Sequence Control; Limit Control - Honeywell AUTOMATIC CONTROL Engineering Manual

— The controller must be located where the condition it
measures is uniformly affected by changes in position
of the multiple valves. If not, the application requires
more than one controller.
A direct- or reverse-acting signal to a three-way mixing or
diverting valve must be selected carefully. Figure 58 shows
that the piping configuration determines the signal required.
DIRECT-
ACTING
SIGNAL
THREE-WAY
MIXING VALVE
HOT WATER
HOT WATER
SUPPLY
RETURN
HOT WATER COIL
Fig. 58. Three-Way Mixing Valve Piping
with Direct Actuators.

SEQUENCE CONTROL

In pneumatic control systems, one controller can operate
several dampers or valves or several groups of dampers or valves.
For example, year-round air conditioning systems sometimes
require heating in the morning and evening and cooling in the
afternoon. Figure 59 shows a system in which a single controller
controls a normally open heating valve and normally closed
cooling valve. The cooling valve is set for an 8 to 13 psi range
and the heating valve, for a 2 to 7 psi range. The controller operates
the two valves in sequence to hold the temperature at the desired
level continually.
DA
SENSOR
CONTROLLER
S M B
M
N.C. COOLING
VALVE
C2357
8-13 PSI
Fig. 59. Pneumatic Sequencing of Two Valves with
Positive Positioning Actuators.
When the temperature is so low that the controller calls for
full heat, the branchline pressure is less than 3 psi. The normally
open heating valve is open and the normally closed cooling
REVERSE-
ACTING
SIGNAL
THREE-WAY
MIXING VALVE
HOT WATER
RETURN
HOT WATER
SUPPLY
HOT WATER COIL
C2377
M M
POSITIVE
POSITIONING
M P M P
ACTUATORS
N.O. HEATING
VALVE
2-7 PSI
PNEUMATIC CONTROL FUNDAMENTALS
valve is closed. As the temperature rises, the branchline
pressure increases and the heating valve starts to close. At
7 psi branchline pressure, the heating valve is fully closed. If
the temperature continues to rise, the branchline pressure
increases until the cooling valve starts to open at 8 psi. The
temperature must rise enough to increase the branchline
pressure to 13 psi before the cooling valve will be full open.
On a drop in temperature, the sequence is reversed.
Valves with positive positioners ensure tight close-off of the
heating valve at 7 psi branchline pressure, and delay opening
of the cooling valve until 8 psi branchline pressure is reached.
Positive positioners prevent overlapping caused by a variation
in medium pressure, a binding valve or damper, or a variation
in spring tension when using spring ranges for sequencing.
A greater deadband can be set on the positioners to provide
a larger span when no energy is consumed. For example, if the
positioners are set for 2 to 7 psi on heating and 13 to 18 psi on
cooling, no energy is used when the controller branchline
pressure is between 7 and 13 psi. The positioners can also be
set to overlap (e.g., 4 to 9 and 7 to 12 psi) if required.
Valve and damper actuators without positioners have various
spring ranges. To perform the sequencing application in Figure
59 without positioners, select a heating valve actuator that has
a 2 to 7 psi spring range and a cooling valve actuator that has
an 8 to 13 psi spring range. Although this method lessens
precise positioning, it is usually acceptable in systems with
lower pressure differentials across the valve or damper and on
smaller valves and dampers .

LIMIT CONTROL

Figure 60 shows a sensor-controller combination for space
temperature control with discharge low limit. The discharge
low limit controller on a heating system prevents the discharge
air temperature from dropping below a desired minimum.
RETURN AIR PRIMARY
SENSOR CONTROLLER (DA)
B S M
M
N.O.
VALVE
Fig. 60. Low-Limit Control (Heating Application).
85
ENGINEERING MANUAL OF AUTOMATIC CONTROL
LOW-LIMIT
CONTROLLER (DA)
B S M
M
LOW-PRESSURE
P
SELECTOR
RELAY
B P
EXH
SENSOR
DISCHARGE
AIR
C2380
HEATING COIL