Danfoss VZH052 Manual page 30

Manage off cycle migration
Requirement
System evaluation
30
AB237586440343en-000601
Off -cycle refrigerant migration happens:
R
• when the compressor is located at the coldest
part of the installation, refrigerant vapor
condenses in the compressor.
• or directly in liquid-phase by gravity or pressure
difference.
· Compressor can tolerate occasional flooded
start, but it should remain exceptional situation
and unit design must prevent that this situation
from happening at each start.
· Right after start, liquid refrigerant must not flow
massively to compressor.
Use the table below in relation with the system
charge and the application to quickly define
necessary safeties to implement.
Application
Ensure tightness between condenser & evaporator when system is OFF
• Thermostatic expansion Valve (TXV), Liquid Line Solenoid Valve LLSV** strongly
recommended
All
• Electronic expansion valve (EXV) must close when system stops including in power shut
down situation
Non split No test or additional safeties required
Since each installation is unique, refrigerant charge may vary
• Belt type heater*
Split
• Liquid Line Solenoid Valve**+ pump-down cycle***
• External Non-Return Valve
Charge limit is defined in table below:
Single
When the compressor restarts, the refrigerant
diluted in the oil, or stored in evaporator,
generates poor lubrication conditions, and may
reduce bearings life time. In extreme situations,
this leads to liquid slugging that can damage the
compressor scroll set.
· The charge limit is a threshold beyond with
some protective measures must be taken to
limit risk of liquid slugging and extreme dilution
at start.
BELOW charge limit
Models
VZH052-065
ABOVE charge limit
• Belt type heater*
• External Non-Return Valve
Refrigerant charge limit (kg / lb)
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