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Refrigeration system and cycles:

 

 

Artcle by Dr. L.C.Singal, Prof. (ME Deptt), CEC Landran, (Mohali), Punjab (India), Formerly Prof in Chemical Engg. Deptt., Panjab University, Chandigarh

Refrigeration is a process of producing low temperatures as compared to the surrounding temperatures.

It will be possible only if heat is transferred from the low temperature region to a high temperature region. Obviously it is not possible in the natural manner because heat flows from high temperature to low temperature like fluid flows from high pressure to low pressure/ current flows from high voltage to low voltage/ gas flows from high concentration to the region of low concentration. It means in refrigeration one is trying to go against the natural process as well as against the second law of thermodynamics which states that heat cannot flow from low temperature region to a high temperature region without the use of an external agent. The external agent in refrigeration is the compressor which introduces the most common method of refrigeration.

                 The most commonly used closed vapor compression refrigeration system consists of six main parts namely compressor, condenser, expansion device, evaporator, piping and circulating working substance called the refrigerant.  The other refrigeration systems are

          vapor absorption refrigeration,

          gas refrigeration,  

          Steam jet water vapor refrigeration,

          Vortex tube refrigeration,

          Thermoelectric refrigeration,

          Magnetic cooling.

          Liquefaction of natural gases.

Fig.1 is the schematic diagram of a vapor compression refrigeration cycle, Fig.2 is the schematic diagram of a vapor absorption water chilling unit Fig.3 is the schematic diagram of a gas refrigeration cycle.

 

Fig.1 Simple Vapor Compression Refrigeration System

  

 

 

 

 

Fig.2 Vapor Absorption Water Chilling Unit

 

Fig.3 Gas Refrigeration Cycle

 

VAPOUR COMPRESSION REFRIGERATION CYCLE

There are six main components in a refrigeration system:

.       The Compressor

          The Condenser

          The Metering Device or expansion valve

          The Evaporator

          Piping material

          Refrigerant

           

COMPRESSOR: It is heart of the refrigeration system as it circulates the refrigerant in the system like the heart of a human being circulating the blood in the body.

          Two different pressures exist in the refrigeration cycle.  The evaporator or low pressure, and the condenser, or high pressure.  These pressure areas are divided by the other two components.  On one end, is the metering device which controls the refrigerant flow, and on the other end, is the compressor?

The compressor is the heart of the system.  The compressor does just what its name is.  It compresses the low pressure refrigerant vapor from the evaporator and compresses it into a high pressure vapor.

          The inlet to the compressor is called the “Suction Line”.  It brings the low pressure vapor into the compressor.

          After the compressor compresses the refrigerant into a high pressure Vapor, and the outlet of the compressor is called the “Discharge Line”.

 There are three types of compressors namely reciprocating, rotary and centrifugal. The type of compressor depends on the pressure difference between the high pressure side (condenser) and low pressure side (evaporator) of the refrigeration system. This further depends on the refrigerant selected for the application under consideration.

 

THE CONDENSER

          The “Discharge Line” leaves the compressor and runs to the inlet of the condenser.

          Because the refrigerant was compressed, it is a hot high pressure vapor.

          The hot vapor enters the condenser and starts to flow through the tubes.

          Cool air is blown across the out side of the finned tubes of the condenser (usually air by a fan or water with a pump).

          Since the air is cooler than the refrigerant, heat jumps from the tubing to the cooler air (energy goes from hot to cold – “latent heat”).

          As the heat is removed from the refrigerant, it reaches its “saturated temperature” and starts to change state, into a high pressure liquid.

          The high pressure liquid leaves the condenser through the “liquid line” and travels to the “metering device” through a filter dryer to remove any dirt or foreign particles.

 The condenser can be free air cooled (domestic refrigerator), forced air cooled (window air conditioner), water cooled (Central air conditioning plant in a library, cinema house and evaporative cooled (ice plant unit or a cold storage unit).

EXPANSION DEVICE

          Metering devices regulate how much liquid refrigerant enters the evaporator as per heat load on evaporator.

          Common used metering devices are, small thin copper tubes referred to as “capillary tubes”, thermally controller diaphragm valves” (thermostatic expansion valves, called “TXV’s.  This valve has the capability of controlling the refrigerant flow. If the load on the evaporator changes, the valve can respond to the change and increase or decrease the flow accordingly. The TXV has a sensing bulb attached to the outlet of the evaporator. This bulb senses the suction line temperature and sends a signal to the TXV allowing it to adjust the flow rate. This is important because, if not all, the refrigerant in the evaporator changes state into a gas, there could be liquid refrigerant content returning to the compressor. This can be fatal to the compressor. Liquid can not be compressed and when a compressor tries to compress a liquid, mechanical failing can happen. The compressor can suffer mechanical damage in the valves and bearings. This is called” liquid slugging”.  Normally TXV's are set to maintain 10 degrees of superheat. That means that the gas returning to the compressor is at least 10 degrees away from the risk of having any liquid. The metering device tries to maintain a preset degree of superheat at the outlet openings of the evaporator. As the metering devices regulates the amount of refrigerant going into the evaporator, the device lets small amounts of refrigerant out into the line and looses the high pressure to low pressure.

          Now we have a low pressure, cooler liquid refrigerant entering the evaporative coil.

 These are of five type namely capillary tube (domestic fridge), Automatic expansion valve (ice plant unit), Thermostatic expansion valve (Library refrigeration plant, theatre air conditioning unit and many more), Low side float valve (industrial cooling units) and high pressure float valve (industrial cooling units). These causes the required pressure drop between the high and low pressure sides and also control the flow of refrigerant as per cooling requirements.

THE EVAPORATOR:

The evaporator is where the heat is removed from your house, business or     products to be cooled.

          Low pressure liquid leaves the metering device and enters the evaporator.

          Usually, a fan will move warm air from the conditioned space across the evaporator finned coils.

          The cooler refrigerant in the evaporator tubes, absorb the warm room air. The change of temperature causes the refrigerant to “flash” or “boil”, and changes from a low pressure liquid to a low pressure cold vapor.

          The low pressure vapor is pulled into the compressor and the cycle starts over.

          Evaporators are two types i.e. flooded evaporators necessitating the use of accumulators to permit only vapors to the compressor and dry expansion type evaporators. Flooded types are used in industrial units whereas dry expansion types are used in domestic and commercial refrigeration units.

 

PIPING MATERIALS: Pipe material should have high thermal conductivity, low cost, easy working and inertness with the refrigerant. Till date most commonly used pipe material is soft copper with all refrigerants except ammonia. The pipe material used with ammonia is mild steel as ammonia is highly corrosive to copper.

REFRIGERANT: It is working substance in a refrigeration unit like blood in the human body. Its selection depends on many considerations like temperature to be produced, latent heat, ozone depletion potential, global warming potential, toxicity, inflammability, inertness, corrosion, erosion, action with water and lubricating oil, cost, availability, leak detection and power requirements for a certain amount of cooling needed. Various commonly used refrigerants are halogenated saturated hydrocarbons like R-134, R-22 and inorganic compounds like ammonia and air. Most common previously used refrigerants like R-12 and R-11 has been banned because of their high ozone depletion and global warming potentials. Mixed refrigerants and zoetrope’s are also in use. Refrigerants can be primary, secondary and tertiary type depending where and how these being used are. The same substance, for example, air can be primary in aircraft refrigeration; can be secondary as in a window air conditioner and tertiary in a central air conditioning plant.

 

VAPOUR ABSORPTION REFRIGERATION

Fig.2 Vapor Absorption Water Chilling Unit

VAPOUR ABSORPTION SYSTEM

The vapor absorption refrigeration system consists of:

          Evaporator

          Absorber

          High pressure generator

          Condenser

.

Evaporator

          The refrigerant (water) evaporates at around 4oC under a high vacuum condition of 754 mm Hg in the evaporator.

          Chilled water goes through heat exchanger tubes in the evaporator and transfers heat to the evaporated refrigerant.

          The evaporated refrigerant (vapor) turns into liquid again, while the latent heat from this vaporization process cools the chilled water (in the diagram from 12 oC to 7 oC). The chilled water is then used for cooling purposes.

 

Absorber

          In order to keep evaporating, the refrigerant vapor must be discharged from the evaporator and refrigerant (water) must be supplied. The refrigerant vapor is absorbed into lithium bromide solution, which is convenient to absorb the refrigerant vapor in the absorber. The heat generated in the absorption process is continuously removed from the system by cooling water. The absorption also maintains the vacuum inside the evaporator.

 

High Pressure Generator

          As lithium bromide solution is diluted, the ability to absorb the refrigerant vapor reduces. In order to keep the absorption process going, the diluted lithium bromide solution must be concentrated again. An absorption chiller is provided with a solution concentrating system, called a generator. Heating media such as steam, hot water, gas or oil perform the function of concentrating solutions. The concentrated solution is returned to the absorber to absorb refrigerant vapor again.

 

Condenser

          To complete the refrigeration cycle, and thereby ensuring the refrigeration takes place continuously, the following two functions are required

          To concentrate and liquefy the evaporated refrigerant vapor, which is generated in the high-pressure generator.

          To supply the condensed water to the evaporator as refrigerant (water)

          For these two functions a condenser is installed.

 

Six distinctions between the vapor compression and vapor absorption refrigeration system are.

 

1.       For lower tonnage capacity vapor absorption system is more expansive.

2.       For 50 TR cost becomes the same for the two systems. For TR > 50 tons the vapor absorption refrigeration system becomes more favorable.

3.       Vapor absorption systems are environmentally friendly as there is no GWP and no ODP.

4.       A single unit of 7000 tons capacity is available in VAS where as much less capacity say, 1000 TR single units in VCS.

5.       In VAS, liquid coming from the evaporator has no bad effect whereas in VCS the liquid going to compressor will result in physical breakdown of the compressor.

6.       In the event of lower temperature requirement in VCS the cooling capacity decreases very significantly whereas in VAS the cooling capacity can still be same only by controlling the heating rate in the generator.

 

 

 

 

 

 

 

 

 

Fig.3 Gas Refrigeration Cycle

Air or gas refrigeration system

The components of the air refrigeration system are shown in Fig.. In this system, air is taken into the compressor at point 1 from atmosphere and compressed to condition 2. The hot compressed air is cooled in heat exchanger up to the atmospheric temperature condition3 (in ideal conditions). The cooled air is then expanded in an expander to point 4. The temperature of the air coming out from the expander is below the atmospheric temperature due to isentropic expansion. The low temperature air coming out from the expander enters into the cabin and absorbs the heat. The cycle is repeated again. The working of air refrigeration cycle is represented on T-s diagrams in Fig.

 

COMPARISON OF VAPOUR COMPRESSION AND GAS REFRIGERATION CYCLE:

The comparison of vapor compression and gas compression refrigerating cycles based on following parameters:

1) Type of refrigerant used: In vapor compression cycle liquids like Freon and ammonia are used as the refrigerant. In the gas cycle the gas like air is used as the refrigerant.

2) Heat exchangers: In the vapor compression refrigeration cycle condenser and evaporator are the two heat exchangers where the refrigerant gives up and absorbs latent heat respectively. The refrigerant undergoes change in phase in both the heat exchangers. In the gas cycle the refrigerant exchanges heat in the heat exchangers, but there is no phase change of the gas there is only sensible heat transfer

3) Efficiency of the cycle: The efficiency of the vapor compression cycle is more than that of the gas cycle. For producing the same amount of refrigerating effect in the gas cycle, large volume of gas is required; hence the systems tend to become very large, bulky and expensive, which are not affordable for the domestic applications.

4) Cycle used: The vapor compression cycle works on the reverse Bray ton cycle while the gas compression cycle works on reverse Rankine cycle.

5). NET WORK: Gas refrigeration is used in all types of aircrafts. There is already a compressor and a turbine in each aircraft. Therefore weight of equipment needed for air conditioning is quite small i.e. around 1.3 times weight is required in vapor compression systems. Work of expansion is recovered in gas refrigeration. Therefore net work is employed in finding COP.

6. COP of VCR cycle is much more than that of air (gas) refrigeration cycle.

7. SAFE: Gas refrigeration is safe as compared to that of vapor compression cycle.

 

 

 

 

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