Carrier 48TC*D08 - Manual (Page 79)

79
Table 30 – Available Cooling Stages
NUMBER OF
STAGES
0
1
(ECONOMIZER*)
2 3
Compressor 1 Off Off On On
Compressor 2 Off Off Off On
* If conditions are suitable for economizer operation.
Any time the compressors are running, the PremierLink
controller will lockout the compressors if the SAT
becomes too low. These user configurable settings are
found in the SERVICE configuration table:
Compressor 1 Lockout at SAT < SATLO1 (50 to 65_F)
(default is 55_F)
Compressor 2 Lockout at SAT < SATLO2 (45 to 55_F)
(default is 50_F)
After a compressor is locked out, it may be started again
after a normal time--guard period and the supply air
temperature has increased at least 8_F above the lockout
setpoint.
Dehumidification — The PremierLink controller will
provide occupied and unoccupied dehumidification
control when AUXOUT = 5 in the CONFIG table and is
installed on HVAC units that are equipped with additional
controls and accessories to accomplish this function. This
function also requires a space relative humidity sensor be
installed on the OAQ/IRH input.
When in the occupied mode and the indoor relative
humidity is greater then the Occupied High Humidity
setpoint, then the H3_EX_RV output point will be
energized. When in the unoccupied mode and indoor
relative humidity is greater then the Unoccupied High
Humidity setpoint, then the H3_EX_RV output point and
supply fan output will be energized. There is a fixed 5%
hysteresis that the indoor relative humidity must drop
below the active setpoint to end the dehumidification
mode and deenergize the H3_EX_RV output. If the
PremierLink controller is in the unoccupied mode, then
the fan relay will deenergize if there is no other mode
requiring to the fan to be on. This function will not
energize mechanical cooling as a result of the indoor
relative humidity exceeding either setpoint.
A high humidity alarm will be generated if the indoor
relative humidity exceeds the high humidity setpoint by
the amount configured in the Control Humidity Hysteresis
in the ALARMS table for 20 minutes. The alarm will
return to normal when the indoor relative humidity drops
3% below the active humidity setpoint.
Economizer — The economizer dampers are used to
provide free cooling and indoor air quality if optional CO
2
sensor is installed and when the outside conditions are
suitable. Temperature control is accomplished by
controlling the SAT to a certain level determined by the
Economizer PID Loop by calculating a submaster
reference (ECONSR) value. This algorithm will calculate
the submaster reference temperature (ECONSR) based on
OAT and enthalpy conditions and cooling requirements.
The ECONSR value is then passed to the Economizer
Submaster Loop, which will modulate dampers to
maintain SAT at ECONSR level.
The following conditions are required to determine if
economizer cooling is possible:
S Indoor fan has been on for at least 30 seconds
S Enthalpy is low
S SAT reading is available
S OAT reading is available
S SPT reading is available
S OAT ≤ SPT
S OAT < OATMAX (OATMAX default is 75_F)
S Economizer position is NOT forced
If any of the above conditions are not met, the ECONSR
will be set to its MAX limit of 120_F and the damper will
go to its configured minimum position. The minimum
damper position can be overridden by the IAQ routine
described later in this section.
The calculation for ECONSR is as follows:
ECONSR = PID function on (setpoint -- SPT), where:
setpoint = ((OCSP+STO) + (OHSP+STO))/2 when
NTLO (Unoccupied Free Cool OAT Lockout) < OAT <
68_F
setpoint = (OCSP+STO) -- 1 when OAT ≤ NTLO
setpoint = (OHSP+STO) + 1 when OAT ≥ 68_F
The actual damper position (ECONPOS) is the result of
the following calculation. Values represented in the right
side of the equation can be found in the SERVICE
configuration table descriptions in this manual. Note that
that the OAT is taken into consideration to avoid large
changes in damper position when the OAT is cold:
ECONPOS = SubGain x (ECONSR--SAT) + CTRVAL
where SubGain = (OAT -- TEMPBAND) / (ESG + 1)
If the OAT < DXLOCK (DX Cool Lockout setpoint) then
the damper will be modulated to maintain the SAT at the
ECONSR value.
48TC