We believe that the economics of producing oxygen and nitrogen are primarily affected by the reliability of the machinery employed. The power consumption, labor requirements, and installed cost also affect economics of operation but reliability is the single most important factor. Machinery on air separation plants involves air compressors and, except on the very smallest plants, expansion turbines.
In recent years, centrifugal compressors have become available in small sizes over a wide range of pressures. At American Air Plants, we believe that centrifugal compressors are ideally suited for use on air separation plants, and we use them whenever possible. They deliver oil free air. Also, there is no vibration or out of balance forces so extensive foundations are not required. They have no wearing parts, which keeps maintenance requirements to a minimum.
American Air Plants uses this type compressor on all its plants except the smallest sizes which are high pressure type plants. These plants use reciprocating compressors operating at pressures of up to 3,000 psig. Our plants in larger sizes, if producing liquid products, use centrifugal compressors at a medium pressure of around 450 psig. Centrifugal compressors capable of this pressure are available at power ratings of 1500 hp and above. This size compressor can be used on a plant producing around 50,000 scfh (1,300 Nm3/hr) of liquid products. On a liquid producing plant, there are advantages from the standpoint of power consumption in using a medium pressure process compared to low pressure process.
We also build plants operating at medium pressure using centrifugal machinery where the oxygen or nitrogen is compressed as liquid, but supplied as gas. Refer to the accompanying flow diagram. This eliminates the need for a gaseous oxygen or gaseous nitrogen compressor.
For plants in intermediate sizes producing gaseous or liquid oxygen, we rely on centrifugal compressors operating at around 125 psig. This low pressure type plant offers advantages from the standpoints of safety, reliability, low maintenance costs, and the ability to operate unattended.