Danfoss ADAP-KOOL AK-PC 781, Design Manual

Page: 114 / 142

114 Capacity controller RS8GG602 © Danfoss 2015-03 AK-PC 781
CO2 transcritical system and heat recov-
ery
General
When the refrigerant in the system is CO2, the higher pressure and
temperature make it possible to recover heat for tap water and
heating. The excess heat is removed using a gas cooler.
Regulation is carried out during transcritical and subcritical states
and the controller will control the gas pressure/condensing pres-
sure so that the system achieves the optimum COP when the
recovered heat is taken into account.
The regulation of the heat recovery circuits is done with regard
to the cooling system. In the event of conflict, the safety situation
is that the cooling system has higher priority than the recovery
circuits.
The two heat recovery circuits can be considered as independent
circuits - also with regard to the cooling system.
First, the circuit for hot tap water will take the energy it needs to
use. The remaining energy is then available for use by the next
circuit. This also takes what is available. If there is then any excess
energy this is removed via the gas cooler.
There must be a cooling requirement in order to supply for heat
recovery.
Info
In normal operating conditions, the temperature at Sd will be be-
tween 60 and 70°C - depending on whether it is winter or summer.
If the "Heat reclaim" function is to raise the condensing pressure,
the temperature may increase to 90° or higher.
The Sc3 sensor should be positioned so that it measures the air
intake temperature for the gas cooler. If it measures a temperature
that is too high, the system's COP will become impaired.
The Sgc signal must be stable. If this cannot be done using a sys-
tem sensor, it may be necessary to use an immersion tube sensor.
If the power supply to AK-PC 781 or the high pressure valve Vhp
fails, the system cannot be controlled. We recommend installing
an emergency supply (UPS) for both the controller and the valve
to avoid faults. A relay in the UPS should be incorporated into the
controllers safety circuit so that it can restart safely.
Remember the isolation amplifier
If signals are received from different controls, e.g. heat recovery
for one of the inputs, a galvanically insulated module should be
inserted.
There are safety functions for the individual regulation functions,
for example:
- Boiling at S3, S4 and S8
- A S3 temperature must be lower than the gas temperature that
can be sent into the heat exchanger. If the S3 temperature is
higher, the circuit is not connected.
The pump is kept running for a little while before and after the
gas valves connect. It can take up to 2 minutes for the gas valve to
change position.
Important
This sensor must be placed
immediately at the gas
cooler outlet.