35
OPERATING SEQUENCE
Base Unit Controls
Indoor (Supply) Fan —
The indoor fan contactor (IFC) is remotely located at the fan
coil or fan section. If the thermostat fan oper ation is selected
as Continuous, the IFC is energized and the indoor (supply)
fan motor runs continuously. If the thermostat fan operation
is selected as Au tomatic, the IFC will be energ ized on a call
for Cooling; indoor (supply) fan motor runs. When
thermostat call for Cooling is satisfied , the IF C is
de-ener gized and indoor (supply) fan motor stops.
Cooling, Unit Without Economizer —
38AUZ (Single Circuit)
On a thermostat c all for Cooling, IFC will be energized
and indoor (supply) fan motor runs. Thermostat output Y1
is energized; terminal Y1 at 38AUZ unit receives 24-v.
24-v received at CADM1 terminal Y. If anti-recycle time
delay period has not expired, CADM1 relay will remain
open, de-energizing Solenoid Va lve Relay (SVR) and
preventing compressor start. When safety pressure
switches are closed and CADM1 time delay expires,
CADM1 relay closes, SVR and compressor contactor C1
are energized; liquid line solenoid valve LLSV opens, all
outdoor fan motors start a nd Compressor 1 star ts.
On a thermostat calling for Stage 2 Cooling, thermostat
output Y2 is energized; terminal Y2 at 38AUZ unit
receives 24-v. 24-v received at CADM2 terminal Y. If
anti-recycle time delay period has not expired, CADM2
relay will remain open, preventing Compressor 2 start.
When safety pressure switches are closed and CADM2
time delay expires, CADM2 relay closes, compressor
contactor C2 is energized; Compressor 2 starts.
As spac e cooling l oad is satisfied, thermostat outputs Y2
and Y1 are sequentially de--energized, removing 24-v at
38AUZ terminals Y2 and Y1. Compressor 2 stops on Y2
opening. CADM2 begins it s three-minute anti-recycle
time delay. On Y1 opening, Compressor 1 stops, all
outdoor fan motors stop and SVR rela y is de-energized.
Liquid line solenoid va lve is de-energized and valve
closes. CADM1 begins its three-minute anti-recycle time
dela y.
38AUD (Two Circuit)
On a thermostat c all for Cooling, IFC will be energized
and indoor (supply) fan motor runs. Thermostat output Y1
is energized; terminal Y1 at 38AUD unit receives 24-v.
24-v received at CADM1 terminal Y. If anti-recycle time
delay period has not expired, CADM1 relay will remain
open, de-energizing Solenoid Valve Relay 1 (SVR1) and
preventing compressor start. When safety pressure
switches are closed and CADM1 time delay expires,
CADM1 relay closes, SVR1 and compressor contactor C1
are energized; liquid line solenoid valve LLSV1 opens, all
outdoor fan motors start a nd Circuit 1 compressor starts.
On a th ermo s tat calling fo r Stage 2 Co olin g, thermostat
outp ut Y2 is ener gized ; terminal Y2 at 38A U D unit receives
24-v. 24-v received at CAD M 2 terminal Y. If anti-recycle
time delay perio d has not expired , CADM2 relay will
remain open, de-energizing Solenoid Valve Relay 2 (SVR2)
and preventing compressor start. When safety pressure
sw itch es are closed and CAD M 2 time delay expires,
CADM2 relay closes, SV R2 and compress o r contactor C2
are energized; liquid line solenoid valve LLSV2 opens and
Circuit 2 compressor starts.
As space cooling load is satisfied, thermostat outputs Y2 and
Y1 are de--energized, removing 24-v at 38AUD terminals
Y2 and Y1. Circuit 2 compressor stops on Y2 opening;
SV R 2 is de- energized and LLSV2 closes . CAD M 2 begin s its
three-min u te anti-recycle time delay. On Y1 opening , Circuit
1 compressor stops, all outdoor fan motors stop and SVR1
relay is de-en ergized. Liquid line solenoid valve LLSV1 is
de-energized and valve closes . CADM 1 begins its
three-min u te anti-recycle time delay.
38AUDC, Unit with Hot Gas Bypass (HGBP) —
Fig . 32 (on page 36) illustrates the factory--ins talled hot gas
bypass (HGBP) option for 38AUDC units. The HGBP is set
at about 109 psig (752 kPa) (about 36_F(2_C) satur ation
temperatur e) , to maintain the evapo rato r coil temperatures
above 36_F(2_C). The liquid injection valve (TXV) is set at
appro x imately 17_F(9_C) superheat. The valves on the
HGBP option are preset at the factory, but may be adjusted
in the field.
As the load on the indoor evaporator decreases, the suction
pres s u re will decrease wh ich is detected by the HGBP valve
(constant pressure valve). When the suction pressure
approaches 109 psig (752 kPa), the HGBP valve begins to
open to allow hot gas from the compressor to bypass to the
suctio n pipe in a contro lled manner to attempt to maintain a
constant suction pressure. Since hot gas is too hot for
entering the compressor, the device includes a liquid injector
(or TXV ) w hich feeds a little refrig eran t from the liquid line
through the TXV to de--superheat the hot gas and maintain
an acceptab le refrigeran t quality to not damage the
compressor . When hot gas bypasses, it reduces the mass
flow of refrigerant to the evaporator which reduces its
capacity, and attempts to maintain the suction satur ation
temperature above 32_F(0_C) which minimizes the
poten tial for freeze--up in the evaporato r coil.
The HGBP solenoid valve, as indi cated in the lower left of
Fig. 32, allows HGBP control during first stage cooling
operation.
38AU
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