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Water Side Economizer
A completely factory-installed, factory-piped, and factory-
controlled water side economizer system is available on any
constant or variable air volume system . Whenever the entering
water temperature is more than 5°F to 7°F below the mixed
air temperature to the unit, the control valves modulate to
provide cooling directly from the tower water (adjustable at the
MicroTech III keypad/display) . The economizer system can be
used to provide 100% of the cooling demand or supplement
mechanical cooling by precooling the return air . The
economizer system consists of a water coil and two, two-way,
control valves . The unit's MicroTech III controller modulates
the control valves to satisfy the cooling demand whenever
the entering water is suitable . When the control valves are in
the 90% open position, the unit's compressors are allowed to
stage on to satisfy the cooling load . When the entering water
temperature is no longer suitable, the economizer control valve
closes and the unit is on 100% mechanical cooling .
Two valve control arrangements are available from the factory .
The first maintains full flow through the unit condensers at
all times . This control arrangement is used for systems with
constant pumping systems . For installations with a variable
pumping system, the control valves are sequenced so flow is
removed from the unit whenever cooling is not required .
The economizer system is factory piped and the coil takes
advantage of the same drain pan and condensate connection .
To vent air from the economizer coil, use the uppermost
clean out plug or vent plug on top of the header . The torque
requirement for the clean out plugs is 10 inch-lb (1,13 N-M) .
Condenser Water Head
Pressure Control
An optional condenser head pressure control valve is available
on units without water side economizer . This option permits
operation with entering water temperatures as low as 40°F . The
valve is a two-way regulating valve controlled via MicroTech III
to maintain refrigerant head pressure .
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Variable Frequency Drive
As an option, a variable frequency drive (VFD) is available
for airflow modulation. A manually activated bypass contactor
is provided to allow system operation in the event of drive
service .
Static pressure is controlled by the unit-mounted MicroTech
III controller. Indication of current airflow is available at the
MicroTech III controller . Static pressure is sensed by one or
two factory-mounted duct sensors . The installer provides and
installs the sensor tubing from unit mounted sensor(s) to duct
location(s) . The static pressure set point is keypad adjustable
through the MicroTech III DDC controller .
All variable air volume units include a field-adjustable, duct
high limit safety control to protect ductwork from excessive duct
pressure .
Disconnect Switch
A factory-mounted, non-fused main circuit interrupter is
available for disconnecting the main electrical power . The
switch is located at the front of the control panel and is
accessible without unit penetration . The lug size information is
provided in
Table 8
and
Dual Power Supply
A dual power block is an option for the power supply, which
allows the fan motor and control circuit to be isolated from the
compressor circuit . If the unit has the optional electric heat, it is
circuited with the compressors .
Heating Coil Control
A factory-mounted, heating water coil is available, without a
factory-mounted control valve. The field-provided and installed
heating control valve is controlled by the unit's MicroTech III
controller. Refer to "as built" schematics for field terminals.
Spring Return Actuators
24 VAC powered actuators are used to automatically control
the water valves . The MicroTech III controller unit monitors
conditions and controls the actuators accordingly, using a 0 to 10
VDC analog signal to close and open the valves respectively .
33
IM 1032-2 • Self-Contained Air Conditioning Systems
u
nIT
Table 9 on page
15 .
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