OPERATING INSTRUCTIONS
CO
2
/NH
3
CASCADE UNIT
250 - 1000 KW
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In order to avoid a pressure rise in this area up to 25
bar g.p, while the system is at a standstill and thus to
avoid the refrigerant from being blown out, the
refrigerant is cooled in the CO
2
liquid separator with
the help of an external chiller.
The auxiliary refrigeration unit is part of the cascade
and is completely installed. It is delivered like the
complete CO
2
/ NH
3
cascade without a refrigerant
charge.
It is a small standard refrigeration condenser unit,
which is connected to a pipe coil acting as an
evaporator in the CO
2
liquid separator.
The auxiliary unit is switched on and off depending
on the pressure in the CO
2
liquid separator.
3.5 Safety devices
The cascade is equipped with the necessary safety
devices in accordance with the relevant regulations,
in particular EN 378.
The following safety devices are installed in every
cascade.
3.5.1 CO
2
circuit
• Safety devices to prevent exceeding of the
maximum allowable final discharge pressure
— Pressure transducer (5105)
If the limit value is exceeded, the compressor
control switches off the CO
2
unit.
Limit value final pressure alarm (max) = see
parameter list
However, before the limit value is reached
the compressor's output level is lowered to
prevent exceeding of the limit value.
— Safety pressure switch (5350)
The CO
2
compressor is switched off if the
limit value is exceeded.
Limit value p = 0.9 x p
max
bar
(p
max
= maximum allowable operating
pressure on the high-pressure side)
— Pressure relief device (5340)
The back-pressure independent overflow
valve discharges on the compressor's intake
side when p
max
is reached.
• Pressure relief device of the pressure vessel on
the high-pressure side (6420) (overflow valve to
the low pressure side)
The counter-pressure overflow valve discharges
on the low-pressure side of the plant (liquid
separator) if p
max
= 48 bar g.p. is reached.
• Pressure relief device of the pressure vessel on
the high-pressure side (6230) (discharging safety
valve)
The safety valve discharging into the outside
opens the low-pressure side of the plant if p
NDmax
= 25 bar g.p is reached.
• Safety device to prevent the suction pressure
falling below the allowable limit (pressure
transducer 5100)
If the pressure falls below the limit value the
compressor control switches off the CO
2
unit.
However, before the limit value is reached the
output level of the CO
2
compressor is lowered to
prevent the value falling below the limit value.
• Safety device to prevent the final discharge
temperature from being exceeded (resistance
thermometer 5120)
If the limit value is exceeded, the compressor
control switches off the CO
2
unit.
Limit value t = 80 °C
• Safety device to prevent the oil temperature
from being exceeded (resistance thermometer
5125)
If the limit value is exceeded, the compressor
control switches off the CO
2
unit.
Limit value t = 60°C
• Safety devices of the compressor drive motor
— Rated current limiting control (5016)
realised by the respective compressor
control.
If the rated motor current is exceeded, the
output level of the CO
2
compressor is
lowered until the motor current is back
within the allowable range.
The normal output control then comes back
into force.
— Thermistor (5017) which shuts down the
compressor drive motor if its winding
temperature exceeds the allowable value.
• Level monitoring of the CO
2
liquid separator,
level transmitter (6900)
The CO
2
compressor switches off if the maximum
level is exceeded.
• Differential pressure CO
2
pumps differential
pressure switch (6910)
The control switches off the refrigerant pump if
the differential pressure falls below the
minimum limit value.
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