3
APPLICATION AND OPERATING PRINCIPLE
Free cooling units meet growing demands for energy sav-
ings, since they have been designed to reduce the oper-
ating costs of refrigerating machines that work to serve
process applications or in the IT field.
A strong point of our free cooling units is certainly the con-
trol system that allows maximum use to be made of the
free resource, consisting of outside air, so minimizing the
energy used by the compressors. The controller of the unit
activates the chiller section and the free cooling section,
also in combined mode, based on the actual external air
temperatures, the set point and the required load level.
The free cooling section is hydraulically in series with the
evaporator and this allows a benefit to be obtained from
its activation even when the outside air temperature is
sufficient to carry out only a pre-cooling of the water. The
missing amount of capacity, in any case lower than the
total required, will be provided by the compressors.
As the outside air temperature goes down, the amount of
capacity that the free cooling section will be able to trans-
fer to the water will gradually increase. Consequently, the
amount of capacity that will have to be covered by the
compressors will always be lower.
When the TFT (Total Free-cooling Temperature) is reached,
the free cooling section will be able to fully meet the cool-
ing capacity requirement and therefore the compressors
can be switched off. In this condition, the unit will be able
to provide the system with a cooling capacity equal to that
required at design conditions, but with current drawn by
the fans alone.
How the unit behaves in the various scenarios is explained
briefly below.
ZETA REV HE FC
Chiller only mode
When the ambient temperature is higher than the tem-
perature of the water returning from the system, all the
required cooling capacity must be produced by the com-
pressors.
In this mode, the damper that controls the air flow rate
at the free cooling coil is fully closed, whereas the one in
front of the condensing coil is fully open.
The 3-way valve bypasses the free cooling coil (to pre-
vent unnecessary head losses) and condensation control
is done, when necessary, through modulation of the fans.
Mixed mode
When the ambient temperature is lower than the tempera-
ture of the water returning from the system, the controller
activates the free cooling section.
The controller switches over the 3-way valve to put the
free cooling coil in series with the evaporator, fully opens
the free cooling damper and pushes the fans to maximum
speed. In this way, extraction of maximum cooling capac-
ity from the outside air is ensured.
The water leaving the free cooling coil will be the water
that enters the evaporator. Since the water is "pre-cooled",
the amount of capacity required of the chiller section will
be lower and therefore this will be able to operate with
reduced capacity.
Since the fans must be kept at 100% to ensure the max-
imum capacity at the water coil, condensation control is
done by modulating the opening of the damper situated in
front of the condensing coil.
Free cooling only mode
Below a certain outside air temperature, the unit operates
exclusively in free cooling mode.
The capacity given by the water coil fully meets the re-
quirement of the system, and therefore the damper in
front of the condenser is fully closed and the compressors
are kept switched off.
Also, through fan speed modulation, the controller of the
unit will manage the capacity given by the free cooling
section that, with the lowering of the outside air tempera-
ture, could be excessive.
