SOLID STATE CONTROLS
A Honeywell W973 solid state logic panel controls the
economizer motor and stages of cooling and heating in
response to a signal from a dual set point thermostat located
in the controlled space or a dual set point transmitter with a
remote sensor located in the controlled space. To maintain
stable space temperatures, the logic panel balances the space
thermostat demand signal against the system output. System
output is measured by a temperature sensor located on the
discharge air duct. The combined demand and output signals
determine the economizer position and the number of heating
or cooling stages energized. The discharge sensor also
provides a positive modulating low limit signal to the logic panel,
insuring that the economizer will modulate closed if discharge
air gets too cold.
On a power failure, all stages go off and the damper motor goes
to the 0% outside air position. When power is restored, the
required stages sequence goes on with a time delay between
stages.
The operation of these units can be divided into four systems.
1. Economizer System (optional)
2. Heating System (optional)
3. Cooling System
4. Exhaust Air (optional)
ECONOMIZER SYSTEM
The Economizer System consists of:
1. Outdoor and Return Air Dampers
2. Damper Actuator (Spring Return)
3. Enthalpy Control - 5TH
4. Minimum Outdoor Air Adjuster (Potentiometer) - 2RH
The Economizer System provides the first stage of cooling
whenever the outdoor air is cool and dry enough to satisfy the
internal cooling demand. The outdoor and the return air
dampers are mechanically linked and are mounted by the
spring return damper actuator. As the outdoor air dampers are
opened by the damper actuator the return air dampers are
closed.
When the enthalpy control (5TH) senses outdoor air
temperatures per Table 5, the outdoor air dampers close to their
minimum position. The minimum position of the outdoor air
dampers is determined by the set point of the minimum outdoor
air adjuster (2RH). The minimum outdoor air adjuster is factory
mounted in the top of the damper motor.
The enthalpy control (5TH) is factory set. It senses both
temperature and humidity and varies its setting so as to limit
the introduction of outdoor air. As noted in Table 5, the control
will not allow outdoor air to enter the unit during high humidity
conditions except at a lower outdoor temperature. During dry
conditions, air is introduced at a higher ambient.
The outdoor and return air dampers are controlled by the logic
panel. The discharge sensor acts as a modulating low limit
which senses the temperature of the air leaving the evaporator
coil. If at any time during the cooling cycle the discharge air
temperature drops to 62ÉF, the economizer motor starts to
modulate closed. The economizer motor will be at minimum
position when discharge air temperature is 50ÉF.
If the economizer cannot satisfy the space demand for cooling,
mechanical cooling stages are energized as needed.
DAMPER LINKAGE ADJUSTMENT
After power has been supplied to the unit, the outside and
return air dampers should be checked to make sure they
operate freely and close tightly. It may be necessary to readjust
the linkage between the damper motor and the blades due to
loosening in shipment.
Readjust linkage as follows:
1. Turn the minimum outside air adjuster to the 100% outside
air position. All return air damper blades should be fully
closed. Be sure that the spring-return damper actuator has
completed its stroke (stopped running). If not, loosen the
drive rod bolt and let the damper motor drive the crankarm
until the motor stops running. Retighten the drive rod bolt
to secure the drive rod.
2. Turn the minimum outside air adjuster to the 0% outside air
position. All outside air damper blades should be fully
closed. The linkage connecting the outside air and the
return air dampers must move freely.
3. Return the minimum outside air adjuster to the 100%
outside air position and check for complete freedom of
linkage movement as the return air dampers close.
4. Set the minimum outside air adjuster for the minimum
ventilation requirement of the job.
HEATING SYSTEM
Two different types of heat are offered as factory-mounted
options. Refer to the appropriate instruction for information on
the heating system.
•
530.25-N1.1 - Gas-Fired Heating Option
•
530.25-N1.9 - Electric Heating Option
COOLING SYSTEM
These units have nominal cooling capacities of 30 and 40 tons.
The DSC360 units have one semi-hermetic compressor. Two
cooling stages are accomplished by unloading the compressor.
The DSC480 unit has two semi-hermetic compressors. Two
cooling stages are accomplished by cycling the individual
compressors.
530.25-N3Y
Unitary Products Group
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