54
Static Pressure Control —
Variable air volume (VAV)
air-conditioning systems must provide varying amounts of air
to the conditioned space. As air terminals downstream of the
unit modulate their flows, the unit must maintain control over
the duct static pressure in order to accommodate the needs of
the terminals and meet the varying combined airflow require-
ment.
The static pressure control routine is also used on CV units
with VFD for staged air volume. The fan is controlled at dis-
crete speeds through the VFD by the unit
Comfort
Link controls
based on the operating mode of the unit.
A 48/50A3,A5 unit equipped with a duct pressure control
system is provided with a variable frequency drive (VFD) for
the supply fan. The speed of the fan can be controlled directly
by the
Comfort
Link controls. A transducer is used to measure
duct static pressure. The signal from the transducer is received
by the ECB-2 board and is then used in a PID control routine
that outputs a 4 to 20 mA signal to the VFD.
Generally, only VAV systems utilize static pressure control.
It is required because as the system VAV terminals modulate
closed when less air is required, there must be a means of
controlling airflow from the unit, thereby effectively prevent-
ing overpressurization and its accompanying problems.
A 48/50A2,A4 unit can be equipped with a VFD for staged
air volume control. The speed of the fan is controlled directly
by the
Comfort
Link controls based on the operating mode of
the unit. A 4 to 20 mA signal is sent to the VFD to control the
fan speed.
The four most fundamental configurations for most applica-
tions are
Configuration
SP
SP.CF
, which is the static pres-
sure control type,
Configuration
SP
CV.FD
, used to indi-
cate CV unit with VFD (staged air volume).
Configura-
tion
SP
SP.S
, used to enable the static pressure sensor, and
Configuration
SP
SP.SP
, the static pressure setpoint to be
maintained.
OPERATION — On VAV units equipped with a VFD and a
proper static pressure sensor, when
SP.CF
,
SP.S
and
SP.SP
are
configured, a PID routine periodically measures the duct static
pressure and calculates the error from setpoint. This error is
simply the duct static pressure setpoint minus the measured
duct static pressure. The error becomes the basis for the propor-
tional term of the PID. The routine also calculates the integral
of the error over time, and the derivative (rate of change) of the
error. A value is calculated as a result of this PID routine, and
this value is then used to create an output signal used to adjust
the VFD to maintain the static pressure setpoint.
Static pressure reset is the ability to force a lowering of the
static pressure setpoint through an external control signal.
The unit controls support this in two separate ways, through a 4
to 20 mA signal input wired to the unit’s isolator board input
terminals (third party control) or via CCN.
When employing the CCN, this feature uses the communica-
tions capabilities of VAV systems with ComfortID™ terminals
under linkage. The system dynamically determines and
maintains an optimal duct static pressure setpoint based on the
actual load conditions in the space. This can result in a signifi-
cant reduction in required fan energy by lowering the setpoint to
only the level required to maintain adequate airflow throughout
the system.
OPERATION — On CV units equipped with a VFD (Staged
Air Volume) when
SP.CF
,
CV.FD
,
SP.FN
are configured, the
Comfort
Link controls will control the speed of the supply fan
based on the operating mode of the unit. The VFD speed setting
points are
SP.MN
,
SP.MX
,
HT.VM
. When in LOW COOL
mode and the compressor stage less than 50%, fan will be as
SP.MN
minimum speed. When in HIGH COOL, the fan will be
at
SP.MX
maximum speed. In heating mode, the fan will operate
at
SP.MX
maximum speed when the heating stage is 75% or
greater and at
HT.VM
heating minimum speed when the heating
stage is less than 75%. On units configured for two-stage ther-
mostat operation, the fan will be at
SP.MX
on a call for W2 and
at
HT.VM
on a call for only W1.
SETTING UP THE SYSTEM — The options for static
pressure control are found under the Local Display Mode
Configuration
SP
. See Table 60.
Static Pressure Configuration (
SP.CF
) — This variable is
used to configure the use of
Comfort
Link controls for static
pressure control. There are the following options:
0 (None)
— There will be no static pressure control by
Com-
fort
Link controls. This setting would be used for a constant vol-
ume (CV) application when static pressure control is not re-
quired or for a VAV application if there will be third-party con-
trol of the VFD. In this latter case, a suitable means of control
must be field installed. This setting must be used on CV units
with VFD (staged air volume).
Additionally,
SP.CF
must be set to 0 (None) when a unit is
equipped with optional VFD bypass and is operating in Bypass
mode. Failure to change this configuration in Bypass mode will
result in the indoor fan motor running continuously.
1 (VFD Control)
— This will enable the use of
Comfort
Link
controls for static pressure control via a supply fan VFD.
Constant Vol IDF ia VFD? (
CV.FD
) — This variable enables
the use of a CV unit with VFD for staged air volume control.
Static Pressure Fan Control? (
SP.FN
) — This is automatical-
ly set to Yes when
SP.CF
= 1 or when
CV.FD
is set to Yes.
When the user would like the 4 to 20 mA output to energize
the VFD, as opposed to the fan relay,
SP.FN
may be set to Yes
when
SP.CF
= 0. When the control turns the fan ON, the con-
trol will send the
SP.MX
value of the 4 to 20 mA signal to the
third party VFD control.
Additionally,
SP.FN
must be set to NO when the unit is
equipped with optional VFD bypass and is operating in Bypass
mode. Failure to change this configuration in bypass mode will
result in the indoor fan motor running continuously.
Static Pressure Sensor (
SP.S
) — This variable enables the use
of a supply duct static pressure sensor. This must be enabled to
use
Comfort
Link controls for static pressure control. If using a
third-party control for the VFD, this should be disabled. This is
not used when
CV.FD
is set to Yes.
Static Pressure Low Range (
SP.LO
) — This is the minimum
static pressure that the sensor will measure. For most sensors
this will be 0 in. wg. The
Comfort
Link controls will map this
value to a 4 mA sensor input.
Static Pressure High Range (
SP.HI
) — This is the maximum
static pressure that the sensor will measure. Commonly this
will be 5 in. wg. The
Comfort
Link controls will map this value
to a 20 mA sensor input.
Static Pressure Setpoint (
SP.SP
) — This is the static pressure
control point. It is the point against which the
Comfort
Link con-
trols compare the actual measured supply duct pressure for deter-
mination of the error that is used for PID control. Generally one
would set
SP.SP
to the minimum value necessary for proper op-
eration of air terminals in the conditioned space at all load condi-
tions. Too high of a value will cause unnecessary fan motor pow-
er consumption at part load conditions and/or noise problems.
Too low a value will result in insufficient airflow.
VFD Minimum Speed (
SP.MN
) — This is the minimum speed
for the supply fan VFD. Typically the value is chosen to main-
tain a minimum level of ventilation.
CAUTION
Failure to correctly configure
SP.CF
and
SP.FN
when
operating in VFD Bypass mode will result in the indoor fan
motor running continuously. Damage to unit could result.
Содержание Carrier Weathermaker 48A2
Страница 105: ...105 Fig 20 Typical Main Control Box Wiring Schematic 48 50A2 A3 A4 A5 Units...
Страница 106: ...106 Fig 21 Typical Auxiliary Control Box Wiring Schematic...
Страница 107: ...107 Fig 22 Typical 2 Stage Gas Heat Wiring Schematic Size 060 Units Shown a48 8357...
Страница 108: ...108 TO NEXT PAGE Fig 23 Typical Staged Gas Heat Wiring Schematic Size 060 Units Shown A48 7296...
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Страница 110: ...110 Fig 24 Typical Electric Heat Control Schematic 50 Series Size 060 Units Shown a50 8228...
Страница 111: ...111 Fig 25 Typical Power Schematic 48 50A2 A3 A4 A5 060 Unit Shown...
Страница 112: ...112 Fig 26 Typical Low Ambient Controls Option Wiring...
Страница 113: ...113 Fig 27 Typical Small Chassis Component Location Size 020 035 Units...
Страница 114: ...114 Fig 28 Typical Large Chassis Component Locations Size 040 060 Units...
Страница 118: ...118 Fig 30 Economizer Control Board ECB1 and VAV Control Board ECB2 A48 7706...
Страница 142: ...142 A48 3733 Fig 56 Main Burner Removal...
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