Plant Services - August 2007 - (Page 51) EFFICIENCY Compressors sion can be removed so the air can be discharged at about 176°F. A temperature increase of 10°F leads to 1% to 2% deterioration in specific power, so keep the discharge temperature as low as possible. This is why oil- or fluidcooled screw compressors can serve single-stage compression applications for compression ratios ranging from 4:1 to as high as 16:1. Fluid-cooled screw compressors that use water instead of oil for cooling can achieve even lower discharge temperatures of 104°F to 122°F. However, water has two disadvantages as a cooling medium. Its lower viscosity leads to higher back-flow losses and limits single-stage compression ratios to between 10:1 and 13:1. Also, the water reacts with impurities in the intake air, which means the water either needs to be changed frequently or purified. Compared to oil- or fluid-cooled screw compressors, oil-free compression systems requiring two or more stages are less energy efficient and more expensive. A lower airend temperature also results in a lower air discharge temperature, which ensures better condensate separation and eases the thermal load on the downstream compressed air treatment equipment. The power differences between oilfree compression (with higher operating temperatures) and oil-cooled (with lower operating temperatures) have an effect on the budget. The specific power of a modern oil-free rotary compressor in the 100 hp class at 108 psig is 19.96 kW/100 cfm, whereas the specific power for an oil- or fluidcooled rotary compressor would be 16.12 kW/100 cfm. This difference in specific power amounts to an annual saving of about $13,409, or 23%, based on 6,000 operating hours per year with an electricity price of 10 cents/kWh. the energy consumption for the fans and ignore the costs for the cooling water. A comprehensive comparison reveals the real costs. Air cooling for a 4,000-hp compressor with an air The warmer the air during compression, the greater the energy requirement. flow capacity of 15,000 cfm at 100 psig requires about 45 kW (60 hp) averaged over a year. Cooling an equivalent system with water would consume between 60 kW and 70 kW (80 hp to 94 hp) when taking pumps, fans, cooling towers and other associated equipment into consideration. This difference represents an annual cost saving of approximately $20,000, based on 8,000 operating hours per year. The more cost-effective air-cooled approach for compressors with direct injection cooling also can be used in a wider range of conditions because of the considerably lower air discharge temperature. Oil-free doesn’t save filter costs One myth is that oil separation and filtration downstream of oil/fluidcooled compressors produce energy and maintenance costs that could be avoided with an oil-free compressor. Comparisons are happily provided to support this view. But, unfortunately, these compare oil/ fluid-cooled compressors equipped with a host of filters versus an oilfree system fitted with only a downstream desiccant dryer. These comparisons are misleading and bear no relevance to modern air systems. An oil/fluid-cooled screw Air cooling lowers costs Calculations to compare the cooling costs for air- and water-cooled compressor systems usually consider only August 2007 www.PLANTSERVICES.com 51 http://www.truco-inc.com http://www.truco-inc.com http://www.PLANTSERVICES.com
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