Atlas Copco ZR 200 Instruction Book page 22

During unloading
If the air consumption is less than the air delivery of the compressor, the net pressure increases.
When the net pressure reaches the upper limit of the working pressure (unloading pressure),
solenoid valve (Y1) is de-energised. The plunger of solenoid valve (Y1) moves by spring force
shutting off the control air supply to unloader (UA).
Phase
1
2
3
4
Air delivery is stopped (0%), the compressor runs unloaded.
20
Description
Control pressure is vented from chamber (1) of unloader (UA) through solenoid valve
(Y1) and chamber (3) towards the inlet of compressor element (El).
As there is no longer any pressure difference between chambers (1 and 3), plunger (2)
returns by spring force keeping full-load/no-load valve (TV) closed and unloading valve
(UV) open.
Compressed air between check valve (CV) and valve (UV) is blown off through silencer
(US). Check valve (CV) closes.
As vacuum prevails in intercooler (Ci), diaphragm (4) switches to the left.
Instruction book
AIF999999