Compressor Unloader; Switches And Controls; Introduction; Main Control And Gauge Panel - Carrier DM-4 Power Pack Operation And Service Manual

1.8 COMPRESSOR UNLOADER

a. Unloaded Operation (Refer to Figure 1-5)
Pressure from the discharge manifold (15) passes
through the strainer (9) and bleed orifice (8) to the back of
the piston bypass valve (7). Unless bled away, this pressure
would tend to close the piston (6) against the piston spring
(5) pressure.
With the solenoid valve (1) energized the solenoid valve
stem (2) will open the gas bypass port (3).
Refrigerant pressure will be bled to the suction manifold
(10) through the opened gas bypass port. A reduction in
pressure on the piston bypass valve will take place because the
rate of bleed through the gas bypass port is greater than the
rate of bleed through the bleed orifice (8).
When the pressure behind the piston has been reduced
sufficiently, the valve spring will force the piston bypass
valve back, opening the gas bypass from the discharge
manifold to the suction manifold.
Discharge pressure in the discharge manifold will close
the discharge piston check valve assembly (14) isolating
the compressor discharge manifold from the individual
cylinder bank manifold.
The unloaded cylinder bank will continue to operate
fully unloaded until the solenoid valve control device is
de-energized and the gas bypass port is closed.
2 3
1
8
9
14
15
1. Solenoid Valve
2. Valve Stem
3. Gas Bypass Port
4. Spring Guide
5. Spring
6. Piston
7. Piston Bypass Valve
8. Bleed Orifice
9. Strainer
10. Suction Manifold
Figure 1-5. Compressor Cylinder Head Unloaded
b. Loaded Operation (Refer to Figure 1-6)
Discharge pressure bleeds from the discharge
manifold (15) through the strainer (9) and bleed orifice
4
5
7
11
6
10
12
13
11. Cylinder Discharge
Valve
12. Valve Plate
13. Cylinder Suction
Valve
14. Discharge Piston
Check Valve
Assembly
15. Discharge Manifold
(8) to the solenoid valve stem chamber (2) and the back of
the piston bypass valve (7).
With the solenoid valve (1) de-energized the solenoid
valve stem will close the bypass port (3).
Refrigerant pressure will overcome the bypass valve
spring (5) tension and force the piston (6) forward closing
the gas bypass from the discharge manifold to the suction
manifold.
The loaded cylinder bank will continue to operate fully
loaded until the solenoid valve control device is energized
and the gas bypass port is opened.
2 3
1
8
9
14
15
1. Solenoid Valve
2. Valve Stem
3. Gas Bypass Port
4. Spring Guide
5. Spring
6. Piston
7. Piston Bypass Valve
8. Bleed Orifice
9. Strainer
10. Suction Manifold
Figure 1-6. Compressor Cylinder Head Loaded

1.9 SWITCHES AND CONTROLS

1.9.1 Introduction

Components required for monitoring and controlling
the diesel engine are located on the main control and
gauge panel (located in the Power Pack) and the driver's
control panel.

1.9.2 Main Control and Gauge Panel

a. Switches
1. Rear Control Switch (RCS)
When placed in the FRONT position the unit is
controlled from the driver's switch panel. When placed in
the REAR position the unit is controlled from the main
panel only. When placed in the OFF position the unit
cannot be controlled from either panel.
1-4
4
7
11
5
6
10
12
13
11. Cylinder Discharge
Valve
12. Valve Plate
13. Cylinder Suction
Valve
14. Discharge Piston
Check Valve
Assembly
15. Discharge Manifold