ArtsAutosBooksBusinessEducationEntertainmentFamilyFashionFoodGamesGenderHealthHolidaysHomeHubPagesPersonal FinancePetsPoliticsReligionSportsTechnologyTravel

Fresh Water Generator or Evaporator (Alfa Laval Type)

Updated on April 12, 2010


A considerable amount of fresh water is consumed in a ship. The crew consumes an average 100 liter/head/day. In a steam ship (a ship whose main propulsion unit is steam turbine or a ship which is a large tanker having steam turbine driven cargo oil pumps) the consumption for the boiler can be as high as 30 tonnes/day.

Sufficient potable water may be taken on in port to meet crew and passenger requirement but the quality of this water will be too poor for use in water tube boilers. It is common practice to take on only a minimum supply of potable water and make up the rest by distillation of seawater. It is statutory requirement to have a distillation plant for emergency use if otherwise ship has carried sufficient potable water.

Working principle

Working principle of the low pressure evaporator is that, boiling point of water can be reduced by reducing the pressure of the atmosphere surrounding it. By maintaining a low pressure, water can be boiled at low temperatures say 50 degree Celsius. The source of heat could be waste heat rejected by main engine jacket cooling water.

Hence using energy from a heating coil, and by reducing pressure in the evaporator shell, boiling can takes place at about 40 to 60 degree Celsius. This type of single effect plant is designed to give better economy than obsolete Boiling Evaporators.

Such plants are widely used because of:

  1. Control over type of scale formed.
  2. Heating medium can be at relatively low temperature. (e.g. diesel engine jacket water or waste steam)
  3. Improved heat transfer across the heating element. This is due to higher temperature difference for lower pressures.

Operation of the Plant

Heat from the diesel engine cooling water is used to evaporate a small fraction of the seawater feed in the plate type evaporator. Unevaporated water is discharged as brine (by brine ejector) and that which is evaporated passes through the demister to the plate type vapour condenser, where, after condensation it is discharged to fresh water storage tank by fresh water distillate pump. During entire operation the feed rate to the evaporator is fixed by the orifice plate.


Freshwater Generator (Alfa-Laval Type)
Freshwater Generator (Alfa-Laval Type)

In the event of salinity of freshwater density exceeding a predetermined value (maximum usually 10 ppm) the solenoid controlled dump valve diverts the flow to the bilge, preventing contamination of the made water. Excess salinity could be used by many factors include leakage of seawater at condenser or priming of evaporator or malfunctioning of demister, or many other reasons.

What cannot be condensed at the condenser are called 'uncondensable gases' like air and these gases are continuously ejected out by air ejector. This way the shell is maintained at high vacuum, a must requirement to boil water at low temperatures.


"Basic Marine Engineering" by J.K.DHAR

Read More


    0 of 8192 characters used
    Post Comment
    • profile image

      Mohamed Fizal 

      5 years ago

      What will be the result when the shell temperature of fresh water generator increases

    • jabelufiroz profile imageAUTHOR


      6 years ago from India

      Yes, while at sea we produce up to 40 tonnes/day for both equipments and personnel on board ships.

    • tamarawilhite profile image

      Tamara Wilhite 

      6 years ago from Fort Worth, Texas

      Do you ever have to produce distilled water for equipment?

    • profile image


      7 years ago

      what is the maximum vacuum press to produce fresh water ?

    • profile image


      7 years ago

      jab what is the shell temperature of fresh water generator

    • profile image

      alpesh devghare 

      8 years ago

      nice dud

    • profile image

      jaypee alestre 

      9 years ago

      is verry excillent to post this basic diagram of f.w.g..we can helped to our marine student seacrhers

    • thevoice profile image


      9 years ago from carthage ill

      terrific first rate hub work thanks

    • stars439 profile image


      9 years ago from Louisiana, The Magnolia and Pelican State.

      Very impressive hub. I enjoyed reading this. God Bless You.


    This website uses cookies

    As a user in the EEA, your approval is needed on a few things. To provide a better website experience, uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

    For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at:

    Show Details
    HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
    LoginThis is necessary to sign in to the HubPages Service.
    Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
    AkismetThis is used to detect comment spam. (Privacy Policy)
    HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
    HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
    Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
    CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
    Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the or domains, for performance and efficiency reasons. (Privacy Policy)
    Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
    Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
    Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
    Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
    Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
    VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
    PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
    Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
    MavenThis supports the Maven widget and search functionality. (Privacy Policy)
    Google AdSenseThis is an ad network. (Privacy Policy)
    Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
    Index ExchangeThis is an ad network. (Privacy Policy)
    SovrnThis is an ad network. (Privacy Policy)
    Facebook AdsThis is an ad network. (Privacy Policy)
    Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
    AppNexusThis is an ad network. (Privacy Policy)
    OpenxThis is an ad network. (Privacy Policy)
    Rubicon ProjectThis is an ad network. (Privacy Policy)
    TripleLiftThis is an ad network. (Privacy Policy)
    Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
    Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
    Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
    Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
    ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
    Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)
    ClickscoThis is a data management platform studying reader behavior (Privacy Policy)