The Refrigeration System on Commercial Ships: Principles and Modern Practices

Refrigeration systems on commercial ships play an essential role in the global trade of perishable goods, providing critical temperature control for food, pharmaceuticals, and other sensitive products during transit. This article explores the design, operation, and environmental considerations of marine refrigeration systems, drawing on a variety of technical resources.

 

Design and Components

The core of a ship’s refrigeration system is the vapour compression cycle, which includes four main stages: compression, condensation, expansion, and evaporation. This cycle ensures the continuous circulation of the refrigerant, absorbing heat from the cargo or air inside the refrigeration unit and releasing it to the environment or a water-cooling system outside the unit.

  • Compression: Refrigerant gas is compressed to a high temperature and pressure, significantly reducing its volume.
  • Condensation: The compressed gas then cools as it passes through condenser coils, transforming from a vapor to a liquid while releasing its heat.
  • Expansion: The high-pressure liquid refrigerant goes through an expansion valve, where its pressure abruptly drops, cooling it further.
  • Evaporation: Finally, the cold, low-pressure liquid absorbs heat as it evaporates back into a gas in the evaporator coils, cooling the surrounding area.

Modern refrigeration systems on ships commonly employ advanced compressors, such as screw or centrifugal types, which are suited for large-scale applications and can handle various refrigerants including R-134a, R-404A, and R-407C, chosen for their lower environmental impacts.

 

There are some other components in the refrigeration systems, however, not considered as the main element of the system. As a good exercise, it would be nice if you answer the following questions about the various components and considerations within refrigeration systems:

What is the main role of the L.P. controller?

The Low-Pressure (L.P.) controller in a refrigeration system primarily functions to maintain adequate suction pressure at the evaporator. It helps to prevent the system from operating at too low a pressure, which can lead to freezing of the evaporator coil and insufficient cooling. It regulates the compressor’s operation to ensure that the evaporator pressure does not fall below a set threshold, thereby protecting the compressor and maintaining efficient system performance.

What is the function of the H.P. cut-out?

The High-Pressure (H.P.) cut-out is a safety device designed to protect refrigeration components from excessive pressure. If the pressure in the system exceeds safe limits, the H.P. cut-out will shut off the compressor to prevent damage or failure of system components such as piping, valves, and the compressor itself. It is critical for preventing hazardous conditions and ensuring the longevity of the system.

What is the function of the drier?

The drier in a refrigeration system removes moisture and contaminants from the refrigerant. Moisture in the system can lead to ice formation and corrosion, which can obstruct the flow and damage components. Driers often contain desiccants like silica gel or molecular sieves that absorb moisture from the circulating refrigerant. Regular replacement or maintenance of the drier is necessary to maintain the health and efficiency of the system.

 

Operational Challenges and Maintenance

Maintaining a marine refrigeration system requires regular checks and servicing to ensure efficient operation and prevent failures that could lead to cargo spoilage. Common issues include compressor problems, refrigerant leaks, and condenser malfunctions. Troubleshooting these systems necessitates a deep understanding of their mechanical and electrical components​System operation challenges may include:

System undercharged: If a refrigeration system is undercharged, it will have insufficient refrigerant to absorb and carry heat effectively. This leads to higher temperatures and pressures in the evaporator, reduced cooling capacity, and the potential freezing of the evaporator coil. Undercharging can also cause the compressor to overheat due to a lack of sufficient refrigerant for cooling.

System overcharged: Overcharging a system with too much refrigerant can lead to excess high pressures and temperatures, inefficient cooling, and increased energy consumption. The compressor may also be subjected to greater mechanical stress and wear. In severe cases, liquid refrigerant may return to the compressor (floodback), causing damage or failure.

Understanding these components and their functions can help in maintaining the efficiency and longevity of refrigeration systems while minimizing environmental impact.

 

Environmental Considerations

Currently, Hydrofluorocarbons (HFCs) are the most commonly used category of refrigerant gases in new equipment. Chlorofluorocarbons (CFCs) and Hydrochlorofluorocarbons (HCFCs) have been mostly phased out due to their ozone-depleting properties. The Montreal Protocol and its amendments have significantly restricted the use of CFCs and HCFCs. Some regions may still use HCFCs, but they are on a schedule for phase-out in favor of less harmful alternatives, including HFCs and newer options like Hydrofluoroolefins (HFOs) which have lower global warming potentials.

With increasing focus on sustainability, the maritime industry is under significant pressure to reduce the environmental impact of its refrigeration practices. This includes improving energy efficiency and minimizing refrigerant leaks, which can contribute to ozone depletion and global warming. Newer systems are designed to be more energy-efficient and are equipped with sophisticated leak detection technologies to promptly address any issues. The selection of refrigerants with low global warming potential (GWP) and ozone depletion potential (ODP) is also a critical component of modern marine refrigeration systems.

 

The refrigeration systems on commercial ships are complex but vital for the preservation and safe transport of temperature-sensitive commodities across the globe. As technology advances, these systems are becoming more efficient and environmentally friendly, but they require careful management and regular maintenance to perform optimally. The maritime industry continues to innovate in this area, seeking solutions that balance operational efficiency with environmental responsibility.

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