ENGINEERING CMANUAL OF AUTOMATION CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
247
The following results are obtained:
Item
No. Explanation
5Mixed air condition is 59°F DB and 53°F WB.
6 SA condition is 75°F DB, and 0.0074 pound of
moisture per pound of dry air.
C2567-1
SA 75
°
F DB, 59.5
°
F WB
0.0074 LB H
2
O/LB DRY AIR
MA 59
°
F DB, 53
°
F WB
RA 72°F DB, 56°F WB
LIGHT SENSITIVE LOAD
5
6
OA
55°F DB,
52.5°F WB
FEATURES
1. The outdoor air quantity is modulated from a minimum
to take advantage of free cooling based on space
temperature demand.
2. The SA temperature will not fall below a set minimum.
3. Air is supplied at the temperature necessary to make up
the sensible heat loss of the space.
4. Space relative humidity is maintained at a minimum value
by a space humidity controller controlling the humidifier.
5. Separate setpoints for heating and cooling.
CONDITIONS FOR SUCCESSFUL OPERATION
See FAN SYSTEM START-STOP CONTROL.
1. An appropriate schedule of settings.
2. The low temperature limit controller located to respond
to the lowest temperature in the duct.
LIMITATIONS
This application used in applications that do not require
mechanical cooling.
SPECIFICATIONS
Anytime the supply fan runs, control system shall be enabled.
Anytime the space relative humidity drops below the setpoint,
the space humidity PI loop shall modulate the humidifier,
subject to an SA humidity high limit override set at 88%.
As the space heating load varies from 0 to 100%, the SA
heating PI loop setpoint shall be reset from 55°F to 105°F. The
hot water valve shall be modulated as required to maintain the
SA temperature setpoint.
Anytime the fan runs, the mixing dampers shall position to a
minimum ventilation setting.
As the space cooling load varies from 0 to 100%, the SA
cooling PI loop setpoint shall be reset from 80°F to 55°F. The
outdoor and return (and relief if applicable) air dampers shall
be modulated as required to maintain the SA temperature
setpoint.
Cooling damper control shall be returned to minimum
position anytime the economizer mode is disabled.
Separate space heating and cooling setpoints shall be
provided.
PSYCHROMETRIC ASPECTS
In the following charts it is assumed that:
1. Design outdoor air condition is 0°F DB and 50 percent
relative humidity.
2. SA condition at design load is 90°F DB and 0.0066 pound
of moisture per pound of dry air.
3. Light load outdoor air condition is 55°F DB and 52.5°F
WB.
4. SA condition at light load is 75°F DB and 0.0066 pound
of moisture per pound of dry air.
5. Minimum outdoor air is 25 percent.
6. RA condition is 74°F DB and 35% RH.
Design Load
C2566-1
HEAT
RA 74°F DB,
35% RH
DESIGN LOAD
1
2
3
4
OA
0°F DB,
50% RH
89.5°F DB,
60.5°F WB
MA 54°F DB,
45.5°F WB
MIX
HUMIDIFY
SA 90°F DB,
0.0066 LB H
2
O/LB DRY AIR
The following results are obtained:
Item
No. Explanation
1 Design outdoor air condition is 0°F DB and 50
percent relative humidity.
2Mixed air condition is 54°F DB and 45.5°F WB.
3 Heated air condition is 89.5°F DB and 60.5°F
WB.
4 SA condition is 90°F DB and 0.0066 pound of
moisture per pound of dry air.
Light Sensible Load
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