24
Free Cooling
The economiz er will be enabled for cool i ng (OK TO FREE COOL?
=Yes)if the supply air tem perat ure se nsor reading is valid, the re are
no appl ied loc kouts, and ec onom izer is operational. Economi zer
Operational (ECON OPERA TIONAL?) indicates if a n economizer is
installed (VENT METHOD = ECON) and feedback indicates it is
operational. The thr ee ec onomizer lockouts that determ i ne if free
cooling should be used to hel p with cooli ng are: Dry Bulb Lockout
(DRY BULB LOCKOUT), Entha l py Loc kout (ENTHALPY
LOCKOUT), and Unocc upied Fre e Cooli ng Lockout (UFC
LOCKOUT?). Any one of the se loc kouts wi l l disable econom izer fr ee
cooling. See below for how each lockout occ urs.
Whe n the e conomizer i s a vailable f or f ree c ool ing (OK TO FREE
COOL? = Yes ) and the compression is not on, the damper wil l st art
opening from the damper’s minimum Position in Ef fect
(EFFECTI VE MIN POS) bas ed on the supply ai r temper ature
(SUPPL Y AIR TEMP) to provi de free cooling. A low cool ing
dem and (DEMAND = LOW COOL) will utilize the Low Free
Cooli ng SAT Setpoi nt (LOW COOL SAT SP) as the Free Cooling
Setpoint (FREECOOL SA T SP) to cont r ol the ec onom izer. A
me dium or high cool ing dema nd (DEMAND = HIGH COOL) will
utilize the High Free Cooling SAT Setpoint (HI GH COOL SAT SP)
as the Fr ee Cooling Setpoint ( FREECOOL SA T SP) to control the
economiz er.
During free cooling the fan will start at the dedicated free cooling
speed (FREE COOL IDF SPD). After the economizer (ECON
CMD POSITION) reaches 100% (or Max) for 5 minutes, the fan
will be changed to the High Cool Speed (HIGH COOL IDF
SPD). When a high cooling demand (DEMAND = HIGH COOL)
is active the control will use the High Cool Speed (HIGH COOL
IDF SPD). The compressor will be allowed for use after the fan
and economizer are 100% (or Max) for 5 minutes. Once
compression is turned on the economizer and fan will remain at
100% until the call for cooling is removed or until the unit is no
longer allowed to free cool (OK TO FREE COOL = No).
Dry Bulb Lockout
Dry Bulb Lockout (DRY BULB LOCKOUT) occurs when any of
the following are true:
S The Out door Air Te m perature (OUTDOOR AIR TEMP) is invalid.
S When Differential Dry Bulb Control is disabled (DIFF DRY
BULB CTL = Disable) and the Outdoor Air Temperature
(OUTDOOR AIR TEMP) is greater than the configured Free
Cooling Maximum Temperature (FREE COOL MAX OAT) or
less than the configured Free Cooling Minimum Temperature
(FREE COOL MIN OAT).
S When Differential Dry Bulb Control is enabled (DIFF DRY
BULB CTL = Enable) and the return air temperature (RETURN
AIR TEMP) plus the Differential Dry Bulb deadband (DIFF
DB DEADBAND) is lower than the outdoor air temperature
(OUTDOOR AIR TEMP).
Enthalpy Lockout
The control uses the Outdoor Air Temperature (OUTDOOR AIR
TEMP), Outdoor Relative Humidity (OARH LEVEL),and
Barometric Pressure (BAROMETRIC PRESS) to calculate the
Outdoor Enthalpy (OUTDOOR ENTHALPY). The control uses
the Return Air Temperature (RETURN AIR TEMP),Return
Relative Humidity (RARH LEVEL), and Barometric Pressure
(BAROMETRIC PRESS) to calculate the Return Enthalpy
(RETURN ENTHALPY). Enthalpy Lockout (ENTHALPY
LOCKOUT) occurs when any of the following are true:
S When Differential Enthalpy Control is disabled (DIFF
ENTHALPY CTL = Disable) and the outdoor enthalpy
(OUTDOOR ENTHALPY) is greater than the Maximum
Outdoor Enthalpy limit (ENTHALPY HI LIMIT).
S When Differential Dry Bulb Control is enabled (DIFF DRY BULB
CTL = Enable) and t he outdoor ent halpy (OUTDOOR
ENTHALPY) is greater than the return enthalpy (RETURN
ENTHALPY). The Diffe r ential Entha lpy de adband (ENTHALPY
DEADBAND) i s use in the case of unlocking the Enthalpy loc kout
(ENTHALPY LOCKOUT) .
S The Enthalpy switch input (ENTHALPY SWITCH) is reading
high.
Unoccupied Free Cooling Lockout
Unoc c upied Free Cooling lockout (UFC LOCKOUT?) occurs when
the unit is in the unoc cupied period (OCCUPI ED NOW? = No) and
the Outdoor Air Te mpera t ure (OUTDOOR AIR TEMP) is less than
the Unoc c upied Fre e Cooling low te mperature (UFC LOW TEMP).
Unoccupied Free Cooling
The unoccupied free cooling algorithm attempts to maintain the
building space half wa y bet w een the Occupied Cool Set Point (OCC
COOL SETPOINT) and Occupied Heat Set Point (OCC HEA T
SETPOINT) usi ng only the economizer whe n t he conditions in the
building and the outdoor s are suitable, duri ng unoccupied pe riods.
Three different poi nt s def i ne this algorithm: Unocc upi ed Free Cooling
configuration (WHEN TO UNOCC FC), Fr ee Cooling Preoccupa ncy
T i me c onf igurat ion (UFC PREOCC TIM E), and Free c ool ing allowed
(OK TO FREE COOL?).
WHEN TO UNOCC FC = 0(Disabled)
Free Cooling will only occur if the space exceeds the unoccupied
setpoints.
WHEN TO UNOCC FC = 1 (Preoccupancy)
Unoccupied free cooling can only occur when the time until the
next occupied period is less than the Unoccupied Free Cool
Pre--Occupancy Time (UFC PREOCC TIME) in minutes.
WHEN TO UNOCC FC = 2 (Unoccupied)
Unoccupied free cooling can occur throughout the entire
unoccupied period. The space temperature must be higher then the
mid --point between the occupied cooling and heating setpoints.
Power Exhaust
Power Exhaust is a function used to assist in the building exhaust
air if the barometric relief damper is not enough. It can be one or
two motors which can be controlled independently to provide 2
stages of exhaust. These two power exhaust stages are controlled
by relays on the Main Base board, and therefore need to be
configured on relay channels. To assign the channels set the PE1
RELAY CHANNEL and PE2 RELAY CHANNEL as needed.
NOTE: Factory installed power exhaust is only one channel and is
on Relay 06.
When a power exhaust 1 relay channel is configured, the control
will create a PE1 curve, example shown in Fig. 17. This curve is
created by applying the difference of the power exhaust stage 1 at
maximum fan speed (PE1 POS @ MAX S PD) and the Economizer
minimum at maximum fan speed (MIN POS @ MAX FAN) in
relationship to the minimum position curve. When a power exhaust
2 relay channel is configured, the control will create a PE2 curve,
example shown in Fig. 17. This curve is created by applying the
difference of the power exhaust stage 2 at maximum fan speed
(PE2 POS @ MAX SPD) and the Economizer minimum at
maximum fan speed (MIN POS @ MAX FAN) in relationship to
the minimum position curve.
Power exhaust 1 (PE1 RELAY) and power exhaust 2 (PE2
RELAY) are controlled using their respective curves as a threshold.
When the operating point of the Commanded Fan Speed (IDF
SPEED OUTPUT) and Economizer Commanded Position
(ECON CMD POSITION) is above the power exhaust 1 curve,
the Power exhaust 1 (PE1 RELAY) will be turned on. When the
operating point falls below the curve minus the power exhaust turn
off deadband (PE OFF DEADBAND) the Power exhaust 1 (PE1
RELAY) will be turned off. Power exhaust 2 operates the same as
Power exhaust 1 except using the PE2 curve.
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