Nominal System Water Flow Control; Water Flow Control Procedure - Carrier AquaSnap 30RB Series Installation, Operation And Maintenance Instructions

9 - NOMINal SYSTEM WaTER FlOW CONTROl

The water circulation pumps of the 30RB 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. It is necessary to know the nominal system flow rate to
allow its control via a manual valve either provided in the
water leaving piping of the module (item 9 in the typical
hydronic circuit diagram) or to be installed as shown in the
same diagram (chapter 8.2).
With the pressure loss generated by the control valve in the
hydronic system, the valve can impose the system pressure/
flow curve on the pump pressure/flow curve, to obtain the
desired operating point. The pressure drop reading in the
heat exchanger and its internal piping is used to control
and adjust the nominal system flow rate.
Use this specification for the unit selection to know the
system operating conditions and to deduce the nominal air
flow as well as the pressure drop of the heat exchanger and its
internal piping at the specified conditions. 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 evaporator or with the Electronic Catalogue selection
program for all ∆T conditions other than 5 K in the range
of 3 to 10 K.

9.1 - Water flow control procedure

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 application.
proceed as follows:
Open the valve fully.
Start-up the pump using the forced start command (refer
to the controls manual) and let the pump run for two
consecutive hours to clean the hydronic circuit of the
system (presence of solid contaminants).
Read the filter pressure drop by taking the difference of
the readings of the pressure gauge connected to the filter
inlet and outlet, using valves, if options 116B, C, F, G are used,
if not install a pressure gauge after filter inlet and outlet
(see typical hydronic circuit diagrams), and comparing this
value after two hours of operation.
If the pressure drop has increased, this indicates that the
screen filter must be removed and cleaned, as the hydronic
circuit contains solid particles. In this case close the shutoff
valves at the water inlet and outlet and remove the screen
filter after emptying the hydronic section of the unit.
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Renew, if necessary, to ensure that the filter is not contami-
nated. Purge the air from the circuit using the purge valves in
the hydronic circuit and the system (see typical hydronic
circuit diagram).
When the circuit is cleaned, read the pressures at the
pressure gauge (entering water pressure - leaving water
pressure), expressed in bar and convert this value to kPa
(multiply by 100) to find out the pressure drop of the heat
exchanger and its internal piping.
Compare the value obtained with the theoretical selection
value.
It is essential to carry out systematic filter cleaning at the
initial start-up, as well as after any modification in the
hydronic circuit.
ATTENTION: It is essential to keep the pressure gauge
purge valve open after measuring the pressure (risk of
freezing during winter).
If the pressure drop measured is higher than the value
specified the flow rate in the evaporator (and thus in the
system) 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 one turn for options 116B,
C, F, G 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.
If the system has an excessive pressure drop in relation to
the available static pressure provided by the pump, the
resulting water flow rate will de reduced and the difference
between entering and leaving water temperature of the
hydronic module will be increased.
To reduce the pressure drops of the hydronic system, it is
necessary:
• to reduce the individual pressure drops as much as
possible (bends, level changes, accessories, etc.)
• to use a correctly sized piping diameter.
• to avoid hydronic system extensions, wherever possible.