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AIR HANDLING SYSTEM CONTROL APPLICATIONS
In the following chart it is assumed that:
1. Desired space condition is 24 C DB and a maximum of
50% RH.
2. OA condition is 27 C DB and 26 C WB.
3. Air entering the system is from the ECONOMIZER
CYCLE DECISION application. The system operates on
25 percent OA during the cooling cycle.
4. Cooling coil leaving air temperature is 13 C.
3
COOLING COIL
DISCHARGE
13 C DB
WATER COIL FACE AND BYPASS SYSTEM CONTROL
Functional Description
COOL
HEAT
70
MA
90
Item
No.
Function
1
Control system energizes when fan is turned
on (See FAN SYSTEM START-STOP
CONTROL).
2-4
Space temperature PI control loops have heat
and cool setpoints with deadband.
5,6
Space humidity sensor and setpoint enable
dehumidification.
OA 27 C DB,
26 C WB
2
MA
1
RA 24 C DB,
5
50% RH
4
C3253
3
25
11
CONTROL
23.5
2
PROGRAM
22
4
32
7
8
SUPPLY
FAN
9
The following results are obtained:
Item
No.
Explanation
1
Mixed air temperature at cooling design
condition.
2
Air entering the cooling coil is cooled along a
line of constant moisture content until saturation
is approached. Near saturation the moisture con-
tent is reduced as the air is cooled. This process
involves both latent and sensible cooling.
3
The final leaving air temperature necessary to
satisfy space requirements will be maintained
by reheating along a constant moisture line.
4
Heating coil discharge.
5
This process line represents the increase in
temperature and humidity that occurs due to
the sensible and latent heat gains in the space.
6
50
COOL OR
50
DEHUMIDIFIER
DEMAND
5
0
60
SA
10
100
1
ON
7-9
Heating and cooling valves and face and
bypass dampers position for heating, cooling,
and dehumidifying cycles.
10
Stages chilled water and face-and-bypass
damper loading.
11
Control program coordinates cooling, heating,
dehumidifying, and fan and hot water
interlock control.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
244
10
CHW
FACE
VALVE
DAMPER
0
100
0
100
M15174
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