If outdoor air alone cannot satisfy the cooling requirements of the conditioned space, and the OAT is above the MECH CLG LOCKOUT set point, the EconoMi$er integrates free cooling with mechanical cooling. This is accomplished by the strategies below.
NOTE: Compressors have a two-minute Minimum On and Minimum Off, which are accomplished by the strategies below.
1.If Y1 is energized, and the room thermostat calls for Y2 (2-stage thermostat), the compressor and OFC are energized. The position of the EconoMi$er damper is maintained at its current value.
2.If Y1 is energized for more than 20 minutes, and Y2 is not energized (whether or not a 2-stage thermostat is used), the compressor and OFC are energized. The position of the EconoMi$er damper is maintained at its current value.
3.If Y1 is energized, and compressor no. 1 is already energized (see Step 2) and the room thermostat calls for Y2, compressor no. 1 continues to operate. If Y2 remains energized for more than 20 minutes, com- pressor no. 2 is energized.
NOTE: Compressor no. 2 cannot be energized unless there is a signal for Y2 from the space thermostat.
4.If compressor no. 2 is energized, and the Y2 signal from the thermostat is satisfied, compressors 1 and 2 are deenergized. Re-asserting Y2 will start compres- sor no. 1 and (after a 20-minute interstage delay) compressor no. 2.
5.If compressor no. 1 is energized and the thermostat is satisfied, compressor no. 1, the OFM, and IFM are deenergized and the EconoMi$er modulates closed.
When the OAT is below the MECH CLG LOCKOUT set point, the compressors remain off.
D. Freeze Protection Thermostat(s)
A freeze protection thermostat (FPT) is located on the top and bottom of the evaporator coil. It detects frost build-up and turns off the compressor, allowing the coil to clear. Once frost has melted, the compressor can be reenergized by reset- ting the compressor lockout.
E.Heating, Units With EconoMi$er (If Accessory or Optional Heater is Installed)
When the room thermostat calls for heat, the heating con- trols are energized as described in the Heating, Units With- out EconoMi$er section. The IFM is energized and the EconoMi$er damper modulates to the minimum position. When the thermostat is satisfied, the damper modulates closed.
F. Units With Perfect Humidity™ Dehumidification Package
When thermostat calls for cooling, terminals G and Y1 and/ or Y2 and the compressor contactor C1 and/or C2 are ener- gized. The indoor (evaporator) fan motor (IFM), compressors, and outdoor (condenser) fan motors (OFM) start. The OFMs run continuously while the unit is in cooling. As shipped
from the factory, both Perfect Humidity dehumidification cir- cuits are always energized.
If Perfect Humidity circuit modulation is desired, a field- installed, wall-mounted humidistat is required. If the Perfect Humidity humidistat is installed and calls for the Perfect Humidity subcooler coil to operate, the humidistat internal switch closes. This energizes the 3-way liquid line solenoid valve coils (LLSV1 for circuit 1 and LLSV2 for circuit 2) of the Perfect Humidity circuits, forcing the warm liquid refrig- erant of the liquid line to enter the subcooler coils. See Fig. 29.
As the warm liquid passes through the subcooler coils, it is exposed to the cold supply airflow coming off the evaporator coils and the liquid is further cooled to a temperature approaching the evaporator coil leaving-air temperature. The state of the refrigerant leaving the subcooler coils is a highly subcooled liquid refrigerant. The liquid then enters a thermostatic expansion valve (TXV) where the liquid is dropped to the evaporator pressure. The TXVs can throttle the pressure drop of the liquid refrigerant and maintain proper conditions at the compressor suction valves over a wide range of operating conditions. The liquid proceeds to the evaporator coils at a temperature lower than normal cooling operation. This lower temperature is what increases the latent and sensible capacity of the evaporator coils.
The 2-phase refrigerant passes through the evaporators and is changed into a vapor. The air passing over the evaporator coils will become colder than during normal operation as a result of the colder refrigerant temperatures. However, as it passes over the subcooler coils, the air will be warmed, decreasing the sensible capacity and reducing the sensible heat of the roof- top unit.
As the refrigerant leaves the evaporator, the refrigerant passes a subcooler control low-pressure switch (S-LPS1 for circuit 1 or S-LPS2 for circuit 2) in the suction line. This low- pressure switch will deactivate the Perfect Humidity pack- age when the suction pressure reaches 60 psig. The sub- cooler control low-pressure switch is an added safety device to protect against evaporator coil freeze-up during low ambi- ent operation. The subcooler control low-pressure switch will only deactivate the 3-way 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 subcooler control low-pressure switch. The 3-way solenoid valve and the Perfect Humidity package will be reactivated only when the call for cooling has been satis- fied, the subcooler control low-pressure switch has closed above 80 psig, and a new call for cooling exists. The crank- case heaters on the scroll compressors provide additional protection for the compressors due to the additional refriger- ant charge in the subcooler.
When the humidistat is satisfied, the humidistat internal switch opens, cutting power to and deenergizing the LLSVs. The refrigerant is routed back through the evaporators and the subcooler coils are removed from the refrigerant loops. When the thermostat is satisfied, C1 and C2 are deenergized and the compressors, IFM, and OFMs shut off. If the thermo- stat fan selector switch is in the ON position, the IFM will run continuously.
