Suction Pressure Optimization
The Adaptive Suction Pressure function in your AK-SM makes it
possible to automatically optimize the suction pressure so that it
will be adapted to the systems actual load. During optimization,
data is collected that tells the system which refrigeration
appliances are most heavily loaded. This energy saving function
can make substantial savings directly whilst also saving on
compressor wear and tear and also providing an analysis tool for
refrigeration appliances.
The individual controllers handle the temperature control in
the refrigeration appliances. The load and operating conditions
of each are continuously collected by the AK-SM via the data
communication system. The collected data is computed and the
“most heavily loaded” refrigeration points are identied.
An adaptation can now be made of the Suction Pressure, whilst
ensuring the air temperature at the refrigeration appliance
is maintained. It is the AK-SM that collects data from the
refrigeration appliances and it is the AK-SM that transmits any
oset to the compressor pack control so that suction pressure
reference is changed to suit the needs of the “most heavily loaded”
refrigeration point. It is always the temperature at the appliance
that takes top priority and in fact the suction pressure can oat
down if necessary.
The time during which a refrigeration point has been designated
“the most heavily loaded” will be summed up in the log (history)
within the AK-SM.
The pack suction (Po) pressure is optimized in accordance with
the current refrigeration demand, taking into account short-
term changes (day/night setback/defrost) and long-term eects
(seasonal/weather changes).
In order to get the best eciency out of the Po Optimization
function it is highly recommended that a plant survey is done
before enabling this function. A poorly running site will not
be corrected by Po Optimization and the maximum benets
will not be seen - ensure all plant & evaporator cases are
running close to their designed setpoints and that defrost are
operating correctly. Also ensure any manual plant override
systems are set accordingly to allow the oating up of suction
pressure.
This feature is available on all supported
AK-SM Evap & Pack controllers.
Theory of operation
The Po Optimize function uses a calculation on all controllers to
determine a “Load Factor”. This has been developed by Danfoss
and is available in your AK-SM. By use of the Load Factor the Most
Loaded Case (MLC) is constantly updated.
Floating of the Suction Setpoint is then determined at any given
time by the MLC
1/ The AK-SM continuously receives operating information
from each (Po enabled) controller connected on the network.
The AK-SM is looking for the 'Most loaded case' (MLC).
Each evaporator is analyzed to see if it's current operating
temperature is within a calculated MLC 'target window'.
During defrost and including after defrost recovery, the Po
Optimization will temporarily remove the case from the Po
calculation loop. This ensures that normal system uctuations
(due to defrost etc) do not eect the overall operation of Po
Optimization.
2/ In eect the continuous Po Optimization is looking for the
evaporator that is under the heaviest load (the one that has to
work the hardest to maintain temperature), but still falls within
the MLC target 'window'
3/ Based on the MLC the Po optimization function will then send a
control signal to the Pack controller to optimize the running
suction pressure, i.e. oat the pressure up (based on the max
limit set in the Pack conguration page). As the Pack controller
allows the suction pressure to increase the AK-SM monitors the
complete network and ensures that the refrigerating system as
a whole is stable. This then is a continuous function that once
set runs automatically and ensures the refrigeration system is
running to the best conditions.
AK-SM 850 User Guide Lit. no. USCO.PI.R1.E1.02 © Danfoss 07-2013 41
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