Honeywell AUTOMATIC CONTROL Engineering Manual page 239

FEATURES
1. Preheat coil conditions large quantities of low temperature
OA before it enters the system.
2. Bypass damper controls temperature of air leaving the
face-and-bypass section without endangering the preheat
coil which operates in full valve open mode during
freezing conditions.
3. Upon fan shutdown, valve controls at 100°F leaving air
temperature in freezing weather and positions closed upon
fan shutdown in non-freezing weather.
CAUTION: If steam (or very hot water) valves
position full open during off periods, air
temperatures may melt fire damper
linkages.
CONDITIONS FOR SUCCESSFUL OPERATION
1. Preheat coil sized to raise a maximum quantity of OA
from outdoor design temperature to 50°F.
2. Averaging element sensor positioned in face-and bypass
air to sense average air temperature.
3. Bypass air well mixed with face (and return) air prior to
entering down-stream water coils (additional low
temperature controls may be required for down-stream
water coils).
4. On large 100% OA systems the OA damper opens prior
to fan start up to protect ducting from collapse.
5. The setpoint of air leaving the preheat section is usually
dictated by the downstream AHU temperature controls.
SPECIFICATIONS
See FAN SYSTEM START-STOP CONTROL.
Anytime the supply fan runs, heating control shall be enabled,
and the OA dampers shall open for ventilation requirements.
The heating coil valve shall open upon loss of actuator motive
force, shall close upon fan shutdown if the OA temperature is
above 35°F, and shall control to maintain 100°F leaving air
temperature upon fan shutdown if the OA temperature is below
35°F.
AIR HANDLING SYSTEM CONTROL APPLICATIONS
The heating coil valve shall position from 0 to 100% open as
the OA temperature varies from 50°F to 35°F. The face-and-
bypass dampers shall modulate to maintain an average face-
and-bypass leaving air temperature of 50°F.
PSYCHROMETRIC ASPECTS
In the following chart it is assumed that:
1. The coil has a 50 Fahrenheit degree temperature rise with
full air flow 0°F air and a 70 Fahrenheit degree
temperature rise with reduced air flow as in the example.
2. The coil water flow modulates from closed to open as
OA temperature drops from 50°F to 35°F.
3. OA temperature is 30°F DB.
2
AIR LEAVING
PREHEAT
1
SECTION
OA AND AIR
LEAVING BYPASS
SECTION 30°F DB
The following results are obtained:
Item
No. Explanation
1 Heating of OA occurs along a line of constant
moisture content from 30°F to 100°F.
2 The PID control loop maintains the
temperature of the preheat section leaving air.
229 ENGINEERING CMANUAL OF AUTOMATION CONTROL
AIR LEAVING
HEATING COIL
100°F DB
C2552-2