108
✓
By means of a 4-20 mA input type on the energy
management board. The limit is now based on a
linear interpolation from 0 to 100%, based on the
input signal. The demand limit in night mode is
selectable (if the value is below the one controlled
by the external signal).
5.8 - NIGHT MODE
The night period is defined (see User configuration) by a
start time and an end time that are the same for each
day of the week. During the night period, the number of
fans operating can be reduced, and the unit capacity
may be limited (but a minimum capacity value can be
configured).
5.9 - CAPACITY CONTROL
This function adjusts the number of active compressors
to keep the heat exchanger water temperature at its
setpoint. The precision with which this is achieved
depends on the capacity of the water loop, the flow rate,
the load, and the number of stages available on the unit.
The control system continuously takes account of the
temperature error with respect to the setpoint, as well as
the rate of change in this error and the difference between
entering and leaving water temperatures, in order to de-
termine the optimum moment at which to add or withdraw
a capacity stage. If the same compressor undergoes too
many starts (per hour) or runs below one minute each
time it is started this automatically brings about reduction
of compressor starts, which makes leaving water
temperature control less precise. In addition, the high
pressure, low pressure or defrost unloading functions can
also affect the temperature control accuracy.
Compressors are started and stopped in a sequence
designed to equalise the number of start-ups (value
weighted by their operating time).
5.10 - HEAD PRESSURE CONTROL
The head pressure is controlled by a maximum of six
fans in each circuit, and one of these may have variable
speed. The head pressure is independently controlled
for each circuit, based on the saturated condensing
temperature value.
Fan start-up:
ATTENTION:
The fans can be started periodically (for
cleaning), even if the unit is switched off.
Reset example in cooling mode based on the
outside temperature
5.6.2 - RESET
Reset means that the active setpoint is modified so that
less machine capacity is required (in cooling mode, the
setpoint is increased, in heating mode it is decreased).
This modification is in general a reaction to a drop in the
load. For the Pro-Dialog control system, the source of
the reset can be configured in the User 1 configuration:
it can be provided either by the outdoor temperature
(that gives a measure of the load trends for the building)
or by the return water temperature (delta T that gives an
average building load) either for the reset setpoint based
on a 4-20 mA input (with energy management option) or
based on the room temperature, using a room
temperature sensor (when the energy management option
is used). In response to a drop in the outdoor temperature
or to a drop in delta T, the cooling setpoint is normally
reset upwards in order to optimise unit performance:
In the three cases the reset parameters, i.e. slope, source
and maximum value, are configurable in the Setpoints
menu (see section 4.3.8). Reset is a linear function based
on three parameters.
•
A reference at which reset is zero (outdoor
temperature or delta T - no reset value).
•
A reference at which reset is maximum (outdoor
temperature or delta T - full reset value).
✓
The maximum reset value.
Legend
A
Maximum reset value
B
OAT or delta T for no reset
C
OAT or delta T for full reset
D
Building Load
Reset
value
Outdoor temperature (OAT)
Evaporator Delta T
%
Building
load
5.7 - DEMAND LIMIT
The demand limit is used to restrict the unit power
consumption. The Pro-Dialog control system allows
limitation of the unit capacity, using one of two methods:
✓
By means of user-controlled volt-free contacts. The
units without energy management option only have
one demand limit level (only one contact: control
contact 3). The units equipped with energy
management option permit three demand limit levels
(2 contacts). The unit capacity can never exceed
the limit setpoint activated by these contacts. The
limit setpoints can be modified in the setpoint menu.