Ship Refrigeration System - Purpose, Construction & Working

Ship Refrigeration System – Purpose, Construction & Working

Last Updated on September 24, 2022 by Amit Abhishek

On ship, Refrigeration system is used to carry refrigerated cargo, food and provisions. During which the temperature of the space or its content is reduced considerably below that of the surrounding.

Thus preventing possible growth of harmful bacteria, decay of perishable food, fermentation or damage to cargo.

In small ships like large commercial fishing vessels, refrigeration plat is used for keeping the catch ( fish catch during voyage ) at a desired temperature, to prevent them from spoiling.

While on large reefer ship’s the refrigeration plant is used to reduce, control and maintain the temperature of perishable cargo such as food, liquefied gas and chemicals.

The same plant can also be used to provide / used for provision room on ships carrying food ( fish / meat / vegetables ) for the crew member.

Use of Refrigeration on Ship

  1. Domestic fridge rooms on ships for preserving foodstuffs for the crew.
  2. Air conditioning for accommodation for crew or passengers.
  3. Refrigeration of refrigerated liquid cargo such as LNG, LPG, Chemicals etc.
  4. In reefer controlled Air Dryers used in Engine Rooms.
  5. In dedicated refrigerated containers to carrying food / other perishable goods in small amount.
  6. For reefer ships which have large refrigerated cargo space to carry, food, meat, vegetables in bulk.
  7. To Maintain desired temperature inside the hold.
  8. To cool bulk CO2 used in major fire fighting systems.
  9. Chilling/freezing of the catch ( commercial fishing vessels ).

How a Refrigerator System Works

Principle of Refrigeration

Principle of Refrigeration

A cooling effect is produced when liquid evaporates absorbing heat of the body / medium to convert into gas molecules. This is why applying volatile liquid such as thinner or surgical spirit gives us a cold sensation.

Similarly when water leaked through the porous earthen pot surface evaporates heat is removed lowering the water temperature inside the earthen pot.

Similarly if liquid CO2 is made to vaporize using a coil with an small opening as shown in the above diagram.

The heat is absorbed or taken away from the surrounding ( in above diagram from the water bottle ) by the liquid CO2 to evaporate and escape to the atmosphere.

The CO2 cylinder will soon be empty unless refilled and the water inside the bottle will get chilled cold.

The another principle on which refrigeration process relies on is the Joule–Thomson process also known as throttling effect / process. It states that “a gas or liquid undergo change in temperature when it is forced through an expansion valve in an insulated system” with zero ambient heat transfer.

In simple terms under standard temperature and pressure all gases except hydrogen, neon and helium cool upon expansion following the Joule–Thomson process.

Ship Refrigeration System - Purpose, Construction & Working
Fig: Vapor Absorption Cycle

Vapor Absorption Cycle

In early years of refrigeration system ( early 20th century ), vapor absorption cycle using water ( absorbent ) ammonia ( refrigerant ) system was widely used.

Later due to low coefficient of performance ( i.e about 1/5 th of vapor compression cycle ) it was later replaced by the more efficient vapor compression cycle.

Today the systems is still in use where source of heat is available but electricity is not. Since unlike the vapor compression system that relies on mechanical energy, vapor absorption system rely / work on heat energy.

The system consist of 4 part i.e a generator, condenser, evaporator and absorber. Just like in the vapor compression system the refrigerant gets condensed in the condenser and it gets evaporated in the evaporator.

Thus producing a cooling effect in the evaporator ( as discussed earlier under principle of refrigeration ).

The absorber then dissolves the now gaseous refrigerant in a suitable liquid, passed on to generator with the help of a liquid pump. On heat addition the generator drives off the refrigerant vapor from the high-pressure liquid.

Thus the absorber and generator unit acting as a thermal compressor as a whole.

Vapor Compression Cycle

The vapor compression cycle consist of circulating liquid refrigerant via a closed system consisting of four stages i.e compression, condensation, expansion, evaporation.

The circulating liquid refrigerant alternately get compressed and expanded undergoing phase change.

The refrigerant thus absorbs and removes heat from from the spaces being cooled surrounding the evaporator and rejects the heat elsewhere ( i.e space surrounding condenser ).

In an ideal vapor-compression cycle, the system undergoes a series of four process; Isotropic compression, Isobaric heat rejection in condenser, throttling process in expansion valve, and Isobaric heat addition in evaporator.

Ship Refrigeration System - Purpose, Construction & Working

Compression

In the first stage of the refrigeration cycle, low pressure vapor refrigerant is compressed to a high pressure vapor before it leaves for the next stage.

For an an ideal vapor-compression cycle the gaseous medium ( refrigerant ) is compressed adiabatically to a relatively high pressure and temperature. Here the work required for the compressor is given by WC = H2 – H1.

Condensation

After leaving the compressor, the hot vapor refrigerant goes through condenser where it flow under pressure through a series of coils or S-shaped tubes.

In this stage of the Refrigeration cycle the hot vapor refrigerant cools down and condenses as heat transfer from the refrigerant to the cooler surroundings via coils / tubes.

To further improve the efficiency of heat exchange, cool air is blown across the tubes by a fan as the hot refrigerant flow through the S-shaped tube or coils.

The high pressure liquid refrigerant than lives the condenser for the next stage. Here the net heat rejected in the process can be given by Qre = H3 – H2.

Expansion

One of the main principle on which refrigeration works is the Joule-Thomson process ( throttling effect ); under which a gas or liquid undergo change in temperature when it is forced through an expansion valve.

In third stage of vapor refrigeration cycle the high-pressure liquid refrigerant pass through an expansion valve.

During the process of expansion, pressure drops and a small part of the liquid refrigerant evaporates lowering the overall temperature of the liquid refrigerant.

Evaporation

In the last stage the low pressure liquid vapor mixture of the refrigerant pass through the evaporator coils. It is the stage where heat is taken / extracted from the space which needs to be cool.

The evaporator coil absorb the heat lowering the temperature of the conditioned space. While the flowing refrigerant use that heat to evaporate and convert back to low-pressure vapor to enter the compressor.

Main Components of Ship Refrigeration Plant

Compressor

The role of a compressor in a refrigeration system is to suck in vapor refrigerant from the evaporator, compress it and feed it into the system.

For a a large air conditioning installation that relies on R11 or R12 as their primary refrigerant; a 2 or 3 stage centrifugal compressor is mostly used for such a system.

But for the bulk of the refrigeration requirement on ship a reciprocating compressor is used. These compressor can be of single or two stage with in line, W or V configuration.

These compressor operates similar to that of any reciprocating air compressor; except a rotating shaft is required to avoid leak of the refrigerant out of the system.

Condenser

The role of condenser in a refrigeration system is to cool down the hot vapor to condensed liquid refrigerant. On ship condenser are often water cooled and of shell and tube type.

Unlike in normal condenser where refrigerant flow through tube and air pass through them. In these shell and tube type condensers the refrigerant flow over the tube while the cooling water flow across the tubes.

A typical shell and tube exchanger can have tubes running from one side to another; or bend inside to follow a U-shaped path. They also includes a safety valve and vent.

Receiver

A receiver or reservoir is a space below the condenser or sometimes as a separate unit. It allows the refrigerant liquid to be drained or gauged for maintenance purposes.

Ship Refrigeration System - Purpose, Construction & Working
Image Credit: Marine Aux Machinery by H.D.McGeorge ( Amazon Link )

Filter Drier

Connected just after the receiver in a refrigeration system a filter drier remove any moisture from the refrigerant. It is basically a cartridge with strainer/drier pack filled with silica gel overlaid with cotton wool.

Using a drier avoid water moisture to freeze on the expansion valve thus causing starvation / loss of refrigerant flowing to the evaporator causing the compressor to cut out.

Siphon Glass

Also known as the sigh glass it is fitted just after the drier to visually inspect and ensure that the refrigerant flowing is only liquid refrigerant, and not some liquid/gas mixture.

Some design also have color indicator which react in presence of water / moisture in the system.

Solenoids

A solenoid valve is an electro-mechanical valve used to control the flow of refrigerant into the hold. While separate valves are provided for individual cargo rooms, a master solenoid valve is put on the main line.

Controlled by a thermostat they cut in and cut out at predestined / set temperature.

When the temperature in the room reaches the preset cut-in level, the thermostat open the valve allowing the refrigerant liquid to pass through the expansion valve.

Expansion valve

An expansion valve is responsible for the throttling effect, during which pressure drops and a small part of the liquid refrigerant evaporates lowering the overall temperature.

On board ship they are also used to regulate hold or room temperature. For simple refrigeration plant the length and bore of expansion valve is calculated and set at designed stage to give the required pressure drop.

On other hand most complex systems use thermostatic expansion valves.

Evaporator Unit

In direct refrigeration system, the low pressure liquid vapor mixture of the refrigerant pass through the air coolers / evaporator coils and air from the holds or chambers is circulated by fans.

The evaporator coil absorb the heat from the air lowering the temperature of the conditioned space.

In indirect systems where brine is used as secondary refrigerant, the evaporator unit act as a heat exchanger ( ideally shell and tube type ) cooling the secondary refrigerant.

Here the brine to be cooled is passed through the tubes while the refrigerant flowing over them ( outside the tube ). This involves a high liquid level in the shell, ( utilizing only the lower half for shell and half for vapor chamber ).

So some ships use dry evaporator type heat exchanger where the refrigerant passes through the tubes while brine is circulated over them / through the shell.

Control Unit

This includes various safety and operational units related to the refrigeration plant. Some of the major safety device includes, high and low cut out switch, bursting disc, relief valve, back pressure valve etc.

Conclusion

Be it a small ship like a large commercial fishing vessel or a large vessel like cargo ship, LNG carrier, chemical carriers and so on. They all use and require refrigeration of some short.

The role of ships refrigeration system is thus providing efficient, cheap and safe cooling facility be it for air conditioning, chilling, freezing, maintaining prescribed cargo temperature etc.

Be it a direct or indirect refrigeration system it is governed by the simple laws of refrigeration; vapor compression cycle to be precise for the systems used on ships and large industries.

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