Nominal Evaporator Water Flow Control; Water Flow Control Procedure - Danfoss BW05 Installation, Operation And Maintenance Instructions

8 NOMINAL EVAPORATOR WATER FLOW CONTROL

The water circulation pumps of the BW05 units have been
sized to allow the hydronic modules to cover all pos-sible
configurations based on the specific installation con-ditions,
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 above all absolutely necessary to know the nominal system
flow rate to allow its control via a manual valve provided in
the water leaving piping of the module (item 9 in the typical
hydronic circuit diagram).
With the pressure loss generated by the control valve in the
hydronic system, the valve is able to impose the system
pressure/flow curve on the pump pressure/flow curve, to
obtain the desired operating point (see example).
The pressure drop reading in the plate heat exchanger is used
to control and adjust the nominal system flow rate. The
pressure drop is measured with the pressure gauge
connected to the heat exchanger water inlet and outlet.
Use this specification for the unit selection to know the
system operating conditions and to deduce the nominal air
flow as well as the plate heat exchanger pressure drop 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.
8.1 - 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 provided to obtain the specific flow rate for this
application.
Proceed as follows:
Open the valve fully (approximately 9 turns counter-
clockwise).
Start-up the pump using the forced start command (refer to
the controls manual) and let the pump run for two con-
secutive hours to clean the hydronic circuit of the system
(presence of solid contaminants).
Read the plate heat exchanger pressure drop by taking the
difference of the readings of the pressure gauge connected
to the plate heat exchanger inlet and outlet, using valves (see
diagrams below), and comparing this value after two hours of
operation.
VUGFB102
Entering water pressure reading
F
Leaving water pressure reading
O
Legend
O Open
F Closed
Water inlet
Water outlet
Pressure gauge
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.
Renew, if necessary, to ensure that the filter is not conta-
minated.
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 plate heat exchanger
pressure drop.
Compare the value obtained with the theoretical selection
value. If the pressure drop measured is higher than the value
specified this means that the flow rate in the plate heat
exchanger (and thus in the system) is too high. The pump
supplies an excessive flow rate based on the global pres-sure
drop of the application. In this case close the control valve
one turn 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 tempe-
rature of the hydronic module will be increased.
O
F
F
F
23