How Does The Isave Work In An Ro System - Danfoss iSave 40 Series Operation Manual

Operation guide | iSave 40
5.7.5 How does the iSave work in an RO
system?
The high-pressure (HP) concentrate (7) flows to
the low-pressure (LP) concentrate outlet (3).
The LP seawater (2) flows to the HP seawater
outlet (5).
The rotor, moving between the high-pressure
and low-pressure streams, removes the high-
pressure concentrate (7) and replaces it with feed
water (2).
The flow rate on the HP seawater outlet (5) is
controlled by the iSave alone.
The flow rate on the LP concentrate outlet (3) is
controlled by the seawater feed pump (1) and the
back pressure valve.
During the RO process operation, water is
pumped into the HP membrane feed (6) by the HP
pump (4) and the iSave (5). Almost all water
coming from the HP pump (4) penetrates the
membranes (8). Only a slight amount of the water
is used as lubrication flow in the iSave. The
lubrication flow is measured as the difference
between the HP pump flow rate (4) and the
permeate flow rate (8). The resistance to permeate
in the membrane pressurises the HP loop.
The isobaric pressure exchanger technology in a
SWRO or BWRO changes the HP concentrate into
HP seawater that is fed into the HP membranes.
The iSave energy recovery techno logy thus
significantly reduces flow needed from the main
HP pump (4). Overall energy consumption of a
SWRO or BWRO plant using the iSave depends on
the recovery rate.
180R9239 | AQ299555093252en-001601 | iSave 40 | 09.2022
The figure below illustrates a typical flowpath of
an SWRO or BWRO with an iSave.
This means that changing the LP feed flow (2) will
not affect HP outlet flow (5) and, vice versa, that
changing the HP outlet flow (5) will not affect the
LP outlet flow (3).
As LP seawater (2) is flushing the LP concentrate
to LP outlet (3), it is essential that the flow on the
LP inlet (2) is equal to or slightly higher than the
HP inlet (7). Otherwise there will be an "under-
flush" and higher mixing will occur in the HP
outlet (5). This higher mixing will result in a
slightly higher pressure at the membrane.
The booster pump integrated in the iSave must
only overcome the pressure drop from HP in (7)
to HP out (5 ).
The operator can change the recovery rate to
optimise the RO system performance. Changing
the recovery rate in an RO system equipped with
iSave is easy. Using a VFD, change the speed of the
iSave and thereby the flow in the HP flow rate.
Increased speed = Lowered recovery rate
Decreased speed = Raised recovery rate
Then change the LP feed flow (2) flow to the iSave
to minimise mixing and optimise energy consump-
tion. Make sure that flow and pressure are within
the rated parameter of the iSave in question.
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