Pneumatic Control System Example
70-6925 153
Humidistats
and
Thermostats Controllers Sensors Relays Switches Actuators Valves Accessories
Engineering
Guide
Cross
Reference
PNEUMATIC CONTROL SYSTEM
EXAMPLE
The following is an example of a typical air handling system
(Fig. 74) with a pneumatic control system. The control system is
presented in the following seven control sequences (Fig. 75
through 79):
— Start-Stop Control Sequence.
— Supply Fan Control Sequence.
— Return Fan Control Sequence.
— Warm-Up/Heating Coil Control Sequence.
— Mixing Damper Control Sequence.
— Discharge Air Temperature Control Sequence.
— Off/Failure Mode Control Sequence.
Controls are based upon the following system information and
control requirements:
System Information:
— VAV air handling system.
— Return fan.
— 35,000 cfm.
— 4,000 cfm outside air.
— 3,000 cfm exhaust air.
— Variable speed drives.
— Hot water coil for morning warm-up and to prevent discharge
air from getting too cold in winter.
— Chilled water coil.
— Fan powered perimeter VAV boxes with hot water reheat.
— Interior VAV boxes.
— Water-side economizer.
— 8:00 A.M to 5:00 P.M. normal occupancy.
— Some after-hour operation.
Control Requirements:
— Maintain design outside air airflow during all levels of supply
fan loading during occupied periods.
— Use normally open two-way valves so system can heat or
cool upon compressed air failure by manually running
pumps and adjusting water temperatures.
— Provide exhaust/ventilation during after-hour occupied
periods.
— Return fan sized for 35,000 cfm.
Fig. 74. Typical Air Handling System.
Start-Stop Control Sequence
Fans 1M through 3M (Fig. 75) operate automatically subject to
starter-mounted Hand-Off-Automatic Switches.
The Supply Fan 1M is started and controls are energized by
Electric-Pneumatic Relay 2EP at 0645 by one of the following:
— An Early Start Time Clock 1TC
— A drop in perimeter space temperature to 65F at Night
Thermostat TN
— An after-hour occupant setting the Spring-Wound Interval
Timer for 0 to 60 minutes.
The Supply Fan 1M operation is subject to manually reset
safety devices including Supply and Return Air Smoke
Detectors a heating coil, leaving air, Low Temperature
Thermostat; and a supply fan discharge, duct High Static
Pressure Cut-Out.
Any time the Supply Fan 1M runs, the Return Fan 2M runs.
Any time the Return Fan 2M runs, the Exhaust Fan 3M and the
ventilation controls are energized by the After-Hours Interval
Timer or by the Occupancy Schedule Time Clock 2TC set for
0750.
Both Clocks 1TC and 2TC are set to shut the system down at
1700.
GRAVITY
RELIEF
RETURN
AIR
RETURN FAN
SUPPLY FAN
EAST
ZONE
DISCHARGE
AIR
WEST
ZONE
MIXED
AIR
OUTSIDE
AIR
EXHAUST
M10298
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