Mounted in the condenser corner post (or the special enclosure for a field kit VFD), the VFD allows
the operator to set the high head pressure level (the factory default is 320 psig). To meet the
desired refrigerant pressure setpoint, the VFD controls the condenser fan motor's frequency
(speed). The pressure transducer must be only connected to circuit #1.
The VFD option does not include a condenser fan bypass circuit in the case of a microdrive failure.
The microdrive needs to be replaced if a failure occurs to the drive to put the unit back to normal
operation. In the case of a VFD failure, the condenser fan stops running. However, the unit
compressors continue to run until a low pressure safety trip is activated.
For detailed installation and operation instructions, refer to
Variable Frequency Drive manual (Form
145.13-NO1)
.
Hot gas bypass (HGBP)
To allow for low cooling load operation, a direct-acting pressure-modulating bypass control valve is
installed on the system #1 discharge line. To maintain a desired minimum evaporator pressure, use
this valve to divert high temperature, high pressure refrigerant around the TXV. HGBP is standard
on all units with variable air volume (VAV) and optional on units with constant volume (CV).
Adjusting HGBP setpoint
When the HGBP valve opens, select the coil suction pressure or coil temperature by adjusting the
screw on the HGBP valve. To set the load, run the unit and cool down the evaporator coil. To do
this, either shut the fans or block the airflow until the suction pressure drops at least 5 psi below
the chosen evaporator coil setpoint. Next, allow the bypassed gas to raise the pressure. The screw
spring adjustment can be set until the HGBP valve closes at the chosen setpoint.
The pressure of the evaporator coil is set to maintain an evaporator coil temperature above the
point that frost and coil freeze-up can form.
41
Model DSH024C–120C Horizontal R-410A Air Conditioning Units C-Generation Smart Equipment Controller
Johnson Controls
