Table of Contents
Advertisement
Form QCC2-NM1
Issue Date: 5/20/2021
Refrigerant line sizing
Refrigerant piping systems must be designed to pro-
vide practical line sizes without excessive pressure
drops, prevent compressor oil from being "trapped" in
the refrigerant piping, and ensure correct flow of liquid
refrigerant to the thermal expansion valve. Consider-
ations should be given to:
1. Suction line pressure drop due to refrigerant flow.
2. Suction line refrigerant velocity for oil return.
3. Liquid line pressure drop due to refrigerant flow.
4. Liquid line pressure drop (or gain) due to vertical
rise of the liquid line.
Table 5 on page 39 provides the pressure drops for
given pipe sizes for both liquid and suction lines. The
pressure drops given are per 100 equivalent ft. (30.5 m)
of refrigerant piping. These friction losses do not in-
clude any allowances for strainer, filter drier, solenoid
valve, isolation valve, or fittings.
Nominal pressure drop for solenoids, sight glass, and
driers are shown in Table 3.
Table 1 on page 33 includes approximate equivalent
lengths for copper fittings.
To ensure a solid column of liquid refrigerant to the ex-
pansion valve, the total liquid line pressure drop should
never exceed 50 psi (3.4 bar). Refrigerant vapor in the
liquid line will measurably reduce valve capacity and
poor system performance can be expected.
To allow adequate oil return to the compressor, suc-
tion risers should be sized for a minimum of 1000 FPM
(5.08 m/s) while the system is operating at minimum
capacity to ensure oil return up the suction riser. Refer
to Table 5 under column labeled "Nominal Tons (kW)
Unloaded. For additional details, refer to "ASHRAE
Refrigeration Handbook, Chapter 2"
Evaporator below condensing unit
On a system where the evaporator is located below the
condensing unit, the suction line must be sized for both
pressure drop and oil return. In some cases a double
suction riser must be installed to ensure reliable oil re-
turn at reduced loads. Table 5 indicates when a double
suction riser should be used for listed pipe sizes to pro-
vide adequate oil return at reduced loads. The calculat-
ed information was based on maintaining a minimum
of 1000 fpm (5.08 m/s) refrigerant vapor velocity at
full load.
QUANTECH
Condenser below evaporator
When the condensing unit is located below the evapo-
rator, the liquid line must be designed for both friction
loss and static head loss due the vertical rise. The value
of static head loss of 0.5 psi/ft.(3.4 kPa/30 cm) must be
added to the friction loss pressure drop in addition to
all pressure drops due to driers, valves, etc.
Oil traps
All horizontal suction lines should be pitched at least
1/4 in. per foot (6 mm) in the direction of the refrig-
erant flow to aid in the return of oil to the compres-
sor. All suction lines with a vertical rise exceeding 3 ft
(0.91 m) should have a "P" trap at the bottom and top
of the riser to facilitate oil return. Suction lines with a
vertical rise exceeding 25 ft (7.6 m) should be trapped
every 15 ft (4.6 m).
For more details, refer to "ASHRAE Refrigeration
Handbook, System Practices for Halocarbon Refriger-
ants".
On systems where oil return is a problem, oil separa-
tors may be required.
Refrigerant charge
The condensing unit is charged with a nitrogen hold-
ing charge. The remaining operating charge for the
condensing unit, evaporator coil, and refrigerant pip-
ing must be "weighed-in" after all refrigerant piping is
installed, leak checked, and evacuated.
70% of the calculated charge must be
added before starting a system. Failure
to add 70% of the charge may cause
compressor overheating when the system
is first started.
Final adjustment of the refrigerant charge should be
verified by subcooling values (refer to "Pre-Startup"
in Section 6 for checking subcooling). See Table 2 on
page 33 for Refrigerant Line Charges.
Refrigerant piping reference
For more details, refer to ASHRAE Refrigeration
Handbook, Chapter 2.
Filter driers / sight glasses/ Txv's
Liquid line filter driers, sight glass, and TXV's are
field supplied for each refrigerant circuit.
SECTION 4 – INSTALLATION
35
4
Advertisement
Table of Contents
Troubleshooting
