BASIC CHEMICAL ENGINEERING OPERATIONS: Sidestream Centrifugal Compressors(Capacity range, working cycle)

Sunday, August 15, 2010

Sidestream Centrifugal Compressors(Capacity range, working cycle)

Sidestream Compressors

In multistage refrigeration processes, different mass flows pass through the various refrigeration stages. Sidestreams therefore have to or extracted from the main flow in the compressor at process-dependent pressures and temperatures. Injection or extraction is by means of additional nozzles.

Incase of gas injection, the side stream is mixed with the main stream in the return channel. Mixing takes place over the entire periphery. When a stream is extracted, a separating volute removes part of the main stream. The compressor stages are designed to correspond to the stages of the refrigeration
process. The working media for the refrigeration process are primarily ethylene, propylene, ethane and propane. Refrigeration process usually form a closed cycle, rather less frequently a semi-open cycle.

In centrifugal compressor with two main nozzles pairs, the two process stages can be arranged back to back, i.e., flow in the two process stages is in the opposite directions, or they can be arranged in series. In the double flow version, the compression process of both stages terminates in a common
discharge nozzle.

Back to back arrangement of the first and third stages or series arrangement of the stages is also possible. Typical back to back arrangement is shown in the figure 6.

Tried and proven labyrinth seals separate the individual process stages. The choice of the seal system is dictated by the service. Whereas casings with two main nozzle pairs are widely employed for a variety of media and processes, casing with three process stages are mainly employed for air, oxygen and nitrogen. The medium is normally cooled outside the compressor. Interstage cooling produces an almost isothermal compression process. This requires least compression work. Intercooling also becomes necessary when the temperature produced by compression has to be limited.In most compression systems, the coolers are mounted separately, permitting a high degree of freedom in design and layout. However, compressors with internally arranged coolers are available from some manufacturers and may merit consideration when the ultimate in compactness must be achieved.