York YLUA0248 Series, Установка, эксплуатация и обслуживание

Страница: 40 / 158

JOHNSON CONTROLS
40
FORM 150.73-NM2
ISSUE DATE: 09/07/2020
SECTION 4 – INSTALLATION
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 proper 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 4 on page 39 and Table 5 on page 41 pro-
vide the pressure drops for given pipe sizes for both
liquid and suction lines. The pressure drops given are
per 100 ft. (30.5 m) of refrigerant piping. These fric-
tion losses do not include 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 on page 38 .
Table 1 on page 38 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 40 psi (276 kPa). 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. See
Table 4 on page 39 and Table 5 on page 41 under
column labeled “Nominal Tons (kW) Unloaded.
For
more details,
refer to “ASHRAE Refrigeration Hand-
book, 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 many cases a double
suction riser must be installed to ensure reliable oil re-
turn at reduced loads.
Table 4 on page 39 and Table
5 on page 41 indicate when a double suction riser
should be used for listed pipe sizes to provide adequate
oil return at reduced loads. The calculated information
was based on maintaining a minimum of 1000 fpm
(5.08 m/s) refrigerant vapor velocity at full load.
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” per foot (2 cm/m) in the direction of the refriger-
ant flow to aid in the return of oil to the compressor. All
suction lines with a vertical rise exceeding 3 feet (.91
meters) 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 feet (7.6 meters) should be
trapped every 15 feet (4.6 meters).
For more details,
refer to ASHRAE Refrigeration Hand-
book. System Practices for Halocarbon Refrigerants.
On systems where oil return is a problem,
oil separators may be required. However,
if piping design is poor, even with a sepa-
rator, oil may be lost into the system over
time, which may cause compressor fail-
ure.
REFRIGERANT CHARGE
The condensing unit is charged with nitrogen a hold-
ing charge. The operating charge for the condensing
unit, evaporator coil, and refrigerant piping must be
“weighed-in” after all refrigerant piping is installed,
leak checked, and evacuated.
70% of the calculated charge must be
added prior to starting a system. Failure
to add 70% of the charge may cause com-
pressor overheating when the system is
first started.
Final adjustment of refrigerant charge should be veri-
fied by subcooling values (refer to section on Pre-
Startup for checking subcooling). See Table 2 on page
38 for Refrigerant Line Charges.
FILTER DRIERS/ SIGHT GLASSES/ TXVS
Liquid line filter driers, sight glass, and TXV’s are
field supplied for each refrigerant circuit.