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REFRIGERANT PIPE SIZING
WITH REFRIGERANT (Cont.)
14.3 - Suction pipe sizing
This sizing is the most critical. A distillation process takes
place in the evaporator, during which the refrigerant evapo-
rates until it reaches a balance point. It exists in two phases:
the vapour phase that only contains refrigerant, and the liquid
phase, that is a mix of liquid refrigerant and oil.
The content of refrigerant in this mixture depends on the
pressure. The liquid mixture can only be returned to the
compressor by entrainment, initiated by the vapour velocity.
"Suction piping" shows the different pipe diameters
based on the unit size and the equivalent circuit length.
These recommended diameters allow oil return within the
application range.
IMPORTANT: Siphon traps must be inserted in the vertical
riser pipes:
• at the bottom of the piping,
• every 3 m of vertical length,
• at the top of the piping (counter-siphon).
The trap must be correctly sized to ensure that not too much
of the liquid refrigerant/oil mixture is trapped. The horizontal
pipes must have a slight slope (30 mm/m) between evaporator
and condensing unit in the direction of the compressors.
14.4 - Liquid line sizing
38RM-N compressors are supplied with an oil that is fully
miscible with refrigerant R410A in the liquid phase, and low
refrigerant velocities in the liquid lines are not a problem.
"Liquid line piping" shows the different pipe diameters based
on unit size and equivalent circuit length. These recommend-
ed diameters prevent refrigerant migration problems.
To determine the equivalent liquid line length the pressure
drop generated by the filter drier and the solenoid valves must
be take into consideration. The table above gives the equiva-
lent length for each unit, based on the diameter used.
Special attention must be paid to the liquid line sizing when
the expansion device is positioned higher than the condenser.
If the liquid refrigerant head is very high, it may even be
necessary to increase the subcooling to prevent a phase
change in the liquid line. This can be done e.g. by a liquid-
vapour heat exchanger or an additional coil.
15 - START-UP
15.1 - Preliminary checks
Never be tempted to start the unit without reading fully, and
understanding, the operating instructions and without having
carried out the following pre-start checks:
• Check the air handling units and all other equipment
connected to the unit.
• Refer to the manufacturer's instructions.
• Refer to the wiring diagram supplied with the unit.
• Ensure that there are no refrigerant leaks.
• Confirm that all pipe securing bands are tight.
• Confirm the the electrical connections are secure.
15.2 - Refrigerant charge adjustment
IMPORTANT: It is imperative to empty the holding charge
from the system and evacuate the system before beginning to
charge refrigerant into the unit. Never charge refrigerant into
the low-pressure side of the system.
During charging of refrigerant ensure that the indoor fan is
operating.
With all fans operating, adjust the refrigerant charge. Measure
the pressure at the liquid line service valve. If possible, mea-
sure the liquid line temperature as close as possible to the
service valve.
Add charge until the refrigerant passing through the sight
glass is 'clear': the refrigerant is now only liquid. When the
liquid line temperature is measured, it should now be possible
to calculate an actual subcooling value between 4 and 8 K,
based on the liquid line pressure drops (filter drier, solenoid
valves, special components and possible liquid head).
The actual subcooling is equal to the saturated temperature
at the bubble point, minus the liquid line temperature
measured. If the actual subcooling value is higher than 8 K,
(unit outlet) an excess charge is possible. This excess charge
translates to an excessive condensing pressure and increases
the compressor power input.
15.3 - Oil charge adjustment
If a suction riser is used, the oil charge must be adjusted.
Add the equivalent volume to half of the lower part of each
siphon trap (see figure below), when the unit is empty.
Oil volume for a siphon trap
22
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