Operation guide | iSave 40
14
180R9239 | AQ299555093252en-001601 | iSave 40 | 09.2022
5.7.1 Pressure exchanger function
The pressure exchanger transfers pressure from
the high-pressure (HP) concentrate (HP in) to the
low-pressure (LP) seawater coming from
low-pressure feed pump (LP in).
To separate the HP side from the LP side there is
a sealing zone on both port plates. A single duct
in the rotor is either on the HP side, or on the LP
side or in the sealing zone. A single duct is never
in contact with more than one zone at the time.
When the rotor rotates a duct will go from the LP
zone over the first sealing zone into the HP zone,
and hereafter from the HP zone over the second
sealing zone and back to the LP zone.
The flow through the HP side of the iSave is
forced and controlled by the booster pump.
When the high-pressure concentrate is flowing
into the iSave it pressurizes the seawater in
the duct coming from “LP in”. The pressurized
seawater is then pumped out of “HP out”. Just
before the HP concentrate in the duct comes to
the seawater port plate, the duct goes into the
sealing zone and the flow in the duct stops.
When the duct goes into the LP zone the
concentrate water is de-pressurized. The (LP)
seawater coming from the LP feed pump (LP in)
forces the LP concentrate out of “LP out”.
This pressure exchange process is repeated for
each duct with every rotation of the rotor, and
the ducts are thus continuously filling and
discharging. The flow on the HP side and LP side
of the iSave is nearly constant over time.
There is no physical barrier in the ducts between
the concentrate and seawater. This means that
there will be a small amount of mixing between
the two liquids.
When the iSave is rotating the water always flows
respectively from LP-in to HP-out, AND from
HP-in to LP-out. However, if the feed flow into
LP-in is higher than the flow into HP-in, some of
the LP feed flow will flow directly to LP-out.
When the iSave is not rotating the seawater can
only run directly from LP-in to LP-out.
5.7.2 Booster pump
The booster pump is a positive displacement
pump, which means that the flow is controlled
by the speed of the electric motor; e.g. if the
rotation speed of the electric motor is raised by
10%, the flow will be 10% higher and vice versa.
The required rpm can be calculated based on the
“rated flow” of the particular iSave. Use Danfoss
Selection tool which is availbable on: www.isave.
danfoss.com. At low speeds you may hear some
minor clicking sounds from the pump. This is
normal and is caused by the pins in the vane
pump.
5.7.3 Lubrication flow
To lubricate the moving parts in the pressure
exchanger there is a well defined leak between
port plate/valve plate.
In the booster pump there is a well-defined leak
between rotor/sealing plates.
The lubrication flow inside the iSave always goes
from the HP side to the LP side.
5.7.4 Volumetric mixing
There is no physical barrier in the ducts between
the concentrate and the seawater. This means
that there will be a small amount of mixing
between the two liquids. Because the two liquids
are in contact for a short amount of time, the
mixing is relatively low.
On the RO market the mixing rate is defined at
“balanced flow” when HP-out is equal to LP-in.
The customer can reduce mixing by over-flush-
ing the LP feed with excess feed water. See
mixing curve in section 10, appendix. Optimal
over-flushing is obtained when the energy loss
on LP feed is equal to or less than the energy loss
caused by the excess pressure at the membrane.
The pressure exchanger consists of two
port plates, one at the concentrate side
and one at the seawater side. In between
there is a rotor with several ducts that
connect the concentrate side with the
seawater side.