Q. What Do You Mean By Refrigeration And States Its Purpose?
Ans: Refrigeration Is A Process In Which The Temperature Of A Space Or Its Contents Is Reduced To Below That Of Their Surroundings. It Is Required To Cool Perishable Food Stuff And Other Cargoes In Order To Control Its Quality And To Avoid Any Damage During Storage.
Purpose Of Refrigeration:
1. To Prevent Excessive Growth Of Micro Organism, Bacterial And Fungal
2. To Prevent Changes Due To Oxidation (gives Poor Appearance And Taste)
3. To Prevent Enzyme And Fermentive Process, Cause Rancidity
4. To Prevent Drying Out (dessication)
5. To Control Metabolism And Ripening Process Of Fruits And Vegetables
Q. What Are The Main Components Of Marine Refrigeration Plant Their Functions?
Ans: 1. Compressor: Marine Refrigeration Compressor Are Usually Reciprocating Or Rotary Screw Displacement Type. It May Be Single Or Multi Stage. Compressor In Refrigeration System Raises The Pressure Of The Vaporized Refrigerant, Causes Its Saturation Temperature To Raise, So That It Is Higher Than That Of The Sea Water Or Air Cooling The Condenser. Generally Condensing Temperature Should Be 5 Or 6 Degree C Above The Sea Water Inlet Temperature. It Also Acts As A Pump In The Refrigeration System And Promotes Circulation.
2. Condenser: Shell And Tube Type Condensers Are Widely Used In Marine Refrigeration System. Refrigerant Is Liquefied By Sub Cooling It Below Saturation Temperature By Circulating Sea Water Or Air (in Domestic Refrigerators). Latent Heat Is Transferred To Cooling Medium In Condensers. Marine Condensers Are Very Susceptible To Corrosion Hence Suitable Material Is Used To Lengthen Its Life. With Halogenated Hydrocarbon Refrigerant, Use Of Aluminum-Brass Or Cupro-Nickle Tube And Brass Tube Plate Is Acceptable. If Ammonia Is Used As A Refrigerant It Tends To Corrode These Materials, Hence Stainless Steel Or Bi-metallic Tubes And Clad Tube Plate With Ferrous Metal In The Refrigerant Side And Non Ferrous With Sea Water Side Suits Best.
3. Receiver: It Is The Reservoir And Used To Collect Refrigerant. Its Capacity Should Be Such That It Can Collect All The Refrigerant Of The System During Maintenance. A Gauge Glass Is Fitted To Check The Level Of Refrigerant.
4. Oil Separator: It Is Fitted In Hot Gas Discharge Linefrom Compressor Seprates Lubricating Oil From The Refrigerant. The Reduction In Velocity Of Vapour As It Enters In Larger Area Of The Seperator Allows The Oil Particles, Which Have Greater Momentum To Impinge On The Baffles. Oil Is Then Drained To Compressor Crank Case.
5. Expansion Valve: It Throttles The Liquid Refrigerant And Maintains Pressure Difference Between The Condenser And Evaporator. It Also Regulates Refrigerant Causes It To Flows From Higher Pressure Side Of The System To Low Pressure Side. The Pressure Drops Through Expansion Valve Causes Saturation Temperature To Fall, So That Refrigerant Boils At Very Low Temperature. Modern Expansion Valves Are Thermostatically Controlled Hence Called Thermostatic Expansion Valve.
6. Evaporator: It Is A Plane Or Finned Pipe Grid, Helps In Exchange Of Temperature. Refrigerant Entering In The Evaporator Coils At A Temperature Lower Than That Of Surroundings, Receives Latent Heat Evaporates, Causes Cooling Of Provisions.
7. Drier: Presence Of Even Small Amount Of Water Can Effect The Refregeration Plant Performance. Hence A Drier, Usually Silica Gel Or Activated Alumina Is Used As A Drying Agent To Remove Moisture From The Refrigerant.
Q. What Is A Refrigerant And Mention Its Desired Properties?
Ans: Refrigerant Used In A Refrigeration System Is Medium To Exchange The Temperatures.
Desired Properties Of A Refrigerant Used In Marine Refrigeration System
1. Low Specific Heat Capacity (Desirable As Throttling At The Expansion Valve Causes Liquid Refrigerant To Be Cooled At The Expense Of Partial Evaporation)
2. High Latent Heat (less Refrigerant Required To Perform A Given Duty)
3. Non Corrosive
4. Chemically Stable
5. Non Flammable And Non Explosive
6. Availability, Low Cost
7. Miscible With Oil ( Reduces The Loss Of Lubricant From Compressor And Prevent Accumulation In The System)
8. Environment Friendly
Q. State The Principle Of Refrigeration Plant And Its Working?
Ans: Refrigeration System Works On Vapor Compression Cycle. The Diagram Shows Simple Lay Out Of Refrigeration System.
The Pressure Of Refrigerant Gas Is Increased In The Compressor Due To Which Saturation Temperature Of Refrigerant Increases Which Is 5 To 6 Degrees Above The Sea Water Temperature. When This High Pressure Refrigerant Is Passed In To Condenser Gets Condenses. This Cooled Liquefied Refrigerant Is Then Collected In To The Receiver. When Required This Refrigerant Passes Through Expansion Valve Which Regulates The Flow And Allows It To Pass From Higher Pressure Side To A Lower Pressure Side. The Pressure Drop Through Expansion Valve Causes Saturation Temperature To Fall. Some Refrigerant Takes Latent Heat Of Evaporation And Boils Off Causing Drop In The Temperature Of Remainder Refrigerant. Refrigerant Entering The Evaporator At Temperature Lower That Of Surrounding. Thus It Boils Of By Taking Latent Heat Of Evaporator. The Design Of System And Evaporator Should Be Such That The Entire Liquid Refrigerant Is Boiled Off And Gas Slightly Super Heated Before It Returns To The Compressor At A Low Pressure To The Compressor.
Q. What Are The Safety Devices Fitted In The Refrigeration Plant?
Ans: 1. Compressor High Discharge Pressure Cut Out
2. Low Suction Pressure Cut Out
3. Low Lubricating Oil Pressure Cut Out
4. Relief Valve On The Compressure
5. Air Drier
6. Crank Case Lub Oil Heate
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