Common Interview Questions for Marine Engineers on Fresh Water Generators (FWGs)

When it comes to marine engineering, few systems are as essential as the Fresh Water Generator (FWG). Not only does it provide fresh water for drinking and cooking, but it also plays a critical role in ship operations like boiler feedwater. If you’re preparing for an interview as a marine engineer, understanding FWGs is crucial, as these systems are often discussed in detail.

This blog post highlights some of the most common interview questions related to Fresh Water Generators, providing clear, concise answers to help you prepare and succeed.

What is a Fresh Water Generator, and Why is it Used on Ships?

A Fresh Water Generator (FWG) is a system onboard ships used to convert seawater into potable fresh water using processes like evaporation and condensation. Fresh water generated onboard is vital for:

  • Drinking and Cooking: Ensuring the crew has a safe water supply.
  • Engine Room Operations: Supporting essential processes like boiler feedwater and machinery cleaning.
  • Self-sufficiency: Reducing the need to rely on port-based fresh water supplies during long voyages.

FWGs help ships maintain operational efficiency while supporting the crew’s basic needs.

What are the Types of Fresh Water Generators Used on Ships?

There are four main types of FWGs commonly used in marine applications:

  1. Vacuum Distillation Type
      • Operates under low pressure, reducing the boiling point of seawater.
      • Uses waste heat from the engine’s jacket cooling water for energy efficiency.
  1. Reverse Osmosis (RO) Type
      • High-pressure system that forces seawater through a semi-permeable membrane, filtering out salts and impurities.
  1. Submerged Tube Type
      • Uses heat exchangers submerged in seawater to evaporate and condense water.
  1. Plate Type (Multi-Stage Evaporator)
      • Employs titanium plates for efficient heat exchange, increasing the overall water production capacity.

These FWG types offer unique advantages depending on ship design and operational requirements.

The Working Principle of a Vacuum Distillation Fresh Water Generator

A Vacuum Distillation Fresh Water Generator (FWG) is one of the most common systems used on ships. Here’s how it works:

  1. Pressure Reduction
      • The internal pressure of the FWG is lowered below atmospheric levels using a vacuum pump or air ejector. This reduces the boiling point of seawater to 40-50°C.
  1. Heating and Evaporation
      • Seawater is heated using waste heat from the engine’s jacket cooling water, evaporating the seawater at the reduced boiling point.
  1. Condensation
      • The evaporated steam is condensed back into liquid form by passing it through a seawater-cooled condenser.
  1. Storage and Adjustment
      • Fresh water is collected in a tank, passed through a re-hardening filter to adjust its pH and mineral content, ensuring it meets drinking water standards.

This process ensures efficient water production while utilizing waste heat, making it highly energy-efficient.

What are the Common Problems Faced in a Fresh Water Generator?

While FWGs are highly effective, they can face operational challenges. Key issues include:

  1. Scaling & Fouling
      • Salt deposits can accumulate on heat exchanger surfaces, reducing efficiency.
  1. Insufficient Vacuum
      • Caused by air leaks or malfunctioning vacuum pumps, leading to poor evaporation.
  1. Salinity in Fresh Water
      • Improper evaporation or high seawater ingress may cause salt contamination.
  1. Low Freshwater Production
      • Reduced heat supply, scaling, or vacuum loss can lead to decreased efficiency.
  1. Pump Failures
      • Feed and ejector pump malfunctions can disrupt the entire water generation process.

Preventing Scaling in a Fresh Water Generator

Scaling is one of the most common problems in FWGs, but it can be minimized with these measures:

  1. Chemical Dosing
      • Use anti-scaling chemicals to reduce salt deposition.
  1. Fresh Water Flushing
      • Flush the system with fresh water after each operation to remove residual seawater.
  1. Regular Cleaning & Descaling
      • Perform routine maintenance, including acid cleaning of heat exchangers.
  1. Monitoring Operating Parameters
      • Keep a close eye on vacuum levels, operating temperatures, and seawater flow rates to optimize performance.

The Functions of the Ejector in a Fresh Water Generator

Ejectors are critical components of FWGs, performing two primary functions:

  1. Creating a Vacuum
      • The air ejector removes air and non-condensable gases to maintain low pressure in the evaporator.
  1. Brine Removal
      • The brine ejector expels concentrated seawater from the evaporator, maintaining efficiency.

By maintaining proper ejector function, FWGs can operate efficiently and produce high-quality water.

Monitoring Fresh Water Quality on Board

The quality of fresh water generated onboard ships must meet specific standards. Common methods for monitoring include:

  • Salinity Meter
      • Measures salt content, ensuring it stays below 10 ppm.
  • Chlorine Test
      • Verifies the safety of the water for drinking.
  • pH Test
      • Checks for acidity or alkalinity, with safe levels between 6.5-8.5.
  • TDS (Total Dissolved Solids) Test
      • Measures overall impurities in the water.
  • Regular Sampling & Lab Tests
      • Periodically conducted to ensure compliance with international water safety standards.

Why is the Fresh Water Generator Usually Placed in the Engine Room?

Placing the FWG in the engine room comes with several strategic benefits:

  • Efficient Heat Utilization
      • Jacket cooling water from the main engine supplies the heat needed for evaporation.
  • Ease of Maintenance
      • Engineers can easily monitor and maintain the FWG alongside other vital machinery.
  • Optimized Space
      • The engine room layout accommodates FWG integration without impacting other operations.

Safety Precautions When Operating a Fresh Water Generator

Safety is vital when operating FWGs. Key precautions include:

  • Maintaining Proper Vacuum Levels
      • Prevent overheating and ensure efficient evaporation.
  • Monitoring Salinity Levels
      • Avoid contamination of water storage tanks.
  • Inspecting Pumps & Ejectors Regularly
      • Ensure smooth and efficient component operation.
  • Using PPE During Maintenance
      • Protect against hot surfaces and chemicals.
  • Following Proper Shutdown Procedures
      • Flush with fresh water to prevent corrosion.

Start Your Journey as a Knowledgeable Marine Engineer

A solid understanding of Fresh Water Generators is critical for any aspiring marine engineer. These essential systems not only support a ship’s self-sufficient operation but also highlight the importance of technical expertise in the maritime industry. Whether you’re preparing for an interview or simply want to deepen your knowledge, understanding FWG types, working principles, common challenges, and maintenance practices will set you apart.

Master these concepts, and you’ll be ready to impress during your next marine engineering interview.