Honeywell VP525C, Engineering Manual, Page: 156 / 228

Pneumatic Control Combinations
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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.
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
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.
Fig. 60. Low-Limit Control (Heating Application).
Low-limit control applications typically use a direct-acting
primary controller and a normally open control valve. The direct-
acting, low-limit controller can lower the branchline pressure
regardless of the demands of the room controller, thus opening
the valve to prevent the discharge air temperature from
dropping below the limit controller setpoint. Whenever the low-
limit discharge air sensor takes control, however, the return air
sensor will not control. When the low-limit discharge air sensor
takes control, the space temperature increases and the return
air sensor will be unable to control it.
A similar combination can be used for a high-limit heating
control system without the selector relay in Figure 61. The limit
controller output is piped into the exhaust port of the primary
controller, which allows the limit controller to limit the bleed-
down of the primary controller branchline.
Fig. 61. High-Limit Control (Heating Application).
M M
M
S M B
M P M P
POSITIVE
POSITIONING
ACTUATORS
N.C. COOLING
VALVE
8-13 PSI
N.O. HEATING
VALVE
2-7 PSI
SENSOR
DA
CONTROLLER
C2357
N.O.
VALVE
HEATING COIL
C2380
DISCHARGE
AIR
B S M
M
B P
P
B S M
M
SENSOR
RETURN AIR
SENSOR PRIMARY
CONTROLLER (DA) LOW-LIMIT
CONTROLLER (DA)
LOW-PRESSURE
SELECTOR
RELAY
EXH
C2381
B S M
DISCHARGE
AIR
LIMIT
SENSOR
B S M
M
PRIMARY
CONTROLLER (DA) LOW-LIMIT
CONTROLLER (DA)
N.O.
VALVE
HEATING COIL
M
B S M
PRIMARY
SENSOR