The combination of vacuum enhanced distillation and waste heat recovery is a proven concept that has been used for more than 60 years. This tried-and-true method of desalination is making a comeback. Advances in technology have resulted in evaporators that:
- Are more energy efficient
- Offer a reduction in weight and installed spare requirements
- Are more reliable
- Offer a clear green advantage
How Heat Recovery Evaporators Work
Waste heat evaporators use waste heat from engine jacket water, engine exhaust, steam or other heat sources to transform seawater, brackish water or contaminated feed water into pure potable water that is suitable for human consumption, industrial processes, agriculture and many other applications. Operational efficiency can be further enhanced by adding additional evaporation stages and/or effects to increase the amount of fresh water produced using a reduced amount of heat.
Properly designed heat recovery evaporators provide thermal stability in rough seas, consistency of clean water across all types of feed water and can handle wide ranges of feed water temperatures. Evaporators can function well in river water, brackish bay water or sea water, yielding the same high quality water.
Heat recovery evaporators/watermakers extract high-quality salt free water from seawater using waste heat. Watermakers are used in the commercial shipping industry on oceangoing tankers and cargo vessels. The military has applications on aircraft carriers, destroyers and submarines in which highly reliable water production is critical for the mission. The oil industry also uses this technology on offshore oil and gas drilling rigs, production platforms and supply vessels.
Compared to other desalination technologies, heat recovery evaporators are advantageous for a multitude of reasons. Heat recovery evaporation is the most energy efficient desalination technology. Waste heat, normally discharged to the surrounding atmosphere, is used, which provides the bulk of the process’ energy requirements. Fuel efficiency on vessels is enhanced because the transportation of large tanks of potable water or bottled water is not required. More cargo space is also available when large amounts of water do not need to be stored.
The water produced is high quality, containing less than 4 parts per million (ppm) of total dissolved solids, compared to other desalination technologies that contain 300 to 500 ppm of total dissolved solids of which at least 50 percent is salt. Heat recovery evaporators operate under vacuum conditions at low boiling temperatures, which minimizes the scaling of the heat transfer surfaces and the maintenance associated with cleaning. There are no high-pressure (700 to 1,000 psi) hydraulic components. Low-pressure systems, fewer moving parts and simple designs make these systems reliable. A heat recovery evaporator requires less maintenance than other technologies.
Heat recovery evaporators do not use membranes or filters, which can scale in the presence of certain types of feed water and do not react well to changes in the feed water quality or temperature. Heat recovery evaporators, depending upon the size of the unit, use two pumps—a seawater pump and a fresh water pump. Both pumps are close-coupled, centrifugal pumps. The seawater pump ranges from 2 to 7½ horsepower while the fresh water pump averages ¾ horsepower and depends on the capacity of the watermaker. The seawater pump is used to pump seawater or other liquid feed source into a chamber at the base of the evaporator. The fresh water pump is used to pump the distillate into a clean water holding tank.
The footprint for a heat recovery unit is small, and storage space for replacement membranes and filters is not required since heat recovery evaporators require no replacement parts.
Heat recovery evaporators do not use membranes or filters requiring disposal. More than 10,000 tons of filters and membranes are sent to landfills annually.1 This fact makes them a good choice for companies with the goal of environmental responsibility.
Heat recovery evaporators offer a better return on investment. The up-front capital costs of an evaporator are comparable to other desalination technologies. However, throughout the lifetime of the unit, the cost to operate and maintain it is significantly lower. Heat recovery evaporators average a lifespan of more than 30 years.
Heat recovery evaporators offer an energy efficient and reliable solution to water-making needs. Companies that are cost conscience and good stewards of the environment will find that this green technology will meet their expectations.
1. “New Coalition to Coordinate Research on Membrane Disposal.” Desalination & Water Reuse, July 2012.