Compressed air always has been a major source of energy for work in construction and industrial applications. Both oil-flooded and oil-free compressors provide the right solutions to the many rental applications requiring compressed air. The pressure and volume of air required are key factors in choosing a portable compressor for the application. For many contractors the "quality" of the air provided is also an important consideration. "Quality of air" can be defined by how dry, clean and free from contaminates the air is. Dirt, oil and water can be detrimental to many processes and applications. If a rental store sends out a compressor that doesn’t handle the job requirements properly, it could influence future business. Rental compressors Oil-flooded, or lubricated, screw compressors are among the most popular types of portable compressors for many construction and industrial applications because of their low cost to purchase and operate and their wide range of flows and pressures: typically 90 to 1,800 cubic feet per minute and 100-350 pounds per square inch gauge. They are also among the most popular pieces of equipment in the rental yard. The air produced by these compressors is saturated with water vapor and has a level of oil and particulate carry-over that might not be acceptable for some applications. Oil-free, or dry, screw compressors do not add oil to the compression process. However, the compressed air is only as clean as the inlet air that is ingested through a 10-micron inlet filter and is saturated with water. Because these are diesel-driven compressors, the air around the compressor is highly contaminated with hydrocarbons and particulates. Oil-free compressors typically are used in applications such as pharmaceuticals, electronics and food processing in which the validation process requires their use. These compressors typically cost twice as much as oil-flooded compressors. Clearing the air A combination of aftercoolers, water separators, dryers and filters is used to clean and dry compressed air. It all starts with the air from a compressor, typically registering temperatures from 200 to 300 degrees Fahrenheit. Cooling the air via an aftercooler significantly reduces the amount of moisture in the air. A 20-degree reduction in temperature can condense half of the water vapor in the saturated air from the compressor. Once the water vapor condenses, the water is collected and eliminated with a water separator. Dirt and other solid contaminates are removed through filtration. Oil and additional water in aerosol form are also removed. The filters used to remove these contaminates are combinations of particulate and coalescing filters. Filtration levels typically can be provided down to 0.01 microns in size for particulates and aerosols. Depending on the application, a dryer also might be needed. How dry your air needs to be is determined by the dew point required for the specific application, and this requirement determines the type of dryer needed. The most commonly used types of dryers are deliquescent, refrigerated and desiccant. Dryers might be required with oil-flooded or oil-free compressors. The combination of equipment used with a compressor can provide what industry standards define as instrument-quality air. Instrument-quality air is a viable alternative to oil-free air in many industrial and construction applications, including process air, instrument air, painting and abrasive blasting. Selecting the options This additional equipment required to improve the quality of air with compressors should be an option that rental centers look for in stand-alone units. Compressors are also available with built-in aftercoolers, water separators and particulate and coalescing filters. Some compressors with these built-in features are available with a condensate removal system that provides environmentally friendly operation. The oil and water collected from oil-flooded compressors cannot be dumped on the ground for environmental reasons. Normal collection and disposal of this effluent are time-consuming and expensive. The condensate collection system burns off all effluent by injecting it into the exhaust system, eliminating costly disposal. Read More
