HSO 2024 to HSO 2035 Installation, Operation and Maintenance Manual
Page 14 of 94
Publication 2-119
Issue 1.2 : 03/10
The main rotor and star rotors are housed inside a one-piece, cast-iron
main casing. The inside of the casing has a somewhat complex shape,
but essentially consists of a cylindrical annulus which encloses the main
rotor leaving a small clearance. Part of the annulus is cutaway at the
suction end to allow the star teeth to mesh with the main rotor flutes. The
discharge ports (one for each star), are positioned at the other end of the
annulus. These ports convey the compressed gas into the discharge
manifold, formed by a web cast between the annulus and the walls of the
casing; this web separates the casing into two pressure zones. Except
for the discharge manifold, suction pressure prevails elsewhere in the
main casing.
Side covers are provided to allow easy access to the star rotors, star
rotor shafts and bearings, without disturbing working tolerances. The
discharge end cover can also be removed to inspect the capacity control
mechanism. The compressor is provided with the choice of either top or
bottom discharge; the unused connection is sealed off with a blank cover.
It is necessary to fit a suction strainer immediately before the compressor
suction inlet. The strainer is designed to trap any dirt circulating with the
refrigerant which might otherwise enter and damage the compressor.
To prevent reverse rotation of the compressor at shutdown it is necessary
to fit a non-return valve adjacent to the compressor in the suction and/or
discharge lines; refer to 6.4.2.
3.3.
The Compression Process
In the construction of the HallScrew the helical flutes in the main rotor can
be likened to the cylinders of a reciprocating compressor, the star rotor
teeth taking the place of conventional pistons. Instead of using suction
and discharge valves, gas flow in and out of the flutes (the cylinders) is
controlled by fixed ports.
Gas enters the compressor through the suction connection and fills the
available flutes. Rotation of the main rotor traps the gas in chambers
formed by the flute walls, the cylindrical annular ring housing the main
rotor, and the star teeth. The small clearances around the star teeth are
sealed with oil which is injected into the compressor during operation. As
the main rotor turns, the star teeth act as stationary pistons in the moving
flutes (the cylinders), and the gas is compressed until a discharge port is
uncovered. Each flute is used twice per rotor revolution, i.e. once by one
tooth on each star.
The compression process is illustrated and described in detail in Fig 1.
As the compression process is symmetrical, occurring at the same instant
in each half of the compressor, this results in zero transverse gas
pressure loads on the main rotor bearings. The axial loads are also
minimal because the flutes terminate on the outer surface of the main
rotor. The only bearing loads, apart from the weight of parts, are bending
loads on the star rotor shaft bearings due to high pressure gas acting on
one side of each tooth in mesh. There is also a small axial thrust load on
the main rotor bearings resulting from the main shaft projecting through
the casing, combined with the rotor vent pressure.
Capacity control is effected by slide valves, one for each half of the
compressor. These valves are used to vent part of the gas trapped in the
flutes back to suction, thus effectively shortening the compression length
of the main rotor. Using this method, compressor capacity is infinitely
variable between 100 % and 10 % of full load.
