Control From Preheat Leaving Air - Honeywell AUTOMATIC CONTROL Engineering Manual

AIR HANDLING SYSTEM CONTROL APPLICATIONS

CONTROL FROM PREHEAT LEAVING AIR

Functional Description
3
OPEN
OA
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.
ENGINEERING CMANUAL OF AUTOMATION CONTROL
7
CONTROL
PROGRAM
4
N.O.
N.C.
NORMAL
6
56
RA
56
5
MA
2
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 35°F, and shall control at a leaving air temperature
setpoint of 100°F upon fan shutdown if the OA temperature is
below 35°F.
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 35 degree Fahrenheit temperature
rise.
2. The preheat coil valve modulates to maintain leaving
air temperature.
3. RA condition is 75°F DB and 62.5°F WB.
4. The MA is composed of 50 percent preheated air and 50
percent RA.
5. OA condition is 35°F DB and 29°F WB (design).
230
SA
ON
1
M10463