ELECTRO-MECHANICAL CONTROLS
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. Mixed Air Controller - 1RH.
5. Minimum outside air adjuster (potentiometer) - 2RH.
The Economizer System provides the first stage of cooling
whenever the outside air is cool and dry enough to satisfy the
internal cooling demand. The outside and the return air
dampers are mechanically linked and are modulated by the
spring return damper actuator. As the outside air dampers are
opened by the damper actuator, the return air dampers are
closed.
When the enthalpy control (5TH) senses outside air
temperatures per Table 5, the outside air dampers close to their
minimum position. The minimum position of the outside air
dampers is determined by the set point of the minimum outside
air adjuster (2RH) located in the terminal box 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 outside air. As noted in Table 5, the control
will not allow outside air to enter the unit during high humidity
conditions except at a lower outside temperature. During dry
conditions, air is introduced at a higher ambient.
The outside and the return air dampers are controlled by a
proportional type, mixed-air controller (1RH) which senses the
temperature of the air leaving the evaporator coil. The dampers
can modulate anywhere from their minimum outside position
to 100% open in attempting to maintain the temperature setting
of 1RH. If the indoor space temperature rises, TC2 will close
and the No. 1 compressor will be energized through the 5R
relay contacts to provide second stage cooling.
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.
ECONOMIZER RAIN HOOD OPTION
The outdoor and return air dampers, the spring-return damper
actuator, the linkage, the enthalpy changeover control, the
mixed air temperature control and the minimum position
potentiometer with manual auto switch are factory mounted as
part of the economizer option. The rain hood is shipped in a
separate package and must be assembled to the unit per the
assemby instructions in Form 530.25-N1.4.
CAUTION: Never operate the unit without installing the hood or
moisture will be drawn into the unit.
FIXED OUTSIDE AIR RAIN HOOD
The fixed outdoor air damper with a manual adjustment arm is
factory mounted on all units without the economizer operation.
The rain hood is shipped in a separate package and must be
assembled to the unit per the assembly instructions in Form
530.25-N1.3.
CAUTION: Never operate the unit without installing the hood or
moisture will be drawn into the unit.
EXHAUST AIR RAIN HOOD OPTION
The barometric exhaust dampers, the powered propeller ex-
haust fan and the damper motor end switch are factory installed
and wired as part of the exhaust air option. The rain hood is
shipped in a separate package and must be assembled to the
unit per the assembly instructions in Form 530.25-N1.2.
NOTE: The exhaust hood for the DSC480 is shipped in two
packages.
CAUTION: Never operate the unit without installing the hood or
moisture will be drawn into the unit.
OPERATION
OUTDOOR DRY
BULB TEMP. ÉF
% RELATIVE
HUMIDITY
62
80
67
60
71
40
73
20
74
10
TABLE 5 - ENTHALPY CONTROL (SET POINT "B")
530.25-N3Y
Unitary Products Group
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