Honeywell AUTOMATIC CONTROL, Engineering Manual

Page: 249 / 518

ENGINEERING CMANUAL OF AUTOMATION CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
239
TWO-POSITION CONTROL OF DIRECT EXPANSION COIL SYSTEM—
MODULATING FACE AND BYPASS DAMPER
Functional Description
M10471
7
MA
SA
ON
1
9
8 10
MINIMUM
ON TIME
MINIMUM
OFF TIME
COMPRESSOR
(MINUTES)
8
5
10
4
6
RELAY
RELAY
OPEN
CONTROL
PROGRAM
3
2
76
76
81
SUPPLY
FAN
TO COMPRESSOR
START CIRCUIT
80
30
FACE DAMPER
PERCENT OPEN
SOLENOID
POSITION
OPEN
CLOSED
Item
No. Function
1 Control system energizes when fan is turned
on (See FAN SYSTEM START-STOP
CONTROL).
2,3 Space temperature sensor inputs to PID
control loop to maintain setpoint.
4 Face and bypass damper modulates for
cooling control.
5 Damper position determines compressor mode.
6,7 Refrigerant liquid line solenoid valve cycles
for cooling.
8 Relay energizes compressor.
9 Timers protect compressor from short cycling.
10 Control program coordinates temperature,
compressor, and fan interlock control.
FEATURES
1. The proportions of air passing through and around the
coil are varied by modulating face and bypass dampers.
2. Lowering air velocity through the coil lowers the moisture
content of air leaving the coil, thus producing lower space
RH than systems that only cycle the refrigeration at a
constant air volume.
CONDITIONS FOR SUCCESSFUL OPERATION
1. Capacity control of refrigeration provided to avoid icing
under light load.
LIMITATIONS
1. Direct expansion coils are difficult to control from SA
due to the large and rapid temperature drop when
energized.
2. Compressor operating and safety controls must be
incorporated into the control system.
3. The system may be controlled from SA if necessary, but
only if the sensor is located far enough downstream of
the coil to ensure complete mixing of the air and software
timers are provided to prevent compressor short cycling.
Hot gas bypass may be required.