Honeywell R7426B Throttling Range Xpc / Xph P.10 / P11, Reset Time tr1 / tr2 P.12 / P13, Tr1 Tr2

R7426A,B,C TEMPERATURE CONTROLLER WITH AND WITHOUT REAL TIME CLOCK

limit or cascade sensor (T2) to operate the output device from full open (100%) to full closed (0%) or vice versa.

Xp1 is the throttling range for the main control loop, Xp2 is used if limitation or cascade control (submaster control loop) is active (see Table 11).

Table 11. Throttling range and reset time reference

application	sens.	Xp1	Xp2	Xpc	Xph	tr1	tr2
R7426A Controller
Main Temp.Control	T1	x				x
High or Low Limit	T2		x				x
Temperature Control
Cascade Control
Master	T1	x				x
Submaster	T2		x				x
R7426B,C Controller
Main Temp. Seq. Control
Mixed Air Damper	T1	x				x
Energy Recovery	T1	x				x
Heating	T1				x	x
Cooling	T1			x		x
R7426B,C Controller
Temperature Cascade
Sequence Control
Master	T1	x				x
Submaster
- Mixed Air Dampers	T2		x				x
- Energy Recovery	T2		x				x
- Heating	T2				x		x
- Cooling	T2			x			x

Throttling Range Xpc / Xph (P.10 / P11)

The control parameters Xpc and Xph are only available on R7426B,C controllers and are used to set the cooling and heating throttling ranges for the following applications

•Temperature sequence control with heating, mixed air dampers, and cooling (see Fig. 3 and Table 11)

•Temperature cascade control with heating, mixed air dampers, and cooling (see Table 11)

In applications without cooling, the throttling range Xpc must be set to OFF 100% fresh air supply at actual temperature above the control point is required (outdoor and return air dampers fully open).

Y[%]
100		1/4 Xp1		if Xpc = Off
	Damper
	Y1
	Y3			Y2
MINPOS	Heating			Cooling
				if Xpc = Off
0
				Xwh / Xwc /Xwd
	CTRPH	CTRP1	CTRPC
	Xph	Xp1		Xpc	T2

Fig. 3. Temperature sequence control with heating, mixed

air dampers, and cooling valve

Setting Guidelines for Proportional Band of P and P+I Control

To estimate the proportional band (throttling range Xp) for stable control under all different load conditions, the control or correcting range Xh of the heating or cooling coil must be known. This is the maximum air temperature increase pro- duced by the heating coil or decrease of a cooling coil if the control valve is fully open.

The proportional band Xp for discharge air control can be calculated by using the following rule-of thumb formula:

Xp = Xh 5

For room temperature control, the following rule-of-thumb formula can be used:

Xp = Xh or ∆tmaxdischarge air

1010

The ∆tmax (Xh) of the discharge air for mixed air damper con- trol is the maximum difference between outdoor air (OA) temperature and return air (RA) temperature.

Xh = ϑRA - ϑOAmin

The often-specified accuracy for room control of ±1 (Xp = 2K) allows a discharge air alteration of 20 °C.

In P+I control the same proportional band can be used as for P control. The following rule-of-thumb formulae are used for P+I control:

•	Discharge air control		Xp =		Xh
					4...5
•	Room control	Xp =	Xh	or ∆tmaxdischarge air
			8...10		8 ... 10
Reset Time tr1 / tr2 (P.12 / P13)
	Xw
	0
							t
	Y
					P	P	%/s
						tr
		P =100 X w %
			Xp
			tr				t

Fig. 4. Step change response of P+I control

In the case of combined action including proportional and integral components (P+I control), the 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. 4.

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, e.g. discharge temperature T2 (see Table 11).

If only proportional control is required, parameter tr must be set to OFF.

9 EN1B-0203GE51 R0507C