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6.4.2 - Reset
Reset means the active setpoint is modified so that less
machine capacity is required. This modification is in
general a reaction to a drop in the load.
The reset can be based on the following parameters:
•
OAT that gives the measure of the load trends for the
building
•
Return fluid temperature – the difference between the
entering and leaving fluid temperature on the side of
the evaporator (
∆
T provides the average building load)
•
Space temperature (EMM option)
•
Dedicated 4-20 mA input (EMM option)
The reset source and the reset parameters can be configured
in the Main menu (RESETCFG – Reset Configuration). In
response to a drop in the reset source, the cooling setpoint
is normally reset upwards to optimise unit performance.
The amount of reset is determined by linear interpolation
based on the following parameters:
•
A reference at which reset is zero (no reset value)
•
A reference at which reset is maximum (full reset value)
•
The maximum reset value
6.5 - Capacity limitation
The Touch Pilot control system allows for the constant
control of the unit capacity by setting its maximum
allowable capacity. Capacity limitation is expressed in
percentage, where a limit value of 100% means that no
unit capacity limitation is required.
The unit capacity can be limited with one of the external
orders:
•
By means of user-controlled volt-free contacts. Units
without the energy management module have one
contact. Units with the energy management module
permit three capacity limitation levels (see also
section 3.8.4). The unit capacity can never exceed the
limit setpoint activated by these contacts. The limit
setpoints can be modified in the SETPOINT menu.
•
By lag limit set by the master unit (master/slave
assembly).
•
By night mode limitation control. The demand limit
value in the night mode is selectable if the value is
below the selected limit.
Demand limit can be forced when the unit is operating in
Network mode.
CAUTION: In certain conditions, the unit power
consumption can exceed the capacity limitation
threshold to protect the compressors.
6.6 - Capacity control
This function adjusts the capacity using the compressor
slide valve and the frequency variator to keep the water
exchanger temperature at its setpoint. The control system
continuously takes account of the temperature error with
respect to the setpoint, the rate of change in this error and
the difference between entering and leaving water
temperatures in order to determine the optimal moment at
which to add or withdraw capacity. The master circuit is
leading the capacity control.
Compressor capacity is controlled with the slide valve or
the frequency variator depending on the current compressor
capacity. The capacity is controlled with only one method
at a time. Frequency controller is used when the circuit
capacity is more than 50%. Otherwise, the slide valve is
used to control the capacity of the unit.
6.6.1 - Lead/lag circuit capacity control
If both lead and lag circuits in the unit are running, then
the lead circuit controls the unit capacity. Both circuits are
controlled independently which means that if one of the
circuits fails, another one continues to operate (if operating
conditions permit).
For more information about lead/lag circuits, please refer
to section 6.9.
6.6.2 - Ramp loading
The control provides the ramp loading function used to
minimise the rate at which the water temperature changes.
The minimum speed is calculated based on saturated
condensing temperature and saturated suction temperature.
Ramp loading sequence can be enabled by the user (
Ramp
Loading Select,
GEN_CONF – General Configuration).
6.6.3 - Compressor management
The Touch Pilot control employs a number of functionalities
allowing for the efficient operation of compressors:
•
Short cycle management:
Compressors can be
protected by the short cycle management function
preventing frequent compressor starts (
Short cycle
management
, GEN_CONF – General Configuration).
•
Capacity limitation:
Controlling the maximum
allowable capacity using one of the external commands
(see also section 6.5).
•
Diagnostic function:
All abnormal conditions related
to the operation of compressors are immediately
announced and if necessary a certain action is taken
on the unit (see also section 7).