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JOHNSON CONTROLS
17
FORM 150.63-NM6 (1020)
PWM INPUT
The PWM input allows reset of the discharge air
temperature setpoint (when unit is programmed for
Discharge Air Temperature Control mode) by supply-
ing a “timed” contact closure. Field wiring should be
connected to CTB1 - terminals 13 to 20. A detailed ex-
planation is provided in the Unit Control section. Refer
to Figure 5 and unit wiring diagram.
Load Limit Input
Load limiting is a feature that prevents the unit from
loading beyond a desired value. The unit can be “load
limited” either 33% or 66% on 3 or 6 compressor units,
50% on 2 or 4 compressor units, 40% or 80% on 5
compressor units, depending on the number of com-
pressors on the unit.The field connections are wired to
CTB1- terminals 13 to 21, and work in conjunction with
the PWM inputs. A detailed explanation is provided in
the Unit Control section. Refer to Figure 5 and unit
wiring diagram.
When using the Load Limit feature, the
PWM feature will not function - SIMUL-
TANEOUS OPERATION OF LOAD LIM-
ITING AND TEMPERATURE RESET
(PWM INPUT) CANNOT BE DONE.
COMPRESSOR HEATERS
Compressor heaters are standard on all models. If power
is OFF more than two hours, the crankcase heaters
must be energized for 18-24 hours prior to restarting a
compressor. This will assure that liquid slugging and
oil dilution does not damage the compressors on start.
REFRIGERANT PIPING
General
When the unit has been located in its final position,
the unit piping may be connected. Normal installation
precautions should be observed in order to receive
maximum operating efficiencies. System piping should
conform to the York DX piping guide form 050.40ES2 or
ASHRAE refrigeration handbook guidelines. All piping
design and installation is the responsibility of the user.
YORK ASSUMES NO WARRANTY RESPONSI-
BILITY FOR SYSTEM OPERATION OR FAIL-
URES DUE TO IMPROPER PIPING, PIPING
DESIGN, CONTROL PROBLEMS, OR LACK OF
OIL RETURN.
Filter driers and sight glasses are shipped loose for field
installation on each refrigerant circuit. Field refrigerant
piping can be connected to the condensing unit.
All expansion valves, liquid line solenoid valves, and
refrigerant piping are field supplied and installed. TXV
sizing should be equal in size or slightly smaller than
the capacity of the circuit. If multiple coil sections are
utilized, a TXV for each section, sized accordingly, must
be installed.
Table 4
lists refrigerant line connections sizes per unit
model number.
REFRIGERANT LINE SIZING
Refrigerant piping systems must be designed to provide
practical line sizes without excessive pressure drops,
prevent compressor oil from being “trapped” in the
refrigerant piping, and ensure proper flow of liquid re
-
frigerant to the thermal expansion valve. Considerations
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 & 6
provides 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 pip-
ing. These friction 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 2.
Table 1 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, suction
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 & 6 under column labeled “Nominal Tons
(kW) Unloaded.
1
Summary of Contents for YCUL0045
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