Single-Screw Compressors

The single-screw compressor was developed by a French physicist/engineer, Bernard Zimmern18, who began his work on the compressor in the early 1960s. French and U.S. patents were issued in 1964–1965. Initial sales in the U.S. were of machines compressing air, but refrigeration models followed.

The screw of the single-screw compressor is a cylindrical member with helical grooves, shown in the center of Figure 5.35. Mated in the screw are two flat star wheels on either side of the screw that rotate in opposite directions from one another.

Screw of the single-screw compressor in the center with the two star wheels on either side. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) SCREW COMPRESSORS

These star wheels rotate in the plane of the center of the screw shaft. A gastight housing encloses the screw and star wheels, Figure 5.36, and the screw rotates with a slight clearance in a cylindrical mantel that forms part of the housing.

Top and end sectional views of the single-screw compressor and housing.

The mantel contains two slots in which the star wheels run. Only the screw is driven from the outside, and this screw then drives the two star wheels. Capacity control is provided by a variable-return port controlled by a sliding vane that regulates the position where compression begins.

Compression occurs simultaneously in the upper and lower halves of the compressor. This combined action results in negligible net radial loads on the screw bearings. The only bearing loads in the machine, other than from the weight of the parts, are small loads on the star wheel shafts due to high-pressure gas acting on one side of each tooth during meshing.

The machine, like the twin-screw compressor, has few moving parts—one screw and two star wheels. Manufacurers seek to extend favorable compression efficiencies to smaller sizes than are now appropriate for the twin-screw compressor. In the development of single-screw compressors one of the difficulties has been in discovering materials for the star wheels that resist wear. Currently manufacturers of single-screw compressors seem to prefer a composite of steel and glass-fiber reinforced plastic.

When viewing a screw compressor package, such as the one shown in Fig.5.33, the oil separator stands out as the largest-size component. Were some concept developed to eliminate this component, the size of the package could shrink. One such method is to seal the gaps between the star wheels and the rotor by liquid refrigerant—the same refrigerant that the compressor is pumping. Oil is still required for lubrication, but none is injected for sealing. Compressors designed for sealing by liquid refrigerant are built with smaller clearances between the star wheel and rotor than is true of compressors that are sealed with oil. The concept can be applied to halocarbon refrigerants, such as R-134a and R-22, and for air conditioning applications where the pressure ratio is moderate. The use of liquid refrigerant sealing has not so far been successful in ammonia compressors nor for industrial refrigeration applications.

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