Introduction; Application - Danfoss AK-PC 781A User Manual

Application

AK-PC 781A is complete regulating units for capacity control of
compressors and condensers in refrigeration systems. The con-
troller is with oil management, heat recovery function and CO 2
gas pressure control.
In addition to capacity control the controllers can give signals to
other controllers about the operating condition, e.g. forced clos-
ing of expansion valves, alarm signals and alarm messages.
The controller's main function is to control compressors and
condensers so that operation all the time takes place at the
energy-optimum pressure conditions. Both suction pressure
and condensing pressure are controlled by signals from pressure
transmitters.
Capacity control can be carried out by suction pressure P0, media
temperature S4 or separate control pressure Pctrl (for cascade).
Among the different functions are:
- Capacity control of up to 10 compressors
- Up to 3 unloaders for each compressor
- Oil management. Either shared or individual for all of the com-
pressor's oil valves. Receiver pressure control.
- Speed control of one or two compressors
- Up to 6 safety inputs for each compressor
- Option for capacity limitation to minimize consumption peaks
- When the compressor does not start, signals can be transmitted
to other controllers so that the electronic expansion valves will
be closed
- Regulation of liquid injection into suction line
- Start/stop of liquid injection in heat exchanger (cascade)
- MT/LT - coordination between controllers in cascade control
- Safety monitoring of high pressure / low pressure / discharge
temperature
- Capacity control of up to 8 fans
- Floating reference with regard to outside temperature
- Heat recovery function
- CO2 gas cooler control and receiver control
- Parallel compression on transcritical CO2 system
- Step coupling, speed regulation or a combination
- Safety monitoring of fans
- The status of the outputs and inputs is shown by means of light-
emitting diodes on the front panel
- Alarm signals can be generated via data communication
- Alarms are shown with texts so that the cause of the alarm is
easy to see.
- Plus some completely separate functions that are totally inde-
pendent of the regulation – such as alarm, thermostat ,pressure
and PI-regulating functions.
AK-PC 781A

1. Introduction

Capacity controller RS8HE202 © Danfoss 2017-07
Examples
Traditional capacity control
Booster control with 2 controls
(For pure booster control without MT cooling, the intermediate pressure
must be connected to the receiver to prevent Pmin and Pmax cut-outs
during the start-up).
Heat recovery functions, controlling the condensing pressure and
receiver pressure of a CO2 plant
SW = 1.4x
3