JOHNSON CONTROLS
44
FORM 100.50-NOM12
ISSUE DATE: 04/02/2019
SECTION 3 – START-UP
e. After the system is running and the pressures
have stabilized, measure the temperature at
the liquid and common suction lines near the
Schrader service valves.
f. Calculate evaporator superheat and condens-
ing subcooling. Both should be approxi-
mately 15.0°F.
Refer to the proceeding sec-
tion Checking Superheat and Subcooling for
information on how to calculate evaporator
superheat and condenser subcooling
.
g. Repeat the above process for each of the re-
frigerant systems.
6. With an ammeter, verify that each phase of the
condenser fans, compressors, supply fan, and ex-
haust fan are within the RLA/FLA as listed on the
unit data plate.
Refrigerant Charge
This rooftop unit comes fully charged from the factory
with refrigerant R-410A as standard.
Checking Superheat and Subcooling
An R-410A temperature and pressure chart lists the as-
sociated
saturation
temperature in one column, with
the associated pressure in another column. As a result,
only one temperature/pressure column is needed to
show the relationship.
Subcooling (R-410A)
When the refrigerant charge is correct, there is no va-
por in the liquid sight glass with the system operating
under full load conditions.
The subcooling temperature of each system can be cal-
culated by recording the temperature of the liquid line
at the outlet of the condenser and subtracting it from
the saturation temperature listed in
, for the corresponding discharge pressure.
Example:
On a YPAL070 the liquid pressure is 375 PSIG and the
liquid temperature is 97.0°F.
Saturation Temperature for 375 PSIG
= 112.0°F
Minus the liquid line temp
= 97.0°F
Liquid Line Subcooling of
= 15.0°F
The subcooling should be 15.0°F at design conditions.
Superheat (R-410A)
The superheat should be checked only after steady
state operation of the unit has been established, the dis-
charge air temperature has been pulled down to within
the control range, and the unit is running in a fully
loaded condition.
The superheat is calculated as the difference between
the actual temperature of the refrigerant gas in the suc-
tion line and the temperature corresponding to the Suc-
tion Pressure as shown in
Example:
The suction pressure is 130 PSIG and the suction line
temperature is 57.0°F.
Suction Line Temperature
= 57.0°F
Saturation Temperature for 130 PSIG
= 45.0°F
Evaporator Superheat
= 12.0°F
When adjusting the expansion valve, the adjusting
screw should be turned not more than one turn at a
time, allowing sufficient time (approximately 15 min-
utes) between adjustments for the system and the ther-
mal expansion valve to respond and stabilize.
The superheat setting should be adjusted to 12.0°F at
design conditions.