McQuay RoofPak RAH 047C, Руководство по установке и обслуживанию

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McQuay IM 487-4 55
Sequences of Operation
Fan Operation
When the main control board (MCB) commands the supply
and return fans to start, the unit enters the Startup operating
state. As a result, a 3-minute timer is set, output MCB-BO3
(line 307) energizes, and relay R26 energizes (line 306).
After the 3-minute timer expires, the unit enters the Recirc
operating state. As a result, output MCB-BO1 energizes relay
R67 (line 401). This gives a start signal to supply fan drive
AFD10 (line 445). Four seconds after MCB-BO1 is energized,
output MCB-BO2 energizes relay R68 (line 404). This gives a
start signal to return fan drive AFD20 (line 445).
Within 120 seconds after the fans start, the controller expects
airflow switch PC7 (line 228) to close and thus energize binary
input MCB-BI6. (If MCB-BI6 does not energize, the controller
assumes the fans did not start. It then shuts down the unit and
generates an alarm.)
During the Recirc operating state, the outside air damper is
held closed. The controller does this by energizing output
MCB-BO5 (line 318). On VAV units, output MCB-BO12, the
VAV box output, is also de-energized (line 309) during the
Recirc state.
The supply fan adjustable frequency drive (AFD10) is
modulated to maintain the duct static pressure setpoint. When
energized, output MCB-BO14 (line 407) drives AFD10 toward
increased capacity; MCB-BO13 (line 405) drives it toward
decreased capacity.On VAV units or CAV units equipped with
return fan capacity control, the adjustable frequency drive
(AFD20) is modulated to maintain an acceptable building
static pressure (using either VaneTrol logic or direct
measurement of building pressure; see the appropriate OM for
more information). When energized, output MCB-BO16 (line
409) drives AFD20 toward increased capacity; MCB-BO15
(line 411) drives them toward decreased capacity.
Note:
If the inverter bypass switch S4 (lines426 and 430) is in
the bypass position, MMP30 and MMP40 (line 132 and
144) protect the fans from excessive current draw. If
either the supply or return fan is drawing excessive
current, one of the MMPs triggers an auxiliary contacts
(line 426) and open the circuit, causing both fans to
stop.
Economizer Operation
When the outdoor air is suitable for free cooling, the switch in
enthalpy sensor OAE is in position “3” (line 252) energizing
binary input MCB-BI11. When MCB-BI11 energizes, the
economizer is enabled. (Note: If selected from the keypad, the
enthalpy decision can be made based on outdoor temperature.
In that condition, if the outdoor air temperature is less than or
equal to the changeover set point, the economizer is enabled.)
If cooling is required, the economizer dampers (ACT3) are
modulated to maintain the discharge air temperature setpoint.
When energized, output MCB-BO6 drives the outdoor air
dampers toward the open position; MCB-BO5 drives them
toward the closed (line 318).If the outdoor air dampers are
wide open and more cooling is required, the dampers hold
their positions and mechanical cooling is activated (see
below).
When the outdoor air is not suitable for free cooling, the
switch in enthalpy sensor OAE is in position “1,”
de-energizing binary input MCB-BI11 (Alternatively, the
outdoor air temperature is above the changeover setpoint plus
the economizer changeover differential). When the economizer
is disabled, the dampers are held at their minimum position.
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