Economizer Operation; Mechanical Cooling Operation - Daikin RDT-135c Installation And Maintenance Manual

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.

Mechanical Cooling Operation

4-Compressor/4 Stage Unit
Refer to "VAV control inputs" on page 71 and "RPS 135
condensing unit control (with reciprocating compressors)" on
page 85 as you read this sequence of operation. In this
configuration there are four equally sized compressors and two
cooling circuits. In the following description, compressor #1 is
lead. However, if Auto Lead/Lag Staging is selected on the
keypad, the lead compressor is the one in the lead circuit with
the least number of run hours.
When the unit disconnect is closed, 115 VAC power is
supplied directly from control transformer T1 to the
compressor crankcase heaters, HTR-1, 2, 3, and 4 (lines 815,
848, 820, 853) and motor protectors MP1, 2, 3, and 4 (lines
816, 849, 821, 854). This same 115 VAC source also goes
through:
• System switch, S1 (line 203)
• The optional phase voltage monitors, PVM1 and 2
(lines 203, 802)
• The optional ground fault relays, GFR1 and 2
(lines 203, 802)
• Compressor control switches, CS1 and 2 (lines 823, 856)
• Transformer T9 is also powered (line 802)
IM 738-2
Sequences of Operation: Economizer Operation
Compressor control switches, CS1 and 2 provide 24 VAC from
transformer T9 to compressor control boards CCB1 and CCB2
(lines 805, 838) as well as the following cooling circuit binary
inputs:
• Frost protect FP1 to CCB1-BI8 (line 811)—Optional when
no hot gas bypass is ordered on the unit
• Frost protect FP2 to CCB2-BI8 (line 844)—Optional when
no hot gas bypass is ordered on the unit
• HP relay R1 to CCB1-BI7 (lines 807)
• HP relay R2 to CCB2-BI7 (lines 840)
• Compressor contactor status M1 to CCB1-BI9 (line 808)
• Compressor contactor status M2 to CCB2-BI9 (line 841)
• Compressor contactor status M3 to CCB1-BI10 (line 809)
• Compressor contactor status M4 to CCB2-BI10 (line 842)
• Circuit pump down switch PS1 to CCB1-BI11(line 812)
• Circuit pump down switch PS2 to CCB2-BI11(line 845)
• Cool enable from MCB-B07 to CCB1-BI12 and CCB2-BI12
(lines 813, 846)
When manual pumpdown switches PS1 and PS2 are closed, all
four compressors are fully enabled and ready to start if
commanded to by the MicroTech II control system.
Cross Circuit Loading
If cooling is enabled (MCB-BI3 is energized—line 220) and
mechanical cooling is required, the MCB energizes cool
enable output MCB-BO7 (line 813) to binary inputs CCB1-
BI12 (line 813) and CCB2-BI12 (line 846). The MCB also
sends a digital communications signal to CCB1 and 2 to enable
cooling. CCB1-BO4 output energizes and opens liquid line
solenoid valve SV1 (line 831), allowing refrigerant to flow
into the evaporator coil. As the refrigerant evaporates, the
suction pressure increases until low pressure switch LP1 closes
(line 815) as a binary input to CCB1-BI6. When CCB1 senses
that LP1 has closed, CCB1-BO1 and BO5 energize contactors
M1 and M11 (lines 817 and 832) to start compressor #1 and
condenser fan #11. (The above description applies to units
without the low ambient start option, For a description of low
ambient start, see the "Unit Options" section of this manual,)
Additional condenser fan stages are added as the outdoor air
temperature rises above setpoints input through the keypad.
CCB1-BO6 controls contactor M12 (line 833), which cycles
condenser fan 12. CCB1-BO7 controls contactor M13 (line
834), which cycles condenser fan 13. This is stage 1.
If more cooling is required, CCB2-BO4 energizes and opens
liquid line solenoid valve SV2 (line 864), allowing refrigerant
to flow into the evaporator coil. As the refrigerant evaporates,
the suction pressure increases until low pressure switch LP2
closes (line 848) as a binary input to CCB2-BI6. When CCB2
senses that LP2 has closed, CCB2-BO1 and BO5 energize
contactors M2 and M21 (lines 850 and 865) to start
compressor #2 and condenser fan #21. Additional condenser
fan stages are added as the outdoor air temperature rises above
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