The selection of a compressor depends on the volumetric flow rate, the operating pressures, the compressor efficiency, its capital cost and the cost of energy. Because of their lower installation cost, centrifugal compressors are chosen over reciprocating compressors when volumetric flow rates and pressure allow their use. Axial compressors are used for large volumetric flow rates. Reciprocating machines are used at very high pressures and small volumetric flows.

For most air plants in the size range of 30-3000 tons per day of oxygen, centrifugal compressors containing three or four stages are used for compressing the feed air. Interstage cooling is provided with cooling water to approximate isothermal compression. A large fraction of the water contained in the feed air is condensed in these interstage coolers. Most of these compressors are electrically driven; however, steam or gas turbines are occasionally used when economically justified. Air is passed through one or two stages of filtration to remove particulates prior to entry in a MAC. For plants that are larger than 3000 tons per day of oxygen, a combination of axial and centrifugal compressors is used. At the other end of the scale, for small size plants in the size range of less than 30 tons per day of oxygen, inexpensive screw compressors are used. These guidelines for oxygen plants can easily be translated to nitrogen plants, because for the same production rates, a nitrogen plant requires only 30-50% of the feed air flow required by an oxygen plant.

When gaseous oxygen is to be compressed, a centrifugal compressor is used for low to moderate pressures while a reciprocating compressor is used for higher pressures. When oxygen is needed at fairly high pressures, the initial stages of compression may be centrifugal. The design of an oxygen compressor requires careful selection of materials and seals, and total prevention of rubbing contacts to avoid ignition in the presence of high pressure oxygen. Furthermore, an oxygen compressor is generally enclosed in a building with an external barrier to increase the safety of plant personnel. Special test and start-up procedures are also used for oxygen compressors.

Expanders are used to provide refrigeration by extracting work from an expanding fluid. In the expansion process, the temperature of the expanded fluid is reduced. An air separation or a liquefaction plant generally uses a single-stage radial inflow turbine as a standard. For small plants, the work energy from the expander is either dissipated in an oil brake or through an ambient air blower. For medium to large size separation plants and liquefiers, it is essential that the work energy from an expander be recovered to increase process efficiency. This is done by either loading an expander with an electric generator or a compressor for some other process fluid. When an expander is directly coupled to a compressor, the arrangement is called a compander. As seen from Figure 11, companders are widely used in liquefiers. Expanders used in the cryogenic air separation and liquefaction industry typically have isentropic efficiencies in the range of 85-90%. The dense fluid expanders are essentially reverse-running liquid pumps (Figure 11).

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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