a result of taking intake air from the outside,becomesf reduction W comp,inside W comp,outsideW comp,inside T inside T outsideT insidewhere T inside and T outside are the absolute temperatures (in K or R) of the ambientair inside and outside the facility, respectively. Thus, reducing the absoluteinlet temperature by 5 percent, for example, will reduce the compressorpower input by 5 percent. As a rule of thumb, for a specified amount of compressedair, the power consumption of the compressor decreases (or, for afixed power input, the amount of compressed air increases) by 1 percent foreach 3°C drop in the temperature of the inlet air to the compressor.Compressors are usually located inside the production facilities or inadjacent shelters specifically built outside these facilities. The intake air isChapter 7 | 397 1 T outside(7–95)T inside