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If any of these conditions are met, the appropriate operating
mode, 21 (Circuit A), 22 (Circuit B) or 23 (Circuit C) will be in
effect.
Override #26: Circuit A Operation Outside Compressor
Operating Envelope
Override #27: Circuit B Operation Outside Compressor
operating envelope
Override #28: Circuit C Operation Outside Compressor
Operating Envelope — This override prevents compressor op-
eration outside of its operating envelope.
1. If the mean saturated discharge temperature (SDT) is
greater than 7° F over the limit, the circuit is unloaded
immediately.
2. If the mean SDT is over the limit (but not greater than
7° F over the limit) for 90 seconds, then the circuit is un-
loaded.
3. If the mean SDT is more than the limit minus 2° F, the
circuit is prevented from loading. This override shall re-
main active until the mean pressure goes below the limit
minus 3° F.
Override #34: Circuit A Low Refrigerant Charge
Override #35: Circuit B Low Refrigerant Charge
Override #36: Circuit C Low Refrigerant Charge — The ca-
pacity override attempts to protect the compressor from
starting with no refrigerant in the circuit. This algorithm runs
only when the circuit is not operational, (no compressors ON).
There are several criteria that will enable this override:
1. The Saturated Suction Temperature or Saturated Dis-
charge Temperature is less than –13 F (–10.6 C).
2. All of these conditions must be true:
a. The Saturated Suction Temperature or Saturated
Discharge Temperature is less than Leaving Water
Temperature by more than 5.4º F (3.0º C).
b. Saturated Suction Temperature or Saturated Dis-
charge Temperature is less than 41 F (5 C).
c. Outdoor Air Temperature is less than 32 F (0º C).
d. Saturated Suction Temperature or Saturated Dis-
charge Temperature is less than the Outdoor Air
Temperature by more than 5.4º F (3.0º C).
3. All of these conditions must be true:
a. The Saturated Suction Temperature or Saturated
Discharge Temperature is less than Leaving Water
Temperature by more than 5.4º F (3.0º C).
b. Saturated Suction Temperature or Saturated Dis-
charge Temperature is less than 41 F (5 C).
c. Saturated Suction Temperature or Saturated Dis-
charge Temperature is less than the Brine Freeze
Point (Configuration
than 6º F (3.3º C).
NOTE: The freeze set point is 34 F (1.1 C)
for
fresh
water
SERV
FLUD=1). The freeze set point is Brine
Freeze
Set
Point
LOSP), for Medium Temperature Brine systems
(Configuration
SERV
4. All of these conditions must be true:
a. The Saturated Suction Temperature or Saturated
Discharge Temperature is less than Leaving Water
Temperature by more than 5.4º F (3.0º C).
b. Saturated Suction Temperature or Saturated Dis-
charge Temperature is less than 41 F (5 C).
c. Saturated Suction Temperature or Saturated Dis-
charge Temperature is less than the Outdoor Air
Temperature by more than 9º F (5º C).
SERV
LOSP) by more
systems
(Configuration
(Configuration
SERV
FLUD=2).
If any of these conditions 1, 2, 3 or 4 are met, the appropri-
ate operating mode, 21 (Circuit A), 22 (Circuit B) or 23
(Circuit C) will be in effect.
Override #37: Circuit A Low Superheat
Override #38: Circuit B Low Superheat
Override #39: Circuit C Low Superheat — This override at-
tempts to avoid liquid slugging for the running compressors. It
also protects against operation with excessively high superheat.
No capacity steps will be added to the affected circuit until
there is a superheat greater than 5º F (2.8º C) and less than
45º F (25º C). If the capacity of the machine must be increased,
the control will look to another circuit for additional capacity.
Head Pressure Control —
(MBB) controls the condenser fans to maintain the lowest
condensing temperature possible, and thus the highest unit effi-
ciency. The MBB uses the saturated condensing temperature
input from the discharge pressure transducer to control the
fans. Head pressure control is maintained through a calculated
set point which is automatically adjusted based on actual
saturated condensing and saturated suction temperatures so that
the compressor(s) is (are) always operating within the manu-
facturer's specified envelope (see Fig. 11). Each time a fan is
added the calculated head pressure set point will be raised
25° F (13.9° C) for 35 seconds to allow the system to stabilize.
The control will automatically reduce the unit capacity as the
saturated condensing temperature approaches an upper limit.
See capacity overrides 16-18. The control will indicate through
an operating mode that high ambient unloading is in effect. If
the saturated condensing temperature in a circuit exceeds the
calculated maximum, the circuit will be stopped. For these
reasons, there are no head pressure control methods or set
points to enter. The control will turn off a fan stage when the
condensing temperature is below the minimum head pressure
requirement for the compressor. Fan sequences are shown in
Fig. 12.
LOW AMBIENT TEMPERATURE HEAD PRESSURE
CONTROL OPTION — For low-ambient operation, the lead
fan on a circuit can be equipped with low ambient temperature
head pressure control option or accessory. The controller ad-
justs fan speed to maintain the calculated head pressure set
point.
LOW AMBIENT TEMPERATURE HEAD PRESSURE
CONTROL OPERATING INSTRUCTIONS — The 30RB
low ambient control is a variable speed drive (VFD) that varies
the speed of the lead condenser fan in each circuit to maintain
the calculated head pressure control set point. The fan speed
varies in proportion to the 0 to 10 vdc analog signal produced
by the AUX1 fan board. The display indicates motor speed in
Hz by default. There are two different VFDs used for these
units, a Siemens Micromaster 420/440 and a Schneider Altivar
21. The Schneider Altivar VFD performs the same functions as
Siemens drive. However, there are different control wiring con-
nections and parameter programming.
Siemens VFD Operation — The low ambient temperature
head pressure controller is pre-configured to operate from a
0 to 10 vdc analog input signal present on terminals 3 (AIN+)
and 4 (AIN–). Jumpers between terminals 2 and 4 and termi-
nals 5 and 8 are required for proper operation. The drive is en-
abled based on an increase in the analog input signal above
0 vdc. Output is varied from 0 Hz to 60 Hz as the analog signal
increases from 0 vdc to 10 vdc. When the signal is at 0 vdc the
drive holds the fan at 0 rpm. The head pressure control set point
is not adjustable. The MBB determines the control set point as
required.
Siemens VFD Replacement — If the controller is replaced
the parameters in Table 20 must be configured. See Fig. 13 and
14.
24
The main base board
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