3.1 GENERAL
The capacity control, i.e. the decrease in capacity
at constant speed and under constant operating
conditions, takes place stepwise by successively
cutting out cylinders in pairs.
This is effected by isolating the discharge space
direct above the valves for each individual cylinder
head from the common discharge chamber in the
compressor housing and by putting it into
communication with the common suction
chamber, also located in the compressor housing.
This socalled “internal recirculation control” is,
apart from the flow resistance in valves and
channels, free from losses because the cut out
cylinders do not compress gas but just circulate it
under suction pressure.
3.2 DESCRIPTION OF CAPACITY CONTROL
As shown in figure 3.1, each controllable cylinder
head cover is provided with a by-pass slide valve
(item 9), which can move horizontally forward
and backward in a guide bush (10) and is
connected via a push rod (3) to a hot gas
operated control piston (5) in a horizontal control
cylinder (4). Slide valve, push rod and control
piston form one integral steel part.
Via a three-way solenoid valve (7,8), which is
mounted outside on top of the cylinder head
cover against the sealing cover (6) of the control
cylinder, the space to the left of the control piston
can be communicated either to suction pressure
(via lines c and b, the former connected to the
control cylinder and the latter to the by-pass
chamber 2) or to discharge pressure (via lines a
and c, the former connected to the permanent
discharge chamber 12 in the cylinder head cover).
Actually, only line a, coming from the top
connection of the solenoid valve, is an external
one; the two others b and c are internal bores
provided in the sealing cover of the control
cylinder and in the walls of the cylinder head
cover. These internal bores are not indicated in
the sections of figure 3.1.
3.3 OPERATION OF CAPACITY CONTROL
If the solenoid valve is not energized, line a is
connected via line c to the control cylinder, so that
hot gas from the permanent discharge chamber
enters this cylinder and, as the diameter of the
control piston is substantially larger than that of
the slide valve, the combination piston-rod-valve
will move to the rightmost position, causing the
slide valve to shut off entirely the permanent
discharge chamber from the space direct above
the discharge valves (11) and the latter to be
communicated via a ring of circular openings in
the guide bush with the space between control
piston and slide valve (by-pass chamber), which is
in permanent communication with the common
suction chamber (1) in the compressor housing.
Both cylinders then operate unloaded as a result
of the by-pass thus effected. This situation is
shown in the upper part of figure 3.1.
When the solenoid valve is being energized, line b
is connected via line c to the control cylinder, so
that there is no pressure difference across the
control piston and the combination
piston-rod-valve is kept in its leftmost position by
the pressure prevailing in the permanent
discharge chamber and exerted onto the
righthand face of the slide valve. The space direct
above the discharge valves then communicates,
via the afore mentioned circular openings in the
guide bush of the slide valve, with the permanent
discharge chamber, so that both cylinders operate
normally. This situation is shown in the lower part
of figure 3.1.
For the standard capacity control part-load steps of
the various compressor types and the corresponding
energizing sequence of the solenoid valves, refer to
the figures 3.2, 3.4 and 3.5 for compressor types
RC6 and to figures 3.6, 3.8 and 3.9 for compressor
type RC6W.
3.4 PARTIALLY UNLOADED STARTING
In order to be sure that all controllable cylinder
pairs always operate fully unloaded when there is
no pressure difference between discharge and
suction, a spring is mounted between each sealing
cover and control piston to ensure that in that
case all control pistons and slide valves are kept in
their rightmost position, regardless of whether the
corresponding solenoid valves are energized or
not.
On the other hand, to create sufficient discharge
pressure to operate all available capacity control
slide valves when starting the compressor at zero
pressure difference between discharge and
suction, one cylinder head cover of each
compressor type is non-controllable, i.e. it has no
built-in slide valve and control piston, but is
Refrigeration Division
Grasso
3. DESCRIPTION AND OPERATION
OF CAPACITY CONTROL
v001.99.01.en Installation and Maintenance Manual RC(U)6 page C1.1
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