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13 - NOMINAL SYSTEM WATER FLOW CONTROL
Refer to the chapter ''Water connections'' for all references
points mentioned in this chapter.
The water circulation pumps of unit range have been designed to
allow the hydraulic modules to operate at each possible conditions,
i.e. with chilled water temperature differences at full load from 3
to 10 K.
This temperature difference required between the water inlet and
outlet determines the nominal flow of the system. Use the
specification provided while selecting the unit to determine the
operating conditions of the system.
In particular, collect the data to be used for the control of the system
flow rate:
• Units without hydraulic module : the nominal unit pressure
drop. This is measured with pressure gauges that must be
(field) installed at the inlet and outlet of the unit (item 21).
• Units with fixed speed pumps : nominal flow rate. The
pressure of the fluid is measured by sensors at the inlet of
the pump and outlet of the unit (items 7 and 10).The controls
then calculate the flow rate associated with this pressure
difference and display the result on the user interface. (refer
to unit control manual).
• Units with variable speed pumps : the constant pressure
differential control based on readings at the hydraulic
module inlet and outlet. The buffer tank module option is
not taken into account.
• Units with variable speed pumps : the control on temperature
difference measured at the heat exchanger inlet and outlet.
If this information is not available at the start-up of the system,
contact the technical service department responsible for the
installation to obtain them .
These data can be obtained either from the technical document
with unit performance tables for a Delta T° of 5 K at the evaporator,
or with the help of the ''Electronic Catalog'' selection program for
all conditions of Delta T° different from 5 K in the range of 3 to 10
K.
13.1 - Units without hydraulic module
13.1.1 - General information
The nominal flow of the unit will be set using a manual valve that
should be installed on the outlet of the unit (item 19 on the
schematic hydraulic circuit). Changing the pressure drop of the
valve allows adjustment of the system flow rate to achieve the
design flow rate.
As the total system pressure drop is not known exactly at the
start-up, it is necessary to adjust the water flow with the control
valve to obtain the specific flow of the system.
13.1.2 - Procedure for cleaning the hydraulic circuit
• Open the valve completely (item 22).
• Start-up the system pump.
• Read the pressure drop of the plate heat exchanger as the
difference between the unit inlet and outlet pressures (item
21).
• Let the pump run for 2 hours consecutively to clean up the
hydraulic circuit of the system (presence of contaminating
solids).
• Perform another reading.
• Compare this value to the initial value. A decrease in the
pressure drop value indicates that the filters in 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 then clean the filters (items 20 and 1) after draining
the hydraulic part of the unit (item 6).
• Remove the air from the circuit (items 5 and 17).
• Repeat until the filter remains clean.
38
13.1.3 - Procedure for controlling the water flow
Once the circuit is cleaned, read the pressures on the pressure
gauges (water inlet and outlet pressure) determine the pressure
drop within the unit (plate heat exchanger + internal pipe work).
Compare the value obtained with the design value predicted by
the selection software.
If the pressure drop reading is above the specified value, this
indicates that the flow at the terminals of the unit (and hence in
the system) is too high. In this case, close the control valve and
read the new difference in pressure. Repeat as necessary until
the pressure drop corresponding to the design flow rate is
achieved.
NOTE: If the network has an excessive pressure drop in
relation to the available static pressure delivered by the
system pump, the nominal water flow cannot be obtained
(lower resulting flow) and the difference in temperature
between the water inlet and outlet of the evaporator will be
increased
To reduce the hydraulic system pressure drop:
• Reduce the pressure drops of individual components
(bends, level changes, valves etc.) as much as possible
• Use the correct pipe diameter
• Do not extend the piping system.
Example : Unit with specific nominal flow 3.7 l/s
Water flow rate, l/s
Legend
1
''Unit pressure drop (including internal water piping)/flow rate'' curve
2
With the valve open the pressure drop read (111 kPa) gives point A on the curve.
A Operating point reached with the valve open.
3
With the valve open the flow rate achieved is 4.8 l/s: this is too high, and the
valve must be closed again.
4
If the valve is partially closed, the pressure drop read (65 kPa) gives point B on
the curve.
B Operating point reached with the valve partially closed.
5
Wih the valve partially closed the flow rate achieved is 3.7 l/s: this is the required
flow rate and the valve is in an adequate position.
13.2 - Units with hydraulic module and fixed-speed
pump
13.2.1 - General information
See chapter ''Units without hydraulic module''.
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