ENGINEERING CMANUAL OF AUTOMATION CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
221
SPECIFICATIONS
See FAN SYSTEM START-STOP CONTROL.
Anytime the supply fan runs, the OA, exhaust, and RA
dampers shall position to a minimum ventilation position and
shall be further modulated by an MA PID control loop to
maintain the MA temperature setpoint. Anytime the OA
conditions rise above the economizer setpoint, the MA
temperature control shall be disabled.
PSYCHROMETRIC ASPECTS
In the following chart it is assumed that:
1. The manual positioning setpoint value provides 25
percent minimum OA.
2. The MA controller is set at 55°F.
3. The OA controller closes the OA damper to the minimum
position when OA temperature is 80°F.
4. RA condition is 75°F DB and 59.5°F WB for winter;
80°F DB and 66.5°F WB for summer.
5. OA condition is 35°F DB and 29°F WB for winter; 100°F
DB and 74°F WB for summer.
6. Other components exist in the complete system which
hold the RA at the desired condition.
7. The desired MA temperature can be maintained during
economizer periods until the OA temperature falls below
the temperature at which only minimum OA is admitted
if the OA is less than 55°F.
C2546-1
MA WINTER
RA WINTER
75°F DB, 59.5°F WB
RA SUMMER
80°F DB, 65.5°F WB
MA SUMMER
OA SUMMER
100°F DB, 74°F WB
80°F DB OA
CONTROLLER
SETPOINT
1
2
3
4
OA WINTER
35°F DB, 29°F WB
55°F DB MA
CONTROLLER
SETPOINT
The following results are obtained:
Item
No. Explanation
1At OA temperatures below -5°F DB (25
percent OA set by manual positioning setpoint
value), the MA condition lies to the left of the
55°F DB line.
2 As the OA temperature varies between –5°F
and 55°F, MA conditions lie on the 55°F DB
line.
3 As the OA temperature varies between 55°F
and 75°F, 100 percent OA is admitted and the
MA lies in the area between 55°F and 75°F DB.
4 As the OA temperature rises above 80°F DB,
the system operates on 25 percent OA and the
MA temperature varies from 95°F to 85°F DB.
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