30 November 1998
assigning a portion of the common control signal as a deadband. Each actuator is adjusted so that its full
stroke occurs on either side of the deadband limits outside of the deadband. Examples of the use of
sequencing with a deadband are:
(a) Sequencing of heating and cooling with a deadband between heating and cooling.
(b) Sequencing of heating and outside ventilation air beyond the required minimum quantity
with a deadband between heating and increased ventilation.
(5) Actuators are modulated in parallel by assigning the identical portion of the control signal to
each actuator for its full stroke. Modulation in parallel occurs in air stream mixing applications such as:
(a) Modulation of outside air, return air and relief air dampers for free cooling.
(b) Modulation of multizone hot deck and cold deck dampers in parallel.
d. Current-to-pneumatic transducers. The modulating device for converting a current control signal to
a pneumatic control signal is a current-to-pneumatic transducer (IP). A 140 kPa (20 psig) main air supply to
the IP is the source that develops a 21 - 103 kPa (3 - 15 psig) output signal in a scaled relationship to a 4 -
20 milliampere input signal.
e. Solenoid operated pneumatic valves. The 2-position device for converting an electric contact
closure signal to a pneumatic signal is the solenoid operated pneumatic valve (EP). The EP is a 3-way
valve that connects the normally closed and common ports when the solenoid coil is energized and
connects the normally open and common ports when the solenoid coil is de-energized. The EP is used to
switch 140 kPa (20 psig) main air to the actuators and to exhaust air from the actuators.
f. Positive Positioners.
(1) All modulating control applications of pneumatic actuators require that the actuator be
equipped with a positive positioner (PP). A main air supply is the source of its operating power. The device
throttles main air as required to stroke the actuator to the position dictated by the pneumatic control signal.
However, the positive positioner can exert pressure higher than that of the pneumatic control signal and
thus can maintain the required position against the opposing force of the HVAC system pressure. Piping
system pressures tend to compress the air in the diaphragm chamber of the valve actuator. The
compression causes a shift in the actual operating ranges of the valves. The positive positioner has an
adjustable pneumatic signal start point for the stroke of the actuator and an adjustable pressure span for
the full stroke of the actuator. The stroke is proportional to the pneumatic control signal.
(2) Non-modulating (two-position) control applications where pneumatic actuators are used do not
require positive positioners.
(3) Simultaneous heating and cooling can occur when pneumatic actuators are used, even though
the spring operating ranges are selected without an overlap. The results of this phenomenon are shown in
figure 2-3. Because of this phenomenon, sequencing applications for HVAC systems must have positive
positioners on pneumatic valves and damper actuators, to maintain deadbands between actuator operating
ranges. A control system with positive positioners is illustrated in figure 2-4. When sequencing actuators
from a common control signal, the simultaneous use of heating and cooling can accidentally occur if:
(a) Heating and cooling valve operating ranges overlap.
(b) Heating valve and ventilation damper operating ranges overlap.
(c) Heating valve and cooling air damper operating ranges overlap.