Refrigeration

Turboexpanders may be used in closed cycles with a pure gas such as nitrogen, which is alternately compressed and expanded to provide the required refrigeration through a heat exchanger. Open cycles are also used in which the process stream to be cooled passes through the expander. Liquefaction of natural gas is an example. Open cycles eliminate the need for a low-temperature heat exchanger. Open cycles can also be used to recover condensable products from process streams. Water vapor and C02 must be reduced to very low levels in open cycle systems to prevent solidification and fouling of equipment. A compressor expander is shown in Figure 7-24.

The cryogenic expanders are ideal candidates for magnetic bearings {covered in Chapter 5). In process service, elaborate seals are required to

Figure 7-24. Cryogenic expander-compressor, {Courtesy of Mafi-Trench Corp.)

keep lubricating oil from entering the gas stream. Also the hydrodynamic bearing design is critical because of the high speeds normally required. By use of magnetic bearings, not only can the high speeds be achieved without regard to bearing speed limitations, but the seals can be reduced to the use of simple labyrinth seals in most applications.

Condensation

Radial-inflow turboexpanders can be designed to handle relatively large amounts of condensation with very little loss of efficiency. Axial turboexpanders can also tolerate some condensation, but usually at a loss of efficiency.

Several arbitrary rules of thumb have been suggested to account for the reduction in efficiency when moisture particles are present. One is to multiply the efficiency by the mean vapor content, by weight. For example, if expansion is started in the superheated region and ends with 6% by weight of liquid, the mean vapor content is 97%. If the design efficiency-is 78%, the adjusted value of efficiency is 0.78 (0.97) = 0.757 or 75.7%. Some designers prefer to assume a loss equal to twice the value used in the example.

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