Daikin MicroTech III Operation Manuals page 128

Evaporative Condensing Control (RTU)
The evaporative condensing option for rooftop units uses
the heat absorbed by evaporating water as well as air drawn
across a bank of tubes with refrigerant flowing through them
to condense hot refrigerant to a liquid . Water is pumped from
a sump beneath the condenser tubes to nozzles above the
coil that spray water onto the bank of tubes . The refrigerant in
the tubes is cooled and condensed as some of this water is
evaporated when it strikes the hot tubes and is carried away
by condenser fans . Using this method, the refrigerant can be
cooled to a lower temperature than is the case with a normal
finned condenser that transfers heat directly to the air.
A unit equipped with evaporative condensers cannot operate
in the cooling state with the outdoor air temperature below
40°F . If the OAT Compressor Lockout Temperature on units
with evaporative condensing is lowered below 40°F, it will
immediately be set back up to 40°F .
The first condenser fan on each circuit is turned on and off
via ModBus communications with a single VFD . The speed of
these two fans is controlled via ModBus communications with
the VFD . These two fans always operate at the same speed .
The second and third condenser fans on each compressor
circuit are turned ON and OFF using the two outputs on the
Main Board that are used for standard condenser fans . Relays
are driven by each of these outputs so that fans associated
with a circuit that is not operating will not be turned ON .
Evaporative Condenser Compressor Staging Sequences
There are two cooling staging configurations available for
evaporative condenser units . Both employ reciprocating
compressors and so require pumpdown operation . When there
is a first call for cooling on a circuit, the liquid line solenoid
valve output is first energized and then as soon as the low
pressure switch for that circuit closes the first compressor in
the sequence is turned ON . If the low pressure switch fails to
close within the low pressure compressor delay timer setting
the low pressure alarm for that circuit is generated .
The circuit that has the fewer run hours is the lead circuit if
Lead Circuit is set to Auto . The lead circuit is changed only
when all compressors are off or at the maximum stages for the
unit . If Lead Circuit is set to #1 then circuit #1 always leads if
it is available . If Lead Circuit is set to #2 then circuit #2 always
leads if it is available .
OM 920-6 • MICROTECH UNIT CONTROLLER
The Staging Type can be set to either Standard or Alternate .
Standard:
The two circuits are loaded up and unloaded as evenly as
possible . When a stage up is required, the circuit operating at
the lower stage is staged up . If both circuits are operating at
the same stage, the lead circuit is staged up if it is not at its
maximum stage .
When a stage down is required, the circuit operating at the
higher stage is staged down if it is not at stage zero . If both
circuits are operating at the same stage, the lag circuit is
staged down if it is not at stage zero .
Alternate:
One circuit is loaded completely before the first compressor
in the other circuit is turned ON, and one circuit is unloaded
completely before the other circuit begins to be unloaded .
When a stage up is required and the lead circuit is not at its
maximum stage, the lead circuit is staged up .
When a stage up is required and the lead circuit is already at
its maximum stage, the lag circuit is staged up . When a stage
down is required and the Lag circuit is at a stage greater than
zero, the lag circuit is staged down . When a stage down is
required and the lag compressor is at stage zero, the lead
compressor is staged down if it is not at stage zero .
A disabled circuit remains at stage zero . If the other circuit
is enabled, it is staged up whenever a stage up is required,
and down whenever a stage down is required . If a previously
disabled circuit becomes enabled, the staging is re-aligned
according to the staging tables upon the next call for stage
increase or decrease .
NOTE: During this re-alignment, the cooling stage time
guaranteed ON and OFF times must be observed as
well as a minimum of 10 seconds between starting
more than one compressor
When dehumidification is active staging reverts to Alternate
regardless of the normal Staging Type setting . When
dehumidification is not active staging is according to the
Staging Type setting .
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