Unit installation
CDU Installation Guide Page 35
Cooling Distribution Unit
OPERATION (continued)
Primary Circuit Operation
The primary water temperature is monitored at the inlet to the CDU cabinet. The cooling
performance of the CDU cabinet has been calculated on a water temperature between 5 and
10°C (41 and 50°F).
- If the primary temperature rises to 11°C (52°F), a ‘Primary Water Temp High’ warning
will be generated. This warning will remain present until the temperature falls below
the default 1°C (2°F) hysteresis value.
- If the primary temperature falls to 4°C (40°F), a ‘Primary Water Temp Low’ warning will
be generated. This warning will remain present until the temperature rises above the
default 1°C (2°F) hysteresis value.
- The high and low temperature warnings are ignored for a 20 minute period (default)
on start up to allow the system time to settle without generating nuisance alarms.
The temperature PI control loop will be operational from when the Start/Stop button is pressed
and the pump has ramped up to speed. If the secondary circuit temperature starts to rise
above the set point, then control Valve 1 will start to open to allow more primary cooling water
through the heat exchanger. The control valve will modulate from 0% (closed) to 100% (fully
open) - the valve position can be monitored on the second welcome screen or Status screen
3-2. The demand signal to the valve is compared to a position feedback signal every 15
minutes (default) to check the healthy operation of the valve.
- If the feedback signal is more than 10% different from the demand signal (allowing for
the drive time of the actuator to respond to load changes), then a ‘Valve 1 Fault’
warning will be generated, Valve 1 will be driven closed and the demand signal
transferred to Valve 2.
- Similarly, with Valve 2 if the feedback signal is more than 10% different from the
demand signal (allowing for the drive time of the actuator to respond to load changes),
then a ‘Valve 2 Fault’ warning will be generated, Valve 2 will be driven closed and the
demand signal transferred to Valve 1.
- If both Valve 1 & Valve 2 are in fault condition, then the warning status of each will be
elevated to alarm level (operation will revert back to the default valve and will stay on
that valve until fault is rectified).
During normal healthy running, the control valves will operate on the same duty sharing cycle
as the pumps, i.e. every 7 days (default) the operational valve will ramp down to closed and
other valve will then respond to the demand signal to continue operating for the next 7 days
etc. Changeover default time is set at 03:00 in the morning.
Both control valves have a ‘spring return’ function to ensure the valves close in the event of a
power failure.
A 300μ (50mesh) primary water filter is fitted at the chilled water inlet to the plate heat
exchanger. The differential pressure is monitored across this filter with sensors PS3 & PS4
(see controller Status screen ref. 3-5) to predict when the filter requires cleaning/flushing.
- If the differential pressure exceeds 0.6Bar (9PSI), then a ‘Primary Filter Dirty’ warning is
generated. Filter may be flushed while operational as described in Section 7.5.
A flow meter in the primary water return line monitors flowrate. The flow can be read within the
controller Status screen ref. 3-8. Note: The flow meter is in series with the flow control valves
and will therefore only register a flow when either valve is open or partially open.
- A ‘Primary Low Flow’ warning is generated only if the following conditions are met:-
The Secondary High Temperature warning is active, there is no Primary High
Temperature warning present and the demand to the operational control valve is at
100%.
- A ‘Primary No Flow’ alarm can also be generated when the following conditions are
met:- The Secondary High Temperature warning is active, there is a Primary High
Temperature alarm present and the demand to the operational control valve is at
100%.
- Note: Flows below 20L/M (5USgpm) will not be accurate and may not display at all.