HSO 2024 to HSO 2035 Installation, Operation and Maintenance Manual
Issue 1.2 : 03/10
Publication 2-119
Page 19 of 94
The best part load characteristics are achieved if the design full load V
R
is
maintained as the compressor’s capacity is reduced. The HallScrew
2000 series compressor is fitted with a pair of sliding valves, one for each
half of the symmetrical compression process. These valves reduce
pumping capacity by delaying the sealing of the flute volume together
with the opening of the discharge port, altering the effective length of the
main rotor flutes. The valves not only permit stepless capacity control
down to approximately 10 % of full load (actual minimum value varies
with operating conditions), but also maintain the best possible V
R
over a
wide capacity control range.
Each slide valve is housed in a semicircular slot in the wall of the annular
ring which encloses the main rotor. As the slide valve travels axially from
the full load position it uncovers a port which vents part of the gas
trapped in the main rotor flute back to suction before compression can
begin. When the flute has passed beyond the port, compression
commences with a reduced volume of gas. However, a simple bypass
arrangement without any further refinement would produce an
undesirable fall in the effective volume ratio which in turn causes
undercompression and inefficient part load operation. To overcome this
problem, the slide valve is shaped so that it reduces the discharge port
area at the same time as the bypass slot is created.
Fig 2 shows one of the capacity control slide valves in two positions: fully
loaded and at part load, the arrows on the diagram indicating the flow of
gas. Each half of the compressor is provided with its own slide valve
system, these are operated simultaneously to maintain balanced gas
loads and low bearing loads within the compressor. The position of the
slot at the suction end of the slide and the position of the moving delivery
port can both be chosen to give the desired full load V
R
, the appropriate
ratio being selected according to the operating conditions. The following
volume ratio slides are available for each compressor size: 1.85, 2.2, 2.6,
3.5 and 4.9.
4.1.
Slide Valve Actuation
The capacity control slides valves are joined together by a yoke which is
connected to a hydraulic piston, housed inside a cylinder and mounted
internally at the discharge end of the compressor.
The motive force required to actuate the piston is derived from a supply
of pressurised oil taken from the lubrication system. The flow of oil to the
cylinder is controlled either by a specially adapted 4-way solenoid valve
or by two pairs of solenoid valves connected in parallel. As the
requirement for duty changes, the plant control system energises or de-
energises the solenoid valves, supplying oil pressure to drive the piston
and slide valves in the load or unload direction. The oil on the other side
of the piston is vented to the evaporator side of the suction non-return
valve via a drain line.
The capacity control piston/slide valves speed of travel, and hence how
quickly the compressor loads and unloads, is determined by flow control
valves fitted in the load and unload oil lines to the capacity control
cylinder.
4.1.1.
Minimum Load Interlock
Starting at minimum load minimises motor starting current and starting
torque. This in turn minimises stresses on the motor and mechanical
parts, and also reduces the load on the power supply network.
The control system must be interlocked to prevent the compressor
starting unless the linear variable displacement transducer (LVDT)
provides an ‘at minimum load’ permit start signal.
If the slide valves are not already at minimum load, the pressure
generated by the system oil pump is used to return them to the minimum
load position.
