Condenser Fan Control; Circuit Pumpdown - McQuay MicroTech II Operation Manual

"small" compressors on circuit #1 would cycle on
and off (based on run hours) to maintain the load.
Generic Condensing Unit Staging
When a unit is equipped with a DX cooling coil and is inter-
faced with a field supplied condensing unit, it is equipped
with a generic condenser control board (CCB1) for control-
ling the cooling stages on the condensing unit. In this case,
circuit lead/lag, cross-circuit loading and lead-circuit loading
are not applicable. The outputs on the condenser control
board (CCB1) are simply staged up and down sequentially as
the cooling capacity changes.

Condenser Fan Control

Table 39: Programmable Parameters
Keypad/Display ID
Menu Name Item Name
Cond Fan1 Spt= 0°F
Cond Fan2 Spt= 55°F
Compressor Setup Cond Fan3 Spt= 65°F
Cond Fan4 Spt= 75°F
Cond Fan Diff= 10°F
On units equipped with compressorized cooling, there are up
to four condenser fans per cooling circuit depending on the
size of the unit. The fans in each circuit are started and
stopped based on outdoor air temperature when circuit is
operating. The first condenser fan in each operating circuit is
turned on when the OA Temp= value rises above the First
Condenser Fan Set Point and is turned off when the OA
Temp= value falls below the First Condenser Fan Set Point
by more than the Condenser Fan Differential. The second
condenser fan in each operating circuit is turned on when the
OA Temp= value rises above the Second Condenser Fan Set
Point and is turned off when the OA Temp= value falls below
the Second Condenser Fan Set Point by more than the Con-
denser Fan Differential. The third condenser fan in each
operating circuit is turned on when the OA Temp= value
rises above the Third Condenser Fan Set Point and is turned
off when the OA Temp= value falls below the Third Con-
76
denser Fan Set Point by more than the Condenser Fan Differ-
ential. The fourth condenser fan in each operating circuit is
turned on when the OA Temp= value rises above the Fourth
Condenser Fan Set Point and is turned off when the OA
Temp= value falls below the Fourth Condenser Fan Set Point
by more than the Condenser Fan Differential.

Circuit Pumpdown

Normal Circuit Pumpdown
During normal cooling circuit operation, the circuit is
pumped down when the last compressor in the circuit is shut
down due to normal staging. It is also pumped down if the
entire circuit is shut down due to any alarms other than Hi
Pres-Ckt1, Hi Pres-Ckt2, Comp #1 Alm, Comp #2 Alm,
Comp #3 Alm, and Comp #4 Alm.
When pumdown is required for a circuit, the Liquid Line
Parameter Name
Solenoid Valve for that circuit is closed (output turned off)
and a compressor on that circuit operates until the circuit low
First Condenser Fan
pressure switch opens at which time the compressor is turned
Set Point
off. If the low pressure switch does not open within 180 sec-
Second Condenser onds, pumpdown is terminated by turning off the compressor
Fan Set Point
and the PumpDown-Ckt1 (or PumpDown-Ckt2) problem is
generated.
Third Condenser
Fan Set Point
Manual Circuit Pumpdown
Fourth Condenser
Fan Set Point
Compressors in a Circuit are Operating: Circuit
pumpdown is manually initiated if either the master pump-
Condenser Fan Dif-
down switch (PS3) in the main control panel or the circuit
ferential
pumpdown switch (PS1-Circuit #1 or PS2-Circuit #2) in the
condenser control panel is placed in the Pumpdown (Off)
position.
Compressors in a Circuit are not Operating: Circuit
pumpdown is manually initiated if either the master pump-
down switch (PS3) or the circuit pumpdown switch (PS1-
Circuit #1 or PS2-Circuit #2) is cycled from the Auto (On) to
the Pumpdown (Off) position twice in less than 20 seconds
and the low pressure switch is closed.
When pumdown is initiated for a circuit, the liquid line sole-
noid valve for that circuit is closed (output turned off) and a
compressor on that circuit operates until the circuit low pres-
sure switch opens at which time the compressor is turned off.
If the low pressure switch does not open within 180 seconds,
pumpdown is terminated by turning off the compressor and
the PumpDown-Ckt1 (or PumpDown-Ckt2) problem is gen-
erated.
OM 138