Bryant COMMERCIAL SINGLE PACKAGE ROOFTOP GAS HEAT/ELECTRIC COOLING 581B - Manual (Page 59)

59
CONTROLS (cont)
If the supply-air temperature falls below 52 F, a switch on the
supply-air thermostat is closed between the T1 terminal and the
24 vac terminal. This causes the damper to modulate closed
until the supply-air temperature rises above 55 F or the damper
reaches the minimum position.
When the supply-air temperature is between 55 F and 57 F, the
supply-air thermostat has open switches between the T2 and
24 vac terminals. This causes the economizer damper to
remain in an intermediate open position.
If the outdoor air alone cannot satisfy the cooling requirements
of the conditioned space, economizer cooling is integrated with
mechanical cooling, providing second stage cooling. Compres-
sor no. 1 and condenser fan will be energized, and the position
of the economizer damper will be determined by the supply-air
temperature. Compressor no. 2 is locked out.
When the second stage of cooling is satisfied, the compressor
and condenser fan motors will be deenergized. The damper
position will be determined by the supply-air temperature.
When the first stage of cooling is satisfied, the IFM shuts off
after a 30-second delay. The damper then moves to fully closed
position.
Cooling, Units With EconoMi$er — When the outdoor-air
temperature (OAT) is above the ECON SP set point and the
room thermostat calls for Stage 1 cooling (R to G +Y1), the
indoor-fan motor (IFM) is energized and the EconoMi$er
damper modulates to minimum position. The compressor con-
tactor is energized starting the compressor and outdoor-fan
motor (OFM). After the thermostat is satisfied, the damper mod-
ulates to the fully closed position when the IFM is deenergized.
When the OAT is below the ECON SP set point and the room
thermostat calls for Stage 1 cooling (R to G + Y1), the
EconoMi$er modulates to the minimum position when the IFM
is energized. The EconoMi$er provides Stage 1 of cooling by
modulating the return and outdoor air dampers to maintain a
55 F supply air set point. If the supply-air temperature (SAT) is
greater than 57 F, the EconoMi$er modulates open, allowing a
greater amount of outdoor air to enter the unit. If the SAT drops
below 53 F, the outdoor-air damper modules closed to reduce
the amount of outdoor air. When the SAT is between 53 and
57 F, the EconoMi$er maintains its position.
Heating, Units Without Economizer — When the thermostat
calls for heating, terminal W1 is energized. In order to prevent
thermostat short-cycling, the unit is locked into the Heating
mode for at least 1 minute when W1 is energized. The induced-
draft motor (IDM) is then energized and the burner ignition
sequence begins. The indoor (evaporator) fan motor (IFM) is
energized 45 seconds after a flame is ignited. When additional
heat is needed, W2 is energized and the high-fire solenoid on
the main gas valve (MGV) is energized. When the thermostat is
satisfied and W1 and W2 are deenergized, the IFM stops after a
45-second time-off delay.
Heating, Units With Economizer or Two-Position Damper —
When the thermostat calls for heating, terminal W1 is ener-
gized. In order to prevent thermostat short-cycling, the unit is
locked into the Heating mode for at least 1 minute when W1 is
energized. The induced-draft motor is then energized and the
burner ignition sequence begins. The indoor (evaporator) fan
motor (IFM) is energized 45 seconds after a flame is ignited and
the damper moves to the minimum position. If the accessory
two-position damper is used, the outdoor-air damper opens to
the minimum position whenever the evaporator fan runs. When
additional heat is needed, W2 is energized and the high-fire
solenoid on the main gas valve (MGV) is energized. When the
thermostat is satisfied and W1 and W2 are deenergized, the
IFM stops after a 45-second time-off delay. The economizer
damper then moves to the fully closed position. When using
continuous fan, the damper will remain in the minimum position.
OPERATING SEQUENCE FOR 581B036-150 SIZE UNITS
Units With Perfect Humidity™ Dehumidification Package —
When thermostat calls for cooling, terminals G and Y1 is ener-
gized. The indoor (evaporator) fan motor (IFM), compressor (C),
and outdoor (condenser) fan motor (OFM) start. The OFM runs
continuously while the unit is in cooling. As shipped from the
factory, Perfect Humidity dehumidification circuit is always ener-
gized. If Perfect Humidity circuit modulation is desired, a field-
installed, wall-mounted LC Thermidistat or humidistat are
required.
If the Perfect Humidity humidistat is installed and calls for the
Perfect Humidity subcooler coil to operate, the humidistat inter-
nal switch closes. This energizes and closes the liquid line sole-
noid valve coil (LLSV) of the Perfect Humidity circuit, forcing the
hot liquid refrigerant of the liquid line to enter the subcooler coil.
As the hot liquid passes through the subcooler coil, it is
exposed to the cold supply airflow coming off from the evapora-
tor coil and the liquid is further cooled to a temperature
approaching the evaporator coil leaving-air temperature. The
state of the refrigerant leaving the subcooler coil is a highly sub-
cooled liquid refrigerant. The liquid then enters a thermostatic
expansion valve (TXV) where the liquid is dropped to a lower
pressure. The TXV does not have a pressure drop great
enough to change the liquid to a 2-phase fluid. The TXV can
throttle the pressure drop of the liquid refrigerant and maintain
proper conditions at the compressor suction valve over a wide
range of operating conditions. The liquid then enters a second
fixed restrictor expansion device for a second pressure drop to
a 2-phase fluid. The liquid proceeds to the evaporator coil at a
temperature lower than normal cooling operation. This lower
temperature is what increases the latent capacity of the rooftop.
The 2-phase refrigerant passes through the evaporator and is
changed into a vapor. The air passing over the evaporator coil
will become colder than during normal operation as a result of
the colder refrigerant temperatures. However, as it passes over
the subcooler coil, the air will be warmed slightly.
As the refrigerant leaves the evaporator, the refrigerant passes
a low-pressure switch in the suction line. This low-pressure
switch will deactivate the Perfect Humidity package when the
suction pressure reaches 60 psig. The low-pressure switch is
an added safety device to protect against evaporator coil
freeze-up. The low-pressure switch will only deactivate and
open the liquid line solenoid valve in the Perfect Humidity circuit.
The compressors will continue to run as long as there is a call
for cooling, regardless of the position of the low-pressure
switch. The solenoid valve and the Perfect Humidity package
will be re-activated only when the call for cooling has been sat-
isfied, the low-pressure switch has closed, and a new call for
cooling exists. The crankcase heaters on the scroll compressor
provide additional protection for the compressor due to the
additional refrigerant charge in the subcooler.
When the humidistat is satisfied, the humidistat internal switch
opens cutting power to and opening the LLSV. The refrigerant is
routed back through the evaporator and the subcooler coil is
removed from the refrigerant loop.
When the thermostat is satisfied, C1 is deenergized and the
compressor and OFM shut off. After a 30-second delay, the IFM
shuts off. If the thermostat fan selector switch is in the ON posi-
tion, the IFM will run continuously.