REFRIGERATION AND AIR CONDITIONING11.2.6 (2 hours)

ContentsProperties of refrigerantsDescription of Simple domestic refrigeration plantSimple cargo refrigeration system3.Working principle of air conditioning plant

Refrigeration systems & refrigerantsA refrigeration system is used to maintain the refrigerated area at a lower temperature than the surroundings.For this, a refrigerant gas is circulated through the system.The refrigerant gases used in ships systems are R-22, R-134 etc.These gases can be liquefied at moderate pressures by cooling with sea water. At the same time, these gases have low boiling temperatures at pressures slightly above atmospheric pressure.

Properties of refrigerantsHigh specific enthalpy of vapourisation (to reduce the quantity of refrigerant in circulation and lower machine speeds, sizes, etc.).Low specific volume in vapour state, (reduces size and increases efficiency).Non flammable and non explosive (pure or mixed).Easy leak detection. Non toxic (non poisonous and non irritating).Cheap, easily stored and obtained.

Ozone depletion and global warmingMost modern refrigerants are Chloro-Floro Carbons (CFCs) or Hdro-Chloro-Floro Carbons HCFC)These gases have a damaging effect on the ozone layer.These gases accumulate in the atmosphere and cause global warming.Some of these refrigerants are either banned or being replaced by other alternatives, because of their ill effect on the environment.

Refrigeration systemsA refrigeration system uses the following components:Compressor: To circulate the refrigerant in the system and maintain the necessary pressures in the suction and discharge sides.Condenser: To cool the high pressure refrigerant gas discharged from the compressor so as to condense it.Receiver: To collect the liquid refrigerant after it condenses in the condenser.

Refrigeration systems4.Expansion valve: To throttle the liquid in the receiver as it flows to the evaporator. Throttling causes a drop in pressure, which causes boiling of a part of the liquid. This cools down the remaining liquid.5.Evaporator : The cold liquid and gas mixture from the expansion valve flows through the evaporator coils. The evaporator is a heat exchanger. The heat from the room flows through the walls of the evaporator coils and evaporates all the liquid refrigerant . In turn, the room becomes cold.The gas is then sucked by the compressor to finish the cycle.

Simple refrigeration system

Use of refrigeration systemsRefrigeration systems are used on ships for:Preserving food for the crew: Domestic refrigeration system.Preserving ships cargo: Cargo refrigeration systemAir conditioning of ships accommodation

Domestic refrigeration systemThis system is used on ships to preserve food for the crew.Ship has cold rooms where food is stored.The system uses direct expansion, in which the refrigerant gas is made to expand in coils placed in the cold rooms.There may be three or more than three cold rooms in a ship. For Example: Meat room (-12 to -16 Degrees Celsius)Vegetable room (+5 to +8 Degrees Celsius)Dairy room (+3 to +5 Degrees Celsius)

Domestic refrigeration systemEach room has its own expansion valve.A thermostat controlled solenoid valve shuts when the room temperature reaches the lower limit value, and opens when the room temperature reaches the higher limit value.Back pressure valves are placed in the higher temperature room outlets to throttle the gas pressure to compressor suction pressure.

Domestic refrigeration systemFor automatic operation, an Low pressure (L.P) on/off pressure switch is fitted in the compressor suction. It switches off the compressor when the suction pressure reaches the lower limit value due to all solenoid valves closing off as the temperature of each room reaches the lower limit value. When the solenoid valve opens again, the suction pressure rises, and the L.P switch starts the compressor again.

Domestic refrigeration systemFor safety of the compressor, a High pressure (H.P) cut off switch is fitted in the compressor discharge. This switches off the compressor if a malfunction occurs in the condenser and the discharge pressure becomes high.A Lubricating oil low differential pressure (L.O. d.p) cut off switch is fitted in the discharge side of the lubrication pump of the compressor. This switches off the compressor if a malfunction occurs in the lubricating system of the compressor.

Cargo refrigeration systemIn refrigerated cargo ships, the total volume of the refrigerant system piping is large. Use is made of the secondary refrigerant.The secondary refrigerant is a brine solution of Calcium chloride and water.Secondary refrigerant is used as it is cheaper, and any leak in the system is easily detected and located.The brine is cooled by the refrigerant gas (primary refrigerant) in the brine tank. This cooled brine is circulated by the brine pump through the cargo holds to cool them.

Cargo refrigeration systemThe temperature of each hold is maintained by adjusting the return valve of the hold to control the amount of brine flowing through the room.The concentration of the brine solution is made in accordance with the cargo hold temperature. The lower the temperature, higher is the concentration.

Density of brine

Types of compressorsIn refrigeration systems, the compressor used can be of two types:Reciprocating compressorsRotary compressors

Reciprocating compressor

Rotary compressor

Functions of an A.C systemTo maintain the comfort zone conditions. The air conditioning plant should able to:cool air if required.heat air if required.humidify air if required.de-humidify air if required.

Functions of an A.C system2.To maintain air circulation and air freshness:Workers in confined space felt more uncomfortable than those with better air circulation outdoor.fresh air supply of 5 litres/s per person, per compartment of air, is required for an average persons respiratory requirement.maximum air velocity from duct should be less than 1 m/s (3 ft/s), to avoid direct draft.

Functions of an A.C systemTo maintain cleanliness of the air this is maintained by filters which remove dust particles, allergens etc.

Comfort zone for air conditioning

It is the region of temperature and humidity of air, where most people feel comfortable.

A study has to be carried out by getting feedback for comfort from a very large sample of people.

Feedback of whether the air is comfortable or not, is provided based on various air temperature and humidity.

Feedback is used and plotted against psychrometric chart, as well as air comfort chart.

Comfort zone for air conditioningResults from the feedback reveal that the range of comfort zone is within the boundary of;

30% to 70% of relative humidity29oC and 18oC of dry bulb temperature

A tighter range for comfort, where almost 100% of people feel comfortable, is within;

Summer condition 30% to 70% of relative humidity 23oC and 25oC of dry bulb temperature

Winter condition 30% to 70% of relative humidity 19oC and 22oC of dry bulb temperature

Air-conditioning systemThe plant has an Air Handling Unit (AHU), which is equipped with:Fan to circulate air through the ships accommodationAir cooler: The evaporator of the refrigeration system is installed in the AHU to cool and dehumidify the air.Heater: Steam coils are installed to heat the air.Humidifier: Steam injection nozzles are installed to humidify the air.Filters to remove dirt etc.Air ducts to convey conditioned air to the compartments and to bring back this air back to the AHU for recirculation

Air-conditioning systemIn summers the refrigerating system is started and the air in the AHU is cooled and de-humidified by the refrigerant gas in the evaporator.In winters the refrigerating system is shut off and steam heating is started to heat the air in the AHU. The steam injection is also started in the humidifier to maintain air relative humidity.

Air-conditioning systemTo maintain air freshness, the louvers for fresh air and re-circulating air are adjusted so that the fresh air can enter the systemFilters have to be cleaned periodically to keep the air free from dust.

Air handling unit-Single duct system

Double duct systemThe single-duct system is widely used on cargo ships.The twin (double) -duct system provides increased flexibility and is mainly used on passenger ships, where due to the widely spaced cabins the climatic conditions can be varied in different parts of the ship.A central unit is used with cooled dehumidified air provided through one duct. The other duct is supplied with cooled air that has been reheated. Each treated space is provided with a supply from each duct which may be mixed as required at the outlet terminal. As the mixture control louver in the cabins cannot be shut for both the cooled as well as the reheated air, the fresh air supply to the cabins is never turned off. In cold climates the pre-heater will warm both supplies of air, resulting in a warm and a hot supply to each space.

Double duct system

Introduction. Use of special gases in systems.*Additional desirable properties of refrigerants*Effect of refrigerants on the environment*Components in a ref system*Components in a ref system..(continued)*Explain the layout of the components of the system*Uses of ref system on ships*Domestic ref system using direct expansion*Domestic ref system using direct expansion

*Domestic ref system using direct expansion

*Domestic ref system using direct expansion

*Domestic refrigeration system using direct expansion*Cargo refrigeration system using secondary refrigerant*Cargo refrigeration system using secondary refrigerant*Cargo refrigeration system using secondary refrigerant.*Functions of an air conditioning systems*Functions of an air conditioning systems

*Functions of an air conditioning systems

*Explain comfort zone*Explain comfort zone

*Psychrometric chart*Air-conditioning system*