Honeywell AUTOMATIC CONTROL, Engineering Manual

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ENGINEERING CMANUAL OF AUTOMATION CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
228
PREHEAT CONTROL PROCESSES
2. Accurate sizing of preheat coils is important. They should
be sized so it is possible to allow OA for cooling yet not
overheat the space.
In most of the following illustrations, preheat is shown in the
OA preceding the mixing of OA and RA. In complete HVAC
systems, the final preheat coil leaving air temperature setpoint
should be dictated by demands for heating or cooling.
PREHEAT CONTROL WITH FACE AND BYPASS DAMPERS
Functional Description
M10462
10
9
CONTROL
PROGRAM
CONTROL
PROGRAM
RA
MA SA
ON
3
8
1
11
2
40
50
VALVE POSITION
SCHEDULE
N.O.
N.O.
N.C.
N.C.
OUTDOOR
AIR
TEMPERATURE
VALVE
PERCENT
OPEN
50
50
44
35
0
100
4
6
7
5
66
NORMAL
OPEN
OA
PERCENT
OPEN
TO COIL
The preheat process heats the air in preparation for subsequent
conditioning. Preheat is sometimes necessary when high
percentages of low temperature OA must be handled by the
system. Considerations before a preheat component is installed
in an air conditioning system are:
1. Preheat coils are often exposed to subfreezing
temperatures. Provision to prevent freezing the coils must
be made.
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 (in some cases the OA damper
may be part of a mixed air system).
4 OA temperature determines valve position.
5 Valve controls heating medium flow to coil.
6 Control program coordinates fan interlock and
valve control.
7 Valve position determined by OA
temperature.
8-10 Face-and-bypass dampers controlled to
maintain face-and-bypass leaving air
temperature setpoint.
11 Control program coordinates fan interlock and
face-and-bypass damper control.