Operating Principles
54
RTAF-SVX001K-EN
Condensers are factory proof and leak tested at 525 psig.
Coils can be cleaned with water.
Condenser fans features:
•
Direct-drive vertical-discharge airfoil.
•
Permanent magnet motors with integrated drive to
provide variable speed fan control for all fans.
•
Permanently lubricated ball bearings.
•
Internal temperature and current overload protection.
Evaporator
The evaporator is a tube-in-shell heat exchanger design
constructed from carbon steel shells and tubesheets with
internally and externally finned seamless copper tubes
mechanically expanded into the tube sheets. The
evaporator is designed, tested and stamped in accordance
with the ASME Boiler and Pressure Vessel Code for a
refrigerant side working pressure of 200 psig. The
evaporator is designed for a water side working pressure
of 150 psig.
Note:
When free-cooling option is selected, waterside
working pressure is 90 psig.
Standard water connections are grooved for Victaulic style
pipe couplings, with flange style connections optionally
available. Waterboxes are provided in a 2 pass
configuration on units up to 270 tons. Evaporators for 280
to 520 ton units utilize a 1-pass configuration. Each
waterbox includes a vent, drain and fittings for
temperature control sensors. A factory installed flow
switch is installed on the supply waterbox in the
evaporator inlet connection.
Evaporators are insulated with 3/4 in closed cell insulation.
Evaporator shell heaters and waterbox heaters with
thermostat are provided to help protect the evaporator
from freezing at ambient temperatures down to
-4°F (-20°C).
Note:
A separate field supplied 120V power source is
required to power the evaporator freeze protection.
See
“Installer-Supplied Components,” p. 45
for more information.
Refrigerant Management
Refrigerant management system varies by unit size.
•
Unit sizes 115 to 215 tons with design sequence AE and
earlier use an actuated suction valve.
•
Unit sizes 230 to 520 tons, and sizes 115 to 215 tons and
design sequence AF and later, utilize a receiver tank.
Free-Cooling — Operating Modes
The advantage of optional chiller integrated free-cooling is
the ability to utilize outdoor air temperatures to assist in
making chilled water when appropriate. The unit controls
direct flow through or around the free-cooling coils to
optimize chiller efficiency. Determining the operating
mode depends on four temperatures:
•
Ambient air temperature
•
Evaporator entering fluid temperature
•
Evaporator leaving fluid temperature
•
Chilled water set point
Mechanical Cooling Mode
In this operating mode, ambient temperature is the same
or higher than the temperature of the fluid entering the
evaporator. Free-cooling coils are bypassed, compressors
are running, and the controls modulate compressors and
fans to meet cooling load at optimum efficiency.
Combined Mechanical and
Free-Cooling Mode
If the ambient temperature is below the evaporator
entering fluid temperature, and free-cooling only cannot
satisfy the load, the controls modulate compressors and
fans to meet the remaining cooling load at optimum
efficiency. Fluid will continue to flow through free-cooling
coils, reducing the evaporator entering fluid temperature.
Note:
Depending on load, one or both circuits may
engage mechanical cooling.
Free-Cooling Only Mode
In this operating mode, free-cooling is enabled and
capable of meeting the cooling load without the need for
mechanical cooling. As ambient falls below the
temperature at which full load capacity is provided by free-
cooling only (or the load drops), fan control modulates fan
speed down to a minimum of 15%. If ambient (or load)
continues to drop, valve control provides modulation
between free-cooling coils.
Table 27. Free-cooling operation
Component
Mechanical Cooling
Combined Mechanical and Free-Cooling
Free-Cooling Only
Fan Control
Valve Control
Ambient Air
Greater Than Fluid
Less Than Fluid
Less Than Fluid
Less Than Fluid
Compressors
On — Modulating
On — Modulating
Off
Off
Fans
On — Modulating
On — Modulating
Modulating
15%
Free-Cooling Coil Flow
Off
100%
100%
Modulating
