Trane RAUC-IOM-15, Инструкция по установке и обслуживанию, Страница: 51 / 68

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Sequence of Operation
Condenser Fans
Condenser fan cycling is accomplished through interlocking
the fan contactors with liquid line pressure switches (4S7
and 4S8). Figure 5-3 illustrates the condenser fan locations
with their respective fan and relay designator.
When a cooling command has been initiated (circuit #1, first
step), condenser fans 4B1, 4B2, 4B3, 4B4, 4B5 & 4B6 are
held “Off” by the liquid line pressure switch (4S7) and
normally open interlock contacts 1K5 & 1K9. Once the pres-
sure switch has closed (275 psig), condenser fan relay 1K7
(& 1K10 if applicable) is energized starting fans 4B1 & 4B4
(& 4B3 if applicable). The normally open interlock contacts
1K7 closes, energizing fan contactor 1K9, starting fan 4B2
(& 4B6 if applicable). When the normally open interlock
contacts 1K9 close, they seal 1K9 contactor in the “On” po-
sition until the cooling demand has been satisfied. Con-
denser fan 4B5 is not allowed to start until compressor
contactor 1K5 has energized.
If a second step cooling command is initiated, (circuit #2),
condenser fans 5B1, 5B2, 5B3, 5B4, 5B5 & 5B6 are held
“Off” by the liquid line pressure switch (4S8) and normally
open interlock contacts 1K6 & 1K13. Once the pressure
switch has closed (275 psig), condenser fan relay 1K11 (&
1K14 if applicable) is energized starting fans 5B1 & 5B4 (&
5B3 if applicable). The normally open interlock contacts
1K11 closes, energizing fan contactor 1K13, starting fan
5B2 (& 5B6 if applicable). When the normally open interlock
contacts 1K13 close, they seal 1K13 contactor in the “On”
position until the cooling demand has been satisfied. Con-
denser fan 5B5 is not allowed to start until compressor relay
1K6 has energized.
Figure 5-3
Condenser Fan Locations
Sequence of Operation
Low Ambient Dampers
Low Ambient Dampers are available as a factory installed
option or can be field-installed. Dampers are used to extend
the operation of these units from the standard operational
temperatures to a minimum of 0
o F without hot gas bypass
or 10 o F with hot gas bypass. (These values apply when
wind speed across the condenser coil is less than 5 m.p.h.).
If typical wind speeds are higher than 5 m.p.h., a wind
screen around the unit may be required. By restricting the
airflow across the condenser coils, saturated condensing
temperatures can be maintained as the ambient tempera-
tures change.
The low ambient damper actuator controls damper modula-
tion for each refrigerant circuit in response to saturated con-
densing temperature.
Sequence Of Operation
Compressor Crankcase Heaters
Each compressor is equipped with a crankcase heater and
is controlled by a 600 volt auxiliary switch on the compres-
sor contactor. The proper operation of the crankcase heater
is important to maintain an elevated compressor oil tem-
perature during the “Off” cycle to reduce oil foaming during
compressor starts.
When the compressor starts, the sudden reduction in crank-
case pressure causes the liquid refrigerant to boil rapidly
causing the oil to foam. This condition could damage com-
pressor bearings due to reduced lubrication and could
cause compressor mechanical failures.
When power has been “Off” for an extended period, allow
the crankcase heater to operate a minimum of 8 hours be-
fore starting the unit.
Sequence of Operation
Pump Down
Each circuit will go into a pump down cycle when the last
compressor on that circuit is turned "Off". During pump
down, the solenoid valves are closed, the reset circuit is by-
passed and the compressor will continue to run until the 30
psig pressure switch opens.
Sequence of Operation
Hot Gas Bypass Operation
The HGBP valve regulates evaporator pressure by opening
as suction pressure decreases, to maintain a desired mini-
mum evaporating pressure regardless of a decrease in
evaporator external loading.
When the evaporator (suction) pressure is above the
valve’s setpoint, it remains closed. As suction pressure falls
below the valve’s setpoint, the valve begins to open. The
valve will continue to open at a rate proportional to the suc-
tion pressure drop, thus maintaining evaporator pressure.
Hot gas bypass valves are adjustable and should be set to
begin opening at approximately 58 psig suction pressure
and reach the full open position at 51 psig for DX coil appli-
cations. For EVP chiller applications, the regulator should
be adjusted to begin opening at approximately 69 psig suc-
tion pressure and reach full open position at 61 psig.
System Start-Up (Continued)