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minimum “Off” time has been satisfied (Point A). The
minimum “fast response” time and the time delay
between staging for the W7100G is set for 60 seconds.
If the water temperature remains above the upper
control band limit (Point B), the next available stage of
cooling will be energized when the minimum time
delay between stages has elapsed. As the water
temperature decreases below the lower control band,
the last stage that was turned “On” will be cycled “Off”
(Point C) when the minimum “On” time for that stage
has elapsed. As the load on the water increases due to
cooling stages being cycled “Off”, the controller will
maintain its current position, i.e., no staging of cooling
“On” or “Off”, as long as the temperature remains
inside the control band. When the temperature
increases above the upper control band limit (Point D),
mechanical cooling stages will be sequenced “On” in
the same manner as before. As a rule, any time the
water temperature is above the upper control band
limit, a stage of cooling will be “added” and anytime
the water temperature decreases below the lower
control band limit, a stage of cooling will be
“Subtracted”.
Figure 66.
W7100G staging sequence
Discharge
Solution
Temperature
Upper Control
Band Limit
Lower Control
Band Limit
Adjustable
Control Band
0°F to 10°F
W7100
Setpoint
A
B
C
D
Thermostatic Expansion Valve
The reliability and performance of the refrigeration
system is heavily dependent upon proper expansion
valve adjustment. Therefore, the importance of
maintaining the proper superheat cannot be over
emphasized. See Thermal Expansion Valve
Manufacturer Settings tables in Start-Up chapter for
proper superheat adjustment. Accurate measurements
of superheat will provide the following information.
•
Effectiveness of expansion valve control of
refrigerant flow.
•
Efficiency of the evaporator coil.
•
Amount of protection compressor is receiving
against flooding or overheating.
The expected range for superheat is 14 to 18°F at full
load conditions. At part load, expect a properly
adjusted expansion valve to control to 8 to 12°F
superheat. Systems operating with lower superheat
could cause serious compressor damage due to
refrigerant floodback.
The outdoor ambient temperature must be between
65ºF and 105ºF and the relative humidity of the air
entering the evaporator must be above 40 percent.
When the temperatures are outside of these ranges,
measuring the operating pressures can be
meaningless.
Condenser Fans
20 to 60 Ton Specs
Condenser fan cycling is accomplished through
interlocking the fan operation with compressor
operation (1K1 & 1K3), liquid line pressure switches
(4S11 & 4S12). When the low ambient option is
applied, ambient temperature thermostats (1S36 &
1S37) are used to provide additional fan cycling
control.
lists the condenser fan
sequencing data. See
for
condenser fan locations and designators.
When a cooling command has been initiated, the first
compressor for circuit 1 is energized. An auxiliary
contact on the compressor contactor is closed,
energizing contactor 1K6 and turning on fan 2B2. When
the liquid line pressure exceeds 255 psig the fan
pressure switch (4S11) will close and energize
contactor 1K5, turning on fan 2B1.
