Danfoss VZH052 Manual page 39

Control logic
Short cycle protection
Defrost cycle logic
Compressor ON
4WV Heating
EXV 100%
Pump-down logic
recommendations
Short cycling protection requirements need to be
implemented in OEM unit controller:
Meantime, the factory default setting needs to
be disabled( 28-00 short cycle protection change
from default setting "enable" to "disable").
- 3 minutes minimum running time: in order to
get oil return back from circuit to compressor
sump
In reversible systems, the defrost logic can be
worked out to limit liquid flood back effect by:
1. Running full load during defrost to share liquid
refrigerant between all compressors.
The following defrost logic combines both
advantages:
* EXV Opening degree and time have to be set
to keep a minimum pressure for 4 way valve
moving.
Pump down is initiated prior to shutting
down the last compressor on the circuit by
de-energizing a liquid line solenoid valve
or closing electronic expansion valve. When
suction pressure reaches the cut-out pressure,
compressor is stopped, and liquid solenoid valve
or electronic expansion valve remains closed.
Two types of pump-down exist:
- 12 starts maximum per hour: to avoid threaten
the life time of motor and other mechanics due
to frequent starts, OEM needs to limit the starts
cycles within 12 times per hour.
- 10s minimum OFF time: to make sure discharge
valve is closed and motor is stopped before next
start, OEM needs to set the minimum off time as
10 seconds.
2. Transferring liquid refrigerant from one
exchanger to the other one thanks to pressures.
Danfoss recommend above defrost cycle logic,
but the control logic is also system specified.
In any case, defrost logics must respect
requirements and tests described in "Manage
superheat" and "Manage operating envelope".
• One shot pump down (preferred): when
last compressor of the circuit stops, suction
presssure is decreased 1.5 bar below nominal
evaporating pressure. Even if suction pressure
increases again, the compressor will not restart.
• Continuous pump-down: traditional pump-
dow, Compressor restarts automatically when
suction pressure increases up to 4 cycles
maximum. A non-return valve in the discharge
line is recommended.
AB237586440343en-000601
39