Units With Hydraulic Module And Variable Speed Pump - Pressure Differential Control - Carrier AQUASNAP 30RBM Series Installation, Operation And Maintenance Instructions

9.3 - Units with hydraulic module and variable speed
pump – Pressure differential control
The system flow rate has not been set to a nominal value.
The flow rate will be adjusted, by varying the pump speed, to
maintain a system pressure differential value defined by the
user.
This is checked by the pressure sensor at the water exchanger
outlet (item 10 on the main water circuit diagram).
The system calculates the measured pressure difference,
compares it with the setpoint value set by the user and then
modulates the pump speed module, resulting in:
• an increase in the flow rate if the measurement is below
the setpoint,
• a decrease in the flow rate if the measurement exceeds
the setpoint.
This modulation is limited only by the maximum and minimum
flow rates for the unit and by the maximum and minimum
allowable pump speeds.
The maintained pressure difference value may, in certain cases,
differ from the setpoint value:
• if the setpoint value is too high (obtained for a flow rate higher
than the maximum value or a frequency greater than the
maximum value), the system will stop once it reaches the
maximum flow rate or maximum frequency, which will result
in a pressure difference below the setpoint,
• if the setpoint value is too low (obtained for a flow rate
lower than the minimum value or a frequency less than
the minimum value), the system will stop once it reaches
the minimum flow rate or minimum frequency, which will
result in a pressure difference greater than the setpoint.
Contact the manufacturer's service department to implement
the procedures described below
Hydraulic circuit cleaning procedure
Before proceeding, it is advisable to remove any possible
contamination from the water circuit.
• Start-up the system pump by using the forced start
command.
• Control the frequency to the maximum value to generate
a higher flow.
• If there is a ''Maximum flow exceeded'' alarm, reduce the
frequency until an acceptable value is reached.
• Read the value of the flow on the user interface.
• Let the pump run for 2 hours continuously to clean up
the system's hydraulic circuit (presence of contaminating
solids).
• Perform another reading of the flow and compare this
value with the initial value. A reducing value of the flow
rate indicates that the filters on the system need to be
removed and cleaned. In this case, close the shut-off valves
on the water inlet and outlet (item 19) and remove the
filters (items 20 and 1) after draining the hydraulic part
of the unit (items 6).
• Remove the air from the circuit (items 5 and 17).
• Repeat until all fouling is removed from the filter
Procedure for controlling the pressure differential setpoint
Once the circuit is cleaned, place the water circuit in the
configuration for which the unit selection was performed
(generally, this will be all valves open and all cooling coils
active)
Read the value of the flow on the user interface and compare
it with the theoretical value of the range:
• If the flow rate read is greater than the preset value, reduce
the pressure differential setpoint on the user interface to
reduce the flow rate value;
• If the value read is lower than the preset value, increase
the pressure differential setpoint on the user interface to
increase the flow rate value
Repeat until the unit's nominal flow rate at the required
operation point is achieved.
Stop the forced operation of the pump and proceed to the
configuration of the unit for the required control mode. Modify
the control parameters:
• Set water flow control to 'pressure differential'
• Set the value of the required pressure differential.
The unit's default factory configuration is the minimum speed
(frequency: 30 Hz).
NOTE:
If during adjustment, the low or high frequency limits are
reached before reaching the specified flow rate, keep the
pressure differential value at its lower or higher limit as the
control parameter value.
If the user knows in advance the pressure differential value
at the unit outlet to be maintained, this value can be entered
directly as data to be declared. You should not, however, omit
the water circuit cleaning sequence
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