Receiver Control - Danfoss AK-PC 772A User Manual

Receiver control

The receiver pressure can be controlled so that it is kept at the
requested reference point. This control requires the installation of
a receiver valve Vrec (fx type CCM) and a pressure transmitter. It is
possible to regulate using two parallel valves.
Regulation method
Pressure regulation can be carried out with one of the following
definitions:
• Fixed setpoint
• Displacement of the setpoint with an external signal
• Optimisation, where the IT circuit maintains pressure at the most
energy-efficient level
• Delta P, where the reference is a pressure difference between the
receiver and Po on the MT circuit
P-band
There are two safety functions for the receiver.
They are only available for gas-cooled regulation.
A P-belt must be installed to be able to regulate the function, but
both are standard set to zero, which makes the function inactive.
Receiver pressure's max. limit
Set a max. receiver pressure that will typically be the maximum
receiver pressure. If the controller records that the receiver pres-
sure slips into the P-band, shutdown of the high-pressure valve
Vhp commences. The opening degree will be linear through the
p-band so that the Vhp will be totally closed by pressing 'set max.
receiver pressure' (A complete shutdown assumes that the confi-
guration for the high-pressure valve - "Min. OD% is set to 0%)
Receiver pressure's min. limit:
A minimum receiver pressure limit can be set. If the controller
register receiver pressure below the set value, the Vhp valve will
be opened. The opening degree will be linear through the p-band,
and the maximum permitted opening degree of the Vhp will be
present by pressing 'set min. receiver pressure' minus 'set-p-band' .
If the setting of the valve's opening degree is limited and it cannot
be fully opened, the set opening degree value will be at the pres-
sure 'set min. receiver pressure' minus 'set p-band' .
When the suction pressure for MT is increased (e.g. by Po opti-
misation), it will be necessary to maintain a certain differential
pressure so that the refrigeration units can continue to be regu-
lated. The setting "delta P MT" will ensure the necessary receiver
pressure.
AK-PC 772A Capacity controller RS8HF102 © Danfoss 2017-07
Regulation limitations of the receiver pressure
Note
The PI regulation of the receiver pressure must have space to
regulate without restrictions.
This means that there should be sufficient space for the PI regula-
tion to move around the reference, i.e. at least 2-3 bars – both over
and below the reference.
The value is very dependent on the tuning of the PI regulation
and the system dynamics.
An example may be a 40-bar plant in which the receiver's refer-
ence pressure is set to 35 bars. Here the system can interfere
with normal regulation because the high pressure limit is very
stringent.
Hot gas dump
The controller has a function that can turn on the hot gas to the
receiver if the pressure becomes lower than the set value. The hot
gas will shut off again when the pressure exceeds the difference.
Stop of compressors
If the MT compressors are stopped via the function "External com-
pressor stop", the reference for the receiver control will be to the
setting "Max. receiver pressure" minus the P-band.
If IT compressors are in operation, the IT compressors will conti-
nue and the reference for the receiver control will be set to Max.
minus the P-band
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