JOHNSON CONTROLS
97
SECTION 6 – COMMISSIONING
FORM 150.27-NM1
ISSUE DATE: 09/30/2015
6
CHECKING SUPERHEAT AND SUBCOOLING
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 liquid line saturation temperature at the liquid stop
valve (liquid line saturation temp. is converted from a
temperature/pressure chart).
Example:
Liquid line pressure =
325 PSIG converted to temp. 101 °F
minus liquid line temp. - 89 °F
Subcooling = 12 °F
The subcooling should be adjusted to 12 °F at design
conditions.
1. Record the liquid line pressure and its corre-
sponding temperature, liquid line temperature
and subcooling below:
SYS 1 SYS 2
Liq Line Press = _______ _______ PSIG
Saturated Temp = _______ _______ °F
Liq Line Temp = _______ _______ °F
Subcooling = _______ _______ °F
After the subcooling is verified, the suction superheat
should be checked. The superheat should be checked
only after steady state operation of the chiller has been
established, the leaving water temperature has been
pulled down to the required leaving water temperature,
and the unit is running in a fully loaded condition. Cor-
rect superheat setting for a system is 10 °F – 15 °F (5.56
°C – 8.33 °C) 18” (46 cm) from the heat exchanger.
Superheat should typically be set for no
less than 10 °F with only a single com-
pressor running on a circuit.
The superheat is calculated as the difference between
the actual temperature of the returned refrigerant gas
in the suction line entering the compressor and the
temperature corresponding to the suction pressure as
shown in a standard pressure/temperature chart.
Example:
Suction Temp = 46 °F
minus Suction Press
105 PSIG converted to Temp - 34 °F
Superheat = 12 °F
When adjusting the expansion valve (TXV only), the
adjusting screw should be turned not more than one
turn at a time, allowing sufficient time (approximately
15 minutes) between adjustments for the system and
the thermal expansion valve to respond and stabilize.
Assure that superheat is set at a minimum of 10 °F (5.56
°C) with a single compressor running on each circuit.
2. Record the suction temperature, suction pres-
sure, suction saturation temperature, and super-
heat of each system below:
SYS 1 SYS 2
Suction Temp = _______ _______ °F
Suction Pressure = _______ _______ PSIG
Saturation Temp = _______ _______ °F
Superheat = _______ _______ °F
LEAK CHECKING
1. Leak check compressors, fittings, and piping to
ensure no leaks.
If the unit is functioning satisfactorily during the ini-
tial operating period, no safeties trip and the compres-
sors cycle to control water temperature to setpoint, the
chiller is ready to be placed into operation.
