Microprocessor Controller for DOAS
5
Sequence of Operation
Sequence of Operation
setpoint. When a modulating compressor is installed
(Digital or Inverter Scroll), the compressor modulates
to maintain the supply air setpoint. Mechanical
cooling is available in the following configurations:
-
Packaged DX:
Unit with compressors and
condensing section located within the same unit.
This unit may have lead standard, lead digital
scroll, or lead inverter scroll compressors.
-
Split DX:
Unit with compressors located in the
unit and utilizes a remote condenser section.
This type of unit may have lead standard, or lead
digital scroll compressors.
Active Head Pressure Control
Packaged DX mechanical systems will maintain head
pressure control by utilizing transducers on each
refrigerant circuit. The pressure reading from the
transducer is converted to a saturated discharge
temperature for each circuit. The temperature, or
maximum temperature when two circuits are present, is
compared to a setpoint.
The following sequences are based on the type of
condenser fan modulation installed in the unit.
•
No Modulating Fans (All AC)
: Condenser fans
are staged using digital outputs and the saturated
discharge temperature. The first fan stages on with
the start of the first compressor. Each additional
stage turns on based on the saturated temperature
reaching setpoint plus an offset and turns off when
the temperature falls below setpoint. Built-in delays
between stages assist in staging fans off or on too
quickly.
• Lead Modulating Fan:
A unit with this option has
one modulating condenser fan per fan bank. The
modulating condenser fan utilizes an analog output
to vary the speed of the fan. The modulating fan
turns on with the start of the first compressor. When
the saturated temperature is above setpoint, the
modulating fan speed will increase to maintain head
pressure. When below setpoint, the fan speed will
decrease.
Additionally, non-modulating fans are staged using
digital outputs and an offset. Each additional stage
turns on based on the saturated temperature
reaching setpoint plus an offset and turns off when
the temperature falls below setpoint. Built-in delays
between stages assist in staging fans off or on too
quickly.
• All Modulating Fans:
A unit with this option
has all modulating condenser fans. One analog
signal modulates all fans in a bank. The first fan
stages on with the start of the first compressor.
The fans modulate to maintain the saturated
discharge temperature setpoint. When the saturated
temperature is above setpoint, the fan speed will
increase to maintain head pressure. When below
setpoint, the fan speed will decrease.
Sliding Head Pressure Control
The head pressure control setpoint changes based
on the outside air temperature and an offset. As the
outside temperature increases so does the control
setpoint for the condenser fans. This feature is active in
cooling and dehumidification modes unless disabled in
the controller. Sliding head pressure control is enabled
by default.
Air Source Heat Pump
When a unit is configured as an ASHP, compressors are
used for cooling and heat pump heating. A reversing
valve is energized when the unit is in heating mode to
reverse the flow of the refrigerant. The ASHP is only
available as a packaged unit with an inverter scroll as
the lead compressor.
•
Cooling:
Mechanical cooling operates the same as
any other unit with compressors by controlling the
compressors to maintain the supply air temperature
set point in cooling mode and to maintain the cooling
coil temperature in dehumidification mode.
•
Heat Pump Heating
:
When heat is required, the
reversing valve is switched, and the compressors
are staged to maintain the supply air temperature set
point.
•
Heat Pump Heating Lockout:
Heat pump heating
may be locked out for any of the following reasons:
- Defrost is initiated 3 times in one hour.
- Supply Air temperature is 5ºF below set point
for more than 10 minutes and secondary heat is
available as backup only.
- Outside ambient temperature is below the HP
ambient lockout set point(10ºF).
•
Resetting HP Heating Lockout
:
One of the
following conditions must occur to return to HP
heating:
- The outside temperature increases by 5ºF.
- The outside humidity decreases by 20%RH, if
humidity sensor is installed.
- The unit has been locked out for more than 2
hours when a humidity sensor is not installed and
not locked out on low ambient condition.
•
Defrost:
Periodically, the ASHP need to initiate a
defrost cycle to remove accumulated frost from the
outside coil when operating in heating mode. The
saturated suction temperature, the outside ambient
temperature and/or the outside humidity determine
when a defrost initiates and terminates.
