30 November 1998
Figure 2-3. Simultaneous heating and cooling with pneumatic actuators without positive positioners.
Figure 2-4. Control system with positive positioners to avoid simultaneous heating and cooling.
g. The choice between pneumatic and electric actuators. All terminal unit control systems will have
electric or electronic actuators. For all other control system applications, the designer will make an
estimate of the total cost of actuators required for all control systems in the project. The designer will take
into account the cost of multiple actuators on large dampers and the cost of larger actuators required for
higher torques to operate large valves. The total installed cost estimate of pneumatic actuators will include:
The cost of the compressed air system.
The total installed cost estimate of electric actuators will include consideration of:
Loop driving circuits as explained in this manual.
h. Existing compressed air source. If sufficient air is available from an existing temperature control
compressed air system, it may be used as the air source for additional control systems.
i. Life cycle cost. After the installed cost estimates are prepared, a life cycle cost estimate will
determine the choice between pneumatic and electric actuators. Some manufacturers' catalogs provide
guidelines to assist in estimating the cost benefits of using electric versus pneumatic actuators.
j. Sequencing actuators. The actuators that control valves and dampers are sequenced when HVAC
applications require that the process variables be sensed at a common location and controlled from a
common modulating signal. The objective of sequencing is to avoid energy waste by preventing the
following opposing processes from acting simultaneously:
(1) Heating and cooling.
(2) Humidification and dehumidification.
k. Design requirement in regard to actuator sequencing ranges. The designer will show the actuator
sequencing ranges in the equipment schedule when standard control signals apply.
(1) Pneumatic actuators are sequenced by connecting the signal input connections of the
actuators' positive positioners to the same pneumatic control signal and adjusting the positioners' starting
points and spans to achieve the required sequence. For example, two valves can be operated in sequence
if their positive-positioner spans are set at 28 kPa (4 psig) and their starting points are set at 21 kPA (3 psig)
and 62 kPa (9 psig) respectively. This results in ranges of valve full-stroke operation of 21 to 48 kPa (3 to 7
psig) and 62 to 90 kPa (9 to 13 psig), with a 14 kPa (2 psig) deadband between the ranges of operation.
(2) Some electric actuators have starting points and span adjustments similar to those of the
pneumatic actuator's positive positioner. This is sometimes an optional feature, and must be specified if
required for sequencing. In this case, the starting points and spans are adjusted in milliampere values.
When electric actuators are sequenced, the modulating control circuit will be designed within a 600 ohm