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13 - NOMINal SYSTEM WaTER flOW CONTROl
The water circulation pumps of the 30RQS/RQSY units
have been sized to allow the hydronic modules to cover all
possible configurations based on the specific installation
conditions, i.e. for various temperature differences between
the entering and the leaving water (∆T) at full load, which
can vary between 3 and 10 K.
This required difference between the entering and leaving
water temperature determines the nominal system flow rate.
Use this specification for the unit selection to find the
system operating conditions.
In particular, collect the data to be used for the control of
the system flow rate:
•
Unit without a hydronic module: nominal unit pressure
drop (plate heat exchanger + internal piping),
•
Unit with fixed-speed pump: nominal flow rate,
•
Unit with variable-speed pump, controlled by a diffe-
rence in constant pressure between the unit inlet and
outlet: nominal flow rate,
•
Unit with variable-speed pump, controlled by the heat
exchanger temperature difference: nominal
temperature difference at the heat exchanger.
If this information is not available at the system start-up,
contact the technical service department responsible for the
installation to get it. These characteristics can be obtained
from the technical literature using the unit performance
tables for a ∆T of 5 K at the water heat exchanger or with
the Electronic Catalogue selection program for all ∆T
conditions other than 5 K in the range of 3 to 10 K.
13.1 - Units without hydronic module
The nominal system flow rate is controlled by a manual
valve that must be on the water leaving piping of the
system (item 20 in the typical hydronic circuit diagram).
Due to the pressure drop it generates in the hydronic system
this flow control valve permits adjustment of the pressure/
system flow rate curve in accordance with the pump pressure/
flow rate curve to obtain the nominal flow rate at the required
operating point (see example for unit size 30RQS 078).
The pressure drop reading in the unit (plate heat exchanger
+ internal piping) is used as means of control. This reading
can be taken with pressure gauges that must be installed at
the unit inlet and outlet (item 19). As the total system
pressure drop is not known exactly at the start-up, the
water flow rate must be adjusted with the control valve to
obtain the specific flow rate for this system.
hydronic circuit cleaning procedure
•
Open the valve fully (item 20).
•
Start-up the system pump.
•
Read the plate heat exchanger pressure drop by taking
the difference of the readings of the pressure gauge
connected to the unit inlet and outlet (item 19).
•
Let the pump run for two consecutive hours to clean
the hydronic circuit of the system (presence of solid
contaminants).
•
Take another reading.
30
•
Compare this value to the initial value.
•
If the pressure drop has decreased, this indicates that
the screen filter must be removed and cleaned, as the
hydronic circuit contains solid particles. In this case
close the shut-off valves at the water inlet and outlet
(item 17) and remove the screen filter (item 18) after
emptying the hydronic section of the unit (item 6).
•
Purge the air from the circuit (items 5 and 15).
•
Renew, if necessary, to ensure that the filter is not
contaminated.
Water flow control procedure
When the circuit is cleaned, read the pressures at the
pressure gauges (entering water pressure - leaving water
pressure), to find out the unit pressure drop (plate heat
exchanger + internal water piping).
Compare the value obtained with the theoretical selection
value. If the pressure drop read is higher than the value
specified the unit flow rate (and thus system flow rate) is too
high. The pump supplies an excessive flow rate based on the
global pressure drop of the application. In this case close the
control valve and read the new pressure difference.
Proceed by successively closing the control valve until you
obtain the specific pressure drop that corresponds to the
nominal flow rate at the required unit operating point.
NOTE: If the system has an excessive pressure drop in
relation to the available static pressure provided by the
system pump the nominal water flow rate cannot be obtained
(the resulting flow rate is lower) and the temperature
difference between the water heat exchanger entering and
leaving water will increase.
To reduce the pressure drops of the hydronic system:
•
to reduce the individual pressure drops as much as
possible (bends, level changes, options, etc.).
•
to use a correctly sized piping diameter.
•
to avoid hydronic system extensions, wherever possible.
Example: 30RQS 078 at Eurovent conditions of 3.7 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.
Water flow rate, l/s
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