The Spectra Pearson Pump
Spectra's Pearson Pump was developed to be used as the high pressure feed water pump in small Reverse Osmosis desalination systems where the best possible energy efficiency is required. Models are available with capacities to produce from 1000 gallons fresh water per day up to 5000gpd. Energy consumption will be as low as 11 watt- hours per gallon in sea water applications and even lower in brackish water.
Principle Of Operation
In conventional Reverse Osmosis systems the high pressure needed in the membranes is created and regulated by a back pressure regulating valve in the high pressure concentrate (brine) discharge line. All of the potential energy in the high pressure concentrate is lost in the back pressure regulator, and the power required to produce that energy is wasted.
The Spectra Pearson pump is a positive displacement three cylinder reciprocating high pressure pump with the same motors and crankcases used in conventional RO system feed pumps. The Pearson Pump head delivers water to the membranes in the same way as conventional feed pumps but is capable of recovering the energy in the concentrate. This is done by returning the concentrate to the Pearson pump at high pressure, where it flows into the pump cylinders on the undersides of the pistons, transferring its energy to the feed water being discharged to the membranes. The energy recovered from the concentrate reduces the load on the pump motor, reducing the electrical consumption dramatically.
Fixed Recovery Ratios
If the volume of the cylinder under the piston were the same as the volume of the cylinder above the piston all of the water being discharged from the Pearson pump could return to the pump and no pressure would be developed. However, part of the volume in the underside cylinder is taken up by the ceramic plunger, reducing the space available for the returning concentrate. Because feed water is being forced out of the cylinder by the upper side of the piston, and only a portion of that water will be able to return to the underside, a “Hydraulic Lock” is created, and flow is only possible if the water displaced by the plunger has somewhere else to go. Because the electric motor is forcing the piston upwards, and that water has no where to go, pressure begins to build up. When the pressure rises high enough water will be forced through the membrane as permeate and flow will begin.
The proportion of the feed water discharged by the Pearson pump which will permeate the membrane and become product water is fixed by the percentage of the volume of the underside cylinder taken up by the plunger. If the plunger takes up half the volume of the cylinder, then only 50% of the feed water discharged by the Pearson Pump can return to the pump, and the other 50% must leave the system as permeate. Thus, a Pearson Pump based system will be a fixed recovery ratio system, and the operating pressure will vary with pump speed, feed water salinity, and feed water temperature but the percentage of feed recovered as permeate will always be constant. Conventional systems are constant pressure systems, where the back pressure regulator keeps the system at a fixed pressure, and the recovery ratio will vary according to operating conditions.
Because in a conventional system the energy in the concentrate is lost, recovery ratios and operating pressures must be kept as high as possible to reduce the concentrate flow rates to the lowest possible amount. This results in high concentrate densities and the resultant scaling and other problems related to high concentrate density. In a system using a Spectra Pearson Pump, there is little difference in energy efficiency with different recovery ratios because nearly all the energy is recovered. This means that recovery ratios and operating pressures can be kept lower and antiscaling treatments reduced or eliminated, with resulting improvements in maintenance costs.