Handbook of air conditioning and refrigeration / Shan K

consumption than proportional control mode. A wider proportional band and properly selected
proportional and integral gains for the controller provide a stable and effective operation.
Cold discharge air temperature reset for the perimeter zone is energy-efficient when PI or proportional-integral-derivative
(PID) control mode is used.
● The use of an air economizer cycle is energy-efficient and provides adequate outdoor ventilation
air for occupants.
● A sensor should be located where the value it senses represents the controlled variable of the
whole zone to be controlled. A sensor should be exposed and located in a place that has sufficient
air movement. However, it should not be directly under a supply air jet, in which the air temperature
and relative humidity are quite different from those in the occupied zone. It also should not
be located in a stratified airstream.
● To measure the duct static pressure, if a long section of straight duct is available (say, a length of
duct section greater than or equal to 10 duct diameters), a single point, Pitot-tube type of duct
static pressure sensor can be used. Otherwise, a multipoint Pitot-tube array of flow-measuring
station with airflow straighteners should be used for velocity pressure or static pressure measurement.
The small holes used to measure static pressure in a Pitot tube should never be directly opposite
an airstream with a velocity pressure that can affect the reading. A reference pressure
should be picked up at a point with low air velocity outside the duct, at a point served by the same
air system, or in the ceiling plenum.
● A space pressure sensor should be located in an open area of the conditioned space where the air
velocity is less than 40 fpm (0.2 m/s) and where its reading is not affected by the opening of
doors. The reference pressure pickup is best located at the rooftop, at a level 10 ft (3.0 m) above
the building and shielded from the influence of wind gusts.
● If a simpler control system can do the same job as a more complex system, the simpler system is
always the first choice.
A sufficiently clear, well-followed operations manual and a well-implemented maintenance
schedule are key factors for an effective control system.
Interaction between Controls
AIR SYSTEMS: MINIMUM VENTILATION AND VAV SYSTEM CONTROLS 23.29
In a typical multizone VAV system, each of the zone temperature control, discharge air temperature
control, minimum ventilation control, duct static pressure control, space pressurization control, and
warm-up or cool-down controls may consist of a single closed control loop, or several closed control
loops combined together. The interaction between these control loops can be categorized as follows:
● Sequence control. For this kind of control action, several individual control loops operate in sequence
in order to maintain a particular controlled variable within predetermined limits to prevent
simultaneous cooling and heating. For example, the discharge air temperature control in an AHU or
PU consists of an air economizer control loop as the first-stage cooling, a cooling coil control loop,
a mixed air temperature control loop, and a heating coil control loop in sequence. The two cooling
control loops can be actuated simultaneously, whereas the cooling coil, mixed air, and heating coil
control loops must be operated in sequence to prevent simultaneous cooling and heating.
● Override. If two control loops operate on the same control device, one control loop must override
the control action of the other control loop. One must have a priority to actuate the control device.
The predetermined precedence of one control signal over another in the DDC controller is called
override. For example, the minimum ventilation control and the air economizer control both
actuate the minimum outdoor air damper, economizer dampers, and recirculating dampers. When
the DDC controller fully opens the economizer and minimum outdoor dampers, the air
economizer control overrides the minimum ventilation control because the outdoor air intake
during the 100 percent air economizer cycle is far greater than the minimum outdoor air intake
during minimum ventilation control.