T-CU19-JAN24-1
Page 21
System Charge
Never start the compressor under vacuum.
Ensure an adequate liquid charge has been
introduced to the high side of the system
before starting to ensure a minimum
operating pressure on the suction side of 0.5
bar is maintained, otherwise overheating of the scrolls
and subsequent damage may occur. Refrigerant blend
must be charged in liquid form to avoid change of
chemical properties.
Condensing unit must not run, and service valves must be
closed for initial refrigerant charge. The service port on the
receiver outlet rotolock valve (liquid line) can be used for
initial charge. Charge refrigerant as close as possible to the
nominal system charge (will varying depend on the tube
size and lengths) before the compressor with all service
valves turn to OPEN position.
Compressor Charge Limit = 6kgs
For the adjustment of refrigerant charge until the
installation reaching a level of stable nominal working
condition, slowly throttling liquid in through schrader valve
on the suction service valve. Charge system until reaching
suction superheat 6~12K at desired evaporating
temperature. Suction superheat, suction, and condensing
pressures (temperatures) could be found on the info screen
of the controller. While optimizing charging, ensure oil sight
glass doesn’t start foaming.
A proper refrigerant charge should secure stable condition
at minimum and maximum heat load within the limits of the
condensing unit’s application envelope.
•
Minimum heat load conditions, which occurred during
winter. Condenser should not be flooded by liquid
refrigerant, receiver and liquid line should be able to
contain remaining charge at this condition.
•
Maximum heat load conditions, which occurred during
summer. All evaporators are working with maximum
air/liquid flow via evaporators and refrigerant charge
should be enough to feed to all evaporators.
Refrigerant charge by judging the liquid sight
glass does not guarantee as 100% correct
way.
To avoid system overcharging, which can cause higher
energy consumption and low compressor sump superheat
.
Compressor sump superheat = temperature of (compressor
shell-bottom at 30mm above foot shell
–
condensing
saturated vapor at that time)
•
Ensure compressor sump superheat more than 10K.
The system approximate maximum charge can be
calculated as below:
Approximate Maximum Charge = (Receiver Volume
+ Internal volume of Liquid Line) * 0.9
When the compressor operates under stabilized conditions,
the oil level must be visible in the sight glass (floating ball).
Foam filling the oil sight glass indicates presence of large
concentration of liquid to the compressor.
No additional oil is required for installation with good oil
returns, line runs up to 30m. Additional oil might be
required if lines exceeded 30m, with minimum oil level must
not lower than ¼ of sight glass. Top-up the oil while
compressor is idle, via suction schrader connector with a
suitable pump.
Pipe Size Selection
Do not assume that the suction/liquid
connections sizes on the condensing unit are
the correct sizes to run your interconnecting
refrigeration pipes.
As a piping design concept, convenience store application
will operate at part load condition. It is a concern that too
big suction cross section area will cause refrigerant do not
obtain sufficient velocity to carry oil returned to compressor.
Table 15
provides a guideline for quick selection of suction
pipe size.
To find optimum suction pipe size for oil return.
Example:
1. Calculate % of refrigerant flow for each indoor
•
Total cooling load = 6.5+6.5 = 13kW = 100%
•
Flow for IDU 1 = 6.5/13 = 50%
•
Flow for IDU 2 = 6.5/13 = 50%
2. Select suction pipe size using
Table 15
based on optimum
minimum speed not lower than 30rps. Refer
Figure 18
for
the selected suction pipe size.
3. Conclusion: Minimum running speed of compressor for oil
return with this combination of piping is 21rps.
Figure 18: Suction Pipe Layout for 1 outdoor to
2 indoors
