COOLING SYSTEM
The cooling section is a complete factory package utilizing an
air-cooled condenser. The system is factory-charged with
Refrigerant-22.
The compressors are hermetically sealed, internally sprung
and base-mounted with rubber-insulated hold-down bolts.
Compressors have inherent (internal) protection. If there is an
abnormal temperature rise in a compressor, the protector will
open to shut down the compressor.
PRELIMINARY OPERATION COOLING
After installation has been completed, energize the crankcase
heaters for at least four hours before operating the unit. After
the initial installation, the compressors should be given three
false starts (energized just long enough to make a few
revolutions) with 5-7 minutes delay between each start, before
being put into full time service.
NOTE: Prior to each cooling season, the crankcase heaters
must be energized at least 8 hours before system is
put into operation.
COOLING SEQUENCE OF OPERATION
NO OUTDOOR AIR OPTIONS - When the room thermostat
calls for “first-stage” cooling, the low voltage control circuit from
“R” to “G” and “Y1" is completed to energize compressors #1/2
and #5/6 (66% capacity), condenser fan motor #1, condenser
fan motor #2 (if the ambient temperature is above 60
°
F), and
the supply air blower motor (if the fan switch on the room
thermostat is set in the ”AUTO" position).
When the thermostat calls for “second-stage” cooling, the low
voltage control circuit from “R” to “Y2" is completed to energize
compressor #3/4.
NOTE: Unit is factory wired for two-stage cooling operation at
66/33% capacity, respectively. This can be altered for
two-stage cooling at 33/66% capacity or three-stage
cooling at 33/33/33% capacity per instructions on the
unit wiring label.
After the thermostat is satisfied and opens, all components will
stop simultaneously. The blower motor will continue to operate
if the fan switch on the room thermostat is set in the “ON”
position.
ECONOMIZER WITH SINGLE ENTHALPY SENSOR - When
the room thermostat calls for “first-stage” cooling, the low
voltage control circuit from “R” to “G” and “Y1" is completed.
The ”R" to “G” circuit energizes the blower motor (if the fan
switch on the room thermostat is set in the “AUTO” position)
and drives the economizer dampers from fully closed to their
minimum position. If the enthalpy of the outdoor air is below the
setpoint of the enthalpy controller (previously determined), “Y1"
energizes the economizer. The dampers will modulate to
maintain a constant supply air temperature as monitored by the
discharge air sensor. If the outdoor air enthalpy is above the
setpoint, ”Y1" energizes compressors #1/2 and #5/6 (66%
capacity), condenser fan motor #1, and condenser fan motor
#2 (if the ambient temperature is above 60
°
F).
When the thermostat calls for “second-stage” cooling, the low
voltage control circuit from “R” to “Y2" is completed. If the
enthalpy of the outdoor air is below the setpoint of the enthalpy
controller (i.e. first stage has energized the economizer), ”Y2"
will energize compressors #1/2 and #5/6 (66% capacity). If the
outdoor air is above the setpoint, “Y2" will energize compressor
#3/4.
Regardless of the enthalpy setpoint, if three stages of cooling
are utilized, Y3 will energize compressor #5/6.
After the thermostat is satisfied and opens, all components will
stop simultaneously. The blower motor will continue to operate
if the fan switch on the room thermostat is set in the “ON”
position.
ECONOMIZER WITH DUAL ENTHALPY SENSORS - The
operation with the dual enthalpy sensors is identical to the
single sensor except that a second enthalpy sensor is mounted
in the return air. This return air sensor allows the economizer
to choose between outdoor air and return air, whichever has
the lowest enthalpy value, to provide maximum operating
efficiency.
E CO N OM IZ ER (S IN GL E O R D UA L ) W IT H P OW ER
EXHAUST - This system operates as specified above with one
addition. The power exhaust motor is energized whenever the
economizer is chosen by the enthalpy sensor for first stage
cooling, “Y1". As always, the ”R" to “G” connection provides
minimum position but does not provide power exhaust
operation.
MOTORIZED OUTDOOR AIR DAMPERS - This system
operation is the same as the units with no outdoor air options
with one exception. When the “R” to “G” circuit is complete, the
motorized damper drives open to a position set by the damper
motor adjustment. When the “R” to “G” circuit is opened, the
damper spring returns fully closed.
CONTINUOUS BLOWER - Continuous blower operation is
possible by closing the R to G circuit on the thermostat.
SAFETY CONTROLS
Each refrigerant system is equipped with the following safety
controls:
1. A Suction Line Freezestat to protect against low evaporator
temperatures due to a low air flow or a low return air
temperature.
2. A High Pressure Cutout Switch to protect against excessive
discharge pressures due to a blocked condenser coil or a
condenser motor failure.
3. A Low Pressure Switch to protect against loss of refrigerant
charge.
If either one of the above safety controls opens, that individual
refrigerant system will be locked out. The other refrigerant
system will continue in operation unless it too is effected by the
same fault. The lock out of either system can be reset by
opening the 24V circuit either at the room thermostat or at the
unit disconnect.
ELECTRIC HEATING - SEQUENCE OF OPERATION
WITH POWER TO UNIT AND THERMOSTAT IN THE
HEATING MODE
Single-stage heating: (applies only to 18 KW heater, all other
heaters MUST use a two-stage thermostat:)
a) If the fan switch is in the “ON” position, the evaporator
blower motor contactor (3M) will be energized through
terminal G to provide continuous blower operation. If the
fan switch is in the “AUTO” position, the blower will operate
only when there is a call for heating by the thermostat.
NOTE: All 240 & 480V heaters are provided with manual
reset backup protection limits. These will de-ener-
gize the heaters should the primary limit fail to open
or the contactors fail to open in a failure mode.
b) Upon a call for heat by the thermostat, the heater contactor
(6M) will be energized.
c) The thermostat will cycle the electric heat to satisfy the
heating requirements of the conditioned space.
Two-stage heating: (applies to all heaters except 18 KW):
a) If the fan switch is in the “ON” position, the evaporator
blower motor contactor (3M) will be energized through
terminal G to provide continuous blower operation. If the
fan switch is in the “AUTO” position, the blower will operate
only when there is a call for heating by the thermostat.
b) Upon a call for first-stage heat by the thermostat, the heater
contactor (6M) (6M & 7M on 72 KW, 240V) will be
energized.
If the second stage of heat is required, heater contactor
(7M) will be energized. Note that on the 54 KW, 240V
heater, heater contactors (7M & 8M) will be energized and
OPERATION
530.18-N11Y
Unitary Products Group
15
