Water. Desalination + reuse
Issue link: https://fhpublishing.uberflip.com/i/946410
March 2018 Water. desalination + reuse Tech Quarterly 23 some cases, the equipment is de- signed around these maximum conditions, and does not ac- count for the fact that the pump is going to operate far from that maximum for the majority of the time. Therefore, you will see very oversized machines. Then when they work at normal operating conditions, the efficiencies are not that high," says Rubio. The way to overcome these concerns is to establish with the client what will be the normal, everyday conditions, to design for these; and then to flex the system during peak times. "Sometimes it's difficult, because it's a very complex system, but it's really worth paying attention to the whole range of operation, and know at what point the pump, the equipment, will mostly have to be working on the plant," Rubio says. The desalination industry has to some extent adopted the use of variable frequency drives as a way to deal with variable flow rates, which provide some additional energy savings. Go smaller One notable innovation has been the development of new positive displacement pumps for desalination that are applicable to smaller, de-centralised RO in - stallations. Whereas in the past, these type of higher efficiency pumps were only applicable for RO units of up to about 1,000 m3/d, the latest ranges can span up to around 15,000 m3/d. "Instead of these big centralised infrastructures, you can have a more de-centralised system, closer to the supply point, with medium size plants, at the highest efficiency as in the big infrastructures," explains Joan Galtes, sales manager for high pressure pumps at Danfoss. The pumps have supported installation of modular, decentralised desalination units to serve local communities of 5,000 to 10,000 people, including tourist islands and coastal resorts with high seasonable variability in demand for water. "In the past few years, we have developed a technology that enables us to increase significantly the size of the plants we support. That was not feasible before in medium size plants because the equipment was not as energy efficient as in the big plants. Our approach is to implement pumps which have almost 90 per cent efficiency, no matter what the size is," says Galtes. The cost of water produced by medium size plants can now begin to match that of a larger infrastructure, which in the past had the advantage of economies of scale. The economics of smaller plants now stack up more easily in terms of operating efficiency, and, because they are closer to the communities that they supply, capital expenditure. In one project, Danfoss supplied positive displacement pumps for an RO plant of 20,000 m3/d, built in four trains of 5,000 m3/d. "They implemented our technology in the pump system, and in the energy recovery system. We were able to make savings of 0.2 KwH per m3 of produced water. This is 10 per cent of energy savings compared to a centrifugal pump. In terms of long-term operation, this represents a significant amount of money," says Galtes. In another project, in the Canary Islands, Danfoss helped to reduce energy consumption by 20 per cent by replacing centrifugal pumps with positive displacement pumps. "You can put this high efficient technology and you get a really good specific energy consumption," Galtes adds. A third project called for the pumps to handle wide flexibility in the pressure requirement conditions, owing to salt intrusion into the water supply during winter rains. "The salinity of the water is completely different from winter to summer, and the pressure requirement can rise from 22 bar to 63 bar. That's a huge change in operating conditions, and is very challenging for a centrifugal pump, which most likely cannot run with the entire pressure operating range; and so in the end they lose efficiency," says Galtes. "With positive displacement pumps you don't have this problem, because no matter what the pressure, your pump will run at the optimal point." TQ Our approach is to install pumps which have almost 90 per cent efficiency