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ENGINEERING MANUAL OF AUTOMATIC CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
230
CONTROL FROM PREHEAT LEAVING AIR
Functional Description
Item
No.
Function
1-2
Control system energizes when fan is turned
on (See FAN SYSTEM START-STOP
CONTROL).
3
OA damper opens on fan startup, closes upon
fan shutdown.
4-6
Heating valve modulates to maintain heating
coil leaving air temperature setpoint.
7
Control program coordinates fan interlock and
valve control.
FEATURES
1. A preheat coil conditions large quantities of low
temperature OA before it enters the system.
2. A fixed amount of OA for ventilation is delivered
whenever the fan is on.
CONDITION FOR SUCCESSFUL OPERATION
1. The temperature is limited to a value low enough to have
the coil valve full open at freezing temperatures.
2. Water temperature reset and/or a recirculating pump is
very helpful in keeping high flow at varying OA
temperatures. If a large temperature rise is required, use
an arrangement of multiple inline coils.
3. The control network sends a command to start the hot
water system anytime the hot water valve is not closed.
NOTE: If a variable flow hot water system exists, it may be
preferable to have the hot water valve position to
approximately 25% open (rather than full open) upon
fan shutdown in freezing weather. This saves pump
energy and often prevents the need for multiple pumps.
LIMITATIONS
If too high a temperature rise is used, the valve may short cycle
or slow down the water in the coil and allow the coil to freeze.
SPECIFICATIONS
See FAN SYSTEM START-STOP CONTROL.
Anytime the supply fan runs, heating control shall be enabled,
and the OA dampers shall open for ventilation requirements.
The heating coil valve shall be modulated by a PID control
loop to maintain the coil leaving air temperature setpoint.
The heating coil valve shall open upon loss of actuator motive
force, shall close upon fan shutdown if the OA temperature is
above 1.5
°
C, and shall control at a leaving air temperature
setpoint of 38
°
C upon fan shutdown if the OA temperature is
below 1.5
°
C.
PSYCHROMETRIC ASPECTS
The coil discharge and MA conditions can be distributed over
considerable area on the psychrometric chart depending on the
entering air condition and the temperature rise through the coil.
In the following chart it is assumed that:
1. The preheat coil has a temperature rise of 19 kelvins.
2. The preheat coil valve modulates to maintain leaving
air temperature.
3. RA condition is 24
°
C DB and 17
°
C WB.
4. The MA is composed of 50 percent preheated air and 50
percent RA.
5. OA condition is 1.5
°
C DB and –1.5
°
C WB (design).
M15165
5
6
CONTROL
PROGRAM
RA
MA
SA
ON
3
4
1
7
2
13
N.O.
N.C.
13
NORMAL
OPEN
OA
Summary of Contents for AUTOMATIC CONTROL SI Edition
Page 1: ...AUTOMATIC CONTROL for ENGINEERING MANUAL of COMMERCIAL BUILDINGS SI Edition ...
Page 4: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL iv ...
Page 6: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL vi ...
Page 46: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL CONTROL FUNDAMENTALS 36 ...
Page 66: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL PSYCHROMETRIC CHART FUNDAMENTALS 56 ...
Page 128: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL ELECTRIC CONTROL FUNDAMENTALS 118 ...
Page 158: ...MICROPROCESSOR BASED DDC FUNDAMENTALS 148 ENGINEERING MANUAL OF AUTOMATIC CONTROL ...
Page 208: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL BUILDING MANAGEMENT SYSTEM FUNDAMENTALS 198 ...
Page 493: ...INDEX ENGINEERING MANUAL OF AUTOMATIC CONTROL 483 INDEX ...
Page 506: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL INDEX 496 NOTES ...
Page 507: ...INDEX ENGINEERING MANUAL OF AUTOMATIC CONTROL 497 NOTES ...
Page 508: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL INDEX 498 NOTES ...
Page 509: ...INDEX ENGINEERING MANUAL OF AUTOMATIC CONTROL 499 NOTES ...
Page 510: ...ENGINEERING MANUAL OF AUTOMATIC CONTROL INDEX 500 NOTES ...
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