Deadband Multiplier; Head Pressure Control; Cooler Freeze Protection; Low Saturated Suction Protection - Carrier AQUASNAP 30RAP010-150 Controls, Start-Up, Operation, Service, And Troubleshooting

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LEGEND
LWT — Leaving Water Temperature
First Stage Override — If the current capacity stage is zero,
the control will modify the routine with a 1.2 factor on adding
the first stage to reduce cycling. This factor is also applied
when the control is attempting to remove the last stage of
capacity.
Slow Change Override — The control prevents the capacity
stages from being changed when the leaving fluid temperature
is close to the set point (within an adjustable deadband) and
moving towards the set point.
Ramp Loading — Ramp
 
tion SLCT CRMP) limits the rate of change of leaving
fluid temperature. If the unit is in a Cooling mode and config-
ured for Ramp Loading, the control makes 2 comparisons be-
fore deciding to change stages of capacity. The control calcu-
lates a temperature difference between the control point and
leaving fluid temperature. If the difference is greater than 4° F
(2.2° C) and the rate of change (°F or °C per minute) is more
than the configured Cooling Ramp Loading value (CRMP),
the control does not allow any changes to the current stage of
capacity.
Low Entering Fluid Temperature Unloading — When the
entering fluid temperature is below the control point, the
control will attempt to remove 25% of the current stages being
used. If exactly 25% cannot be removed, the control removes
an amount greater than 25% but no more than necessary. The
lowest stage will not be removed.
Minimum Load Control — If equipped, the minimum load
control valve is energized only when one compressor is run-
ning on Circuit A and capacity is decreasing, provided that the
minimum run time for Circuit B compressors has been satis-
fied. If the close control feature is enabled, the minimum load
control valve may be used as needed to obtain leaving fluid
temperature close to set point.
Cooler Freeze Protection — The control will try to prevent
shutting the chiller down on a Cooler Freeze Protection alarm
by removing stages of capacity. If the cooler fluid selected
is Water, the freeze point is 34 F (1.1 C). If the cooler fluid
selected is Brine, the freeze point is the Brine Freeze Point (Set
 
Points FRZ BR.FZ). This alarm condition (A207) only
references leaving fluid temperature and NOT Brine Freeze
point. If the cooler leaving fluid temperature is less than the
freeze point plus 2.0° F (1.1° C), the control will immediately
remove one stage of capacity. This can be repeated once every
30 seconds.
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0 200
STANDARD
DEADBAND
Fig. 19 — Deadband Multiplier
loading (Configura-
DEADBAND EXAMPLE
400 600
TIME (SECONDS)
Low Saturated Suction Protection — The control will try to
prevent shutting a circuit down due to low saturated suction
conditions by removing stages of capacity. These circuit alert
conditions (T116, T117) compare saturated suction tempera-
ture to the
 
Points FRZ
configurable value that must be left at 34 F (1.1 C) for 100%
water systems. A lower value may be entered for systems with
brine solutions, but this value should be set according to the
freeze protection level of the brine mixture. Failure to properly
set this brine freeze point value may permanently damage the
brazed plate heat exchanger. The control will initiate Mode 7
(Circuit A) or Mode 8 (Circuit B) to indicate a circuit's capaci-
ty is limited and that eventually the circuit may shut down.
Head Pressure Control —
(MBB) controls the condenser fans to maintain the lowest
condensing temperature possible, and thus the highest unit
efficiency. The MBB uses the saturated condensing tempera-
ture input from the discharge pressure transducer and outside
air temperature sensor to control the fans. Head pressure con-
trol is maintained through a calculated set point which is auto-
matically adjusted based on actual saturated condensing and
saturated suction temperatures so that the compressor(s) are
always operating within the manufacturer's specified enve-
lope (Fig. 20A and 20B). If OAT is greater than 70 F before a
circuit is starting, then all condenser fan stages will be ener-
gized. A fan stage is increased based on SCT (saturated con-
densing temperature). When the highest SCT is greater than
the Fan On Set Point (Set Points
additional stage of fan will be added to the current fan stage.
Fan On Set Point (F.ON) equals Head Set Point (Set

Points HEAD
when the Head Set Point is increased by Fan Stage Delta (Set

Points HEAD
SCTs of both circuits are less than the Fan Off Set Point (Set

Points HEAD
the number of fan stages, contactors energized and the fans that
are on during the fan stage. Unit sizes 035 to 060 have com-
mon fan control. Figure 21 shows the location of each fan and
control box within the unit.
30
2 STARTS
800
3 STARTS
MODIFIED
DEADBAND
configured Brine Freeze
BR.FZ). The Brine Freeze point is a user-
The main base board
 
HEAD

H.DP) except after a fan stage increase

F.DLT). A fan stage is decreased when the

F.OFF) for two minutes. Table 10 shows
1000
Point (Set
F.ON), then an