HSO 2024 to HSO 2035 Installation, Operation and Maintenance Manual
Page 20 of 94
Publication 2-119
Issue 1.2 : 03/10
4.2.
Capacity Control Solenoid Valve Arrangements
As discussed in 7. Oil Circulation, if the system suction/discharge
pressure differential is sufficient to generate oil pressure for injection and
lubrication, an oil circulation pump can be dispensed with altogether.
However, as there is no other oil pressure source, special precautions
must be taken to automatically return the capacity control piston to the
minimum load position when the compressor stops to provide a
‘compressor on minimum load’ interlock permit start signal for the next
start. Therefore, instead of a 4-way solenoid valve, it is necessary to use
two pairs of solenoid valves connected in parallel as illustrated in Fig 3.
In this arrangement the solenoid valves in the unload path are reverse
acting, i.e. energise to close. Hence when the compressor stops for
whatever reason, and especially in the event of power failure, these
valves will always de-energise (open) and unload the compressor.
The arrangement using separate pairs of solenoid valves must be used if
the compressor is located in a Zone 1 or Zone 2 hazardous area where
explosion-proof coils are a mandatory requirement. These coils cannot
be fitted to the 4-way valve. The various capacity control solenoid valve
options are summarised in Table 1.
A
PPLICATION
C
APACITY
C
ONTROL
S
OLENOID
V
ALVE
A
RRANGEMENT
No oil pump
Use separate pairs of solenoid valves
Oil pump runs at start-up only
A 4-way valve
or
separate pairs of solenoid valves can be
used
Continuously running or demand oil pump
All oil pump applications - compressor located in a Zone 1 or
Zone 2 hazardous area
Use separate pairs of solenoid valves fitted with Valvex
explosion-proof coils
Table 1 Capacity Control Solenoid Valve Options
4.3.
Capacity Control Arrangement Using Separate Pairs of Solenoid
Valves
This arrangement, shown in Fig 3, provides continuously variable
capacity control between minimum and maximum load. There are two
pairs of solenoid valves connected in parallel:
•
Solenoid valves X and Y are normally open (NO), energise
to close.
•
Solenoid valves W and Z are normally closed (NC), energise
to open.
4.3.1.
Compressor Loading
With all the solenoid valves energised, valves W and Z are open, X and Y
are closed. Pressurised oil flows through valve W into the load side of
the capacity control cylinder, forcing the piston and slide valves in the
load direction and increasing compressor pumping capacity. Oil on the
unload side of the piston is forced through valve Z and drains to suction.
4.3.2.
Compressor Unloading
With all the solenoid valves de-energised, valves W and Z are closed, X
and Y are open. Pressurised oil flows through valve X into the unload
side of the capacity control cylinder, forcing the piston and slide valves in
the unload direction and decreasing compressor pumping capacity. Oil
on the load side of the piston is forced through valve Y and drains to
suction.
4.3.3.
Hold Slide Valve Position
When solenoid valves X and Y are energised, valves W and Z de-
energised, all the valves are closed, hydraulically locking the capacity
control piston and slide valves at the desired load position.
