Carrier Weathermaster 48P2030-100, Управление, пуск, эксплуатация, обслуживание и устранение неисправностей.

Страница: 69 / 252

69
counteract this low supply-air temperature. This is referred to
as the tempering mode.
Setting up the System
The relevant set points for tempering are located at the local
display under Setpoints :
Operation
First, the unit must be in a vent mode, a low cool, or a high
cool HVAC mode to be considered for a tempering mode. Sec-
ondly, the tempering mode is only allowed when the rooftop is
configured for staged gas, modulating gas, SCR electric heat,
or hydronic heating ( Configuration

HEAT

HT.CF =3 or 4).
Also, if OAT is above the chosen tempering set point, temper-
ing will not be allowed. Additionally, tempering mode is
locked out if any stages of mechanical cooling are present.
If the control is configured for staged gas, modulating gas,
SCR electric heat, or hydronic heating, and the control is in a
vent, low cool, or high cool HVAC mode, and the rooftop con-
trol is in a situation where the economizer must maintain a
minimum position/minimum cfm, then the evaporator dis-
charge temperature (EDT) will be monitored. If the EDT falls
below a particular trip point then tempering mode may be
called out.
HVAC mode = “Tempering Vent”
HVAC mode = “Tempering LoCool”
HVAC mode = “Tempering HiCool”
The decision making/selection process for the tempering trip
set point is as follows:
If an HVAC cool mode is in effect, then the tempering cool
point is SASP – T.CL .
If not in effect and unit is in a pre-occupied purge mode ( Oper-
ating Modes

MODE

IAQ.P =ON), then the trip point is
T.PRG .
If not in effect and unit is in an occupied mode ( Operating
Modes

MODE

IAQ.P =ON), then the trip point is
TEMPVOCC.
For all other cases, the trip point is TEMPVUNC.
NOTE: The unoccupied economizer free cooling does not qualify
as a HVAC cool mode as it is an energy saving feature and has its
own OAT lockout already. The unoccupied free cooling mode
(HVAC mode = Unocc. Free Cool) will override any unoccupied
vent mode from triggering a tempering mode.
A minimum amount of time must pass before calling out any tem-
pering mode. In effect, the EDT must fall below the trip point val-
ue –1°F continuously for a minimum of 2 minutes. Also, at the
end of a mechanical cooling cycle, a 10-minute delay will be en-
forced before considering a tempering during vent mode in order
to allow any residual cooling to dissipate from the evaporator coil.
If the above conditions are met, the algorithm is free to select
the tempering mode (MODETEMP).
If a tempering mode becomes active, the modulating heat source
(staged gas, modulating gas, SCR electric heat, or hot water) will
attempt to maintain leaving-air temperature (LAT) at the temper-
ing set point used to trigger the tempering mode. The technique
for modulation of set point for staged gas, modulating gas, SCR
electric heat, and hydronic heat is the same as in a heat mode.
More information regarding the operation of heating can be ref-
erenced in the Heating Control section on page 60.
Recovery from a tempering mode (MODETEMP) will occur
when the EDT rises above the trip point. On any change in
HVACMODE, the tempering routine will re-assess the temper-
ing set point which may cause the control to continue or exit
tempering mode.
Static Pressure Control
Variable air volume (VAV) air-conditioning systems must pro-
vide varying amounts of air to the conditioned space. As air
terminals downstream of the unit modulate their flows, the unit
must simply maintain control over duct static pressure in order
to accommodate the needs of the terminals, and therefore to
meet the varying combined airflow requirement. The unit de-
sign includes two alternative optional means of accommodat-
ing this requirement. This section describes the technique by
which this control takes place.
A unit intended for use in a VAV system can be equipped with
an optional variable frequency drive (VFD) for the supply fan.
The speed of the fan can be controlled directly by the Com-
fort Link controls. A transducer is used to measure duct static
pressure. The signal from the transducer is received by the
RXB board and is then used in a PID control routine to deter-
mine the required fan speed. The required speed is then com-
municated 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 preventing overpres-
surization and its accompanying problems.
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.
The four most fundamental configurations for most applica-
tions are Configuration

SP

SP.CF , which is the static pres-
sure control type, Configuration

SP

SP.SV , used to indi-
cate CV unit with VFD, staged air volume control, Configura-
tion

SP

SP.S , used to enable the static pressure sensor, and
Configuration

SP

SP.SP , the static pressure set point to be
maintained.
OPERATION FOR VAV
On VAV units equipped with a VFD and a proper static pres-
sure 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 set point. This error at any point in
time is simply the duct static pressure set point minus the mea-
sured duct static. 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 set point.
Static pressure reset is the ability to force a lowering of the
static pressure set point through an external control signal. Ex-
plained in further detail below, the control supports this in two
separate ways; through a 4 to 20 mA signal input wired to
TB202 terminals 6 and 7 (thereby facilitating third party con-
trol), or via CCN controls.
In the latter case, this feature leverages the communications ca-
pabilities of VAV systems employing ComfortID™ terminals
under linkage. The system dynamically determines and main-
tains an optimal duct static pressure set point based on the actu-
al load conditions in the space. This can result in a significant
reduction in required fan energy by lowering the set point to
only the level required to maintain adequate airflow throughout
the system.
ITEM EXPANSION RANGE UNITS
CCN
POINT
DEFAULT
T.PRG
Tempering
Purge SASP –20 to 80 dF TEMPPURG 50
T.CL
Tempering in
Cool Offset 5 to 75 ^F TEMPCOOL 5
T.V.OC
Tempering Vent
Occ SASP –20 to 80 dF TEMPVOCC 65
T.V.UN
Tempering Vent
Unocc. SASP –20 to 80 dF TEMPVUNC 50