The liquid enters the evaporator coil at a temperature lower than in standard cooling operation. This lower temperature increases the latent capacity of the rooftop unit. The refrigerant passes through the evaporator and is turned into a vapor. The air passing over the evaporator coil will become colder than during normal operation. However, as this same air passes over the subcooling coil, it will be slightly warmed, partially reheating the air.
Subcooling mode operates only when the outside air temperature is warmer than 40_F. A
The scroll compressors are equipped with crankcase heaters to provide protection for the compressors due to the additional refrigerant charge required by the subcooling/reheat coil.
| When in subcooling mode, there is a slight decrease in system | |
| total gross capacity (5% less), a lower gross sensible capacity | |
| (20% less), and a greatly increased latent capacity (up to 40% | |
551B,C | ||
more). | ||
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C06135
Fig. 42 --- Perfect Humidityt Normal
Design Cooling Operation
C06136
Fig. 43 --- Perfect Humidity Subcooling
Mode Operation
C06137
Fig. 44 --- Perfect Humidity Hot Gas
Reheat Mode Operation
Hot Gas Reheat Mode
When the humidity levels in the space require humidity control, a hot gas solenoid valve (specific to hot gas reheat mode only) will open to bypass a portion of hot gas refrigerant around the condenser coil. (See Fig. 44.)
This hot gas will mix with liquid refrigerant leaving the condenser coil and flow to the subcooling/reheat dehumidification coil. Now the conditioned air coming off the evaporator will be cooled and dehumidified, but will be warmed to neutral conditions (72_F to 75_F) by the subcooling/reheat dehumidification coil.
The net effect of the rooftop when in hot gas reheat mode is to provide nearly all latent capacity removal from the space when sensible loads diminish (when outdoor temperature conditions are moderate). When in hot gas reheat mode, the unit will operate to provide mostly latent capacity and extremely low sensible heat ratio capability.
Similar to the subcooling mode of operation, hot gas reheat mode operates only when the outside air temperature is warmer than 40_F. Below this temperature, a factory installed outside air temperature switch will lockout this mode of operation.
See Table 34 for the Perfect Humidity adaptive dehumidification system sequence of operation.
SERVICE
!WARNING
ELECTRICAL SHOCK HAZARD
Failure to follow this warning could cause personal injury or death.
When sevicing unit, shut off all electrical power to unit and install lockout tag.
Step 1 —Cleaning
Inspect unit interior at the beginning of heating and cooling season and as operating conditions require.
Evaporator Coil
1.Turn unit power off, tag disconnect. Remove evaporator coil access panel.
2.If economizer or
3.Slide filters out of unit.
50
