Honeywell R7426B Product Data page 18

R7426A,B,C TEMPERATURE CONTROLLER
Table 16. Throttling Range and Reset Time Reference
Application
R7426A
Main Temperature Control
High or Low Limit Temperature
Control
Master Cascade Control
Submaster Cascade Control
R7426B,C (Main Temperature Sequence Control)
Mixed Air Damper
Energy Recovery
Heating
Cooling
R7426B,C (Temperature Cascade Sequence Control)
Master
Submaster
- Mixed Air Dampers
- Energy Recovery
- Heating
- Cooling
To estimate the throttling range (Xp) for stable control under
all different load conditions, you must know the control or
correcting range (Xh).
Xh is the maximum air temperature increase (heating) or
decrease (cooling) produced by the coil, with a fully open
control valve.
Calculate Xp for discharge air control using the following:
X
h
X = ----- -
p
5
For room temperature control use the following:
∆t
X
h max
X = ----- - or ------------- -
p
10 10
NOTE: ∆t
(Xh) of the discharge air for mixed air damper
max
control is the maximum difference between OAT and
return air temperature.
The often specified accuracy for room control of ±2°F (where
Xp is 4.0°F) allows a discharge air alteration of 36°F.
P+I and proportional control can use the same throttling
range. Use the following formula for P+I control:
• Discharge air control: x ranges from 4 to 5.
• Room control: x ranges from 8 to 10.
63-2564—1
I01 x x
I02 x x
I01 x x
I02 x x
I01 x x
I01 x x
I01 x x
I01 x x
I01 x x
I02 x x
I02 x x
I02 x x
I02 x x
X
h
X = ----- -
p
x
18
Reset Time: tr1/tr2 (P.12/P13)
With P+I control, reset time (tr) is defined as the required time
after which the integral part is equal to the change due to the
proportional action for a predetermined step change in the
input variable. See Fig. 20.
The control parameter tr1 sets the reset time of the P+I main
temperature control loop.
For limit or submaster cascade control the control parameter
tr2 sets the reset time of these control loops (such as
discharge temperature I02. See Table 16.)
NOTE: If only proportional control is required, set tr to Off.
Xw
0
O
100
Xw (%)
P=
Xp
Fig. 20. Step change responses of
proportional and P+I controls.
SETTING GUIDELINES FOR P+I CONTROL RESET TIME
Adjust reset time (tr) to 2 to 3 times the response time (Tu).
NOTE: Tu is the time interval between the beginning of a
sustained disturbance (such as a rapid step change
of valve position) and the instant when the resulting
output signal change reaches a specified fraction of
its final steady-state value, either before or in the
absence of overshoot.
• Discharge air control: Tu normally ranges from 0.1 to 0.6
min. Therefore tr adjustment can range from 0.2 to 2 min.
• Room control: Tu ranges from 0.5 to 5 minutes. This
results in a tr setting of 1 to 15 minutes.
Minimum Position: MINPOS (P.14)
Adjustment of parameter MINPOS is available only with
R7426B,C controllers. MINPOS determines the outdoor air
damper actuator minimum position. It maintains the minimum
outdoor air damper setting even when temperature conditions
call for a fully closed position.
NOTE: Controller Off mode for plant/system shut off
overrides minimum position. When switched to Off,
actuators are driven fully-closed.
PROPORTIONAL CONTROL
t
P + I CONTROL
P
(%/S)
tr
P
t
tr
M17457