Honeywell AUTOMATIC CONTROL Engineering Manual page 259

temperature setpoint.
17 Free cooling demand varies SA temperature
setpoint.
18 Chilled water cooling demand varies SA
temperature setpoint.
19 Chilled water valve modulates to maintain SA
temperature cooling setpoint.
20 MA temperature sensor for operator
information.
21 OA temperature sensor for operator
information.
22 Control program coordinates space and supply
cooling, heating, and ventilation control, and
fan interlock.
FEATURES
1. Use of space control resetting SA temperature control
adds stability. The MA sensor is required to prevent
freeze-up if the free cooling setpoint is lowered in freezing
weather.
2. SA temperature is maintained only as high or low as
required to meet space requirements.
3. Free cooling cycle selection determined by the
economizer control program to minimize load on
mechanical cooling system.
4. Optimum comfort temperature provided during free
cooling cycle with energy conserving deadbands for
heating and cooling.
CONDITIONS FOR SUCCESSFUL OPERATION
1. Sensor locations must be selected to measure
representative air conditions.
2. SA temperature control loops should provide PID control
to assure stability with minimum offset.
NOTE: In cold climates this unit would most often have
the heating coil ahead of the cooling coil and the
low temperature switch after the heating coil.
Control would not change. In the configuration
shown, it would be possible to add a
dehumidification cycle with reheat (software) at a
later time.
SPECIFICATIONS
See FAN SYSTEM START-STOP CONTROL.
Anytime the supply fan runs, control system shall be enabled,
and mixing dampers shall position to minimum ventilation
position during scheduled occupancy periods.
Anytime the economizer decision program determines that
OA is suitable to assist in cooling, the temperature controls
shall be enabled to override the dampers minimum ventilation
AIR HANDLING SYSTEM CONTROL APPLICATIONS
position for free cooling as required.
The space temperature shall have a free cooling PID loop
setpoint selected to provide optimum occupant comfort
temperature. The space temperature shall have a chilled water
cooling PID loop setpoint adjustable to no lower than 1.5
degrees (minimum) above the free cooling setpoint. The space
temperature shall have a heating PID loop setpoint adjustable
to no higher than 1.5 degrees (minimum) below the free cooling
setpoint.
As the space heating load varies from 100 to 0%, an SA
heating PID control loop setpoint shall vary from 105°F to 55°F.
The hot water valve shall modulate to maintain the heating SA
temperature setpoint, except that anytime the space temperature
is greater than one degree above the free cooling space
temperature setpoint, the hot water valve control PID setpoint
shall be 52.5°F.
As the space free cooling load varies from 0 to 100%, an SA
free cooling PID control loop setpoint shall vary from 75°F to
57°F. The mixing dampers shall modulate to maintain the free
cooling SA temperature setpoint.
As the space chilled water cooling load varies from 0 to 100%,
an SA chilled water cooling PID control loop setpoint shall
vary from 75°F to 55°F. The chilled water valve shall modulate
to maintain this cooling SA temperature setpoint.
PSYCHROMETRIC ASPECTS
In the following chart it is assumed that:
1. OA condition is 64°F and 53% RH.
2. System is in the economizer mode.
The following results are obtained:
Item
COOLING COIL
DISCHARGE 59°F
2
1
COIL
100% OA
COOLING
64°F DB, 53% RH
LOAD
No. Explanation
1 100% economizer air is entering the cooling
coil.
2 The chilled water coil cools the entering air
to its 59°F setpoint (removing little moisture
because the water flow is low and the OA
moisture content is not high).
3 Space temperature is 76°F and 38% RH.
249 ENGINEERING CMANUAL OF AUTOMATION CONTROL
SPACE
3
76°F DB, 38% RH
M10465