Unit installation

CDU Installation Guide Page 34
Cooling Distribution Unit
OPERATION (continued)
If either sensor goes open circuit, then a ‘Sec Temp Out of Range’ alarm will be raised and the
controller will read the remaining working sensor only.
The secondary temperature should correspond to the desired set point temperature – default
fixed set point is 18°C (65°F) as displayed on the welcome screen and is used by the control
loop to regulate the primary water control valve position to achieve and maintain the set point.
The control valve position can be monitored on the second welcome screen (see Section 6-1)
or screen 3-2 under the Status title. High and low temperature warnings are set at a default
value of 2°C (4°F) either side of set point (floating with set point) when either ‘Fixed SP’ or
‘Fixed SP + Dew Point override’ control is selected from screen 6-3-3, with a default
hysteresis of 1°C (2°F).
- If the secondary temperature deviates by more than 2°C (4°F) above set point for 2
minutes or more, a ‘Secondary Water Temp High’ warning will be generated. This
alarm will remain present until the temperature falls below the hysteresis value.
- If the secondary temperature deviates by more than 2°C (4°F) below set point for 2
minutes or more, a ‘Secondary Water Temp Low’ warning will be generated. This
alarm will remain present until the temperature rises above the hysteresis value.
- The high and low temperature warnings are ignored for a period of 20 minutes
(default) on start up to allow the system time to settle without generating nuisance
alarms.
In ‘Fixed Sp + Dew Point override’ control mode, the set point can be overridden by a Dew
Point condition – i.e. where there is a risk of condensation occurring at the RDHx’s.
- Dew Point Override (flashing ‘dw o/r --.-°C (°F)’ on welcome screen display when
activated) – Room temperature and relative humidity are constantly monitored by
remote sensor and used by the controller to calculate the anticipated dew point within
the room. If there is a risk of condensation within the RDHx’s, the controller will
override the set point to a default value of 2°C (4°F) above dew point. The Dew Point
override will then show the new set point value as a flashing display on the second
line of the welcome screen (& also on the Status screen 3-2, which will alternate
between fixed set point & override set point).
Alternatively, if ‘Dew Point Tracking’ is the chosen control mode, then the set point will
continually re-adjust to maintain the supply temperature at a default of 2°C (4°F) above the
current dew point.
- Dew Point Tracking (flashing ‘dw tracking’ on display) – Room temperature and
relative humidity are constantly monitored by remote sensor and used by the
controller to calculate the anticipated dew point within the room. The controller will
then calculate a set point to maintain the water temperature at a default setting of 2°C
(4°F) above this value. The welcome screen will then show the new set point value
with ‘DW Tracking’ as a flashing display on the second line (new set point also visible
on the Status screen 3-2).
Note: Although the controller will display the new Dew Point Tracking set point, this may not always be
achievable as it will often fall outside the cooling capacity of the unit, i.e. if the dew point was 7°C
(44°F), then the controller would re-adjust the set point to 9°C (48°F), however, the plate heat
exchanger is sized for a minimum temperature differential between primary water inlet and secondary
outlet temperatures of 8°C (14.5°F). Therefore, if the primary water temperature was 10°C (50°F), then
a secondary set point of 9°C (48°F) would be unachievable and the ‘actual’ temperature displayed
would be unlikely to fall below 18°C (64.5°F). For this reason it is not possible to operate a Secondary
High/Low temperature warning as with the ‘Fixed SP’ or ‘Fixed SP + Dew Point override’ control
modes that floats with the set point, as the unit could be constantly in warning condition. In this
instance, a fixed Secondary Water Temp High warning is set at a default value of 20°C (70°F) and the
Secondary Water Temp Low warning is only enabled when the temperature drops to or below the dew
point for a period of 3 minutes or more.