RC(U)6 Installation and Maintenance Manual - Marine Refrigeration ...

Refrigeration Division
Grasso
provided instead with only a simple vertical
partition to keep the common suction and
discharge chambers in the compressor housing
apart from each other.
Consequently, when the discharge pressure is
higher than the suction pressure, only partially
unloaded starting is possible.
Therefore, it is recommended to always check the
electric drive motor in this respect, depending on
compressor design speed and starting conditions,
the procedure of which is set out in paragraph 4.7.
3.5 ALTERNATIVES TO CAPACITY CONTROL
The four-cylinder compressor types RC46 can also be
delivered with an alternative capacity control,
including three part-load steps (25, 50 and 75%)
instead of only one (50%) in the standard version.
For the switch sequence of cylinders and solenoid
valves of this alternative capacity control, refer to
figure 3.3 and figure 3.7 for compressor types
RC46 and RC46W respectively.
These additional steps are obtained by replacing
the non-controllable cylinder head cover of
cylinders nr. 1 and 3 (RC46 in fig. 3.3) or cylinders
nr. 2 and 4 (RC46W in fig. 3.7) by a partly
controllable one of special design, which means
that it has a partition in the middle of the space
direct above the discharge valves to separate the
two adjacent cylinders entirely from each other,
thereby causing the built-in control piston and
slide valve to be effective for cylinder nr. 1 (RC46)
or cylinder nr. 2 (RC46W) only and the discharge
valve of cylinder nr. 3 (RC46) or cylinder nr. 4
(RC46W) to be always in communication with the
permanent discharge chamber in the cylinder head
cover and the common discharge chamber in the
compressor housing.
So, only this one cylinder nr. 3 (RC46) or cylinder nr.
4 (RC46W) is always in operation, which has the
extra advantage of also reducing the starting load
from 50 to 25%.
On special order, just by adding the symbol Z after
the type designation, each standard compressor
type can also be delivered without any capacity
control. In that case not only one but all cylinder
head covers are of the non-controllable type, i.e. not
provided with built-in control piston and by-pass
slide valve.
However, the consequence of such a (cheaper)
choice is that either fully loaded compressor
3. DESCRIPTION AND OPERATION
OF CAPACITY CONTROL
starting has to be accepted, inevitably resulting in
a more powerful electric motor to be installed, or
special provisions are to be made by the installer
in the field to overcome this inconvenience, for
instance by applying an external line with solenoid
valve (only open, so energized only during
starting) between the suction and discharge line
of the installation, combined with a non-return
valve in the same discharge line.
Important installation requirement!
To prevent, during prolonged compressor
stand-still, a gradual pressure equalisation
between condenser and evaporator due to
slow leakage of refrigerant gas via the
slide valve, a non-return valve should all
the times be mounted in the external
discharge line.
page C1.2 Installation and Maintenance Manual RC(U)6 v001.99.01.en