Compressed air is air kept under a pressure that is greater than atmospheric pressure. Compressed air is an important medium for transfer of energy in industrial processes. Compressed air is used for power tools such as air hammers, drills, wrenches and others. Compressed air is used for a wide range of commercial and industrial applications. As it is so widely employed throughout industry, it is sometimes considered to be the “fourth utility” at many facilities after electricity, natural gas and water.