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Controls |
VAV Units Only
Sequence of Operation
1
Supply Air Pressure Control
• Inlet Guide Vane Control
Inlet guide vanes are driven by a modulating 2-10 vdc
signal from the VAV Module. A pressure transducer measures duct static pressure, and the
inlet guide vanes are modulated to maintain the supply air static pressure within an
adjustable user-defined range. The range is determined by the supply air pressure setpoint
and supply air pressure deadband, which are set through a unit mounted potentiometer.
Inlet guide vane assemblies installed on the supply fan
inlets regulate fan capacity and limit horsepower at lower system air requirements. When
in any position other than full open, the vanes pre-spin intake air in the same direction
as supply fan rotation. As the vanes approach the full-closed position, the amount of
"spin" induced by the vanes increases at the same time that intake airflow and
fan horsepower diminish. The inlet guide vanes will close when the supply fan is shut
down.
• Supply Air Static Pressure Limit The opening
of the inlet guide vanes and VAV boxes are coordinated, with respect to time, during unit
start up and transition to/from Occupied/Unoccupied modes to prevent overpressurization of
the supply air ductwork. However, if for any reason the supply air pressure exceeds the
fixed supply air static pressure limit of 3.5" W.C., the supply fan is shut down and
the inlet guide vanes are closed. The unit is then allowed to restart four times. If the
overpressurization condition occurs on the fifth time, the unit is shut down and a manual
reset diagnostic is set and displayed at any of the remote panels with LED status lights
or communicated to the Integrated Comfort system.
• Variable Frequency Drives (VFD) Control Variable
frequency drives are driven by a modulating 0-10 vdc signal from the VAV module. A
pressure transducer measures duct static pressure, and the VFD is modulated to maintain
the supply air static pressure within an adjustable user-defined range. The range is
determined by the supply air pressure setpoint and supply air pressure deadband, which are
set through a unit mounted potentiometer. Variable frequency drives provide supply fan
motor speed modulation. The drive will accelerate or decelerate as required to maintain
the supply static pressure setpoint. When subjected to high ambient return conditions the
VFD shall reduce its output frequency to maintain operation. Bypass control is offered to
provide full nominal airflow in the event of drive failure.
2
Supply Air Temperature Controls
• Cooling/Economizer During occupied cooling
mode of operation, the economizer (if available) and primary cooling are used to control
the supply air temperature. The supply air temperature setpoint is user-defined at the
unit mounted VAV Setpoint Panel or at the remote panel. If the enthalpy of the outside air
is appropriate to use "free cooling," the economizer will be used first to
attempt to satisfy the supply setpoint.
On units with economizer, a call for cooling will modulate
the fresh air dampers open. The rate of economizer modulation is based on deviation of the
discharge temperature from setpoint, i.e., the further away from setpoint, the faster the
fresh air damper will open. Note that the economizer is only allowed to function freely if
ambient conditions are below the enthalpy control setting or below the return air enthalpy
if unit has comparative enthalpy installed. If outside air is not suitable for
"economizing," the fresh air dampers drive to the minimum open position. A field
adjustable potentiometer on the Unitary Economizer Module, Tracer®, or a remote
potentiometer can provide the input to establish the minimum damper position.
At outdoor air conditions above the enthalpy control
setting, primary cooling only is used and the fresh air dampers remain at minimum
position. If the unit does not include an economizer, primary cooling only is used to
satisfy cooling requirements.
• Supply Air Setpoint Reset
Supply air reset can be used to adjust the supply air
temperature setpoint on the basis of a zone temperature, return air temperature, or on
outdoor air temperature. Supply air reset adjustment is available on the unit
mounted VAV Setpoint Panel for supply air cooling control.
a
reset based on outdoor air temperature
Outdoor air cooling reset is sometimes used in
applications where the outdoor temperature has a large effect on building load. When the
outside air temperature is low and the building cooling load is low, the supply air
setpoint can be raised, thereby preventing subcooling of critical zones. This reset can
lower usage of primary cooling and result in a reduction in primary cooling energy usage.
There are two user-defined parameters that are adjustable
through the VAV Setpoint Panel: reset temperature setpoint and reset amount. The amount of
reset applied is dependent upon how far the outdoor air temperature is below the supply
air reset setpoint. The amount is zero where they are equal and increases linearly toward
the value set at the reset amount input. The maximum value is 20 F. If the outdoor air
temperature is more than 20 F below the reset temperature setpoint the amount of reset is
equal to the reset amount setpoint.
b
reset based on zone or return temperature
Zone or return reset is applied to the zone(s) in a
building that tend to overcool or overheat. The supply air temperature setpoint is
adjusted based on the temperature of the critical zone(s) or the return air
temperature. This can have the effect of improving comfort and/or lowering energy usage.
The user-defined parameters are the same as for outdoor air reset.
Logic for zone or return reset control is the same except
that the origins of the temperature inputs are the zone sensor or return sensor
respectively. The amount of reset applied is dependent upon how far the zone or return air
temperature is below the supply air reset setpoint. The amount is zero where they are
equal and increases linearly toward the value set at the reset amount potentiometer on the
VAV Setpoint panel. The maximum value is 3 F. If the return or zone temperature is more
than 3 F below the reset temperature setpoint the amount of reset is equal to the reset
amount setpoint. |
