Operating instructions

50
capacity will be held at current capacity. If the compressor cur-
rent is greater than 85% MTA then capacity will be reduced by
repositioning the slide valve until the current is less than 85%
MTA (Must Trip Amps, MTA.X).
Override #44: Circuit A High Suction Superheat at Part Load
Override #45: Circuit B High Suction Superheat at Part Load
Override #46: Circuit C High Suction Superheat at Part Load
— If the compressor of the circuit is on, the compressor current
is no more than 30% of the MTA, main EXV is more than 90%
open and the suction superheat is higher than the superheat
control point for more than 5 minutes, then the circuit will be
shut down.
Override #50: Circuit A MCHX MOP Control Override
Override #51: Circuit B MCHX MOP Control Override
Override #52: Circuit C MCHX MOP Control Override —
This override shall prevent the compressor from increasing ca-
pacity when saturated suction temperature is greater than the
MOP setpoint and saturated condensing temperature is greater
than the maximum condensing temperature setpoint minus
15 F on units equipped with MCHX condenser option. This is
to avoid high pressure alarm and operation outside the com-
pressor envelope.
Override #53: Circuit A Delay for Unloading the Slide Valve
Override #54: Circuit B Delay for Unloading the Slide Valve
Override #55: Circuit C Delay for Unloading the Slide
Va l v e — This override prevents the compressor from re-start-
ing with locked rotor failure after being shutdown due to an
alarm or power cycle. The delay varies depending on the size
of the compressor. Refer to Table 38 for compressor nominal
capacities. A delay of 20 minutes will elapse for 165 and
185 ton compressors, a delay of 8 minutes will elapse for 90
and 120 ton compressors, and 5 minutes will elapse for 45 and
50 ton compressors. The delay allows the slide valve of the
compressor to move back to its fully unloaded position. The
delay is adjusted according to the percent of the compressor
running capacity before it is shut down. If the compressor is
stopped normally, no delay will be applied. If the compressor is
shut down by the locked rotor alarm, a full delay will be ap-
plied before the compressor is allowed to re-start. See Table 38
for compressor nominal capacity.
Override #56: Circuit A Delay for Refrigeration Isolation
Valve to Open
Override #57: Circuit B Delay for Refrigeration Isolation
Valve to Open
Override #58: Circuit C Delay for Refrigeration Isolation
Valve to Open — This override allows the discharge motor-
ized ball valve to open before the compressor starts. The delay
is 2 minutes and 30 seconds. (Does not apply to units with DX
cooler option.)
Override #59: Circuit A Low Oil Level
Override #60: Circuit B Low Oil Level
Override #61: Circuit C Low Oil Level — This override is
only effective when the circuit is not running. The override will
prevent the circuit from starting up with a low oil level. If this
override occurs three times, the low oil level alarm will be
tripped.
Override #62: Circuit A High Motor Temperature Override
Override #63: Circuit B High Motor Temperature Override
Override #64: Circuit C High Motor Temperature Override
— This override prevents the compressor motor temperature
from rising above the high temperature limit, but still allows
the chiller to run close to the high temperature limit by unload-
ing the compressor. If the motor temperature is greater than
214 F (101.1 C), the compressor will not load. This override
will remain active until the temperature drops below 214 F
(101.1 C). If the motor temperature is greater than 225 F
(107.2 C) for 60 seconds, the circuit capacity will decrease by
one stage. If the motor temperature is greater than 228 F
(108.9), the circuit capacity will decrease by one stage immedi-
ately.
Override #77: Boostload Function
— This override can be
present when boostload function is enabled. It is set in the fol-
lowing conditions :
if cool_lwt > ctrl_pnt + 5.4 and cool_ewt > ctrl_pnt + 9.0
and demand limit > 99%.
Head Pressure Control — The Main Base Board
(MBB) controls the condenser fans to maintain the lowest
condensing temperature possible, and thus, the highest unit ef-
ficiency. 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. 31). 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 ex-
ceeds 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. 31. See Table 38 for compressor nominal capacity.
LOW AMBIENT TEMPERATURE HEAD PRESSURE
CONTROL OPTION — Units will start and operate down to
32 F (0° C) as standard. Operation to –20 F (–29 C) requires
optional low ambient head pressure control as well as wind
baffles (field fabricated and installed on all units for operation
below 32 F [0° C]) if wind velocity is anticipated to be greater
than 5 mph (8 kp/h). Inhibited propylene glycol or other suit-
able corrosion-resistant anti-freeze solution must be field sup-
plied and installed in all units for unit operation below 34 F
(1.1 C). Solution must be added to fluid loop to protect loop
down to 15° F (8.3° C) below minimum operating ambient
temperature. Concentration should be based on expected mini-
mum temperature and either “Burst” or “Freeze” protection
levels. At least 6 gal per ton (6.5 l/kW) of water volume is the
recommended minimum for a moderate system load.
For low-ambient temperature operation, the lead fan on a
circuit can be equipped with low ambient temperature head
pressure control option or accessory. The controller adjusts fan
speed to maintain the calculated head pressure set point.
LOW AMBIENT TEMPERATURE HEAD PRESSURE
CONTROL OPERATING INSTRUCTIONS — The 30XA
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.
Table 38 — 30XA Compressor Nominal Capacity
30XA
UNIT SIZE
080,
082
090,
092
100,
102
110,
112
120,
122
140,
142
160,
162
180,
182
200,
202
220,
222
Compressor Nominal Capacity (tons)
Circuit A 45 45 50 60 60 90 100 90 100 120
Circuit B 45 45 50 50 60 50 60 90 100 100
Circuit C ——————————
30XA
UNIT SIZE
240,
242
260,
262
280,
282
300,
302
325,
327
350,
352
400 450 500
Compressor Nominal Capacity (tons)
Circuit A 120 165 165 185 165 185 120 185 185
Circuit B 120 100 120 120 165 165 120 90 120
Circuit C — — — — — — 165 185 185